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Article

:THE COSMOLOGICAL OTOC: Formulating New Cosmological Micro-Canonical Correlation Functions for Random Chaotic Fluctuations in Out-Of-Equilibrium Quantum Statistical Field Theory

by
Sayantan Choudhury
Quantum Gravity and Unied Theory and Theoretical Cosmology Group, Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Mühlenberg 1, 14476 Potsdam-Golm, Germany
Symmetry 2020, 12(9), 1527; https://doi.org/10.3390/sym12091527
Submission received: 7 July 2020 / Revised: 28 August 2020 / Accepted: 1 September 2020 / Published: 16 September 2020
(This article belongs to the Special Issue New Advances of Cosmology and Astrophysics)

Abstract

:
The out-of-time-ordered correlation (OTOC) function is an important new probe in quantum field theory which is treated as a significant measure of random quantum correlations. In this paper, using for the first time the slogan “Cosmology meets Condensed Matter Physics”, we demonstrate a formalism to compute the Cosmological OTOC during the stochastic particle production during inflation and reheating following the canonical quantization technique. In this computation, two dynamical time scales are involved—out of them, at one time scale, the cosmological perturbation variable, and for the other, the canonically conjugate momentum, is defined, which is the strict requirement to define the time scale-separated quantum operators for OTOC and is perfectly consistent with the general definition of OTOC. Most importantly, using the present formalism, not only one can study the quantum correlation during stochastic inflation and reheating, but can also study quantum correlation for any random events in Cosmology. Next, using the late time exponential decay of cosmological OTOC with respect to the dynamical time scale of our universe which is associated with the canonically conjugate momentum operator in this formalism, we study the phenomenon of quantum chaos by computing the expression for the Lyapunov spectrum. Furthermore, using the well known Maldacena Shenker Stanford (MSS) bound on the Lyapunov exponent, λ 2 π / β , we propose a lower bound on the equilibrium temperature, T = 1 / β , at the very late time scale of the universe. On the other hand, with respect to the other time scale with which the perturbation variable is associated, we find decreasing, but not exponentially decaying, behaviour, which quantifies the random quantum correlation function out-of-equilibrium. We have also studied the classical limit of the OTOC and checked the consistency with the large time limiting behaviour of the correlation. Finally, we prove that the normalized version of OTOC is completely independent of the choice of the preferred definition of the cosmological perturbation variable.

Contents
1Introduction4
2Formulation of OTOC in Cosmology18
2.1General Remarks on OTOC………………………………………………………………………………………………18
2.2Eigenstate Representation of OTOC in Quantum Statistical Mechanics……………………………………………20
2.3Constructing OTOC in Cosmology………………………………………………………………………………………23
2.3.1For Massless Scalar Field………………………………………………………………………………………29
2.3.2For Partially Massless Scalar Field………………………………………………………………………………33
2.3.3For Massive Scalar Field………………………………………………………………………………………34
3Quantum Micro-Canonical OTO Amplitudes and OTOC in Cosmology36
3.1Computational Strategy………………………………………………………………………………………………37
3.2Classical Mode Functions in Cosmology………………………………………………………………………………38
3.3Quantum Mode Function in Cosmology………………………………………………………………………………41
3.4Canonical Quantization of Cosmological Hamiltonian: Classical to Quantum Map………43
3.5Cosmological Two-Point and Four-Point “In-In" OTO Micro-Canonical Amplitudes………44
3.5.1OTOC Meets Cosmology………………………………………………………………………………………44
3.5.2Fourier Space Representation of the Commutator Bracket: Application to Two-Point OTOC………46
3.5.3Fourier Space Representation of Square of the Commutator Bracket Application to Four-PointOTOC………………………………………………………………………………………………48
3.6Cosmological Thermal Partition Function: Quantum Version………………………………………………………52
3.6.1Quantum Vacuum State in Cosmology………………………………………………………………………52
3.6.2Quantum Partition Function in Terms of Rescaled Field Variable………………………………………52
3.6.3Quantum Partition Function in Terms of Curvature Perturbation Field Variable………………………………………53
3.7Trace of Two-Point “In-In" OTO Amplitude for Cosmology………………………………………………………………………54
3.8OTOC from Regularised Two-Point “In-In" OTO Micro-Canonical Amplitude: Rescaled Field Version………………………………………………………………………………………55
3.9OTOC from Regularised Four-Point “In-In" OTO Micro-Canonical Amplitude: Curvature Perturbation Field Version………………………………………………………………………………………………………………56
3.10Trace of Four-Point “In-In" OTO Amplitude for Cosmology………………………………………………………………58
3.11OTOC from Regularised Four-Point “In-In" OTO Micro-Canonical Amplitude: Rescaled Field Version………………………………………………………………60
3.11.1Without Normalization………………………………………………………………60
3.11.2With Normalization………………………………………………………………64
3.12OTOC from Regularised Four-Point “In-In" OTO Micro-Canonical Amplitude: Curvature Perturbation Field Version………………64
3.12.1Without Normalization……………………………………………………………………………64
3.12.2With Normalization……………………………………………………………………………64
4Numerical ResultsI: Interpretation of Two-Point Micro-Canonical OTOC in Cosmology65
5Numerical ResultsII: Interpretation of Four-Point Micro-Canonical OTOC in Cosmology73
6Lyapunov Spectrum and Quantum Chaos in Cosmology86
7Numerical ResultsIII: Interpretation of Cosmological Lyapunov Spectrum89
8Classical Limit of Micro-Canonical OTO Amplitudes and Related OTOC in Cosmology93
8.1Computational Strategy………………………………………………………………………………93
8.2Classical Limit of Cosmological Two-Point “In-In" OTO Micro-Canonical Amplitudes………………94
8.3Classical Limit of Cosmological Four-Point “In-In" OTO Micro-Canonical Amplitudes………………95
8.4Cosmological Micro-Canonical Partition Function: Classical Version………………………………………………98
8.4.1Classical Micro-Canonical Partition Function in Terms of Rescaled Field Variable98
8.4.2Classical Micro-Canonical Partition Function in Terms of Curvature Perturbation Field Variable……………………………………99
8.5Classical Limit of Cosmological Two-Point Micro-Canonical OTOC: Rescaled Field Version99
8.6Classical Limit of Cosmological Two-Point Micro-Canonical OTOC: Curvature Perturbation Field Version…………………………………101
8.7Classical Limit of Cosmological Four-Point Micro-Canonical OTOC: Rescaled Field Version………………………………………101
8.7.1Without Normalization………………………………………………………………………………101
8.7.2With Normalization………………………………………………………………………………103
8.8Classical Limit of Cosmological Four-Point Micro-Canonical OTOC: Curvature Perturbation Field Version………………………………………104
8.8.1Without Normalization………………………………………………………………………104
8.8.2With Normalization………………………………………………………………………………104
9Summary and Outlook105
AAsymptotic Behaviour of the Cosmological Mode Functions108
BWKB Solution of the Cosmological Mode Functions for Time Dependent Protocols115
CQuantum Two-Point OTO Micro-Canonical Amplitudes for Cosmology116
C.1Definition of Micro-Canonical OTO Amplitude Δ ^ 1 ( K 1 , K 2 ; τ 1 , τ 2 ) ………………………………116
C.2Definition of Micro-Canonical OTO Amplitude Δ ^ 2 ( K 1 , K 2 ; τ 1 , τ 2 ) ………………………………117
DQuantum Four-Point OTO Micro-Canonical Amplitudes for Cosmology117
D.1Definition of Micro-Canonical OTO Amplitude T ^ 1 ( K 1 , K 2 , K 3 , K 4 ; τ 1 , τ 2 ) ………………………117
D.2Definition of Micro-Canonical OTO Amplitude T ^ 2 ( K 1 , K 2 , K 3 , K 4 ; τ 1 , τ 2 ) ………………………118
D.3Definition of Micro-Canonical OTO Amplitude T ^ 3 ( K 1 , K 2 , K 3 , K 4 ; τ 1 , τ 2 ) ………………………119
D.4Definition of Micro-Canonical OTO Amplitude T ^ 4 ( K 1 , K 2 , K 3 , K 4 ; τ 1 , τ 2 ) ………………………120
EComputation of Classical Limit of Four-Point “In-In” OTO Micro-Canonical Amplitudes for Cosmology121
FComputation of Quantum Micro-Canonical Partition Function in Cosmology124
F.1Quantum Micro-Canonical Partition Function in Terms of Rescaled Field Variable………124
F.2Quantum Micro-Canonical Partition Function in Terms of Curvature Perturbation Field Variable………………………………………125
GComputation of Classical Micro-Canonical Partition Function in Cosmology126
G.1Classical Micro-Canonical Partition Function in Terms of Rescaled Field Variable………126
G.2Classical Micro-Canonical Partition Function in Terms of Curvature Perturbation Field Variable…………………………………………127
HComputation of the Trace of the Two-Point Amplitude in Micro-Canonical OTOC128
IComputation of the Trace of the Four-Point Amplitude in Micro-Canonical OTOC130
JTime Dependent Two-Point Amplitude in Micro-Canonical OTOC135
KTime Dependent Four-Point Amplitudes in Micro-Canonical OTOC137
K.1Computation of I 1 ( τ 1 , τ 2 ) ………………………………………………………………………………………………137
K.2Computation of I 2 ( τ 1 , τ 2 ) ………………………………………………………………………………………………151
K.3Computation of I 3 ( τ 1 , τ 2 ) ………………………………………………………………………………………………151
K.4Computation of I 4 ( τ 1 , τ 2 ) ………………………………………………………………………………………………151
K.5Computation of I 5 ( τ 1 , τ 2 ) ………………………………………………………………………………………………152
K.6Computation of I 6 ( τ 1 , τ 2 ) ………………………………………………………………………………………………152
K.7Computation of I 7 ( τ 1 , τ 2 ) ………………………………………………………………………………………………152
K.8Computation of I 8 ( τ 1 , τ 2 ) ………………………………………………………………………………………………153
K.9Computation of I 9 ( τ 1 , τ 2 ) ………………………………………………………………………………………………153
LComputation of the Normalization Factor in Four-Point Micro-Canonical OTOCC153
L.1Normalization Factor of Four-Point Micro-Canonical OTOC Computed from Rescaled Field Variable………………………………………………………………………………………………………153
L.2Normalization Factor of Four-Point Micro-Canonical OTOC Computed from Curvature Perturbation Field Variable………………………………………………………………………………………156
MComputation of the Normalization Factor in Classical Limit of Four-Point Micro-Canonical OTOC157
M.1Normalization Factor of the Classical Version of Four-Point Micro-Canonical OTOC Computed from Rescaled Field Variable…………157
M.2Normalization Factor of the Classical Version of Four-Point Micro-Canonical OTOC Computed from Curvature Perturbation Field Variable……………158
NThermal Trace Operation in Terms ofWave Function of the Universe in Cosmology159
References160

