Special Issue "Cosmology and the Quantum Vacuum"

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (30 November 2017)

Special Issue Editors

Guest Editor
Dr. Diego Sáez-Chillón Gómez

Instituto de Ciencias del Espacio (ICE), Consejo Superior de Investigaciones Científicas (CSIC) and Institut d'Estudis Espacials de Catalunya (IEEC), Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra (Barcelona), Spain
E-Mail
Interests: theoretical cosmology; dark energy; modified gravity; inflation
Guest Editor
Prof. Dr. Emilio Elizalde

Consejo Superior de Investigaciones Científicas, Instituto de Ciencias del Espacio (CSIC), Institut d'Estudis Espacials de Catalunya (IEEC/CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra (Barcelona), Spain
Website | E-Mail
Phone: 630835179
Fax: +34 93 581 4363
Interests: zeta functions; regularization of infinite sums; spexial functions of mathemtatical physics; vacuum fluctuations; theoretical cosmology

Special Issue Information

Dear Colleagues,

This Special Issue will collect contributions from leading scientists working on problems at the interface between modern cosmological theories and the quantum nature of the vacuum; in particular, topics related to modified theories of gravity, dark energy and inflation. In addition, a description of the universe’s evolution in these theories, and the compulsory modification of these within the theoretical framework of quantum gravity, with loop quantum cosmology being an elegant theory that may provide a singularity free cosmic evolution. Description and understanding of the quantum vacuum is also a challenge to all these theories, in particular its manifestation as a Casimir effect.

A non-exhaustive list of topics:

  • Cosmological models: modified gravities, f(R) theories and the like, non-local models
  • Possibility of observing modified gravity traces at the astrophysical level
  • Quantum Gravity
  • Quantum Cosmology and Loop Quantum Cosmology
  • Quantum vacuum and the Casimir Effect
  • The cosmological constant problem
  • Mathematical physics techniques in quantum vacuum studies

Dr. Emilio Elizalde
Dr. Diego Sáez-Gómez
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Galaxies is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 350 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Published Papers (19 papers)