1. Introduction

The out-of-time ordered correlation (OTOC) [1,2,3,4,5,6,7,8,9] functions in the context of quantum field theory at a finite temperature is considered as a very strong probe of any kind of stochasticity, randomness and quantum mechanical chaos in the present day research of theoretical physics. Earlier it was only studied in the various condensed matter systems where out-of-equilibrium phenomena play significant role. The concept of OTOC was first introduced in the computation of superconductivity to describe the vertex correction of current [10]. However, for the past few years theoretical physicists have been applying this idea to explore various unknown out-of-equilibrium features of various quantum field theories at finite temperatures and in the context of bulk gravitational theories. From the very common understanding, one can physically interpret OTOC as a quantum mechanical analogue of the classical version of sensitiveness against tiny random fluctuations in the initial conditions, and, particularly if we get exponential growth in the time dynamics of the OTOC, then it is treated as a very strong probe of quantum mechanical chaos (in terms of Schrödinger time evolution it is very difficult to describe such quantum phenomena in quantum mechanical systems). In terms of traditional physics, one can think of this concept as the theoretical indicator of energy gaps, and in that connection it is very interesting to investigate that whether or not the OTOC can be treated as a better probe of stochastic randomness and quantum mechanical chaos at out-of-equilibrium.
To describe this in a more technical way, let us consider two quantum operators X ( t ) and Y ( 0 ) , which are separated by time, and using them, OTOC is defined by the following expression [1]:
How to define OTOC ? ̲ C ( t ) : X ( t ) , Y ( 0 ) 2 β = 1 Z Tr exp β H X ( t ) , Y ( 0 ) 2 ,
where the thermal partition function is defined as:
Partion function : ̲ Z = Tr exp β H where β = 1 T with k B = 1 .
Specifically, in this context, the commutator of the two time-separated operators in quantum mechanics describe the effect of perturbation by the operator Y on the measurement of the operator X on later time scales, and the converse statement is also true here. In this construction, we also assume that these two operators have zero one point functions. Now, if we fix these operators as two canonically conjugate operators, i.e., in terms of position operator X ( t ) = q ( t ) and momentum operator Y ( 0 ) = p ( 0 ) for a quantum system, then in the semi-classical limit, one can replace the commutator bracket of these two operators, q ( t ) , p ( 0 ) , will be replaced by the Poisson bracket, i q ( t ) , p ( 0 ) PB = i ( q ( t ) / q ( 0 ) ) (in the natural unit system, where we take, = 1 ). For classical system to have chaotic description, ( q ( t ) / q ( 0 ) ) exp ( λ t ) , where λ is the Lyapunov exponent, which appears as an outcome of the sensitiveness of the initial conditions. Consequently, if we compute the expression of OTOC out of these two canonically conjugate quantum operators, then it will scale with respect to time scale, as C ( t ) exp ( 2 λ t ) , to achieve a quantum chaotic description from this set up and here λ is treated as quantum Lyapunov exponent. It is expected from this discussion that if we are able to quantize the classical chaotic system properly then it can provide a positive numerical value of the quantum Lyapunov exponent within the framework of OTOC. Following this discussion, one can further distinguish between the classical and quantum chaotic systems, which come from the computation of OTOC, and the time evolution of this shows that in quantum mechanics OTOC is a quantity which does not grow with the evolutionary time scale of the quantum theory but at the late time scales saturates to a constant value and in this literature this time scale is identified as the Ehrenfest time scale. One can also describe this characteristic time scale as a critical scale after which the quantum mechanical wave function of the theory spreads over the whole system under consideration for this description (in a very rough sense, very crudely also this characteristic time scale is identified as the transition time scale which describes the phase transition from a particle description to a wave description of the quantum mechanical wave function).
As we have already mentioned in the present day research the concept of OTOC getting more attention due to the fact that it can be served the purpose of a strong theoretical probe of possible bulk gravitational dual theories, in terms of the framework of AdS/CFT correspondence [11,12]. One can quote here many many examples to feature the importance of the OTOCs in the context of AdS/CFT. One of the famous examples are the study of the existence of shock waves in the black hole physics [13,14,15,16,17,18] which can be described by various types of geometries and this specific study finally led to maximum saturation bound of the quantum version of the Lyapunov exponent ( λ ), given by the following expression [1]:
M ( aldacena ) S ( henker ) S ( tanford ) bound : ̲ λ 2 π β with = 1 , c = 1 , k B = 1 .
which appears in the expression for OTOCs at a finite temperature. This well known bound was established by Maldacena, Shenker, Stanford, which is commonly known as the MSS bound on quantum chaos these days [1]. In a gravitational paradigm, this saturation bound is physically interpreted as the red shift factor near the black hole event horizon having a finite Hawking temperature. In this description, the Sachdev–Ye–Kitaev (SYK) model [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37] is the most famous example at present which can able to saturate the MSS bound very successfully, which describes the quantum mechanical model of Majorana fermions in presence of infinitely long disorder. In the context of SYK model the saturation of the MSS bound corresponds to a quantum mechanical description of the black hole paradigm within the framework of AdS/CFT correspondence. Most significantly, it is important to note that the appearance of a quantum Lyapunov exponent in the computation of OTOC allows to consider the connection with the bulk gravitational physics in the quantum mechanical regime by applying the understanding from AdS/CFT correspondence. From the detailed past study in this literature we already know that any bulk gravitational physics which have their own dual description are described by the strongly quantum mechanical description. So from the present discussion it is evident that to connect this description with the phenomenon of quantum mechanical chaotic picture, we explicitly need to have an analogous description of quantum Lyapunov exponent, which will mimic the role of the well known Lyapunove exponent as appearing in the description of chaotic dynamical classical systems and to serve this purpose successfully, OTOC is the only strongest probe which can be treated as the physical discriminator of the classical and quantum mechanical description in the present context of discussion.
Now, we will talk about something very unusual from the perspective of applying the framework of the computation of OTOC in a completely different framework in which we are mostly interested in this paper. This is nothing but a cosmological application of OTOC from our own universe. Initially anyone can think that this is just a crazy idea and the final result will not give any physically relevant information regarding cosmological OTOC. We are completely ok with this initial thought. However, gradually once we proceed in this paper by detailed computation we will try to convince the general readers regarding the strong applicability of OTOC within the framework of Cosmology. Before going to discuss this feature in detail we will first start with some basic understanding from the study of Cosmology, which will surely helps to understand the background physical motivation of this crazy computation of OTOC in cosmological paradigm. In the quantum mechanical description of Cosmology, which is mostly used to describe the early universe physics the most significant quantity that we study are the N-point correlation functions of the cosmological scalar fluctuations or the tensor fluctuations or the mixture of them [38,39,40,41,42,43,44,45,46,47,48]. These quantum fluctuations are originated from a geometrical and a very fundamental quantity of the study of Cosmology, which is the well known classical gravitational background metric. To describe the observationally relevant universe in the cosmological scales it is a very general practice to consider the Friedmann–Lemaître–Robertson–Walker (FLRW) background metric of our space-time which is an exact solution of Einstein’s field equations within the framework of the General Theory of Relativity and describes a homogeneous, isotropic and expanding universe. However, the most general solution of the metric contains an additional curvature parameter, which one can fix to be zero by considering the present to observational data obtained from various observational probes of Cosmology. So this implies that a spatially flat FLRW metric is sufficient enough to describe most of the observationally consistent aspects within the framework of Cosmology [49]. For this reason we will stick to this background metric only for the rest of the discussions of this paper. Using this metric one can study the cosmological perturbation theory from which one can explain the origin of previously mentioned scalar and tensor fluctuations, which are treated to be quantum to describe the physics of early universe and for the late time scale it is considered to be classical in nature. These quantum fluctuations are very fundamental objects in the context of Cosmology from which all N-point correlation functions can be computed in Fourier space and these results can be used to probe the physics of early universe as well as to comment on its impact on the present day galaxy and cluster formation in large scale structure non-linear cosmological perturbation theory. Apart from having a great understanding the computation of these correlation functions of the quantum fluctuations at thermodynamic equilibrium, we have until now have a lot of constraints and limitations from the observational probes. From cosmological observations until now we have information regarding the amplitude of the primordial power spectrum from scalar mode fluctuations (this is actually represent the amplitude of the cosmological two-point correlation function of scalar mode quantum fluctuations in Fourier space) [50] and about its nearly scale invariance feature with respect to momentum scale (this constraint will helps us to determine the feature of the primordial power spectrum for scalar modes in all cosmological momentum scales. Until now from observational probes only the information regarding the spectral index (which is represented by the logarithmic derivative of the logarithm of the power spectrum at the cosmological horizon crossing scale) with high statistical accuracy is available. Any further information, such as running and the running of the running of the scalar spectral index are not available with any significant statistical accuracy). Additionally, we have information regarding upper bound on the tensor-to-scalar ratio (it represents the ratio of the amplitude of the power spectrum from tensor and scalar modes fluctuation and in terms of quantum description it represents the ratio of the two-point cosmological correlations. This is a very important observable in Cosmology the determination of which with high statistical accuracy will directly confirm the existence of gravitational waves in primordial cosmology and can also further able to fix the origin of primordial fluctuations by exactly determining the scale of inflation) at the cosmological horizon scale from the observational probes. No other information regarding the higher point (three and four-point etc.) cosmological correlation functions are available until now with significant statistical accuracy which can also be treated as the probe of new physical phenomenon through non-Gaussianity in the primordial Cosmology. This implies that just using the present observational probes one cannot able to distinguish amongst various possible origin of primordial quantum mechanical fluctuations and rule out models which describes inflationary paradigm within the framework of Cosmology. So, one can immediately ask about a questions regarding the possible options left to explore the physics of primordial quantum fluctuations:
  • Possibility I:
    The first possibility pointing towards the future observational aspects which can be probed by different ongoing and upcoming experiments to verify various theoretical features of primordial Cosmology. The most significant quantity which is possible to understand the underlying quantum field theory origin of the primordial physics is the tensor-to-scalar ratio. Detection of this with high statistical accuracy will provide us with the information regarding the generation of primordial gravitational waves, which will further confirm the exact origin of the primordial quantum mechanical fluctuations by exactly estimating the scale of inflation. Not only discriminating different frameworks of inflationary paradigm, but also the existence of alternatives to inflationary frameworks, i.e., bouncing cosmology [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90], cyclic cosmology [91,92,93,94,95,96], ekpyrotic scenario [97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112], etc., can also be verified by the confirmation of the primordial gravity waves. The next important quantity within the framework of primordial Cosmology is the study the existence of non-Gaussian features in the quantum mechanical fluctuations and the probability distribution profile of its related correlation functions. For this purpose, the study of bispectrum and trispectrum, which are basically representing the momentum-dependent amplitude of the three-point and four-point correlation function are very important. In the near future, through the upcoming cosmological missions, if it is possible to detect these non-Gaussian amplitudes with high statistical accuracy then more stringent constraints on the primordial physics are put. This is because of the fact that the probability distribution of such quantum fluctuations in the primordial universe is almost following Gaussian profile and a small but significant deviation from such Gaussianity will be extremely helpful to discriminate amongst various possible theoretical models of inflation which can be able to generate a significant amount of non-Gaussian amplitude in the context of three-point and four-point correlation functions. We are very hopeful for the detection of these important observables in near-future cosmological experiments. For this purpose, one needs to upgrade the present experimental tools and techniques or have to wait for upcoming future advanced experiments, which can able to measure these mentioned observables with high statistical accuracy. Now, we all know that for a single field inflationary paradigm the amplitude of the three point function is directly related to scalar spectral index, given by [38] (in ref. [38], the author have considered an additional contribution which is given by the following expression:
    f NL = 5 12 ( 1 n s f ( k ) n t ) ,
    where n t is the spectral index of the tensor power spectrum which is appearing from the perturbation of the primordial gravitational waves and not yet observed through CMB observations. On the other hand the scale-dependent factor, f ( k ) , is basically capturing some information regarding the bispectrum and the numerical value of this factor will lie within the window, 0 f 5 6 . Since n t = 2 ϵ from inflationary paradigm and the contribution from the factor f n t is very small, then one can easily neglect this terms from the full expression for the non-Gaussian amplitude of the three point scalar fluctuations. In the future, if we can able to detect the primordial gravitational waves at CMB B-modes and from that can able to extract the individual contribution of the tensor perturbation then adding the contribution of such terms will be more relevant in the present context of discussion):
    f NL = 5 12 ( 1 n s ) ,
    which in this literature is treated as a very strong theoretical probe, commonly known as Maldacena’s consistency relation. Now just from theoretical understanding of inflationary paradigm from this relation the expectation is that the amplitude of the three point non-Gaussian amplitude has to be very small for single filed slow-roll inflation. To detect this very small value, very high precision from observation is required. On the other hand, if the cosmological power spectrum have any additional characteristic features, like the running and running of spectral index then theoretically using the mentioned consistency relationship one can translate the statement in terms of the running or the certain scale dependence of the amplitude of the three point function. This fact can be expressed by the two new derived consistency conditions, which are given by,
    d f NL ( k ) d ln k = 5 12 d n s ( k ) d ln k = 5 12 α s ( k ) ,
    d 2 f NL ( k ) d ln k 2 = 5 12 d 2 n s ( k ) d ln k 2 = 5 12 d α s ( k ) d ln k = 5 12 β s ( k ) ,
    which further allows us to write a generic expansion for a scale-dependent three point non-Gaussian amplitude as:
    f NL ( k ) = f NL ( k CMB ) k k CMB Δ NL ( k ) f NL ( k CMB ) ,
    where the scale-dependent index Δ ( k ) is defined as:
    Δ ( k ) : = 5 12 { n s ( k CMB ) 1 + α s ( k CMB ) ln k k CMB + β s ( k CMB ) 2 ln 2 k k CMB + } .
    Here, it is important to note that k CMB is the momentum scale on which the CMB observation is performed and f NL ( k CMB ) is computed in terms of scalar spectral index using the previously mentioned Maldacena’s consistency relation. All these three relations can be further written in terms of the slow-roll parameters, which can be constructed from the field derivative of the inflationary potentials or the time derivative from the Hubble parameter. Now as we have mentioned, if we can verify this relation through cosmological observation with high statistical accuracy, then not only the appearance of non-Gaussian fluctuations in the primordial cosmological perturbation can be immediately tested, but also the appearance of the scale dependence of primordial power spectrum through the running and running of the scalar spectral index can also be verified by considering the above mentioned consistency relations.
  • Possibility II:
    The second possibility is pointing towards the probing of new physical concepts by incorporating additional but significant features within the framework of early universe Cosmology. Until now all of the features are studied by considering the fact that the quantum fluctuations appearing in the primordial universe is at thermal equilibrium. However, if the quantum mechanical systems which we use to study to describe the early universe Cosmology are not in thermal equilibrium then quantifying the quantum correlation functions within the framework of finite temperature out-of-equilibrium quantum field theory of Cosmology is extremely difficult to compute and until now in the literature of Cosmology no such framework is available using which one can pursue this specific computation. On the other hand, anybody can ask us here why at all computation and quantification of all such quantum mechanical correlation function for the primordial Cosmology describing an out-of-equilibrium is important at all and if these ideas can be provided theoretically then in which stages of the evolutionary scale of our universe this can be really implemented? The answer is very simple and it is already hidden there in the study of the early universe Cosmology. In the following we now explicitly mention about these phenomena where this methodology can be applicable:
    • Stochasticity and particle production during inflation:
      During the epoch of inflation [40,49,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142], we describe the physics of inflationary paradigms with scalar field which have a negligible mass compared to the Hubble scale, which is treated as the reference characteristic scale of early universe Cosmology. However, during inflation many particle produces in a stochastic manner which have masses either of the order of Hubble scale or have mass very heavier than the Hubble scale. These particles are commonly known as partially massless or heavy scalar fields whose origin can be explained from the randomness appearing in the quantum mechanical stochastic fluctuation appearing in the early universe where various unknown out-of-equilibrium features play significant role. However, it is extremely difficult to quantify or describing this phenomenon correctly at the out-of-equilibrium regime of quantum field theory which consistently describe a quantum mechanical theory of primordial Cosmology. Additionally it is important to note that during the stochastic particle production during inflation due to the presence of randomness noise plays significant role to describe the quantum mechanical phenomenon during this particular epoch in the cosmological evolutionary time scale. However, we have a very small understanding regarding the time-dependent random noise within the framework of out-of-equilibrium quantum field theory until now. Only in some specific situations if we provide some additional constraints on the two-point and the one-point correlation of the time-dependent noise, which have the Gaussian probability distribution profile one can deal with noise. However, such analysis within the framework of quantum field theory is not exactly correct which in turn could not able to gives us the correct predictions from the quantum theory of early universe. One the other hand, we really do not know about the nature of the time-dependent noise which we are studying in the present context. This means we do not know that whether the time-dependent noise have Gaussian or non-Gaussian probability distribution profile, which information can be directly understandable if we really know the exact computation of N-point correlation functions of time-dependent noise in cosmology. A lot of attempts have been made within the framework of quantum-field theory to study the quantum mechanical features and the related N-point correlation functions to study the exact nature, behaviour and magnitude of the noise. However, apart from its rigor, not much is known about the detailed quantum filed theory structure from which one can reliably compute such correlation functions as quantum mechanics. Here lies one of the prime motivations to write this paper. Our expectation from the presented computation of this paper is that the cosmological version of OTOC is defined in a specific quantum mechanical vacuum state, which actually describes the initial condition in early universe Cosmology, which describes the randomness and stochastic features of quantum mechanical fluctuations in the out-of-equilibrium regime in a perfectly correct fashion. This is just not an arbitrary claim, but also one can appear at such conclusion by considering the basic understandings of the background physical phenomenon which describes the particle production mechanism during the epoch of inflation. Throughout our paper, we have explicitly established the framework for the first time in the Cosmology literature using which one can explicitly perform the computation of the cosmological correlation functions of the random quantum mechanical fluctuations in the out-of-equilibrium regime of quantum field theory. Instead of studying the quantum correlations with the time ordered or the anti-time ordered physics, in the present context cosmological OTOC functions are playing significant role to describe the underlying hidden features of out-of-equilibrium physics to describe the stochastic randomness during the particle production mechanism during inflation (here it is important to note that the Random Matrix Theory is an another alternative framework using which one can compute these quantum mechanical cosmological correlation functions within the framework of out-of-equilibrium quantum field theory [143,144]).
    • Reheating / Warm Inflation:
      Reheating is an epoch in the evolutionary time scale of our universe which appears just after inflation. During inflation the inflaton field started slowly rolling through the valley of the inflationary potential under consideration and it is expected that after a certain time the inflaton field will reach the stable minimum of the potential and the inflationary mechanism just stops there. However, is the the end of the story? Obviously it is not the end. Once the inflation ends, the inflation field started oscillating around the stable minimum of the potential and started interacting with the valley of the potential as well as some other field, which we identify as the reheaton, the field who is mostly responsible for reheating (it is important to note that in the Cosmology literature there exists a few models of inflation where reheating is not required and without having any reheating phase in those models one can able to describe the genesis of the dark matter candidate theoretically. However, these models are very small in number and the construction of these models relies on various underlying assumptions, which may not be completely correct as far as the technical rigour of the quantum field theory for Cosmology is concerned. So for our discussion we stick to the general models of inflations which are developed from more reliable version of UV complete theories at very high energy scale and these models has to have the reheating phenomenon in the evolutionary cosmological time scale of our universe). As a consequence of such interactions enormous amount of heat is generated and the quantum mechanical system that we are studying immediately goes to its out-of-equilibrium phase. From the previous understanding of the subject it was extremely difficult to determine the quantum effects in terms of studying the quantum correlations in this out-of-equilibrium phase. However, it is expected from our basic understanding of quantum statistical mechanics that if we wait for long cosmological time scale then the system reaches the equilibrium where it is possible to associate an equilibrium reheating temperature associated with the system. After reaching equilibrium the reheaton field started decaying to some new particle contents which are responsible to describe the genesis of dark matter in the early universe Cosmology. That means without explaining the detailed quantum aspects throughout the total cosmological time scale in detail it is impossible to study the genesis of the dark matter contents of our universe and this actually motivates us to study this hidden unexplored phenomenon in this paper. Until now we have very less description available regarding the quantum mechanical aspects during the epoch of reheating. Only the quadratic and quartic inflationary models are studied phenomenologically in this context and it is until unknown how to quantify and estimate the quantum mechanical correlations and study the detailed quantum field theory aspects of reheating. On the other hand, as we have already mentioned, it is extremely important to determine the quantum mechanical correlations during this epoch to study the genesis of the dark matter contents where the principles of out-of-equilibrium quantum field theory will play significant role. However, until now there are no such significant inputs available from the theory side which can be able to provide us the relevant tools and techniques to compute such quantum mechanical correlation function and study various other quantum field theory aspects related to reheating at out-of-equilibrium. In this paper, we have established a detailed quantum field theory framework at the out-of-equilibrium phase which helps us to quantify the extremely relevant quantum mechanical correlation functions in terms of OTOCs. The methodology presented in this paper not only helps us to quantify the correlation at the out-of-equilibrium phase, but also provides us a detailed understanding about the equilibrium behaviour of the correlation functions at a large time limit in the cosmological time scale. Additionally, this detailed study through the cosmological generalization of OTOCs helps us to determine a lower bound on the reheating temperature in a completely model independent way. From the previously available understanding of the subject it was possible to give an estimate of the reheating temperature in a completely model-dependent way after knowing about the relativistic degrees of freedom which are participating during the epoch of reheating. However, apart from having a model-dependent expression, that expression actually relies on the scale of inflation completely, which is not known from the observation side until now; only an upper bound on the scale of inflation in terms of the tensor-to-scalar ratio is known from the observational probes. So the previously described method of determining the reheating temperature is not very reliable and only helps us to know about the upper bound on the reheating temperature in a completely model-dependent way. On the other hand, the present study helps us to determine the exact time-dependent behaviour of the reheating temperature and additionally provides a lower bound of reheating temperature which we have derived in a completely model independent way. As we proceed through the subject material of the paper, one can have a clear understanding about each of our big claim which we have explicitly established with detailed analysis. Apart from this, one can consider another well known framework, which is described by the warm inflationary paradigm [145,146], where the radiation appears due to the stochastic particle production concurrently with inflationary expansion in the early universe evolutionary time scale. Sometimes this framework can be treated as the finite temperature generalization of usual well known zero temperature inflationary paradigm, which is mostly studied in cosmology literature. The presence of the effect of radiation during the epoch of inflation implies that there is a possibility exists on which the epoch of inflation could smoothly end directly into a radiation-dominated epoch, particularly without having a separate reheating epoch. This is one of the early universe phenomena apart from reheating where the effect of out-of-equilibrium physics is significantly dominating and the detailed quantum field theoretic phenomena have not been studied yet in detail. Until now, from the previous work, it is very clear that this phenomenon is very model-dependent and only a few models have studied this possibility. This is because of the fact that handling the out-of-equilibrium dynamics in the quantum regime is extremely complicated. However, we are again very hopeful that, similar to reheating, OTOC at finite temperature studied in this paper is able to describe the quantum correlation functions for the warm inflationary scenario, which is able to describe the out-of-equilibrium dynamical phenomenon in the quantum regime in a very elegant and model-independent way. Though it is true that our computation performed in this paper will be applicable to the warm inflationary scenario, we will restricted our analysis here to describe only the quantum effects during the epoch of reheating particularly in the very early time scale of evolutionary scale of our universe where the contribution of the out-of-equilibrium physics play pivotal role to control the dynamics.
    • Stochastic inflation:
      Stochastic inflationary [147,148,149,150] paradigm is a very important aspect of the early universe Cosmology whose technical construction is completely different from the usual inflationary paradigm. To have inflation from a model which is derived from some UV complete high energy quantum field theories we do not need any additional source, time-dependent scalar inflaton field slowly rolls down through an inflationary potential and participate in the quantum field theory using which one can explicitly compute the N-point correlation functions within the framework of early universe Cosmology. However, in the framework of stochastic inflationary paradigm a time-dependent stochastic random source function play significant role to study the background construction of the quantum field theory. More precisely, here we have two fields, the inflaton and a stochastic random time-dependent noise field, where both of them are participating to carry forward a correct quantum field theory construction of inflationary paradigm within the framework of early universe Cosmology. To construct a correct and consistent version of these type of quantum field theories one needs to start with a theory where the time-dependent inflaton and the stochastic random time-dependent field coupled to each other. In most of the previous literature, during the construction of these type of quantum field theories the correlation between the time-dependent noise using which one can compute the N-point correlation function using the time-dependent inflation to study the role of quantum mechanical fluctuations in the early universe Cosmology. In this theoretical construction, the correlation between the noise acts as a source of the correlation between the inflaton. However, cannot say concretely about the exact behaviour of the noise at the starting point of the computation. Most of the computation until now have performed in this literature by assuming the underlying Gaussian behaviour of the probability distribution of the time-dependent stochastic noise function. As a consequence of this assumption one can now expect that the one-point function and any odd point correlation function of the time-dependent stochastic noise vanish trivially. On the other hand the two-point function has to de proportional to a time translationally invariant Dirac Delta function and the proportionality constant actually determine the amplitude of two-point correlation function, which we identify as the power spectrum of the time-dependent stochastic noise in the context of early universe Cosmology. One can also compute any other higher point even correlation functions from this construction, where one can explicitly show that the connected part of these correlations are factorizable in terms of the two-point function or the Green’s function of the theory which we are considering to describe the background quantum field theory of early universe Cosmology. If we believe that the initial assumption regarding having a Gaussian probability distribution of time-dependent stochastic noise is perfectly correct then everything we have mentioned above are the automatic consequence of that and using these information one can determine the N-point correlation function from the inflaton field consistently. Here the type of the noise we have pointed which follow the Gaussian probability distribution is commonly known as the white noise within the framework of quantum field theory. However, unfortunately we really do not have any idea if the assumption that we have taken at the starting point is correct at all or not when one can think of any arbitrary stochastic randomness within the framework of quantum field theory. This allows to think about considering non-Gaussian noise, which commonly identified as the coloured noise in the present context. However, if we have some time-dependent stochastic coloured non-Gaussian noise then it is extremely complicated to determine the quantum correlations between the noise and hence the N-point quantum correlation for inflaton field which is sourced by the coloured noise time-dependent profile. Furthermore, it is expected that the random quantum fluctuations in the stochastic noise is the prime source for which the background equilibrium quantum filed theory set up goes to its out-of equilibrium phase, where we have very less information regarding the cosmological correlation functions within the framework of quantum field theory of early universe Cosmology. This actually motivates us to think about some alternative construction of computing the N-point quantum correlation functions due to the presence of coloured time-dependent noise profile and in this paper by constructing the cosmological version of OTOC we have tried to address this crucial issue in an alternative way within the framework of out-of-equilibrium quantum field theory.
    • Quantum quench in Cosmology:
      The study of quantum mechanical quench in the presence of time-dependent random coupling and its consequences in quantum correlation functions in Cosmology is a very important topic of study in the context of theoretical physics. Until now, this has not been very well understood and studied in the context of Cosmology. In the earlier literature for various statistical mechanical and condensed matter systems quantum quench has been studied rigorously, but its extension in the framework of early universe Cosmology will provide us the understanding of the thermalization phenomenon and the details of the achieving thermal equilibrium from an out-of-equilibrium phase in the presence of a random time-dependent coupling parameter. One can start with various possibilities here where in each case, theories are minimally coupled with classical FLRW conformally flat cosmological metric in a minimal fashion. The first and the simplest possibility appears in free scalar field theory where we consider a time-dependent mass, which is a random coupling parameter within the framework of quench. The second possibility appears within the framework of an interacting quantum field theory where one can consider a situation where two scalar fields with constant mass are interacting with each other in presence of a random time-dependent coupling parameter. If we treat the interaction term between the two scalar fields as quadratic, then constructing the effective theories of each scalar field becomes simpler after performing the path integration over the other unwanted scalar field for the description. In the quantum description, sometimes this is identified to be the partial trace operation when we are describing everything in terms of density matrix and similar approach have been followed in the context of quantum field theories driven by an open quantum system, where the system is non-adiabatically interacting with the environment. One can further generalize this idea for N number of scalar fields which are placed at the thermal bath and interacting with a system which is described by a singe scalar field. In terms of the interaction here one can consider the quadratic or any other non linear interactions. This description can be identified as the quantum field theory generalization of the well known Feynman–Vernon model of influence functional theories or the Caldeira Leggett model which describes the quantum dissipation phenomenon in the Quantum Brownian motion picture within the framework of early universe Cosmology. Considering these mentioned frameworks, one can explicitly compute the OTOCs using the methodology presented in this paper to study the quantum mechanical N-point out-of-time ordered correlation functions in presence of a quantum mechanical quench within the framework of out-of-equilibrium version [151,152,153,154] of open quantum field theory of Cosmology [155,156,157,158].
In Figure 1, I have added the dedication note of this work to Professor Freeman J. Dyson to whom I had a chance to meet personally and discuss about this work during my visit at Institute for Advanced Studies (IAS), Princeton in December, 2019. Further in Figure 2, UI have explicitly pointed the prime motivation of writing this paper and its connection with various other interesting aspects of quantum field theories. Finally, in Figure 3, I have explicitly depicted the background philosophy of this paper.
Now, we mention the prime highlights of our obtained results in this paper, which we strongly believe will further help to know about many more unexplored features of cosmological quantum correlations within the framework of the out-of-equilibrium version of quantum statistical field theory:
  • Highlight I:
    The methodology presented in this paper helps us to quantify the quantum mechanical correlation functions within the framework of Cosmology in the presence of random quantum fluctuations. In this connection, we have computed the expressions for the two-point and four-point cosmological OTOC in the quantum regime which will provide the behaviour of the correlation functions in the out-of-equilibrium regime of the quantum field theory of early universe Cosmology.
  • Highlight II:
    We have additionally studied the classical limiting version of the two-point and four-point cosmological OTOCs which will provide the decaying large time behaviour of the correlation function, which perfectly matches with the expectations from the chaotic phenomenon in the classical regime of the field theory.
  • Highlight III:
    The large time limiting behaviour of the four-point OTOC helps us to comment on the equilibrium behaviour of the quantum correlations and to determine the lower bound of the equilibrium temperature. Using this concept, one can further determine the lower bound on the reheating temperature within the framework of early universe Cosmology.
  • Highlight IV:
    We have also studied the quantum Lyapunov spectrum for Cosmology and computed the associated quantum Lyapunov exponent to have a consistent chaotic description in the quantum regime from the four-point cosmological OTOC derived in this paper.
  • Highlight V:
    We have explicitly proved from our detailed computation that the results obtained from the normalized version of the four-point cosmological OTOC is completely independent of the choice of the time-dependent perturbation variable as appearing in the specific scheme of the cosmological perturbation theory.
By seeing the length of this paper one may feel very scared. Do not worry at all. After reading this paper we strongly believe that the readers can get to know about something very interesting which was not presented earlier in any Cosmology paper. The study material and the obtained results of this paper are organized as follows:
  • In Section 2, we discuss how one can formulate the OTOC in the context of Cosmology and what exact quantity we have to compute for this study.
  • In Section 3, we discuss about the detailed computation of quantum micro-canonical two-point and four-point OTO amplitudes and the related OTOC in Cosmology.
  • In Section 4 and Section 5, we present the numerical results obtained from the two-point and four-point OTOC and also discuss about its physical significance in Cosmology.
  • In Section 6 and Section 7, we present the detailed computation for Lyapunov spectrum, study the quantum chaotic phenomenon, numerically study the obtained results and its physical impact in the context of Cosmology.
  • In Section 8, we discuss about classical limit of micro-canonical two-point and four-point OTO amplitudes and the related OTOC in Cosmology.

2. Formulation of OTOC in Cosmology

2.1. General Remarks on OTOC

In this section, my prime objective is to study the out-of-equilibrium physics in the inflationary patch of de Sitter space time. In the context of quantum field theory the time dynamics of the out-of-equilibrium physics is described by the out-of-time ordered correlation (OTOC) function, which is typically defined by the following expression:
Thermal OTOC : C ( t ) : X ( t ) , Y ( 0 ) 2 β ,
where β represents the thermal average which is taken using one parameter family α vacua and Bunch–Davies quantum vacuum state in de Sitter space. Here, X ( t ) and Y ( t ) are quantum operators are defined at time scale t in the Heisenberg representation. For any quantum operator O ( t ) the thermal average is technically defined as:
Thermal average : O ( t ) β : 1 Z Tr e β H O ( t ) ,
where Z is the thermal Partition Function, which is defined as:
Thermal partion function : Z = Tr e β H .
Here, H is the quantum system Hamiltonian under consideration.
Furthermore, using Equations (11) and (12) in Equations (10), we get the following simplified expression for the out-of-time ordered correlation (OTOC) function:
C ( t ) : Tr e β H X ( t ) , Y ( 0 ) 2 Tr e β H = Tr e β H Z X ( t ) , Y ( 0 ) 2 = Tr ρ X ( t ) , Y ( 0 ) 2 ,
where we have used the fact that the thermal density matrix is defined by the following expression:
Thermal density matrix : ρ = e β H Z .
Here it is important to note that the OTOC is defined in terms of the square of the quantum mechanical commutator bracket of two quantum operators separated by a time scale t because its connection to the classical Poisson bracket and the exponentially divergent trajectories expected in the context of classical description of chaos.
The thermal average of the quantum mechanical commutator bracket of two quantum operators is not allowed to describe chaos in the present context. To understand the actual physical reason behind this fact let us assume that the commutator bracket is replaced by the Poisson bracket by considering the semi-classical limit. In this situation the Poisson bracket shows an exponential growth with respect to time, e λ L t , where λ L represents the Lyapunov exponent which quantify chaos. Now if we take the thermal average of the commutator bracket representing two point OTOC function then both the contributions are cancelled in the semi-classical limit and will not finally contribute to quantum chaos. On the other hand, from the quantum field theory point of view the two point thermal averaged function captures the effect of correlation between the two quantum Hermitian operators which decay in the large time limit and cannot characterise the chaotic behaviour at all. Instead of that if we consider the square of the commutator bracket, which actually represents the four-point function, after transforming it to the Poisson bracket in the semi-classical limit it takes a positive signature, which implies no cancellation for both the contributions. The thermal average of this non trivial contribution further quantifies quantum chaos. Similarly, in the quantum picture the four-point thermal averaged function, not decays exponentially with respect to time at the large time leading order limiting result.
Similarly, to define the quantum chaos the thermal average of the three point as well as any odd point correlation function of the quantum mechanical operators are also not allowed to define OTOC. This can be easily verified using the well known Kubo–Martin–Schwinger (KMS) condition, which can be demonstrated by applying Schwinger–Keldysh formalism of the closed time path formulation of real time finite temperature field theory. After applying KMS to any odd point thermal averaged function, one can explicitly show that each of the contributions from the odd point function vanish trivially and consequently will not contribute to quantify quantum chaos in terms of odd point OTOC.
A quantum mechanical system is treated as a chaotic system if the quantum mechanical commutator squared exponentially grow with time, which is technically expressed as:
C ( t ) 1 N 2 e 2 λ L t ,
where the exponential growth is characterised by the Lyapunov exponent λ L . Furthermore, N represents the number of degrees of freedom of the system under consideration. In more technical ground, this phenomenon of the exponential growth is related to the fast scrambling in the present context. Here the time scale corresponding to fast scrambling is expected to lie within the interval, t d t t * , where
Dissipation time : t d β = 1 T ,
is the dissipation time scale which is the inverse of the temperature in de Sitter space and the upper bound of the scrambling time is defined as:
Scrambling time : t * 1 λ L log N .
On the physical ground, time scale associated to scrambling represents the associated time scale for a perturbation involving a few degrees of freedom to spread over all the degrees of freedom of the quantum mechanical system under consideration. In this connection it is important to note that any quantum mechanical operations performed after the time interval for scrambling for a certain number of degrees of freedom cannot able to re-track the quantum information associated with the perturbation.
Furthermore, expanding the right hand side of the Equation (10) we get the following simplified expression for the OTOC, as given by:
C ( t ) = X ( t ) Y ( 0 ) Y ( 0 ) X ( t ) β + Y ( 0 ) X ( t ) X ( t ) Y ( 0 ) β 2 Re Y ( 0 ) X ( t ) Y ( 0 ) X ( t ) β .
It is important to note that the first two terms representing two different thermal averaged four-point function appearing in the above expression for OTOC can be factorized in terms of the two point thermal averaged function over the dissipation time scale t d β as given by:
X ( t ) Y ( 0 ) Y ( 0 ) X ( t ) β X ( t ) X ( t ) β Y ( 0 ) Y ( 0 ) β + O e t / t d ,
Y ( 0 ) X ( t ) X ( t ) Y ( 0 ) β X ( t ) X ( t ) β Y ( 0 ) Y ( 0 ) β + O e t / t d .
Consequently, over the dissipation time scale t d β the full expression for the OTOC can be factorized as:
C ( t ) = 2 X ( t ) X ( t ) β Y ( 0 ) Y ( 0 ) β Re Y ( 0 ) X ( t ) Y ( 0 ) X ( t ) β + O e t / t d .
Here X ( t ) X ( t ) β represents the thermal two point function of the quantum operator X ( t ) which is actually perturbed by the insertion of the quantum operator Y ( 0 ) in terms of the thermal two point function Y ( 0 ) Y ( 0 ) β . It is important to note that if the insertion energy of the quantum operator Y ( 0 ) is small enough, then the quantum state will relax with respect to the time scale and the corresponding expectation value (two point function) of the quantum operator Y ( 0 ) will approach to the thermal expectation value multiplied by the norm of the quantum mechanical state (it is a very well known fact that the time ordered correlation functions decay over the dissipation time scale t d β to the products of the expectation value of the quantum operator with respect to the thermal quantum mechanical state). Consequently, beyond the dissipation time scale t t d β , the normalized OTOC can be expressed by the following expression:
C ( t ) = C ( t ) X ( t ) X ( t ) β Y ( 0 ) Y ( 0 ) β 2 1 Re Y ( 0 ) X ( t ) Y ( 0 ) X ( t ) β X ( t ) X ( t ) β Y ( 0 ) Y ( 0 ) β + O e t / t d .
Additionally, it is important to note that late time vanishing behaviour of the OTOC for quantum mechanical systems are equivalent to the saturation of the square of the normalized thermal expectation value of the square of the commutator and this can be expressed by the exponential time-dependent growth e λ L t , where λ L is the Lyapunov exponent. Consequently we get:
C ( t ) 2 1 1 N 2 e λ L t + O 1 N 4 λ L 1 t ln N 2 Re Y ( 0 ) X ( t ) Y ( 0 ) X ( t ) β X ( t ) X ( t ) β Y ( 0 ) Y ( 0 ) β ,
where the number of degrees of freedom N scaled as:
N 1 G N = 8 π in M P = 1 ,
which is a very large number in terms of the energy scale and of the order of the cut-off of the quantum gravity cut-off scale, i.e., Planck scale.
Here, the Lyapunov exponent, λ L , satisfy the following saturation bound for quantum chaos:
Bound on Lyapunov exponent : λ L 2 π β = 2 π T where β = 1 T with = 1 = c .
This implies the exponential time-dependent growth of the real part of the time-dependent thermal correlation function, which is given by the following upper bound:
Bound on normalised four point function : Re Y ( 0 ) X ( t ) Y ( 0 ) X ( t ) β X ( t ) X ( t ) β Y ( 0 ) Y ( 0 ) β 1 N 2 e 2 π t β .

2.2. Eigenstate Representation of OTOC in Quantum Statistical Mechanics

Now, instead of discussing further about the general definition of OTOC, we now concentrate on the eigenstate representation OTOC, using which many quantum systems can be analysed very easily. In this eigenstate representation we start with two canonically conjugate operators, q ( t ) and p ( 0 ) , which are separated in time scale. In this context, at finite temperature the OTOC is defined as:
C ( t ) = q ( t ) , p ( 0 ) 2 β .
Here β = 1 / T is the temperature of the quantum system under consideration. Next considering the energy eigenstates as the required basis of the Hilbert space, we can further rewrite the OTOC as:
C ( t ) = 1 Z Thermal partition function n e β E n Thermal Boltzmann factor g n ( t ) Microcanonical OTOC Thermal OTOC ,
where the time-dependent coefficient g n ( t ) is defined by the following expressions:
Microcanonical OTOC : g n ( t ) n | q ( t ) , p ( 0 ) 2 | n .
Here | n is the energy eigenstate of the quantum system under consideration, which satisfy the following eigenvalue equation:
Time independent Schr o ¨ dinger equation : H | n = E n | n ,
where H is in general any time-independent Hamiltonian of the quantum system under consideration. This is basically the fixed energy eigenstate representation of OTOC for which in this context the time-dependent coefficients g n ( t ) for a given energy level is identified to be the OTOC computed in the microcanonical statistical ensemble. On the other hand C ( t ) represents OTOC at finite temperature or thermal OTOC, as in the eigenstate representation an additional Boltzmann factor is involved. This implies that once we compute the expression for the OTOC for a microcanonical type of statistical ensemble then one can easily obtain further the expression for the OTOC at finite temperature after taking the thermal average of g n ( t ) .
Now to simplify the expression for microcanonical OTOC, g n ( t ) it is better to express this in terms of the matrix elements of the previously mentioned canonically conjugate operators, q ( t ) and p ( 0 ) , respectively. To implement this strategy we need to use the following completeness relations of the energy eigenstates:
n | n n | = 1 .
Consequently, the microcannical OTOC can be expressed in terms of the required matrix elements as:
g n ( t ) = m I n m ( t ) I n m * ( t ) ,
where the matrix element I n m ( t ) is defined as:
I n m ( t ) i n | q ( t ) , p ( 0 ) | m .
Here one can note a basic property of this matrix I ( t ) is that it is Hermitian, which implies:
I n m * ( t ) = I m n ( t ) .
Consequently, the expression for the microcannical OTOC can be further simplified as:
g n ( t ) = m I n m ( t ) I m n ( t ) .
Furthermore, considering the operator representation in Heisenberg picture one can write the time-dependent operator q ( t ) as:
q ( t ) = e i H t q ( 0 ) e i H t .
Consequently, the matrix element I n m ( t ) can be computed as:
I n m ( t ) = i k e i Δ E n k t q n k ( 0 ) p k m ( 0 ) e i Δ E k m t p n k ( 0 ) q k m ( 0 ) ,
where we define Δ E n m , q n m ( 0 ) and p n m ( 0 ) by the following expressions:
Δ E n m = E n E m ,
q n m ( 0 ) = n | q ( 0 ) | m ,
p n m ( 0 ) = n | p ( 0 ) | m .
As we know any kind of general any N particle Hamiltonian of a quantum system can be represented by the following expression:
H = i = 1 N p i 2 + U ( q 1 , , q N ) .
Here we have assumed that each of the N particle have the same mass, m i = 1 / 2 i = 1 , , N . Using the above mentioned general form of the Hamiltonian one can further simplify the expression for the matrix element I n m ( t ) , which is given by:
I n m ( t ) = 1 2 k q n k ( 0 ) q k m ( 0 ) E k m e i Δ E n k t E n k e i Δ E k m t ,
where I have used the following fact:
p m n ( 0 ) = m | p ( 0 ) | n = i 2 m | H ( 0 ) , q ( 0 ) | n = i 2 E m n q m n ( 0 ) .
Consequently, the microcanonical OTOC is computed as:
g n ( t ) = 1 4 m k s q n k ( 0 ) q k m ( 0 ) q m s ( 0 ) q s n ( 0 ) × E k m e i Δ E n k t E n k e i Δ E k m t E s n e i Δ E m s t E m s e i Δ E s n t = 1 4 m k s q n k ( 0 ) q k m ( 0 ) q m s ( 0 ) q s n ( 0 ) × E k m E s n e i Δ E n k m s ˜ t + E n k E m s e i Δ E k m s n ˜ t E n k E s n e i Δ E k m m s ˜ t E k m E m s e i Δ E n k s n ˜ t .
where we define new energy shifts, Δ E n k m s ˜ , Δ E k m s n ˜ , Δ E k m m s ˜ and Δ E n k s n ˜ as:
Δ E n k m s ˜ = Δ E n k + Δ E m s = E n + E m E k E s ,
Δ E k m s n ˜ = Δ E k m + Δ E s n = E k + E s E m E n ,
Δ E k m m s ˜ = Δ E k m + Δ E m s = E k E s
Δ E n k s n ˜ = Δ E n k + Δ E s n = E s E k .
Finally the thermal OTOC can be computed as:
C ( t ) = 1 Z Thermal partition function n e β E n Thermal Boltzmann factor g n ( t ) Microcanonical OTOC Thermal OTOC = 4 n e β E n 1 × n m k s e β E n q n k ( 0 ) q k m ( 0 ) q m s ( 0 ) q s n ( 0 ) × E k m E s n e i Δ E n k m s ˜ t + E n k E m s e i Δ E k m s n ˜ t E n k E s n e i Δ E k m m s ˜ t E k m E m s e i Δ E n k s n ˜ t .
Once we determine the shifts in the energy eigen values and the matrix elements of the canonically conjugate variable q ( 0 ) in the energy eigen basis, we can explicitly compute the expression for the thermal OTOC in the energy eigen basis itself.