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Research

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Open AccessFeature PaperArticle
Towards New Constraints in Extended Theories of Gravity: Cosmography and Gravitational-Wave Signals from Neutron Stars
Received: 29 January 2018 / Revised: 23 February 2018 / Accepted: 24 February 2018 / Published: 27 February 2018
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Abstract
Combined cosmological, astrophysical and numerical tests may shed some light on the viability of theories of gravity beyond Einsteinian relativity. In this letter, we present two different techniques providing complementary ways of testing new physics beyond the ΛCDM cosmological paradigm. First, we [...] Read more.
Combined cosmological, astrophysical and numerical tests may shed some light on the viability of theories of gravity beyond Einsteinian relativity. In this letter, we present two different techniques providing complementary ways of testing new physics beyond the Λ CDM cosmological paradigm. First, we shall present some of the latest progress and shortcomings in the cosmographic model-independent approach for several modified gravity theories using supernovae catalogues, baryonic acoustic oscillation data and H ( z ) differential age compilations. Second, we shall show how once the Einsteinian paradigm is abandoned, the phenomenology of neutron stars changes dramatically since neutron-star masses can be much larger than their General Relativity counterparts. Consequently, the total energy available for radiating gravitational waves could be of the order of several solar masses, and thus a merger of these stars constitutes a privileged wave source. Unfortunately at the present time our persisting lack of understanding in the strong interaction sector does not allow to distinguish the alternative theories from the usual General Relativity predictions. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle
“All that Matter … in One Big Bang …”, &Other Cosmological Singularities
Received: 2 January 2018 / Revised: 25 January 2018 / Accepted: 8 February 2018 / Published: 21 February 2018
Cited by 2 | PDF Full-text (313 KB) | HTML Full-text | XML Full-text
Abstract
The first part of this paper contains a brief description of the beginnings of modern cosmology, which, the author will argue, was most likely born in the year 1912. Some of the pieces of evidence presented here have emerged from recent research in [...] Read more.
The first part of this paper contains a brief description of the beginnings of modern cosmology, which, the author will argue, was most likely born in the year 1912. Some of the pieces of evidence presented here have emerged from recent research in the history of science and are not usually shared with the general audiences in popular science books. In particular, the issue of the correct formulation of the original Big Bang concept, according to the precise words of Fred Hoyle, is discussed. Too often, this point is very deficiently explained (when not just misleadingly) in most of the available generalist literature. Other frequent uses of the same words, Big Bang, as to name the initial singularity of the cosmos, and also whole cosmological models, are then addressed, as evolutions of its original meaning. Quantum and inflationary additions to the celebrated singularity theorems by Penrose, Geroch, Hawking and others led to subsequent results by Borde, Guth and Vilenkin. Additionally, corresponding corrections to the Einstein field equations have originated, in particular, R 2 , f ( R ) , and scalar-tensor gravities, giving rise to a plethora of new singularities. For completeness, an updated table with a classification of the same is given. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessFeature PaperArticle
Cosmological Constant and Renormalization of Gravity
Received: 23 October 2017 / Revised: 20 December 2017 / Accepted: 13 February 2018 / Published: 18 February 2018
Cited by 2 | PDF Full-text (201 KB) | HTML Full-text | XML Full-text
Abstract
In arXiv:1601.02203 and arXiv:1702.07063, we have proposed a topological model with a simple Lagrangian density and have tried to solve one of the cosmological constant problems. The Lagrangian density is the BRS exact and therefore the model can be regarded as a topological [...] Read more.
In arXiv:1601.02203 and arXiv:1702.07063, we have proposed a topological model with a simple Lagrangian density and have tried to solve one of the cosmological constant problems. The Lagrangian density is the BRS exact and therefore the model can be regarded as a topological theory. In this model, the divergence of the vacuum energy coming from the quantum corrections from matters can be absorbed into the redefinition of the scalar field. In this paper, we consider the extension of the model in order to apply the mechanism to other kinds of divergences coming from the quantum correction and consider the cosmology in an extended model. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessFeature PaperArticle
Unitary Issues in Some Higher Derivative Field Theories
Received: 30 November 2017 / Revised: 22 January 2018 / Accepted: 2 February 2018 / Published: 14 February 2018
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Abstract
We analyze the unitarity properties of higher derivative quantum field theories which are free of ghosts and ultraviolet singularities. We point out that in spite of the absence of ghosts most of these theories are not unitary. This result confirms the difficulties of [...] Read more.
We analyze the unitarity properties of higher derivative quantum field theories which are free of ghosts and ultraviolet singularities. We point out that in spite of the absence of ghosts most of these theories are not unitary. This result confirms the difficulties of finding a consistent quantum field theory of quantum gravity. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle
The Avoidance of the Little Sibling of the Big Rip Abrupt Event by a Quantum Approach
Received: 29 November 2017 / Revised: 19 January 2018 / Accepted: 22 January 2018 / Published: 6 February 2018