2.3. Constructing OTOC in Cosmology

One can further map this idea to Quantum Field Theory of Curved Space Time as well. To show this mapping let us start with a theory of N scalar fields in an arbitrary curved gravitational background which is described the following ( d + 1 ) dimensional action:
S = d d + 1 x g 1 2 a = 1 N b = 1 N g μ ν G a b μ ϕ a ν ϕ b U ( ϕ a , ϕ b ) Lagrangian density in curved space L ( ϕ a , ϕ b , μ ϕ a , ν ϕ b , g μ ν , g ) a , b = 1 , , N ,
where in the above action d represents the number of spatial dimension and the gravity is minimally coupled with N scalar fields in an arbitrary background. In general one can consider any arbitrary class of gravitational metric for this calculation. However to avoid mathematical complication for any unwanted reason we restrict ourselves in the class of gravitational metrics which can be expressed in diagonal form. Here G a b takes care of all possible interaction between N scalar fields in the kinetic term. On the other hand, one can consider between all possible interactions in the interaction potential U ( ϕ a , ϕ b ) . In a more generalised physical situation, a = b always, so G a b = δ a b , which means that all the off-diagonal components of the matrix is zero. In that specific situation, the above action can be simplified to the following simplified form:
S = d d + 1 x g 1 2 a = 1 N g μ ν μ ϕ a ν ϕ a U ( ϕ a ) Lagrangian density in curved space L ( ϕ a , μ ϕ a , g μ ν , g ) a = 1 , , N .
Here, the first term in the above action represents a very simplest kinetic term for N scalar fields and the second term U ( ϕ a ) corresponds to the simplest form of the self interacting potential for the N scalar fields.
Now from this most generalised action one can compute the canonically conjugate momenta of the each N scalar fields, which is given by the following expression:
Π ϕ c = L ϕ ˙ c = 1 2 g g 00 a = 1 N b = 1 N G a b δ a c ϕ ˙ b + δ b c ϕ ˙ a = g g 00 a = 1 N G c a ϕ ˙ a c = 1 , , N .
Furthermore, in the simplest situation where G a c = δ a c we can further simplify the expression for the canonically conjugate field momenta as:
Π ϕ c = g g 00 a = 1 N δ c a ϕ ˙ a = g ϕ ˙ c ϕ ˙ c = Π ϕ c g g 00 c = 1 , , N .
Consequently, for the simplest case non-interacting N scalar fields the Hamiltonian density can be written as:
H = c = 1 N Π ϕ c ϕ ˙ c L ( ϕ a , μ ϕ a , g μ ν , g ) , = 1 g g 00 c = 1 N Π ϕ c 2 g 1 2 a = 1 N g μ ν μ ϕ a ν ϕ a U ( ϕ a ) = 1 2 1 g g 00 c = 1 N Π ϕ c 2 + 1 2 g g i i c = 1 N i ϕ c 2 + g U ( ϕ a ) .
Now we consider a specific situation where space-time is such that the scalar field is independent on space, but only function of time. This type of situation one usually consider in the context of Cosmology. In this situation the general structure of the ( d + 1 ) dimensional metric can be expressed using the following ansatz:
d s d + 1 2 = g 00 d t 2 + i = 1 d g i i d x i d x i .
Particularly for ( d + 1 ) dimensional de Sitter space the infinitesimal line element in the planar inflationary coordinate is described by:
d s d + 1 2 = d t 2 + a 2 ( t ) i = 1 d d x i d x i = d t 2 + a 2 ( t ) d x d 2 ,
where we have fixed the diagonal component of the metric as:
g 00 = 1 , g i i = a 2 ( t ) .
Furthermore, the scale factor a ( t ) is defined as:
a ( t ) = e H t ,
where H is the Hubble parameter in the present context.
In this coordinate system, the above mentioned Hamiltonian density of N non-interacting scalar fields can be further simplified as:
H = c = 1 N Π ϕ c 2 ˜ + U ( ϕ a ) ˜ ,
which is exactly similar like the previously discussed Hamiltonian for N non-interacting system in Quantum Mechanics.
It is important to note that here we have used the following redefinition:
Π ϕ c 2 ˜ = 1 2 a d ( t ) Π ϕ c 2 ,
U ( ϕ a ) ˜ = a d ( t ) U ( ϕ a ) .
In the flat Minkowski space limit one can fix a ( t ) = 1 .
Now in the present context the Hamiltonian of the N non-interacting scalar fields can be expressed as:
H = d d x H = d d x c = 1 N Π ϕ c 2 ˜ + U ( ϕ a ) ˜ , .
However in Quantum Field Theory we do not have any sort of eigenstate representation of the Hamiltonian similar like Quantum Mechanics. So for Quantum Field Theoretic systems representing the thermal OTOC in terms of the microcanonical OTOC is not very straight forward just like Quantum Mechanics. In Quantum Field Theory best possible way is to express the Hamiltonian is in the Fourier space in normal ordered form, as given by:
: H : = d d k ( 2 π ) d E k a k a k ,
where the energy E k can be computed from the dispersion relation. Here a k and a k are the creation and annihilation operators in the present context. In the flat space, the corresponding vacuum is known as Minkowski vacuum which is unique. On the other hand, the corresponding curved space vacuum is not unique in nature. In the context of de Sitter space we use Bunch–Davies and α vacua for the computation which are SO(1,4) invariant in nature.
Furthermore, in the further computation instead of using the usual time coordinate we use the conformal coordinate in the above mentioned ( d + 1 ) dimensional de Sitter metric, which can be expressed using the following ansatz:
d s d + 1 2 = a 2 ( τ ) d τ 2 + i = 1 d d x i d x i = a 2 ( τ ) d τ 2 + d x d 2 .
Here we have introduced the concept of conformal time which can be expressed in planar de Sitter space as:
τ = τ d t a ( t ) = 1 H a ( τ ) Conformal time during inflation < ø < ø inf a ( τ ) = 1 H τ Scale factor during inflation .
Here H is the Hubble parameter defined in the planar inflationary patch of de Sitter space. This result can be used during stochastic particle production during inflation.
For the reheating case there is no closed form expression exists in literature. When the inflaton oscillates around the potential minimum, the equation of state is that of pressureless matter, i.e., p = 0 , so the scale factor would behave accordingly. It is nothing but the non-relativitic matter with equation of state parameter w = p / ρ = 0 . Then, as new light particles are produced, the equation of state will switch to p = ρ / 3 , and the scale factor will expand according to a radiation dominated phase. In this case the equation of state parameter takes the form, w = p / ρ = 1 / 3 . The first approximation is then to just match the scale factor (and the conformal time) at these transitions. So it implies that during reheating the scale factor is lying within the window, 0 w r e h 1 / 3 . Consequently the conformal time on this epoch can be explicitly computed as:
τ = 0 τ d t a ( t ) = 3 ( 1 + w r e h ) ( 1 + 3 w r e h ) a ( τ ) ( 1 + 3 w r e h ) 2 Conformal time during reheating 0 < ø < ø reh a ( τ ) = ( 1 + 3 w r e h ) 3 ( 1 + w r e h ) τ 2 ( 1 + 3 w r e h ) Scale factor during reheating .
In the present context, during the computation of OTOCs both of the scale factors computed during inflationary epoch (for stochastic particle production) and reheating epoch are useful. Here it is important to note that in general the equation of state parameter during the reheating epoch can be expressed as a function of conformal time in general. If we assume that the energy momentum tensor can be expressed using a perfect fluid with pressure p r e h and energy density ρ r e h , then for a time-dependent scalar field in de Sitter FLRW background can be written as:
Equation of state parameter for reheating : 0 w r e h ( τ ) = p r e h ( τ ) ρ r e h ( τ ) 1 3 .
For N interacting scalar field the equation of state parameter can be computed as:
w r e h ( τ ) = 1 2 a 2 ( τ ) g 00 a = 1 N b = 1 N G a b τ ϕ a τ ϕ b U ( ϕ a ) 1 2 a 2 ( τ ) g 00 a = 1 N b = 1 N G a b τ ϕ a τ ϕ b + U ( ϕ a ) ,
where the pressure p r e h and energy density ρ r e h for N interacting scalar field can be written as:
Pressure : p r e h ( τ ) = 1 2 a 2 ( τ ) g 00 a = 1 N b = 1 N G a b τ ϕ a τ ϕ b U ( ϕ a ) ,
Density : ρ r e h ( τ ) = 1 2 a 2 ( τ ) g 00 a = 1 N b = 1 N G a b τ ϕ a τ ϕ b + U ( ϕ a ) .
Similarly for N non-interacting scalar field the equation of state parameter can be computed as:
w r e h ( τ ) = 1 2 a 2 ( τ ) g 00 a = 1 N ( τ ϕ a ) 2 U ( ϕ a ) 1 2 a 2 ( τ ) g 00 a = 1 N ( τ ϕ a ) 2 + U ( ϕ a ) ,
where the pressure p r e h and energy density ρ r e h for N non-interacting scalar field can be written as:
Pressure : p r e h ( τ ) = 1 2 a 2 ( τ ) g 00 a = 1 N ( τ ϕ a ) 2 U ( ϕ a ) ,
Density : ρ r e h ( τ ) = 1 2 a 2 ( τ ) g 00 a = 1 N ( τ ϕ a ) 2 + U ( ϕ a ) .
For a single scalar field ( N = 1 ) the equation of state parameter can be further simplified as:
w r e h ( τ ) = 1 2 a 2 ( τ ) g 00 ( τ ϕ ) 2 U ( ϕ ) 1 2 a 2 ( τ ) g 00 ( τ ϕ ) 2 + U ( ϕ ) ,
where the pressure p r e h and energy density ρ r e h for N non-interacting scalar field can be written as:
Pressure : p r e h ( τ ) = 1 2 a 2 ( τ ) g 00 ( τ ϕ ) 2 U ( ϕ ) ,
Density : ρ r e h ( τ ) = 1 2 a 2 ( τ ) g 00 ( τ ϕ ) 2 + U ( ϕ ) .
For FLRW case the time component of the metric g 00 = 1 = g 00 (statutory warning: from the above mentioned results it is important to note that when we are dealing with massless scalar field then we have U ( ϕ ) = 0 and consequently the equation of state parameter reduces to w = 1 w r e h . However, since 0 w r e h 1 3 any values w r e h > 1 3 is not physically allowed for the reheating phenomenon. So massless scalar field is not at allowed to describe the reheating phenomenon as it predicts w r e h = 1 which is strictly not allowed. So in the context of reheating, partially massless and massive scalar fields are allowed to describe the physical phenomenon. This is because of the fact that in both the cases, due to presence of mass term in the effective potential contribution, the equation of the state parameter computed from this setup is expected to lie within the allowed window, 0 w r e h 1 3 . Because of this reason during the computation of OTOCs during reheating we only restrict on partially massless and massive scalar fields. The details of these issues can be found in the next subsections. However, once we introduce the gauge invariant perturbation instead of field variable and translate the whole problem in terms of that new language, then it is explicitly possible to show that an effective mass m e f f 2 = 1 z ( τ ) d 2 z ( τ ) d τ 2 1 a ( τ ) d 2 a ( τ ) d τ 2 and spatial gradient of the gauge invariant perturbation term will be induced. Here z = 2 ϵ a is Mukhanov–Sasaki time-dependent variable which controls the mathematical structure of the effective mass term depending on the background physical phenomenon in which we are interested in. These phenomena are stochastic particle production during inflation and reheating respectively. There are many more; however, we restrict to these two fact only. At the leading order one can show that the effective mass term in the gauge invariant description can be expressed in terms of scale factor of our Universe and its double time derivative as a function of conformal time. Here a ( τ ) is the conformal time-dependent scale factor and ϵ = 1 a ( τ ) H d H d τ captures the rate of Hubble expansion in Cosmology, which also shows that a little bit deviation from constant Hubble parameter as appearing in de Sitter space. This deviation is very small at the particle production during inflation and becomes united at the end of inflation. On the other hand, at the epoch of reheating this factor becomes large. Due to these all facts, it is expected that the equation of state parameters during the epoch of reheating for massless scalar field in the gauge invariant description will lie within the window, 0 w r e h 1 3 . We will show details of the computation in the next subsections written for massless, partially massless and heavy scalar field in the gauge invariant perturbed description).
Now we will comment on some of the important future predictions from our analysis for massless, partially massless and massive scalar field theory in the context of computing OTOC which we will explicitly define in the following subsections. These predictions are appended below:
  • We are dealing with commutative version of the quantum field theory, i.e., all the field momenta and the fields are commutative amongst themselves. Things will change when one deals with the non commutative version of the quantum field theory which means in that in that case all the field momenta and the fields are non-commutative amongst themselves. Usually in both of the versions of quantum field theories we mostly consider the equal time commutation relations at different space points (ETCR) and the unequal time commutation relations (UETCR). In the Fourier transformed versions which can be translated as computing commutation relations at different momenta with same time for ETCR and with different time for UETCR. However, if we look into the specific mathematical structure of OTOC, then we see that the commutators are either ETCR or UETCR with fixed momentum or position.
  • If we get divergence to compute OTOC, then like the previous cases we deform the contour of integration in Schwinger–Keldyshpath integral formalism by introducing one or more regulators in the theory. In the technical ground, introducing the regulator cut-off means one needs to deform the Schwinger–Keldyshpath integral contour at finite temperature by introducing a single or more than one parameter. Consequently, in that context we will get a finite regulated result. On the other hand, in the context of non-commutative version of the quantum field theory it is expected that the form OTOC (the mathematical definition of OTOC is provided in the next subsections explicitly for massless, partially massless and massive scalar field theories) we get non vanishing physically relevant answers. During the computation of the OTOC if we get some divergences then like previous case here we also deform the contour of integration by introducing a cut-off to get finite results.
  • The explicit time-dependence of the OTOC is very important in the present context, particularly the study the physics of out-of-equilibrium. If the second term of the OTOC grows exponentially with time then one can quantify quantum chaos from the present computation and check whether the final result saturated the chaos bound on the Lyapunov exponent or not. On the other hand, if instead of getting an exponentially growing time-dependent behaviour if we get some other non-trivial characteristic behaviour as a function of time that is also very important to study in the present context to study the out-of-equilibrium behaviour of quantum mechanical systems considered in the framework of Cosmology. Particularly in Cosmology, studying the non trivial time-dependent behaviour of the OTOC for stochastic particle production during inflation, reheating are very useful to extract physical information from the quantum mechanical systems studied. Since we are trying to compute the OTOC for quantum field theory, it is expected to get some interesting physical outcomes from the correlation functions. However, it is also expected from the studies of the OTOC that at very large time limit the correlation functions reach thermal equilibrium and saturates to a finite value. This may not indicate the saturation of the quantum chaos, but this will also play significant role to study the time-dependent behaviour of OTOC. In this context, one can use this information as a limiting boundary condition of OTOC. Apart from that the early time behaviour of OTOC is also good to know. The early time behaviour is physically important as it gives the idea about the initial condition on the time-dependent behaviour of the OTOC. Initial condition OTOC actually fixes from which point in the time scale the quantum system goes to out-of-equilibrium. This information in the context of Cosmology is very useful because it will give a physical picture about a particular out-of-equilibrium phenomenon in which we are interested.
  • In this computation, we are considering massless ( m < < H ), partially massless ( m = 2 H H ) and massive ( m > > H ) scalar field for the computation of OTOC in the context of Cosmology. In the context of Cosmology, the fundamental characteristic scale is described by Hubble scale, i.e., H. Now compared to this characteristic scale we define massless, partially massless and massive scalar field in the de Sitter gravitational background. In the context of massive scalar field theory one usually consider various time-dependent protocols to describe the stochastic particle production phenomenon during inflation and reheating process in the context of primordial Cosmology. For a few types of time-dependent protocols one can exactly analytically solve the field equations for the scalar field, which are commonly used in the study of quantum quench. In general exact solution of the field equation is very difficult so solve and for this purpose one use the WKB approximation method to find out the analytical solution of the scalar field. These solutions are extremely important to study in de Sitter cosmological background as it provide the key ingredient to compute the previously mentioned OTOC in the present context. In the context of stochastic particle production one consider Dirac Delta type of scatterer as a time-dependent protocol, which is nothing but the white noise in the present context. In some cases, one also considers coloured noise time-dependent protocol. In the quantum description, the white noise is identified to be non-Markovian and coloured noise is considered as Markovian at the level of correlation function (quantum analogue: at the level of quantum correlation function for the white and coloured noise are described by the following correlation functions:
    White noise : m ( τ ) m ( τ ) = A ( τ ) δ ( τ τ ) Non Markovian , Coloured noise : m ( τ ) m ( τ ) = B ( τ ) K ( τ τ ) Markovian ,
    where A ( τ ) and B ( τ ) are the amplitude for the white and coloured noise respectively as a function of conformal time. Here δ ( τ τ ) represents a localised white noise function at τ = τ time scale. On the other hand, the coloured noise is represented by the general time-dependent Kernel K ( τ τ ) which is not at all localised at τ = τ time scale) (classical analogue: at the classical level Delta type of scatterer mimics the role of white noise appearing in the quantum regime. On the other hand, other non trivial time-dependent functions mimics the role of coloured noise at the quantum level. For N number of scatterers at the classical level one can then write:
    White noise : m 2 ( τ ) = j = 1 N A ( τ j ) δ ( τ τ j ) Classical Non Markovian , Coloured noise : m 2 ( τ ) = j = 1 N B ( τ j ) K ( τ τ j ) Classical Markovian ,
    where A ( τ j ) and B ( τ j ) are the amplitudes respectively as a function of conformal time. Here δ ( τ τ j ) represents a localised function at the point of j-th scatterer τ = τ j j = 1 , , N time scale. On the other hand, the coloured noise is represented by the general time-dependent Kernel K ( τ τ ) which is not at all localised at τ = τ time scale. However, any reliable physically significant time-dependent protocols are allowed in this context to extract the unknown physical information from the OTOC computation).
In the next subsections, we will explicitly define the expressions for the thermal OTOC in the context of the massless, partially massless, heavy scalar field and additionally for the stochastic scalar field which is used to study the particle production procedure in Early Universe Cosmology.

2.3.1. For Massless Scalar Field

In this context we will give the definition of OTOC computed from a massless scalar field. Here the terminology massless is used to consider scalar fields which have mass less than the characteristic Cosmological scale, i.e., the Hubble scale ( m < < H ). So here we are not considering exactly massless scalar fields. However, due to the approximation m < < H one can easily neglect such contribution in the effective action. A massless N number of scalar fields can be generalized by the following simplified action:
S = 1 2 d d + 1 x g a = 1 N b = 1 N g μ ν G a b μ ϕ a ν ϕ b .
In the simplest non-interacting situation, one can write G a b = δ a b and in that case the action for the N massless scalar field can be simplified as:
S = 1 2 d d + 1 x g a = 1 N g μ ν μ ϕ a ν ϕ a .
From this action one can further compute the Hamiltonian density, which is given by the following expression:
H = c = 1 N Π c 2 ˜ where Π c 2 ˜ = 1 2 a d ( t ) Π c 2 .
In this case the following normalised OTOC for the non-interacting case are the interesting one, which are given by:
2 - point OTOC : Y c c ( t , τ ) = ϕ c ( t ) , Π c ˜ ( τ ) β ,
4 - point OTOC : C c c ( t , τ ) = ϕ c ( t ) , Π c ˜ ( τ ) 2 β a = 1 N ϕ a ( t ) ϕ a ( t ) β b = 1 N Π b ˜ ( τ ) Π b ˜ ( τ ) β .
For the interacting case this OTOC is further generalised to the following expressions:
2 - point OTOC : Y c m ( t , τ ) = ϕ c ( t ) , Π m ˜ ( τ ) β ,
4 - point OTOC : C c m ( t , τ ) = ϕ c ( t ) , Π m ˜ ( τ ) 2 β a = 1 N n = 1 N ϕ a ( t ) ϕ n ( t ) β b = 1 N s = 1 N Π b ˜ ( τ ) Π s ˜ ( τ ) β .
For a single field ( N = 1 ) case this action can be further simplified as:
S = 1 2 d d + 1 x g g μ ν μ ϕ ν ϕ = 1 2 d d + 1 x g ϕ 2 ,
where we have expressed the Hamiltonian in ( d + 1 ) dimensional de Sitter background, which is given by the following expression:
H = Π 2 ˜ where Π 2 ˜ = 1 2 a d ( t ) Π 2 .
In this case the following normalised OTOC for the non-interacting case are the interesting one, which are given by:
2 - point OTOC : Y ( t , τ ) = ϕ ( t ) , Π ˜ ( τ ) β ,
4 - point OTOC : C ( t , τ ) = ϕ ( t ) , Π ˜ ( τ ) 2 β ϕ ( t ) ϕ ( t ) β Π ˜ ( 0 ) Π ˜ ( τ ) β .
During inflation (when we fix d = 3 ) we actually consider massless scalar field, i.e., m < < H . In this context one needs to consider the following perturbation in the scalar field in the de Sitter background:
Field after perturbation : ϕ ( x , t ) = ϕ ( t ) Background + δ ϕ ( x , t ) Perturbation
to express the whole dynamics in terms of a gauge invariant description through a variable:
Curvature perturbation : ζ ( x , t ) = H ( t ) ϕ ˙ ( t ) Background δ ϕ ( x , t ) perturbation First order contribution + Higher order contribution .
For the further analysis we restrict ourself only up to the first order contribution and will going to neglect all other higher order contributions as they are sufficiently small enough compared to the first order contribution as appearing in the above equation. In this context, geometrically the curvature perturbation ζ ( x , t ) measures the spatial curvature of constant hypersurface, which is represented by the following equation:
Spatial curvature : R ( 3 ) = 4 a 2 ( t ) 2 ζ ( x , t ) = 4 a 2 ( t ) H ( t ) ϕ ˙ ( t ) 2 δ ϕ ( x , t ) + .
One can explicitly show that at the level of perturbations of de Sitter metric if we fix the following gauge condition:
δ ϕ ( x , t ) = 0 ,
g i j ( x , t ) = a 2 ( t ) e 2 ζ ( x , t ) δ i j + h i j ( x , t ) a 2 ( t ) 1 + 2 ζ ( x , t ) δ i j + h i j ( x , t ) ,
i h i j ( x , t ) = 0 = h i i ( x , t ) .
which actually fixes the time and spatial reparameterizations for the dynamical fields. In this gauge the spatial curvature of constant hypersurface becomes zero, i.e., R ( 3 ) = 0 , which implies comoving curvature perturbation ζ ( x , t ) is conserved on superhorizon scales.
It can be explicitly shown that by doing ADM analysis that the second order perturbed action for scalar modes can be expressed by the following action after gauge fixing (it is important to note that here if we start with a scalar field theory with non-canonical kinetic term (such as general P ( X , ϕ ) theory, where X = 1 / 2 g μ ν μ ϕ ν ϕ is the kinetic term) then we get an additional factor of square of the sound speed appearing in the action:
S = d t d 3 x a 3 ( t ) ϕ ˙ 2 ( t ) 2 H 2 c S 2 ζ ˙ 2 ( x , t ) c S 2 a 2 ( t ) i ζ ( x , t ) 2 .
If we further fix P ( X , ϕ ) = X for the massless scalar field case then we can get back the sound speed c S = 1 as a special case):
S = d t d 3 x a 3 ( t ) ϕ ˙ 2 ( t ) 2 H 2 ζ ˙ 2 ( x , t ) 1 a 2 ( t ) i ζ ( x , t ) 2 .
Now we further define a gauge invariant variable, which is in the cosmology literature sometimes known as Mukhanov variable, as given by:
Mukhanov variable : f ( x , t ) z ( t ) ζ ( x , t ) , where z ( t ) = a ( t ) ϕ ˙ ( t ) H = 2 ϵ a ( t ) ,
which serves the purpose of field redefinition in terms of gauge invariant perturbation variable. Here ϵ is known as the slow-roll parameter, which is defined as, ϵ = H ˙ / H 2 . In this context, H ˙ < 0 and ϵ < 1 during inflation and ϵ = 1 at the end of inflation.
Translating in terms of the conformal time and applying integration by parts the above mentioned action can be recast in the following form:
S = d τ d 3 x 1 2 τ f ( x , τ ) 2 i f ( x , τ ) 2 + 1 z ( τ ) d 2 z ( τ ) d τ 2 f ( x , τ ) 2 .
However, dealing this problem in the coordinate space is very difficult as we are further interested in OTOCs in the present context. On the other hand, dealing this problem in Fourier space gives many good underlying physical features for which we do the following Fourier transform:
Fourier transform : f ( x , τ ) = d 3 k ( 2 π ) 3 f k ( τ ) e i k · x ,
using which the previously mentioned action can be further simplified as:
S = d τ d 3 k 1 2 τ f k ( τ ) 2 + ω k 2 ( τ ) f k ( τ ) 2 Lagrangian density of time dependent oscillator L ( f k ( ø ) , ø f k ( ø ) ) ,
where we define the effective time-dependent frequency ω k ( τ ) as:
Time dependent frequency : ω k 2 ( τ ) k 2 1 z ( τ ) d 2 z ( τ ) d τ 2 .
This means that after gauge fixing in terms of gauge invariant scalar curvature perturbation the problem is transformed to a time-dependent parametric oscillator problem.
From the above mentioned action one can compute the canonically conjugate momentum, which is given by the following expression:
Momentum density : Π k ( τ ) = L ( f k ( τ ) , τ f k ( τ ) ) τ f k ( τ ) = τ f k ( τ ) .
Consequently the Hamiltonian of the system can be expressed in terms of the gauge invariant variables as:
H = d 3 k 1 2 Π k 2 ( τ ) Kinetic term + ω k 2 ( τ ) f k 2 ( τ ) Potential term ,
Additionally, it is important to mention here that the following constraint have to be satisfied:
f k ( τ ) = f k ( τ ) = f k * ( τ ) .
Consequently we have used simplified notations in the above mentioned Hamiltonian in Fourier space, which are given by the following expression:
Π k 2 ( τ ) = Π k ( τ ) Π k ( τ ) = ( τ f k ( τ ) ) ( τ f k ( τ ) ) = f k ( τ ) f k ( τ ) = | f k ( τ ) | 2 = | Π k ( τ ) | 2 ,
f k 2 ( τ ) = f k ( τ ) f k ( τ ) = | f k ( τ ) | 2 .
In the present context one can interpret the time-dependent effective frequency of the gauge fixed field as the effective time-dependent mass of that field:
Time - dependent mass : m E 2 ( τ ) = ω k 2 ( τ ) .
where we explicitly compute the expression for the second term as:
1 z ( τ ) d 2 z ( τ ) d τ 2 = 1 a ( τ ) d 2 a ( τ ) d τ 2 + 1 2 ϵ d 2 ϵ d τ 2 1 2 ϵ 2 d ϵ d τ 2 + 1 a ( τ ) ϵ d ϵ d τ d a ( τ ) d τ Sub leading contributions .
for the massless scalar field. Now for inflation in the de Sitter background and at the epoch of reheating one can further simplify this above mentioned expression by neglecting the contributions from the sub-leading correction terms as:
1 z ( τ ) d 2 z ( τ ) d τ 2 = ν 2 1 4 τ 2 = 2 τ 2 de Sitter 2 ( 1 3 w r e h ) ( 1 + 3 w r e h ) 2 1 τ 2 , where 0 w r e h 1 3 Reheating
where the parameter ν is defined as:
ν = 3 2 de Sitter 1 4 + 2 ( 1 3 w r e h ) ( 1 + 3 w r e h ) 2 , where 0 w r e h 1 3 Reheating
Now, using this setup in terms of the gauge invariant perturbation variables the following interesting OTOC can be computed explicitly:
2 - point OTOC : Y ζ ( τ 1 , τ 2 ) = ζ ( τ 1 ) , Π ζ ( τ 2 ) β ,
4 - point OTOC : C ζ ( τ 1 , τ 2 ) = ζ ( τ 1 ) , Π ζ ( τ 2 ) 2 β ζ ( τ 1 ) ζ ( τ 1 ) β Π ζ ( τ 2 ) Π ζ ( τ 2 ) β .
On the other hand, if we want to express all the OTOC in terms of the gauge invariant perturbation variables and the corresponding rescaled field content and momenta, then one can use the following equivalent definition of OTOC in the present context, which are given by the following expressions:
2 - point OTOC : Y f ( τ 1 , τ 2 ) = f ( τ 1 ) , Π f ( τ 2 ) β ,
4 - point OTOC : C f ( τ 1 , τ 2 ) = f ( τ 1 ) , Π f ( τ 2 ) 2 β f ( τ 1 ) f ( τ 1 ) β Π f ( τ 2 ) Π f ( τ 2 ) β .
For the computation of all of these OTOC we use the SO(1,4) isommetric α vacua and the well known Bunch–Davies vacuum state as the quantum vacuum state.

2.3.2. For Partially Massless Scalar Field

Partially massless fields share some intermediate features of massive and massless fields in flat space. On the one hand, it is important to note that they carry more than two degrees of freedom in the context of quantum field theory. In this context we are interested in the partially massless scalar fields because it is expected that may survive until the end of inflation and therefore it is expected that during the stochastic particle production mechanism it plays significant role. Here the terminology partially massless is used to consider scalar fields which have mass approximately same with the characteristic Cosmological scale, i.e., the Hubble scale ( m H ). To serve this purpose we consider scalar field which is conformally coupled with the gravity and mass of such scalar fields can be written as, m = 2 H . A massless N number of scalar fields can be generalized by the following simplified action:
S = 1 2 d d + 1 x g a = 1 N b = 1 N g μ ν G a b μ ϕ a ν ϕ b ( m 2 ) a b ϕ a ϕ b .
In the simplest non-interacting situation one can write, G a b = δ a b , ( m 2 ) a b = c 2 H 2 δ a b and preferably we consider, c = 2 . In general c should not be a very large number. In this case the action for the N partially massless scalar field can be simplified as:
S = 1 2 d d + 1 x g a = 1 N g μ ν μ ϕ a ν ϕ a c 2 H 2 ϕ a 2 .
From this action one can further compute the Hamiltonian density, which is given by the following expression:
H = a = 1 N Π a 2 ˜ + U ( ϕ a ) ˜ with Π a 2 ˜ = 1 2 a d ( t ) Π a 2 , U ( ϕ a ) ˜ = a d ( t ) c 2 H 2 ϕ a 2 .
It is important to note that here all the previously mentioned definition of OTOCs for interacting and non-interacting N number of scalar fields in case of partially massless case look like similar except the thermal Botzmann factor appearing in the thermal average over ensembles. The prime reason is the expression for the Hamiltonian is different for the partially massless case compared to the massless case discussed in the just previous subsection.
For a single field ( N = 1 ) case this action can be further simplified as:
S = 1 2 d d + 1 x g g μ ν μ ϕ ν ϕ c 2 H 2 ϕ 2 = 1 2 d d + 1 x g ϕ 2 ( c H ϕ ) 2 ,
where we have expressed the Hamiltonian in ( d + 1 ) dimensional de Sitter background, which is given by the following expression:
H = Π 2 ˜ + U ( ϕ ) ˜ with Π 2 ˜ = 1 2 a d ( t ) Π 2 , U ( ϕ ) ˜ = a d ( t ) c 2 H 2 ϕ 2 .
To write down everything in terms of gauge invariant perturbations all expressions are similar as appearing in the previous subsection, except the following one:
1 z ( τ ) d 2 z ( τ ) d τ 2 = 1 a ( τ ) d 2 a ( τ ) d τ 2 + 1 2 ϵ d 2 ϵ d τ 2 1 2 ϵ 2 d ϵ d τ 2 + 1 a ( τ ) ϵ d ϵ d τ d a ( τ ) d τ Sub leading contributions .
for the partially massless scalar field. Now for inflation in the de Sitter background and at the epoch of reheating one can further simplify this above mentioned expression by neglecting the contributions from thesub-leading correction terms as:
1 z ( τ ) d 2 z ( τ ) d τ 2 = ν 2 1 4 τ 2 = 2 c 2 1 τ 2 , where c 2 de Sitter 2 ( 1 3 w r e h ) ( 1 + 3 w r e h ) 2 1 τ 2 , where 0 w r e h 1 3 Reheating
where the parameter ν is defined as:
ν = 9 4 c 2 , where c 2 de Sitter 1 4 + 2 ( 1 3 w r e h ) ( 1 + 3 w r e h ) 2 , where 0 w r e h 1 3 Reheating
For the conformally coupled partially massless scalar field ( c = 2 ) it is clearly observed from the above mentioned expressions that for the de Sitter case we get, 1 z ( τ ) d 2 z ( τ ) d τ 2 = 0 and ν = 1 / 2 . This will be very useful for the further computation of the OTOCs. Apart from the the mathematical definitions of the OTOCs in coordinate space or in momentum space are exactly same apart from the exponential thermal Boltzmann factor where the expression for the Hamiltonian will change in the partially massless scalar field case.

2.3.3. For Massive Scalar Field

In this section, we discuss the outcomes from massive scalar field theory. Here the terminology massive is used to consider scalar fields which have mass much heavier the characteristic Cosmological scale, i.e., the Hubble scale ( m > > H ). To serve this purpose we consider scalar field which have constant and time-dependent profile both. A massless N number of scalar fields can be generalized by the following simplified action:
S = 1 2 d d + 1 x g a = 1 N b = 1 N g μ ν G a b μ ϕ a ν ϕ b ( m 2 ( t ) ) a b ϕ a ϕ b .
In the simplest non-interacting situation one can write, G a b = δ a b , ( m 2 ) a b = m 2 ( t ) δ a b and in general m 2 ( t ) is a function of time coordinates. However, for some special cases one can consider constant time-independent profiles as well for the mass function. We will choose this simplest possibility for the further computation. In this case the action for the N massive scalar field can be simplified as:
S = 1 2 d d + 1 x g a = 1 N g μ ν μ ϕ a ν ϕ a m 2 ϕ a 2 .
From this action one can further compute the Hamiltonian density, which is given by the following expression:
H = a = 1 N Π a 2 ˜ + U ( ϕ a ) ˜ with Π a 2 ˜ = 1 2 a d ( t ) Π a 2 , U ( ϕ a ) ˜ = a d ( t ) m 2 ϕ a 2 .
It is important to note that here all the previously mentioned definition of OTOC for interacting and non-interacting N number of scalar fields in case of massive case look like similar except the thermal Boltzmann factor appearing in the thermal average over ensembles.
For a single field ( N = 1 ) case this action can be further simplified as:
S = 1 2 d d + 1 x g g μ ν μ ϕ ν ϕ m 2 ( t ) ϕ 2 = 1 2 d d + 1 x g ϕ 2 ( m ϕ ) 2 ,
where we have expressed the Hamiltonian in ( d + 1 ) dimensional de Sitter background, which is given by the following expression:
H = Π 2 ˜ + U ( ϕ ) ˜ with Π 2 ˜ = 1 2 a d ( t ) Π 2 , U ( ϕ ) ˜ = a d ( t ) m 2 ϕ 2 .
To write down everything in terms of gauge invariant perturbations all expressions are similar as appearing in the previous subsection, except the following one:
1 z ( τ ) d 2 z ( τ ) d τ 2 = 1 a ( τ ) d 2 a ( τ ) d τ 2 + 1 2 ϵ d 2 ϵ d τ 2 1 2 ϵ 2 d ϵ d τ 2 + 1 a ( τ ) ϵ d ϵ d τ d a ( τ ) d τ Sub leading contributions .
for the massive scalar field. Now for inflation in the de Sitter background and at the epoch of reheating one can further simplify this above mentioned expression by neglecting the contributions from the sub-leading correction terms as:
1 z ( τ ) d 2 z ( τ ) d τ 2 = ν 2 ( τ ) 1 4 τ 2 = m 2 ( τ ) H 2 2 1 τ 2 , where m > > H de Sitter 2 ( 1 3 w r e h ) ( 1 + 3 w r e h ) 2 1 τ 2 , where 0 w r e h 1 3 Reheating
where the parameter ν is defined as:
ν ( τ ) = i m 2 H 2 9 4 , where m > > H de Sitter 1 4 + 2 ( 1 3 w r e h ) ( 1 + 3 w r e h ) 2 , where 0 w r e h 1 3 Reheating
This will be very useful for the further computation of the OTOCs. Apart from the the mathematical definitions of the OTOCs in coordinate space or in momentum space are exactly same apart from the exponential thermal Boltzmann factor where the expression for the Hamiltonian will change in the massive scalar field case.
If the massive scalar field is conformal time-dependent then one can use the following conformal time-dependent protocols for the computation of OTOCs with the massive scalar fields:
m 2 ( τ ) = δ ( τ τ ) Non - Markovian White noise K ( τ τ ) Markovian Coloured noise
For the simplicity here we have fixed the amplitudes A ( τ ) = 1 = B ( τ ) for both the white and coloured noise conformal time-dependent effective mass profile protocols. Now for the Markovian coloured noise case if we fix τ = 0 then then for the phenomenological purpose one can consider the following sets of protocols, which have huge applications in the context of quantum quench, information theory and condensed matter field theory:
m 2 ( τ ) H 2 = K ( τ ) H 2 = ( a ± b tanh ( q τ ) ) Protocol I Θ ( τ ± τ 0 ) Protocol II sech 2 ( q τ ) Protocol III a τ τ 0 1 2 + b Protocol IV tanh 2 ( q τ ) Protocol V ( a ± b sech 2 ( q τ ) ) Protocol VI
These Markovian coloured noise profiles are frequently used in the context of quantum mechanical quench in the context of condensed matter field theory and quantum information theory. Since for the above mentioned time-dependent protocols the mode function for the scalar perturbations are not exactly solvable the WKB approximated mentioned is very useful to compute the expression for for cosmological OTOC in such cases.Though in this paper we have not computed explicitly OTOC using WKB approximated method, but have explicitly derived the general solution of the WKB approximated solution of general conformal time-dependent mass profile in the Appendix A, Appendix B, Appendix C, Appendix D, Appendix E, Appendix F, Appendix G, Appendix H, Appendix I, Appendix J, Appendix K, Appendix L and Appendix M, which one can use further to compute the expression for the cosmological OTOC from the present set up. In the next section, we explicitly compute the expressions for the OTOCs that we have defined in this section for massless, partially massless and massive scalar (with time-independent mass) field theory.

3. Quantum Micro-Canonical OTO Amplitudes and OTOC in Cosmology

In this section we will derive and interpret the results for OTOC introduced and defined in the previous section for massless, partially massless and massive scalar fields in the context of Cosmology, which are useful to study the physical implications of the stochastic process of particle production during the epoch of inflation and also during the reheating epoch as well. We will show that how are derived result will be very useful to provide new information regarding the above mentioned two processes appearing in the context of Cosmology of Early Universe or Primordial Cosmology. Instead of computing the expressions for the usual time ordered or the anti-time ordered correlation functions, which is commonly studied in the context of Cosmology, here we are actually interested to study the expressions for the OTOC because in the present context we want to study the physical outcome of the out-of-equilibrium phenomenon driven Quantum Field Theory aspects of Cosmology. The stochastic particle production and reheating phenomenon are two most important significant facts during which the quantum system studied in this context goes to out-of-equilibrium, and after waiting for a sufficient time scale all such quantum system reaches equilibrium and it is expected in all those cases that the time-dependent behaviour of the quantum correlation function saturates for large late time scale in Cosmology. It is expected that when the system goes to the out-of-equilibrium quantum state the behaviour of the quantum correlation function will be solely controlled by the OTOC that we have defined in the earlier sections. Since these type of correlations are not studied in the context of Cosmology, it is expected that many unexplored features of Cosmology at out-of-equilibrium phase can be studied from the outcomes of the OTOC, which we will compute for massless, partially massless and massive heavy scalar field theories. Apart from the application in the context of stochastic particle production during inflation and reheating, the present computation can be applicable to the Dark Matter bound-state formation at higher order and many more other places in the evolution of our Universe. Though we have not studied this possibility in this paper, but one can think for doing such calculation in the context of Dark Matter. Additionally it is important to note that during the computation of OTOC the quantum system studied in the context of the time evolution of our Universe is closed or isolated or adiabatic quantum system. Here the terminology closed is used to describe a quantum system of our Universe is not thermally or any other way interacting with any kind of environment. So it is obvious here that when the system is interacting with the environment thermally or any other way the corresponding quantum system is identified to be an open or non-adiabatic system. So it might be a nice option to carry forward the computation for OTOC for Cosmology with open quantum systems.

3.1. Computational Strategy

The steps of computing the OTOC are appended below point-wise:
  • At first, we need find out the analytic solution of the equation of motion of the scalar field in the corresponding FLRW flat spatial background.
  • From the above mentioned solution we need to find out the canonically conjugated field momentum and using that we need to compute the square of the quantum mechanical commutator bracket. It is proved during performing this operation we need to quantize the field content in terms of the creation and annihilation operators.
  • From the above mentioned result we need to further compute the thermal average value of the square of the commutator bracket of the field and its canonically conjugate momenta, which physically represents the four-point out-of-time ordered correlation function.
  • Next, we need to compute the partition function the quantum system under consideration by computing the thermal Boltzmann factor and the trace of the corresponding thermal Boltzmann factor. To serve this purpose we need to also compute the expression for the Hamiltonian from the quantum mechanical system under consideration.
  • Furthermore, we need to compute the thermal two point out-of-time ordered correlation functions from the field content and the corresponding canonically conjugate momentum. This will help us to find out the expression for the normalised OTOC which is defined in the earlier sections explicitly.
  • Furthermore, instead of computing these description in the coordinate space one can do a similar computation of OTOC in Fourier space as well. For this purpose one needs to compute all of the required operators in the Fourier transformed space. Sometimes doing the computation in Fourier transformed space is simpler than the computation in coordinate space. Most importantly, doing the computation in Fourier space is really very helpful in the context of Cosmology. The primary reason is that doing the computation in momentum space (which is the Fourier transformed version of the coordinate space) in the context of Cosmology is more technically interesting. Actually, in momentum space most of the quantum correlation functions in the context of Cosmology preserves the conformal symmetry under conformal transformations. In quasi de Sitter space, this conformal symmetry is slightly broken in Fourier space by the amount of slow-roll parameters, which we have taken into account in our computations.
  • In the present context it is better to perform the whole computation in a preferred choice of gauge, which is commonly performed in the context of Cosmological Perturbation Theory. Following the same strategy here we work on δ ϕ = 0 gauge, which makes the computation of OTOC simpler than doing the computation doing without the choice of a preferred gauge.
  • It is important to mention here that doing the computation of OTOC for N interacting scalar field analytically are extremely complicated. On the other hand, doing the computation with N non-interacting multi scalar field is also not very simple. For this reason we try in this paper to restrict ourself to the non-interacting single field and N field case. For the single field case we also provide the computation of OTOC in the previously mentioned preferred gauge choice.
  • Last but not least, doing the computation of OTOC in any arbitrary dimensions is not at all possible analytically. So we have to restrict our computation in this article in d = 3 spatial dimension which is compatible with our spatially flat 3 + 1 dimensional space time in FLRW cosmological background.

3.2. Classical Mode Functions in Cosmology

After varying the gauged action written in δ ϕ = 0 gauge with respect to the redefined field we get the following equation of motion in the Fourier transformed space:
d 2 d τ 2 + k 2 ν 2 1 4 τ 2 f k ( τ ) = 0 ,
where the parameter ν is defined for the massless, partially massless and massive heavy scalar field case as:
ν = 3 2 DS + massless 9 4 c 2 , where c 2 ( m H ) DS + partially massless i m 2 H 2 9 4 , where m > > H DS + heavy 1 4 + 2 ( 1 3 w r e h ) ( 1 + 3 w r e h ) 2 , where 0 w r e h 1 3 Reheating
Here for the third case the explicit form of the conformal time-dependent mass functions are mentioned in the earlier section.
One can further generalise the expression for the mass parameter ν if we introduce a non-minimal coupling parameter ξ to the Ricci scalar curvature term in the action, by replacing the mass term m 2 ϕ 2 with the new rescaled term ( m 2 + ξ R ) ϕ 2 , i.e.,
m 2 ϕ 2 m n e w 2 ϕ 2 = ( m 2 + ξ R ) ϕ 2 .
When we consider the de Sitter space, there the Ricci scalar R is constant and given by R = 12 H 2 . In this case the the new mass parameter can be expressed as,
ν ν n e w = 9 4 m n e w 2 H 2 = 9 4 m 2 + 12 ξ H 2 H 2 = 9 4 12 ξ + m 2 H 2 .
Consequently, we will get same expressions everywhere with a newly rescaled mass parameter ν where non-minimal coupling have been introduced explicitly. So even in the most generalised case where the effective mass term is conformal time-dependent, there is also we replace the term m 2 / H 2 with the 12 ξ + m 2 / H 2 , i.e.,
m 2 H 2 m n e w 2 H 2 = 12 ξ + m 2 H 2 .
In the expression for ν then in the approximated WKB solutions that fact will be propagated. It is important to note that the same prescription is followed in the context of Higgs inflation where the quadric term in the Higgs field is non-minimally coupled to the gravitational sector through the scalar Ricci scalar curvature term. This is a necessary requirement to have inflation from the Higgs sector within the framework of Einstein gravity. Now one can consider a bit more complicated phenomenological situation where the non minimal coupling in the FLRW spatially flat background itself dependent on conformal time and in principal any arbitrary form of the conformal time-dependent functions are allowed for that computation. This will further modify the analytical WKB approximated expression for the mode functions for the scalar perturbation in that case for a given conformal time-dependent specific form of the effective mass m 2 ( τ ) and the non-minimal coupling parameter ξ ( τ ) and in that case we have to consider the following rescaling:
m 2 ( τ ) H 2 m n e w 2 ( τ ) H 2 = 12 ξ ( τ ) + m 2 ( τ ) H 2 .
In this case the the new conformal time-dependent mass parameter can be expressed as,
ν ( τ ) ν n e w ( τ ) = 9 4 m n e w 2 ( τ ) H 2 = 9 4 m 2 ( τ ) + 12 ξ ( τ ) H 2 H 2 = 9 4 12 ξ ( τ ) + m 2 ( τ ) H 2 .
Now the most general solution of the mode equation for any constant mass profile (massless, partially massless and heavy scalar production during inflation and during reheating) is given by the following expression:
f k ( τ ) = τ C 1 H ν ( 1 ) ( k τ ) + C 2 H ν ( 2 ) ( k τ ) ,
where C 1 and C 2 are two arbitrary integration constants which are fixed by the choice of the initial quantum vacuum state necessarily needed for this computation. Here H ν ( 1 ) ( k τ ) and H ν ( 1 ) ( k τ ) are the Hankel functions of first and second kind with order ν .
In the general context the mass parameter ν may be a complex parameter. In this case, the solution for the rescaled scalar perturbation mode can be recast as:
f k ( τ ) = τ D 1 J ν ( k τ ) + D 2 Y ν ( k τ ) ,
where the redefined two new arbitrary integration constants, D 1 and D 2 are defined in terms of the previously defined two new arbitrary integration constants, C 1 and C 2 as, D 1 = C 1 + C 2 , and D 2 = i C 1 C 2 . For the non-integer value of the mass parameter ν , the solution for the rescaled scalar perturbation mode can be recast as:
f k ( τ ) = τ E 1 ( ν ) J ν ( k τ ) + E 2 ( ν ) J ν ( k τ ) ,
where the redefined two new arbitrary integration constants, E 1 and E 2 are defined in terms of the previously defined two new arbitrary integration constants, C 1 and C 2 as:
E 1 ( ν ) = C 1 + C 2 + i C 1 C 2 cot ν π = D 1 + D 2 cot ν π ,
E 2 ( ν ) = i C 1 C 2 cosec ν π = D 2 cosec ν π .
In particular, when ν is an integer or not. In this case, the solution for the rescaled scalar perturbation mode can be recast as:
f k ( τ ) = τ G 1 ( ν ) J ν ( k τ ) + G 2 ( ν ) J ν ( k τ ) ,
where the redefined two new arbitrary integration constants, G 1 and G 2 are defined in terms of the previously defined two new arbitrary integration constants, C 1 and C 2 as:
G 1 ( ν ) = C 1 + C 2 + i C 1 C 2 cot ν π = D 1 + D 2 cot ν π E 1 ( ν ) ,
G 2 ( ν ) = i cosec ν π C 2 C 1 = D 2 cosec ν π E 2 ( ν ) .
The corresponding most general canonically conjugate momentum can be further computed from this derived solution as:
Π k ( τ ) = τ f k ( τ ) = 1 2 τ C 1 k τ H ν 1 ( 1 ) ( k τ ) H ν ( 1 ) ( k τ ) k τ H ν + 1 ( 1 ) ( k τ ) + C 2 k τ H ν 1 ( 2 ) ( k τ ) H ν ( 2 ) ( k τ ) k τ H ν + 1 ( 2 ) ( k τ ) .
Furthermore, one can express the Bessel function of the first kind in terms of the Confluent Hypergeometric limit functions in the present context, using which the most general solution of the above mentioned equation of motion is given by the following expression:
f k ( τ ) = E 1 ( ν ) τ Γ ( ν + 1 ) k τ 2 ν 0 F 1 ν + 1 ; ( k τ ) 2 4 E 2 ( ν ) τ Γ ( 1 ν ) k τ 2 ν 0 F 1 1 ν ; ( k τ ) 2 4 .
Furthermore, the asymptotic solution for the rescaled scalar perturbation can be expressed within the window 0 < ( k τ ) < ν + 1 , as:
f k ( τ ) = D 1 + D 2 cot ν π Γ ( ν + 1 ) k τ 2 ν D 2 Γ ( ν ) π k τ 2 ν if ν > 0 integer D 1 ( 1 ) ν ( ν ) ! k τ 2 ν + D 2 ( 1 ) ν + 1 Γ ( ν ) π k τ 2 ν , if ν < 0 integer
For large real arguments lying within the window, ( k τ ) > > ν 2 1 4 , one cannot write an actual asymptotic form for the Bessel functions of the first and second kind (unless in the situation where ν is a half-integer) because they have zeros all the way out to infinity, which would have to be matched exactly by any asymptotic expansion. However, for a given value of arg ( k τ ) < π , one can write an equation containing a term of order of | k τ | 1 , given by the following expressions:
f k ( τ ) = 2 π k C 1 exp ( i k τ ) exp i π 2 ν + 1 2 C 2 exp ( i k τ ) exp i π 2 ν + 1 2 + 2 π k exp Im ( k τ ) O 1 | k τ | D 1 D 2 .
However, from the general structure of the obtained solution for the rescaled field and for the canonically conjugate momentum it is very difficult to extract the physical information out of that. For this reason the asymptotic solutions are really helpful for physical interpretation in different cosmological scales. These asymptotic limits are k τ 0 and k τ , where we need to determine the behaviour of the Hankel functions of the first and second kind of order ν . Here k τ 0 and k τ asymptotic limiting results are used to describe the superhorizon ( k τ < < 1 ) and subhorizon ( k τ > > 1 ) limiting results in the context of primordial cosmological perturbation scenario. The transition point from the subhorizon to superhorizon regime is identify by k τ = 1 , which in Cosmology is known as the horizon exit and plays a pivotal role to measure various observables of primordial Universe from different theoretical models.
Combining the behaviour in both the superhorizon and subhorizon limiting region we get following asymptotic most general solution for the rescaled field and momentum variable computed for the arbitrary quantum initial vacuum can be expressed as:
f k ( τ ) = 2 ν 3 2 i τ 1 2 k 3 2 ( k τ ) 3 2 ν Γ ( ν ) Γ 3 2 × C 1 ( 1 + i k τ ) exp i k τ + π 2 ν + 1 2 C 2 ( 1 i k τ ) exp i k τ + π 2 ν + 1 2 ,
Π k ( τ ) = 2 ν 3 2 i ( k τ ) 3 2 ν 2 k 5 2 Γ ( ν ) Γ 3 2 [ C 1 1 2 ν ( 1 + i k τ ) k 2 τ 2 + 1 exp i k τ + π 2 ν + 1 2 C 2 1 2 ν ( 1 i k τ ) k 2 τ 2 + 1 exp i k τ + π 2 ν + 1 2 ] ,
These general asymptotic expressions are extremely important to compute the expressions for the OTOCs in the later subsections. To server this purpose we need to promote both of these classical solutions to the quantum level.

3.3. Quantum Mode Function in Cosmology

Now, as we have mentioned earlier to explicitly compute OTOC we need to map the classical solution of the equation of motion of the dynamical conformal time-dependent rescaled field and the corresponding canonically conjugate momentum to the quantum operator description. Consequently, the classical poisson bracket will be replaced by the quantum mechanical commutator bracket in the present context. The representative map is given by the following expressions:
f k ( τ ) Classical to quantum map f ^ k ( τ ) ,
Π k ( τ ) Classical to quantum map Π ^ k ( τ ) , Classical quantities Quantum operators
f k ( τ ) , Π k ( 0 ) Classical to quantum map f ^ k ( τ ) , Π ^ k ( 0 ) ,
f k ( τ ) , f k ( 0 ) Classical to quantum map f ^ k ( τ ) , f ^ k ( 0 ) ,
Π k ( τ ) , Π k ( 0 ) Classical to quantum map f ^ k ( τ ) , Π ^ k ( 0 ) , Poisson bracket Commutator bracket
Here, in this context, the rescaled field operator and the corresponding canonically conjugate momentum in the quantum regime can be expressed as:
f ^ k ( τ ) = f k ( τ ) a k + f k * ( τ ) a k ,
Π ^ k ( τ ) = Π k ( τ ) a k + Π k * ( τ ) a k .
Here a k and a k are the annihilation and creation operators of the conformal time-dependent parametric oscillators after performing canonical quantization, which satisfy the following commutation relation relations:
a k , a k = ( 2 π ) 3 δ 3 ( k + k ) , a k , a k = 0 = a k , a k .
Consequently the curvature perturbation and the corresponding momentum operator in the quantum regime can be re-expressed as:
ζ ^ k ( τ ) = f ^ k ( τ ) z = f k ( τ ) a k + f k * ( τ ) a k z = ζ k ( τ ) a k + ζ k * ( τ ) a k , Π ^ ζ , k ( τ ) = τ f ^ k ( τ ) z = Π ^ k ( τ ) z ζ ^ k ( τ ) z ( τ ) d z ( τ ) d τ
= Π ^ k ( τ ) z ζ ^ k ( τ ) d ln a ( τ ) d τ + 1 2 d ln ϵ ( τ ) d τ Sub leading .
Now neglecting the sub-leading contribution we get the following simplified expression for the canonically conjugate momentum of the quantum curvature perturbation operator:
Π ^ ζ , k ( τ ) = Π ζ , k ( τ ) a k + Π ζ , k * ( τ ) a k ζ k ( τ ) a k + ζ k * ( τ ) a k 1 a ( τ ) d a ( τ ) d τ ,
where the scale factor a ( τ ) is different for de Sitter inflationary patch and during the epoch of reheating, which is given by the following expression:
a ( τ ) = 1 H τ de Sitter ( 1 + 3 w r e h ) 3 ( 1 + w r e h ) τ 2 ( 1 + 3 w r e h ) with 0 w r e h 1 3 Reheating
Using the above mentioned expressions for the scale factors the corresponding parameter ϵ for both of the cases can be calculated as:
ϵ = 1 a ( τ ) H 2 d H d τ = 2 τ g ( τ ) τ g ( τ ) g ( τ ) 1 de Sitter 3 2 ( 1 + w r e h ) Reheating
Furthermore, the expression for Mukhanov-Sasaki varibale is can be computed for both of the cases as:
z ( τ ) = 2 ϵ a ( τ ) = 2 H g ( τ ) τ τ g ( τ ) g ( τ ) 1 de Sitter 3 ( 1 + w r e h ) ( 1 + 3 w r e h ) 3 ( 1 + w r e h ) τ 2 ( 1 + 3 w r e h ) Reheating
Here, it is important to note the following crucial facts which are helpful for the further computations of OTOCs:
  • In the de Sitter case with FLRW background g ( τ ) is a slowly varying conformal time-dependent function in general, which physically represents a very slight deviation from the de Sitter solution, where the Hubble parameter H = Λ / 3 is dominated by Cosmological Constant Λ > 0 in Planckian units. It is observed from the computation ϵ ( τ ) and g ( τ ) that both of the parameters capture the effect of such deviation and for this reason sometimes one refers this as the quasi de Sitter space-time.
  • Another important fact is that since we strictly do not know the explicit conformal tie dependence of the reheating equation of state w r e h ( τ ) , we have assumed that the time-dependence is very slow and for this purpose, this crucial parameter exactly behaves like a constant with the restriction that 0 w r e h 1 / 3 . This fact is extremely important to mention as it fixes the form of the scale factor a ( τ ) , parameter ϵ ( τ ) (the slow-roll parameter ϵ = H ˙ / H 2 = H / ( a H 2 ) is commonly used in the context of quasi de Sitter inflationary phase where the scale factor can be expressed as a ( τ ) = 1 H τ ( 1 + ϵ ) 1 H τ ( 1 + g ( τ ) ) . During inflation ϵ < < 1 and at the end of inflationary phase ϵ = 1 , which is very useful to determine the corresponding field value at the end of inflation. So during stochastic particle production during the inflationary phase, we will respect these constraints on the slow-roll parameter ϵ . On the other hand, since reheating happened after inflation then it is expected that the magnitude of the parameter ϵ > 1 during reheating, where the scale factor can be expressed as a ( τ ) = ( 1 + 3 w r e h ) 3 ( 1 + w r e h ) τ 2 ( 1 + 3 w r e h ) 1 1 ϵ τ 1 ( ϵ 1 ) . This statement can be justified very easily in the context during the epoch of reheating. From the previously derived expression for the parameter ϵ in the context of reheating we have found that ϵ 3 2 ( 1 + w r e h ) , where the equation of state parameter 0 w r e h 1 / 3 . This directly implies that during the epoch of reheating the parameter ϵ will lie within the window, 3 / 2 ϵ 2 (alternatively one can say that the Hubble velocity is lying within the window 3 2 H 2 v H = | H ˙ | 2 H 2 in the Planckian units), which is obviously a very strong and specific information we get from the from the present computation) and the Mukhanov–Sasaki variable z ( τ ) in terms of conformal time τ as well as the equation of state parameter w r e h in the present context.
We will strictly follow the above mentioned two facts for the rest of the computations that we have performed to mathematically quantify the expressions for the OTOCs in the context of stochastic particle production during inflation, which is dominated by quasi de Sitter space-time and during the epoch of reheating. This further implies that the physical outcomes of the out-of-equilibrium studied in terms of OTOCs respect both of the above mentioned assumptions.

3.4. Canonical Quantization of Cosmological Hamiltonian: Classical to Quantum Map

Now we have to promote the classical Hamiltonian that we have derived earlier to the quantized Hamiltonian. This can be expressed as:
H Classical to quantum map H ^ = d 3 k 1 2 Π ^ k 2 ( τ ) Kinetic term + ω k 2 ( τ ) f ^ k 2 ( τ ) Potential term ,
Additionally, it is important to mention here that the following constraint has to be satisfied:
f ^ k ( τ ) = f k ( τ ) a k + f k * ( τ ) a k = f k ( τ ) a k + f k * ( τ ) a k = f ^ k ( τ ) ,
Π ^ k ( τ ) = Π k ( τ ) a k + Π k * ( τ ) a k = Π k ( τ ) a k + Π k * ( τ ) a k = Π ^ k ( τ ) .
Consequently, we have used a simplified notation in the above mentioned Hamiltonian in Fourier space, which is given by the following expression:
Π ^ k 2 ( τ ) = Π ^ k ( τ ) Π ^ k ( τ ) = | Π k ( τ ) a k + Π k * ( τ ) a k | 2 = Π k ( τ ) a k + Π k * ( τ ) a k Π k ( τ ) a k + Π k * ( τ ) a k = Π k ( τ ) a k + Π k * ( τ ) a k Π k ( τ ) a k + Π k * ( τ ) a k = 2 a k a k + 1 2 δ 3 ( 0 ) | Π k ( τ ) | 2 = 2 a k a k + 1 2 δ 3 ( 0 ) Π k 2 ( τ )
f ^ k 2 ( τ ) = f ^ k ( τ ) f ^ k ( τ ) = | f k ( τ ) a k + f k * ( τ ) a k | 2 = f k ( τ ) a k + f k * ( τ ) a k f k ( τ ) a k + f k * ( τ ) a k = f k ( τ ) a k + f k * ( τ ) a k f k ( τ ) a k + f k * ( τ ) a k = 2 a k a k + 1 2 δ 3 ( 0 ) | f k ( τ ) | 2 = 2 a k a k + 1 2 δ 3 ( 0 ) f k 2 ( τ ) .
Consequently, we get the following simplified form of the quantized Hamiltonian:
H ^ = d 3 k a k a k + 1 2 δ 3 ( 0 ) Π ^ k 2 ( τ ) + ω k 2 ( τ ) f ^ k 2 ( τ ) ,
After introducing the normal ordering one can neglect the contribution from the zero point energy, which actually gives the divergent contribution. This further simplifies the the expression for the Hamiltonian, which is given by:
: H ^ : = d 3 k a k a k Π ^ k 2 ( τ ) + ω k 2 ( τ ) f ^ k 2 ( τ ) .
Here, the expression for the time-dependent frequency which has already been mentioned earlier, which can be further expressed in terms of the particle production during inflation in quasi de Sitter phase and during the epoch of reheating. The above mentioned expressions for the Hamiltonians are extremely useful as it will appear in the expression for the thermal Botzmann factor of all OTOCs.

3.5. Cosmological Two-Point and Four-Point “In-In" OTO Micro-Canonical Amplitudes

In this section our prime objective is to compute the explicit expression for the OTOC.

3.5.1. OTOC Meets Cosmology

To compute this explicitly we need the following information in our hand:
  • Information I:
    First of all, one need the quantum operators corresponding to the rescaled field and its canonically conjugate momenta, which is actually written in terms of the classical solution obtained from the mode equation and the creation and annihilation operators. Actually, in the context of Cosmology one can write down the Hamiltonian of the system in terms of an Harmonic oscillator with conformal time-dependent frequency in a Fourier space and after constructing this Hamiltonian one needs to integrate over all possible momenta. Using this prescription and using the canonical quantization procedure one can easily quantize the system Hamiltonian in the framework of Cosmology.
  • Information II:
    Next, one needs to compute the expression for the expression for the square of the commutator bracket in the background of FLRW space-time in curved space quantum field theory in coordinate space. This is the prime component which will fix the final expression for OTOC in the framework of Cosmology. We will explicitly show that this contribution can be written in terms of four parts where each of the parts in Fourier transformed space mimics the role of some kind of scattering amplitudes after taking its trace, which are functions of four momenta and two time scales as appearing in the definition of OTOC. Precisely these four momenta are coming due to the fact that in each contribution, we are dealing with four quantum operators. Now if we are careful about the technicality then we have to say these four-point scattering amplitudes are basically the four-point time-dependent correlation functions in the framework of Cosmology as during performing the trace operation we have to use the same quantum vacuum state, which means that the initial and final state both are the same, and identified as the “in” state. So it is basically an in-in amplitude rather than an usual “in-out” amplitude. The usual “in-out” amplitude can be usually computed using the idea of S matrix, which is a Schwinger Dyson series in the context of quantum field theory and this is an unitary matrix in the context of a closed system in our Universe when the system under consideration is not exchanging any information with the surroundings. On the other hand, in the Cosmological set-up we deal with “in-in” amplitudes which can be computed using the well known Schwinger–Keldyshformalism. Instead of calling this quantity which we want to evaluate as “in-in” amplitude we call these contributions “in-in” quantum correlation functions in the framework of Cosmology.
  • Information III:
    Furthermore, we have to fix the definition of trace in the context of quantum field theory in a classical gravitational background, specifically in the context of FLRW Cosmology. To serve this purpose, we need to first use a standard definition of the quantum wave function of our Universe. For this purpose, the most common choice is to use the definition of standard Bunch–Davies vacuum state, which is basically a thermal ground state in the framework of Cosmology. Sometimes this vacuum is identified to be the Hartle–Hawking or Cherenkov vacuum state in quantum field theory theory of curved space-time with Cosmological background. Apart from that, another useful vacuum is the α vacuum, which is mostly used in the context of interacting quantum field theory in curved space-time with cosmological background. Here in the construction of the α vacuum it appears as a one real parameter family which can take any continuous value starting from the zero value. The zero value is a special case of the α vacuum which is basically the well known Bunch–Davies quantum vacuum ground state. One the other hand, for the other values of α one can construct infinite number of states which can be considered as some excited states which can be expressed in terms of the known Bunch–Davies vacuum state using the Bogoliubov transformation.
  • Information IV:
    Using the above mentioned α vacuum or the Bunch–Davies quantum vacuum one can further compute the numerator of OTOC, which is the trace of the square of the commutator bracket of the rescaled field variable and its canonically conjugate momenta along with the thermal Botzmann factor in which the system Hamiltonian is appearing explicitly. Furthermore, one can compute the denominator of the OTOC which represents the expression for the trace of the only thermal Boltzmann factor, which is physically identified to be the thermal partition function in the Cosmological framework. Then putting a negative sign in front of the computed object, one can derive the expression for the OTOC in un normalized form. This overall negative sign is very important as it makes the un normalized OTOC positive and growing with the combined time scale in which both of the above mentioned quantum extended operators are defined. In this computation, the thermal partition function and the trace of the commutator bracket square at finite temperature are evaluated using semi classical approximation, which means that we treat the fluctuation of scalar modes appearing from cosmological perturbation theory in the primordial universe. Precisely the perturbation in the metric in the FLRW background can be written in terms of scalar, vector and tensor modes in Fourier transformed space. This is commonly known as the SVT decomposition in Cosmology. In this computation, we have restricted our attention to scalar modes which can promoted to be a quantum operator during the computation of OTOC. It is obvious that the canonically conjugate momenta computed from the scalar modes can also be promoted to be a quantum mechanical operator. However, if we look into this problem very carefully, then we observe that the origin of the quantum fluctuations from the scalar modes are coming from metric perturbation in the FLRW background, which we actually treat purely classically. For this reason we will do a semi-classical (not purely quantum or classical) computation for the computation of OTOC in the framework of primordial cosmology.
  • Information V:
    Last but not least, we have to fix the normalization of OTOC. This can be perfectly done using the previously mentioned thermal trace operation in presence of α vacua or Bunch–Davies quantum vacuum state in the context of Cosmology. After normalization we can able to construct a dimensionless cosmological four-point “in-in" amplitude or quantum OTOC. We have computed the normalization factor in the Appendix A, Appendix B, Appendix C, Appendix D, Appendix E, Appendix F, Appendix G, Appendix H, Appendix I, Appendix J, Appendix K, Appendix L and Appendix M in detail. Please look into the technical details in the Appendix C, Appendix D and Appendix M.2 respectively.

3.5.2. Fourier Space Representation of the Commutator Bracket: Application to Two-Point OTOC

Here our job is to compute the following commutator bracket, given by:
f ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) = f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) Γ 1 ( x , ϕ 1 , ϕ 2 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) Γ 2 ( x , ϕ 1 , ϕ 2 ) ,
Now we use the following convention for the Fourier transformation, which is given by:
f ^ ( x , τ 1 ) = d 3 k ( 2 π ) 3 exp ( i k · x ) f ^ k ( τ 1 ) ,
Π ^ ( x , τ 1 ) = τ 1 f ^ ( x , τ 1 ) = d 3 k ( 2 π ) 3 exp ( i k · x ) τ 1 f ^ k ( τ 1 ) = d 3 k ( 2 π ) 3 exp ( i k · x ) Π ^ k ( τ 1 ) ,
which will be very useful for rest of the computation of this paper.
Here we can see that the commutator bracket splits the commutator into two parts which we have written in terms of the perturbation field variable and its canonically conjugate momentum, which are appearing in the context of cosmological perturbation theory performed in a specific scheme. In this computation instead of considering the perturbed variable space independent, we start with space-time-dependent perturbed cosmological operators as after performing cosmological perturbations, all the quantum operators are inhomogeneous and anisotropic in general. Though at the end after performing all the computation we will show that even one start with space-time-dependent perturbed cosmological operators, the ultimate result will be independent of space coordinates and in the OTOC only the information regarding the time coordinates will appear explicitly as from the starting point we have started with time scale separated quantities. The obtained two individual contributions actually represent the two point OTO amplitudes in coordinate space which will finally contribute in the computation of the two-point micro-canonical OTO amplitudes in Fourier space after performing the Fourier transformation following the above mentioned convention. Finally, using this formalism one can explicitly compute the expression for the two point micro-canonical OTOC in the context of Cosmology, which will be only functions of the two conformal time scales in which the quantum operators are associated in the cosmological perturbation theory.
Next, we explicitly compute the expressions for these individual quantum mechanical operators, Γ i ( x , τ 1 , τ 2 ) i = 1 , 2 , which can be expressed in Fourier space as:
Γ 1 ( x , τ 1 , τ 2 ) = f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 1 ( 2 π ) 3 exp ( i ( k 1 + k 2 ) · x ) f ^ k 1 ( τ 1 ) Π ^ k 2 ( τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 1 ( 2 π ) 3 exp ( i ( k 1 + k 2 ) · x ) Δ ^ 1 ( k 1 , k 2 ; τ 1 , τ 2 ) ,
where we have introduced a momentum and conformal time-dependent quantum mechanical operator Δ ^ 1 ( k 1 , k 2 ; τ 1 , τ 2 ) , which is defined as:
Δ ^ 1 ( k 1 , k 2 ; τ 1 , τ 2 ) = f ^ k 1 ( τ 1 ) Π ^ k 2 ( τ 2 ) = D 1 ( k 1 , k 2 ; τ 1 , τ 2 ) a k 1 a k 2 + D 2 ( k 1 , k 2 ; τ 1 , τ 2 ) a k 1 a k 2 + D 3 ( k 1 , k 2 ; τ 1 , τ 2 ) a k 1 a k 2 + D 4 ( k 1 , k 2 ; τ 1 , τ 2 ) a k 1 a k 2 ,
where we have introduced momentum and time-dependent two-point OTO amplitudes, D i ( k 1 , k 2 ; τ 1 , τ 2 ) i = 1 , 2 , 3 , 4 , which are explicitly defined in the Appendix A, Appendix B, Appendix C, Appendix D, Appendix E, Appendix F, Appendix G, Appendix H, Appendix I, Appendix J, Appendix K, Appendix L and Appendix M.
Γ 2 ( x , τ 1 , τ 2 ) = Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) = d 3 k 1 ( 2 π ) 3 d 3 k 1 ( 2 π ) 3 exp ( i ( k 1 + k 2 ) · x ) Π ^ k 1 ( τ 2 ) f ^ k 2 ( τ 1 ) = d 3 k 1 ( 2 π ) 3 d 3 k 1 ( 2 π ) 3 exp ( i ( k 1 + k 2 ) · x ) Δ ^ 2 ( k 1 , k 2 ; τ 1 , τ 2 ) ,
where we have introduced a momentum and conformal time-dependent quantum mechanical operator Δ ^ 2 ( k 1 , k 2 ; τ 1 , τ 2 ) , which is defined as:
Δ ^ 2 ( k 1 , k 2 ; τ 1 , τ 2 ) = Π ^ k 1 ( τ 2 ) f ^ k 2 ( τ 1 ) = L 1 ( k 1 , k 2 ; τ 1 , τ 2 ) a k 1 a k 2 + L 2 ( k 1 , k 2 ; τ 1 , τ 2 ) a k 1 a k 2 + L 3 ( k 1 , k 2 ; τ 1 , τ 2 ) a k 1 a k 2 + L 4 ( k 1 , k 2 ; τ 1 , τ 2 ) a k 1 a k 2 ,
where we have introduced momentum and time-dependent two-point OTO amplitudes, L i ( k 1 , k 2 ; τ 1 , τ 2 ) i = 1 , 2 , 3 , 4 , which are explicitly defined in the Appendix C.
This implies that one can write down the previously mentioned the commutator bracket along with the thermal Boltzmann factor as:
e β H ^ ( τ 1 ) f ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) = e β H ^ ( τ 1 ) Γ 1 ( x , τ 1 , τ 2 ) Γ 2 ( x , τ 1 , τ 2 ) = e β H ^ ( τ 1 ) d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 exp i k 1 + k 2 · x Δ ^ 1 ( k 1 , k 2 ; τ 1 , τ 2 ) Δ ^ 2 ( k 1 , k 2 ; τ 1 , τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 exp i k 1 + k 2 · x ^ 1 ( k 1 , k 2 ; τ 1 , τ 2 ; β ) ^ 2 ( k 1 , k 2 ; τ 1 , τ 2 ; β ) ,
where we define the new sets of quantum operators, : : ^ i ( k 1 , k 2 ; τ 1 , τ 2 ; β ) : i = 1 , 2 as:
^ i ( k 1 , k 2 ; τ 1 , τ 2 ; β ) = e β H ^ ( τ 1 ) Δ ^ i ( k 1 , k 2 ; τ 1 , τ 2 ) i = 1 , 2
Here the thermal Boltzmann factor can be expressed in terms of creation and annihilation operator as:
e β H ( τ 1 ) = exp β d 3 k a k a k + 1 2 δ 3 ( 0 ) E k ( τ 1 ) ,
where we define E k ( τ 1 ) by the following expressions:
E k ( τ 1 ) : = | Π k ( τ 1 ) | 2 + ω k 2 ( τ 1 ) | f k ( τ ) | 2 .

3.5.3. Fourier Space Representation of Square of the Commutator Bracket Application to Four-Point OTOC

Now we explicitly compute the following square of the commutator bracket, given by:
f ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) 2 = f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) K 1 ( x , τ 1 , τ 2 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) K 2 ( x , τ 1 , τ 2 ) f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) K 3 ( x , τ 1 , τ 2 ) + Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) K 4 ( x , τ 1 , τ 2 )
It is important to note that in the expression for the square of the commutator bracket defined in terms of the rescaled cosmological perturbation variable for scalar mode fluctuation and its canonically conjugate momenta we actually have introduced few space-time-dependent new functions which basically divides the whole expression into four parts. These four quantum operators after taking the thermal average represent the four-point correlation function. However doing the computation in coordinate space is very complicated. For this reason, by making use of the mentioned convention of the Fourier transform of each quantum operators we express all of them in momentum space. Now we mention the explicit structure of these operators, K i ( x , τ 1 , τ 2 ) i = 1 , 2 , 3 , 4 which are expressed in Fourier space as:
K 1 ( x , τ 1 , τ 2 ) = f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x f ^ k 1 ( τ 1 ) Π ^ k 2 ( τ 2 ) f ^ k 3 ( τ 1 ) Π ^ k 4 ( τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x T ^ 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) ,
where the function T ^ 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) is defined as:
T ^ 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) = M 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 5 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 6 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 7 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 8 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 9 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 10 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 11 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 12 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 13 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 14 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 15 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + M 16 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 ,
where we define, M j ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) j = 1 , , 16 , in the Appendix D.
K 2 ( x , τ 1 , τ 2 ) = Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x Π ^ k 1 ( τ 2 ) f ^ k 2 ( τ 1 ) f ^ k 3 ( τ 1 ) Π ^ k 4 ( τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x T ^ 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) ,
where the function T ^ 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) is defined as:
T ^ 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) = J 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 5 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 6 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 7 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 8 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 9 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 10 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 11 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 12 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 13 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 14 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 15 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + J 16 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 ,
where we define, J j ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) j = 1 , , 16 , in the Appendix D.
K 3 ( x , τ 1 , τ 2 ) = f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 1 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x f ^ k 1 ( τ 1 ) Π ^ k 2 ( τ 2 ) Π ^ k 3 ( τ 2 ) f ^ k 4 ( τ 1 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x T ^ 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) ,
where the function T ^ 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) is defined as:
T ^ 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) = N 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 5 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 6 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 7 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 8 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 9 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 10 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 11 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 12 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 13 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 14 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 15 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + N 16 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 ,
where we define, N j ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) j = 1 , , 16 , in the Appendix D.
K 4 ( x , τ 1 , τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x Π ^ k 1 ( τ 2 ) f ^ k 2 ( τ 1 ) Π ^ k 3 ( τ 2 ) f ^ k 4 ( τ 1 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x T ^ 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) ,
where the function T ^ 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) is defined as:
T ^ 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) = Q 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 5 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 6 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 7 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 8 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 9 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 10 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 11 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 12 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 13 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 14 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 15 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 + Q 16 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) a k 1 a k 2 a k 3 a k 4 ,
where we define, Q j ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) j = 1 , , 16 , in the Appendix D.
This implies that one can write down the previously mentioned square of the commutator bracket along with the thermal Boltzmann factor as:
e β H ^ ( τ 1 ) f ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) 2 = e β H ^ ( τ 1 ) K 1 ( x , τ 1 , τ 2 ) K 2 ( x , τ 1 , τ 2 ) K 3 ( x , τ 1 , τ 2 ) + K 4 ( x , τ 1 , τ 2 ) = e β H ^ ( τ 1 ) d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x T ^ 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) T ^ 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) + T ^ 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) T ^ 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x V ^ 1 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) V ^ 2 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) + V ^ 3 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) V ^ 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) ,
where we define the new sets of quantum operators, : V ^ i ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) : i = 1 , 2 , 3 , 4 as:
V ^ i ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) = e β H ^ ( τ 1 ) T ^ i ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) i = 1 , 2 , 3 , 4 ,
where the thermal Boltzmann factor can be expressed as:
e β H ( τ 1 ) = exp β d 3 k a k a k + 1 2 δ 3 ( 0 ) E k ( τ 1 ) ,
where we define E k ( τ 1 ) by the following expressions:
E k ( τ 1 ) : = | Π k ( τ 1 ) | 2 + ω k 2 ( τ 1 ) | f k ( τ 1 ) | 2 .
Now to find the trace of the square of the commutator bracket along with the thermal Boltzmann factor we need to first define the quantum vacuum state. In the present context, the quantum vacuum is fixed by the initial condition that we choose to define the mode functions obtained from the quantum fluctuations during the particle production during inflation and reheating epoch of the primordial cosmology. To find out the expression for the OTOC here we will concentrate on S O ( 1 , 4 ) isometric de Sitter vacua. In this category the most famous example is the α -vacua, which are actually described by a one real parameter family α and is CPT symmetric. If we fix, α = 0 , then we get the well known Bunch–Davies vacuum state. In the context of quantum field theory of curved space time, the Bunch–Davies vacuum actually represents the quantum ground state. On the other hand, the α vacua represent the quantum excited states in the context of quantum field theory of curved space. For α vacua the parameter α mimics the role of a super-selection quantum number associated with a different bipartite Hilbert space. However, it is still an unresolved issue that whether the interaction picture of the quantum field theory with any arbitrary value of the parameter α with any arbitrary super-selection rule are consistent with the underlying physical requirements or not. It might be quite likely that the Hilbert space of α vacua excited states and the Bunch–Davies vacuum coincides with each other in the underlying quantum field theory set up. In such a physical situation it is perfectly consistent to describe quantum field theory of excited states in terms of Bunch–Davies vacuum in the ultraviolet regime. On the other hand, in the infrared regime of the quantum field theory of curved space, due to the non-removal of physical infinities appearing from various types of interaction, explaining the physics of excited states with the Bunch–Davies type of adiabatic vacuum state is not at all a viable approximation. As a consequence one can write an effective field theory description from the present set up in the ultraviolet regime of quantum field theory. This further implies that identifying the more appropriate candidate of quantum α vacuum states which are highly fine tuned. However, this only allows us to consider excited quantum states compared to ground state described by the Bunch–Davies vacuum in the context of quantum field theory. Using this prescription apart from describing the inflationary paradigm in (quasi) de Sitter space, one can use it to describe a lot of unexplored late time physical phenomena, i.e., stochastic particle production phenomenon, the process of reheating etc. in presence of non-standard quantum vacuum state.

3.6. Cosmological Thermal Partition Function: Quantum Version

3.6.1. Quantum Vacuum State in Cosmology

In the context of quantum field theory, one can define the class of all excited α vacuum states in terms of the well known adiabatic Bunch–Davies vacuum state as:
| Ψ α = 1 | cosh α | exp i 2 tanh α d 3 k ( 2 π ) 3 a k a k | Ψ BD ,
which satisfy the following constraint condition:
a k | Ψ α = 0 k , α
Here one can easily observed that if we fix α = 0 then one can easily get back the usual quantum adiabatic Bunch–Davies vacuum state. Another important thing we have to mention here that at the level of solution of the mode function of the quantum fluctuations that we have obtained earlier, one can introduce the concept of these excited α vacuum states in the integration constants C 1 and C 2 as:
C 1 = cosh α , C 2 = sinh α ,
which satisfy the following normalization condition:
| C 1 | 2 | C 2 | 2 = 1 cosh 2 α sinh 2 α = 1 α .
If we fix here α = 0 then we get the following values of the normalization constants for the well known adiabatic quantum Bunch–Davies vacuum state:
C 1 = 1 , C 2 = 0 .
Now, using the definition of the above mentioned excited α vacuum states, which is explicitly written in terms of the well known adiabatic Bunch–Davies vacuum state we compute the expression for previously mentioned OTOC in the present context. For this purpose, we need to first of fix the quantum partition function.

3.6.2. Quantum Partition Function in Terms of Rescaled Field Variable

In presence of these excited α vacuum states the quantum partition function can be expressed as:
Z α ( β ; τ 1 ) = d Ψ α Ψ α | e β H ^ ( τ 1 ) | Ψ α = 1 | cosh α | d Ψ BD Ψ BD | exp i 2 tanh α d 3 k 1 ( 2 π ) 3 a k 1 a k 1 exp β d 3 k a k a k + 1 2 δ 3 ( 0 ) E k ( τ 1 ) exp i 2 tanh α d 3 k 2 ( 2 π ) 3 a k 2 a k 2 | Ψ BD .
Then the quantum partition function for α vacua can be expressed as:
Z α ( β ; τ 1 ) = 1 | cosh α | Z BD ( β ; τ 1 ) ,
where Z BD is the quantum partition function computed from adiabatic Bunch–Davies vacuum as:
Z BD ( β ; τ 1 ) = d Ψ BD Ψ BD | exp β d 3 k a k a k + 1 2 δ 3 ( 0 ) E k ( τ 1 ) | Ψ BD = exp 1 + 1 2 δ 3 ( 0 ) d 3 k ln 2 sinh β E k ( τ 1 ) 2 .
This further implies that the expression for the quantum partition function for α vacua can be simplified as:
Z α ( β ; τ 1 ) = 1 | cosh α | exp 1 + 1 2 δ 3 ( 0 ) d 3 k ln 2 sinh β E k ( τ 1 ) 2 .
Until now we have not derived the expressions for the quantum partition function for the α vacua and the adiabatic Bunch–Davies vacuum state by introducing normal ordering which is used to remove unwanted infinities, like here it is coming from the zero point like energy contribution δ 3 ( 0 ) / 2 , which is a divergent quantity. Another possibility is that one can also use the Dirac Delta regularization to remove the divergent contribution.
The normal ordered quantum partition function for Bunch–Davies vacuum is given by:
: Z BD ( β ; τ 1 ) : = exp d 3 k ( 2 π ) 3 ln 2 sinh β E k ( τ 1 ) 2 .
Then the normal ordered quantum partition function for α vacua can be simplified as:
: Z α ( β ; τ 1 ) : = 1 | cosh α | exp d 3 k ln 2 sinh β E k ( τ 1 ) 2 .

3.6.3. Quantum Partition Function in Terms of Curvature Perturbation Field Variable

In this subsection our prime objective is to find out the expression for the partition function in terms of the curvature perturbation field variable. To serve this purpose the time-dependent dispersion relation can be expressed in terms of the curvature perturbation variable as:
E k ( τ 1 ) = | Π k ( τ 1 ) | 2 + ω k 2 ( τ 1 ) | f k ( τ 1 ) | 2 = z 2 ( τ 1 ) Π k ζ ( τ 1 ) + ζ k ( τ 1 ) 1 z ( τ 1 ) d z ( τ 1 ) d τ 1 2 + ω k 2 ( τ 1 ) | ζ k ( τ 1 ) | 2 = z 2 ( τ 1 ) E k , ζ ( τ 1 ) + Π k ζ ( τ 1 ) ζ k ( τ 1 ) + Π k ζ ( τ 1 ) ζ k ( τ 1 ) 1 z ( τ 1 ) d z ( τ 1 ) d τ 1 Contribution from this term is negligibly small z 2 ( τ 1 ) E k , ζ ( τ 1 ) ,
where we define the time-dependent energy dispersion relation in terms of the curvature perturbation variable as:
E k , ζ ( τ 1 ) : = Π k ζ ( τ 1 ) 2 + ω k 2 ( τ 1 ) + 1 z ( τ 1 ) d z ( τ 1 ) d τ 1 2 | ζ k ( τ 1 ) | 2 .
Now, the thermal partition function for cosmology in terms of curvature perturbation computed for α vacua can be expressed as:
Z α ζ ( β ; τ 1 ) = Z BD ζ ( β ; τ 1 ) | cosh α | .
where Z BD ζ ( β ; τ 1 ) is thermal partition function for cosmology in terms of curvature perturbation for Bunch–Davies vacuum which can be expressed as:
Z BD ζ ( β ; τ 1 ) = exp 1 + 1 2 δ 3 ( 0 ) d 3 k ln 2 sinh β z 2 ( τ 1 ) E k , ζ ( τ 1 ) 2 ,
and can be further simplified in the normal ordered form as:
: Z BD ζ ( β ; τ 1 ) : = exp d 3 k ln 2 sinh β z 2 ( τ 1 ) E k , ζ ( τ 1 ) 2 .

3.7. Trace of Two-Point “In-In" OTO Amplitude for Cosmology

Now, we will explicitly compute the numerator of the two-point OTOC for quantum α vacua, which is given by:
Tr e β H ^ ( τ 1 ) f ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) ( α ) = 1 | cosh α | d Ψ BD j = 1 2 d 3 k j ( 2 π ) 3 exp i k j · x Ψ BD | i = 1 2 ^ i ( k 1 , k 2 ; τ 1 , τ 2 ; β ) | Ψ BD .
Furthermore, our aim is to compute the individual contributions which in the normal ordered form is given by the following expression and computed in Appendix H:
d Ψ BD Ψ BD | : ^ i ( k 1 , k 2 ; τ 1 , τ 2 ; β ) : | Ψ BD = d Ψ BD Ψ BD | : e β H ^ ( τ 1 ) Δ ^ i ( k 1 , k 2 ; τ 1 , τ 2 ) : | Ψ BD i = 1 , 2 .
Furthermore, the trace of sum of these individual two-point “in-in” OTO micro-canonical amplitudes in normal ordered form can be expressed as:
d Ψ BD Ψ BD | i = 1 2 : ^ i ( k 1 , k 2 ; τ 1 , τ 2 ; β ) : | Ψ BD = ( 2 π ) 3 exp d 3 k ln 2 sinh β E k ( τ 1 ) 2 Micro canonical thermal partition function δ 3 ( k 1 + k 2 ) Momentum conservation in OTO amplitude D 2 ( k 1 , k 2 ; τ 1 , τ 2 ) Individual two point OTO amplitude + D 3 ( k 1 , k 2 ; τ 1 , τ 2 ) Individual two point OTO amplitude L 2 ( k 1 , k 2 ; τ 1 , τ 2 ) Individual two point OTO amplitude L 3 ( k 1 , k 2 ; τ 1 , τ 2 ) Individual two point OTO amplitude = ( 2 π ) 3 δ 3 ( k 1 + k 2 ) P ( k 1 , k 2 ; τ 2 , τ 2 ; β ) .
Here we introduce, P ( k 1 , k 2 ; τ 2 , τ 2 ; β ) is the temperature-dependent two-point function, which is defined as:
P ( k 1 , k 2 ; τ 2 , τ 2 ; β ) : = exp d 3 k ln 2 sinh β E k ( τ 1 ) 2 D 2 ( k 1 , k 2 ; τ 1 , τ 2 ) + D 3 ( k 1 , k 2 ; τ 1 , τ 2 ) L 2 ( k 1 , k 2 ; τ 1 , τ 2 ) L 3 ( k 1 , k 2 ; τ 1 , τ 2 ) .

3.8. OTOC from Regularised Two-Point “In-In" OTO Micro-Canonical Amplitude: Rescaled Field Version

The cosmological OTOC without normalization for α vacua can be expressed as:
Y f ( τ 1 , τ 2 ) = 1 Z α ( β ; τ 1 ) Tr e β H ^ ( τ 1 ) f ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) ( α ) = ( 2 π ) 3 d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 exp i k 1 + k 2 · x δ 3 ( k 1 + k 2 ) Momentum conservation in OTO amplitude D 2 ( k 1 , k 2 ; τ 1 , τ 2 ) Individual two point OTO amplitude + D 3 ( k 1 , k 2 ; τ 1 , τ 2 ) Individual two point OTO amplitude L 2 ( k 1 , k 2 ; τ 1 , τ 2 ) Individual two point OTO amplitude L 3 ( k 1 , k 2 ; τ 1 , τ 2 ) Individual two point OTO amplitude = d 3 k 1 ( 2 π ) 3 P ( k 1 , k 1 ; τ 1 , τ 2 ) ,
where the two-point OTO micro-canonical amplitude function is explicitly given by the following expression:
P ( k 1 , k 1 ; τ 1 , τ 2 ) : = D 2 ( k 1 , k 1 ; τ 1 , τ 2 ) + D 3 ( k 1 , k 1 ; τ 1 , τ 2 ) L 2 ( k 1 , k 1 ; τ 1 , τ 2 ) L 3 ( k 1 , k 1 ; τ 1 , τ 2 ) = f k 1 * ( τ 1 ) Π k 1 ( τ 2 ) + f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) Π k 1 * ( τ 2 ) f k 1 ( τ 1 ) Π k 1 ( τ 2 ) f k 1 * ( τ 1 ) .
Here we define:
D 2 ( k 1 , k 2 ; τ 1 , τ 2 ) = f k 1 * ( τ 1 ) Π k 2 ( τ 2 ) ,
D 3 ( k 1 , k 2 ; τ 1 , τ 2 ) = f k 1 ( τ 1 ) Π k 2 * ( τ 2 ) ,
L 2 ( k 1 , k 2 ; τ 1 , τ 2 ) = Π k 1 * ( τ 2 ) f k 2 ( τ 1 ) ,
L 3 ( k 1 , k 2 ; τ 1 , τ 2 ) = Π k 1 ( τ 2 ) f k 2 * ( τ 1 ) .
Now we need to evaluate explicitly by doing the momentum integration over three volume. Now to compute this integral one can express the volume element as:
d 3 k 1 ( 2 π ) 3 = 4 π k 1 2 d k 1 0 < k 1 < L .
Here we have taken care of the fact that the individual contribution appearing in the two-point OTOC momentum integral is isotropic. Furthermore, we have introduced a momentum finite large cut-off to regulate the contribution of this integral.
Consequently, one can write the following simplified expressions for the two-point un-normalized OTOC as:
Y f ( τ 1 , τ 2 ) = 1 2 π 2 B ( τ 1 , τ 2 ) ,
where the conformal time scale-dependent regularized integral, B ( τ 1 , τ 2 ) in the above expression, is defined as:
B ( τ 1 , τ 2 ) : = k 1 = 0 L k 1 2 d k 1 f k 1 * ( τ 1 ) Π k 1 ( τ 2 ) + f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) Π k 1 * ( τ 2 ) f k 1 ( τ 1 ) Π k 1 ( τ 2 ) f k 1 * ( τ 1 ) = ( τ 1 ) 1 2 ν ( τ 2 ) 3 2 ν Z ( 1 ) ( τ 1 , τ 2 ) + Z ( 2 ) ( τ 1 , τ 2 ) Z ( 3 ) ( τ 1 , τ 2 ) Z ( 4 ) ( τ 1 , τ 2 ) ,
where we have introduced the time-dependent four individual amplitudes, Z ( i ) ( τ 1 , τ 2 ) i = 1 , 2 , 3 , 4 :
Z ( 1 ) ( τ 1 , τ 2 ) : = k 1 = 0 L k 1 2 d k 1 f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) ,
Z ( 2 ) ( τ 1 , τ 2 ) : = k 1 = 0 L k 1 2 d k 1 f k 1 * ( τ 1 ) Π k 1 ( τ 2 ) ,
Z ( 3 ) ( τ 1 , τ 2 ) : = k 1 = 0 L k 1 2 d k 1 Π k 1 ( τ 2 ) f k 1 * ( τ 1 ) ,
Z ( 4 ) ( τ 1 , τ 2 ) : = k 1 = 0 L k 1 2 d k 1 Π k 1 * ( τ 2 ) f k 1 ( τ 1 ) ,
which satisfy the following symmetry properties:
Z ( 2 ) ( τ 1 , τ 2 ) = ( 1 ) ( 2 ν + 1 ) Z ( 1 ) ( τ 1 , τ 2 ) ,
Z ( 4 ) ( τ 1 , τ 2 ) = ( 1 ) ( 2 ν + 1 ) Z ( 3 ) ( τ 1 , τ 2 ) ,
using which the simplified form of the momentum integrated time-dependent two-point OTOC can be written as:
Y f ( τ 1 , τ 2 ) = 1 2 π 2 B ( τ 1 , τ 2 ) = ( τ 1 ) 1 2 ν ( τ 2 ) 3 2 ν 2 π 2 1 + ( 1 ) ( 2 ν + 1 ) Z ( 3 ) ( τ 1 , τ 2 ) Z ( 2 ) ( τ 1 , τ 2 ) .
We need to explicitly evaluate this above mentioned integral which will going to fix the final expression for the two-point micro-canonical OTOC for Cosmology. We have presented the detailed computation of this integral in the Appendix A, Appendix B, Appendix C, Appendix D, Appendix E, Appendix F, Appendix G, Appendix H, Appendix I, Appendix J, Appendix K, Appendix L and Appendix M.

3.9. OTOC from Regularised Four-Point “In-In" OTO Micro-Canonical Amplitude: Curvature Perturbation Field Version

Here we need to perform the computation for the two-point OTOC in terms of the scalar curvature perturbation and the canonically conjugate momentum associated with it, which we have found that is given by the following simplified expression:
Y ζ ( τ 1 , τ 2 ) = 1 Z α ζ ( β , τ 1 ) Tr e β H ^ ( τ 1 ) ζ ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) ( α ) = 1 z ( τ 1 ) z ( τ 2 ) Y f ( τ 1 , τ 2 ) .
Now substituting the explicit for of the two-point function that we have derived in the previous section we get the following expression:
Y ζ ( τ 1 , τ 2 ) = 1 2 π 2 B ( τ 1 , τ 2 ) z ( τ 1 ) z ( τ 2 ) = ( τ 1 ) 1 2 ν ( τ 2 ) 3 2 ν 2 π 2 z ( τ 1 ) z ( τ 2 ) 1 + ( 1 ) ( 2 ν + 1 ) Z ( 3 ) ( τ 1 , τ 2 ) Z ( 2 ) ( τ 1 , τ 2 )
Now additionally, few points we have to mention that from the finally obtained answer for the two-point OTOCs obtained from the two different set-ups:
  • First, we have explicitly shown that the definition of the two-point OTOC is completely coordinate-independent and just only depend on the time scale on which the operators in cosmological perturbation theory are time scale separated. Even if we start defining the operators in a specific space point at different times, in the final result all such information is integrated out and we get completely a time-dependent two-point OTOC.
  • Next, it is important to note that for ν = 0 , which implies m / H = 3 / 2 , the two-point OTOC trivially vanishes. However, for partially massless and heavy scalar fields, two-point OTOC becomes non-trivial and carries significant information of the theory.
  • We also have found that the final obtained result of the two-point OTOC is β -independent even though we start with a thermal micro-canonical statistical ensemble during definition the two-point OTOC in the trace formula.
  • Furthermore, we observe that if we fix τ 1 = τ 2 = τ , that means if both the quantum operators representing the cosmological fluctuations are defined at the same time then two-point OTOC explicitly gives diverging contribution in the quantum regime. This is perfectly consistent with the definition of any general OTOC and also supports all the physical requirements to construct the foundational set-up to compute OTOC.
  • If we perform an explicit computation, then it is possible to show that in our set-up the one-point functions in the context of Cosmology are given by the following expressions:
    f ^ ( x , τ ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) f ^ ( x , τ ) ( α ) = 0 , Π ^ ( x , τ ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) Π ^ ( x , τ ) ( α ) = 0 .
    The similar statement is also valid if we define the one-point functions for Cosmology in terms of the curvature perturbation field variable and its canonically conjugate momentum. This result implies that there is no point of performing normalization of the obtained two-point OTOC for Cosmology with respect to the above mentioned one-point functions. So technically, it is sufficient enough to compute the two-point OTOC without normalization in the context of Cosmology.
  • Another important point we have to mention is that not only the one point function but also the three point functions in the present context is trivially zero which can be explicitly proved by making use of the well known Kubo–Martin–Schwinger condition in terms of time translational symmetry of the thermal correlation functions computed for Cosmology. These possibilities are explicitly mentioned below:
    f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) Π ^ ( x , τ 3 ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) f ^ ( x , τ ) Π ^ ( x , τ 2 ) Π ^ ( x , τ 3 ) ( α ) = 0 ,
    Π ^ ( x , τ 1 ) f ^ ( x , τ 2 ) Π ^ ( x , τ 3 ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) Π ^ ( x , τ 1 ) f ^ ( x , τ 2 ) Π ^ ( x , τ 3 ) ( α ) = 0 ,
    Π ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 3 ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) Π ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 3 ) ( α ) = 0 ,
    f ^ ( x , τ 1 ) f ^ ( x , τ 2 ) Π ^ ( x , τ 3 ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) f ^ ( x , τ 1 ) f ^ ( x , τ 2 ) Π ^ ( x , τ 3 ) ( α ) = 0 ,
    f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 3 ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) f ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) f ^ ( x , τ 3 ) ( α ) = 0 ,
    Π ^ ( x , τ 1 ) f ^ ( x , τ 2 ) f ^ ( x , τ 3 ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) Π ^ ( x , τ 1 ) f ^ ( x , τ 2 ) f ^ ( x , τ 3 ) ( α ) = 0 ,
    f ^ ( x , τ 1 ) f ^ ( x , τ 2 ) f ^ ( x , τ 3 ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) f ^ ( x , τ 1 ) f ^ ( x , τ 2 ) f ^ ( x , τ 3 ) ( α ) = 0 ,
    Π ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) Π ^ ( x , τ 3 ) β = 1 Z α ( β ; τ ) Tr e β H ^ ( τ ) Π ^ ( x , τ 1 ) Π ^ ( x , τ 2 ) Π ^ ( x , τ 3 ) ( α ) = 0 ,
    which in principle can be further generalized to any odd N point thermal correlation function for micro-canonical ensemble within the framework of Cosmology.

3.10. Trace of Four-Point “In-In" OTO Amplitude for Cosmology

Now, we will explicitly compute the numerator of the four-point OTOC for quantum α vacua, which is given by:
Tr e β H ^ ( τ 1 ) f ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) 2 ( α ) = 1 | cosh α | d Ψ BD j = 1 4 d 3 k j ( 2 π ) 3 exp i k j · x Ψ BD | i = 1 4 V ^ i ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) | Ψ BD .
Furthermore, our aim is to compute the individual contributions which in the normal ordered form are given by the following expression and computed in Appendix I:
d Ψ BD Ψ BD | : V ^ i ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) : | Ψ BD = d Ψ BD Ψ BD | : e β H ^ ( τ 1 ) T ^ i ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) : | Ψ BD i = 1 , 2 , 3 , 4 .
Furthermore, the trace of sum of these individual four-point “in-in" OTO micro-canonical amplitudes in normal ordered form can be expressed as:
d Ψ BD Ψ BD | i = 1 4 V ^ i ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ; β ) : | Ψ BD = ( 2 π ) 6 exp d 3 k ln 2 sinh β E k ( τ 1 ) 2 i = 1 4 Θ 4 ( i ) ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Individual four point OTO amplitude δ 3 k 1 + k 4 δ 3 k 2 + k 3 + δ 3 k 1 + k 3 δ 3 k 2 + k 4 Momentum conservation in OTO amplitude + Θ 6 ( i ) ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Individual four point OTO amplitude δ 3 k 1 + k 2 δ 3 k 3 + k 4 + δ 3 k 1 + k 4 δ 3 k 2 + k 3 Momentum conservation in OTO amplitude + Θ 7 ( i ) ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Individual four point OTO amplitude δ 3 k 1 + k 2 δ 3 k 3 + k 4 + δ 3 k 1 + k 3 δ 3 k 2 + k 4 Momentum conservation in OTO amplitude + Θ 10 ( i ) ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Individual four point OTO amplitude δ 3 k 1 + k 2 δ 3 k 3 + k 4 + δ 3 k 1 + k 3 δ 3 k 2 + k 4 Momentum conservation in OTO amplitude + Θ 11 ( i ) ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Individual four point OTO amplitude δ 3 k 1 + k 2 δ 3 k 3 + k 4 + δ 3 k 1 + k 4 δ 3 k 2 + k 3 Momentum conservation in OTO amplitude + Θ 13 ( i ) ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Individual four point OTO amplitude δ 3 k 1 + k 3 δ 3 k 2 + k 4 + δ 3 k 1 + k 4 δ 3 k 2 + k 3 Momentum conservation in OTO amplitude , i = 1 ( M ) , 2 ( J ) , 3 ( N ) , 4 ( Q ) .

3.11. OTOC from Regularised Four-Point “In-In" OTO Micro-Canonical Amplitude: Rescaled Field Version

3.11.1. Without Normalization

The cosmological OTOC without normalization for α vacua can be expressed as:
C f ( τ 1 , τ 2 ) = 1 Z α ( β ; τ 1 ) Tr e β H ^ ( τ 1 ) f ^ ( x , τ 1 ) , Π ^ ( x , τ 2 ) 2 ( α ) = ( 2 π ) 6 d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 d 3 k 3 ( 2 π ) 3 d 3 k 4 ( 2 π ) 3 exp i k 1 + k 2 + k 3 + k 4 · x E 4 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Four point OTO amplitude δ 3 k 1 + k 4 δ 3 k 2 + k 3 + δ 3 k 1 + k 3 δ 3 k 2 + k 4 Momentum conservation in OTO amplitude + E 6 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Four point OTO amplitude δ 3 k 1 + k 2 δ 3 k 3 + k 4 + δ 3 k 1 + k 4 δ 3 k 2 + k 3 Momentum conservation in OTO amplitude + E 7 ( k 1 , k 1 , k 3 , k 3 ; τ 1 , τ 2 ) Four point OTO amplitude δ 3 k 1 + k 2 δ 3 k 3 + k 4 + δ 3 k 1 + k 3 δ 3 k 2 + k 4 Momentum conservation in OTO amplitude + E 10 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Four point OTO amplitude δ 3 k 1 + k 2 δ 3 k 3 + k 4 + δ 3 k 1 + k 3 δ 3 k 2 + k 4 Momentum conservation in OTO amplitude + E 11 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Four point OTO amplitude δ 3 k 1 + k 2 δ 3 k 3 + k 4 + δ 3 k 1 + k 4 δ 3 k 2 + k 3 Momentum conservation in OTO amplitude + E 13 ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Four point OTO amplitude δ 3 k 1 + k 3 δ 3 k 2 + k 4 + δ 3 k 1 + k 4 δ 3 k 2 + k 3 Momentum conservation in OTO amplitude = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 E 4 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) + E 4 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) + E 6 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) + E 7 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) + E 10 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) + E 11 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) + E 13 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) + E 13 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) + E 7 ( k 1 , k 1 , k 2 , k 2 ; τ 1 , τ 2 ) + E 10 ( k 1 , k 1 , k 2 , k 2 ; τ 1 , τ 2 ) + E 11 ( k 1 , k 1 , k 2 , k 2 ; τ 1 , τ 2 ) ,
where we have introduced new four-point OTO micro-canonical amplitude functions, E m ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) m = 4 , 6 , 7 , 10 , 11 , 13 , which are defined as:
E m ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) = i = 1 4 Θ m ( i ) ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) = M m ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) J m ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) + N m ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) Q m ( k 1 , k 2 , k 3 , k 4 ; τ 1 , τ 2 ) .
To understand the structure of these functions more clearly one can further write them in terms of the redefined field and its canonically conjugate momenta as:
E 4 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) = f k 1 * ( τ 1 ) Π k 2 * ( τ 2 ) f k 2 ( τ 1 ) Π k 1 ( τ 2 ) Π k 1 * ( τ 2 ) f k 2 * ( τ 1 ) f k 2 ( τ 1 ) Π k 1 ( τ 2 ) + f k 1 * ( τ 1 ) Π k 2 * ( τ 2 ) Π k 2 ( τ 2 ) f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) f k 2 * ( τ 1 ) Π k 2 ( τ 2 ) f k 1 ( τ 1 ) ,
E 4 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) = f k 1 * ( τ 1 ) Π k 2 * ( τ 2 ) f k 1 ( τ 1 ) Π k 2 ( τ 2 ) Π k 1 * ( τ 2 ) f k 2 * ( τ 1 ) f k 1 ( τ 1 ) Π k 2 ( τ 2 ) + f k 1 * ( τ 1 ) Π k 2 * ( τ 2 ) Π k 1 ( τ 2 ) f k 2 ( τ 1 ) Π k 1 * ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 ( τ 2 ) f k 2 ( τ 1 ) ,
E 6 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) = f k 1 * ( τ 1 ) Π k 2 ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 ( τ 2 ) Π k 1 * ( τ 2 ) f k 2 ( τ 1 ) f k 2 * ( τ 1 ) Π k 1 ( τ 2 ) + f k 1 * ( τ 1 ) Π k 2 ( τ 2 ) Π k 2 * ( τ 2 ) f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) f k 2 ( τ 1 ) Π k 2 * ( τ 2 ) f k 1 ( τ 1 ) ,
E 7 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) = f k 1 ( τ 1 ) Π k 2 * ( τ 2 ) f k 1 * ( τ 1 ) Π k 2 ( τ 2 ) Π k 1 ( τ 2 ) f k 2 * ( τ 1 ) f k 1 * ( τ 1 ) Π k 2 ( τ 2 ) + f k 1 ( τ 1 ) Π k 2 * ( τ 2 ) Π k 1 * ( τ 2 ) f k 2 ( τ 1 ) Π k 1 ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 * ( τ 2 ) f k 2 ( τ 1 ) ,
E 10 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) = f k 1 * ( τ 1 ) Π k 2 ( τ 2 ) f k 1 ( τ 1 ) Π k 2 * ( τ 2 ) Π k 1 * ( τ 2 ) f k 2 ( τ 1 ) f k 1 ( τ 1 ) Π k 2 * ( τ 2 ) + f k 1 * ( τ 1 ) Π k 2 ( τ 2 ) Π k 1 ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 * ( τ 2 ) f k 2 ( τ 1 ) Π k 3 ( τ 2 ) f k 4 * ( τ 1 ) ,
E 11 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) = f k 1 ( τ 1 ) Π k 2 * ( τ 2 ) f k 2 ( τ 1 ) Π k 1 * ( τ 2 ) Π k 1 ( τ 2 ) f k 2 * ( τ 1 ) f k 2 ( τ 1 ) Π k 1 * ( τ 2 ) + f k 1 ( τ 1 ) Π k 2 * ( τ 2 ) Π k 2 ( τ 2 ) f k 1 * ( τ 1 ) Π k 1 ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 ( τ 2 ) f k 2 * ( τ 1 ) ,
E 13 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) = f k 1 ( τ 1 ) Π k 2 ( τ 2 ) f k 1 * ( τ 1 ) Π k 2 * ( τ 2 ) Π k 1 ( τ 2 ) f k 2 ( τ 1 ) f k 1 * ( τ 1 ) Π k 2 * ( τ 2 ) + f k 1 ( τ 1 ) Π k 2 ( τ 2 ) Π k 1 * ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 ( τ 2 ) f k 2 ( τ 1 ) Π k 1 * ( τ 2 ) f k 2 * ( τ 1 ) ,
E 13 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) = f k 1 ( τ 1 ) Π k 2 ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 * ( τ 2 ) Π k 1 ( τ 2 ) f k 2 ( τ 1 ) f k 2 * ( τ 1 ) Π k 1 * ( τ 2 ) + f k 1 ( τ 1 ) Π k 2 ( τ 2 ) Π k 2 * ( τ 2 ) f k 1 * ( τ 1 ) Π k 1 ( τ 2 ) f k 2 ( τ 1 ) Π k 2 * ( τ 2 ) f k 1 * ( τ 1 ) ,
E 7 ( k 1 , k 1 , k 2 , k 2 ; τ 1 , τ 2 ) = f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) f k 2 * ( τ 1 ) Π k 2 ( τ 2 ) Π k 1 ( τ 2 ) f k 1 * ( τ 1 ) f k 2 * ( τ 1 ) Π k 2 ( τ 2 ) + f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) Π k 2 * ( τ 2 ) f k 2 ( τ 1 ) Π k 1 ( τ 2 ) f k 1 * ( τ 1 ) Π k 2 * ( τ 2 ) f k 2 ( τ 1 ) ,
E 10 ( k 1 , k 1 , k 2 , k 2 ; τ 1 , τ 2 ) = f k 1 * ( τ 1 ) Π k 1 ( τ 2 ) f k 2 ( τ 1 ) Π k 2 * ( τ 2 ) Π k 1 * ( τ 2 ) f k 1 ( τ 1 ) f k 2 ( τ 1 ) Π k 2 * ( τ 2 ) + f k 1 * ( τ 1 ) Π k 1 ( τ 2 ) Π k 2 ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 * ( τ 2 ) f k 1 ( τ 1 ) Π k 2 ( τ 2 ) f k 2 * ( τ 1 ) ,
E 11 ( k 1 , k 1 , k 2 , k 2 ; τ 1 , τ 2 ) = f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) f k 2 ( τ 1 ) Π k 2 * ( τ 2 ) Π k 1 ( τ 2 ) f k 1 * ( τ 1 ) f k 2 ( τ 1 ) Π k 2 * ( τ 2 ) + f k 1 ( τ 1 ) Π k 1 * ( τ 2 ) Π k 2 ( τ 2 ) f k 2 * ( τ 1 ) Π k 1 ( τ 2 ) f k 1 * ( τ 1 ) Π k 2 ( τ 2 ) f k 2 * ( τ 1 ) ,
All the above mentioned quantities physically signify the momentum and conformal time-dependent amplitude of the OTOC which is written explicitly in terms of the contributions from the four-point correlation function. Here we have to note down few symmetry properties of the above mentioned amplitudes under the exchange of the momenta appearing in the third and fourth position, i.e., if we replace k 2 k 1 and k 1 k 2 , which are given by:
E 4 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) = E 4 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) ,
E 13 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) = E 13 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) .
Using these symmetry properties the OTOC can be further simplified as:
C ( τ 1 , τ 2 ) = d 3 k 1 ( 2 π ) 3 d 3 k 2 ( 2 π ) 3 2 E 4 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) + E 13 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) + E 6 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) + E 7 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) + E 10 ( k 1 , k 2 , k 1 , k 2 ; τ 1 , τ 2 ) + E 11 ( k 1 , k 2 , k 2 , k 1 ; τ 1 , τ 2 ) +