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Abstract
We address the quantisation of a model that induces the Little Sibling of the Big Rip (LSBR) abrupt event, where the dark energy content is described by means of a phantom-like fluid or a phantom scalar field. The quantisation is done in the [...] Read more.
We address the quantisation of a model that induces the Little Sibling of the Big Rip (LSBR) abrupt event, where the dark energy content is described by means of a phantom-like fluid or a phantom scalar field. The quantisation is done in the framework of the Wheeler–DeWitt (WDW) equation and imposing the DeWitt boundary condition; i.e., the wave function vanishes close to the abrupt event. We analyse the WDW equation within two descriptions: First, when the dark energy content is described with a perfect fluid. This leaves the problem with the scale factor as the single degree of freedom. Second, when the dark energy content is described with a phantom scalar field in such a way that an additional degree of freedom is incorporated. Here, we have applied the Born–Oppenheimer (BO) approximation in order to simplify the WDW equation. In all cases, the obtained wave function vanishes when the LSBR takes place, thus fulfilling the DeWitt boundary condition. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessFeature PaperArticle
Time in Quantum Cosmology of FRW f(R) Theories
Received: 1 December 2017 / Revised: 9 January 2018 / Accepted: 9 January 2018 / Published: 17 January 2018
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Abstract
The time problem is a problem of canonical quantum gravity that has long been known about; it is related to the relativistic invariance and the consequent absence of an explicit time variable in the quantum equations. This fact complicates the interpretation of the [...] Read more.
The time problem is a problem of canonical quantum gravity that has long been known about; it is related to the relativistic invariance and the consequent absence of an explicit time variable in the quantum equations. This fact complicates the interpretation of the wave function of the universe. Following proposals to assign the clock function to a scalar field, we look at the scalar degree of freedom contained in f ( R ) theories. For this purpose we consider a quadratic f ( R ) theory in an equivalent formulation with a scalar field, with a FRW metric, and consider its Wheeler-DeWitt equation. The wave function is obtained numerically and is consistent with the interpretation of the scalar field as time by means of a conditional probability, from which an effective time-dependent wave function follows. The evolution the scale factor is obtained by its mean value, and the quantum fluctuations are consistent with the Heisenberg relations and a classical universe today. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle
Supersymmetric M-brane Solution in a Dynamical Background
Received: 22 November 2017 / Revised: 31 December 2017 / Accepted: 3 January 2018 / Published: 16 January 2018
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Abstract
Supersymmetry arises in certain theories of fermions coupled to gauge fields and gravity in a spacetime of 11 dimensions. The dynamical brane background has mainly been studied for the class of purely bosonic solutions only, but recent developments involving a time-dependent brane solution [...] Read more.
Supersymmetry arises in certain theories of fermions coupled to gauge fields and gravity in a spacetime of 11 dimensions. The dynamical brane background has mainly been studied for the class of purely bosonic solutions only, but recent developments involving a time-dependent brane solution have made it clear that one can get more information by asking what happens on supersymmetric systems. In this proceeding, we construct an exact supersymmetric solution of a dynamical M-brane background in the 11-dimensional supergravity and investigate supersymmetry breaking, the geometric features near the singularity and the black hole horizon. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessFeature PaperArticle
Ultra Light Axionic Dark Matter: Galactic Halos and Implications for Observations with Pulsar Timing Arrays
Received: 29 November 2017 / Revised: 15 December 2017 / Accepted: 8 January 2018 / Published: 16 January 2018
Cited by 5 | PDF Full-text (333 KB) | HTML Full-text | XML Full-text
Abstract
The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of [...] Read more.
The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of experimental detection of Weakly Interacting Massive Particle (WIMP) favors alternative candidates such as light axionic dark matter that naturally arise in string theory. Cosmological N-body simulations have shown that axionic dark matter forms a solitonic core of size of ≃150 pc in the innermost region of the galactic halos. The oscillating scalar field associated to the axionic dark matter halo produces an oscillating gravitational potential that induces a time dilation of the pulse arrival time of ≃400 ns/(m B /10 22 eV) for pulsar within such a solitonic core. Over the whole galaxy, the averaged predicted signal may be detectable with current and forthcoming pulsar timing array telescopes. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle
Wavepacket Evolution in Unimodular Quantum Cosmology
Received: 29 November 2017 / Revised: 29 December 2017 / Accepted: 3 January 2018 / Published: 12 January 2018
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Abstract
The unimodular theory of gravity admits a canonical quantization of minisuperspace models without the problem of time. We derive instead a kind of Schrödinger equation. We have found unitarily evolving wave packet solutions for the special case of a massless scalar field and [...] Read more.
The unimodular theory of gravity admits a canonical quantization of minisuperspace models without the problem of time. We derive instead a kind of Schrödinger equation. We have found unitarily evolving wave packet solutions for the special case of a massless scalar field and a spatially flat Friedmann universe. We show that the longterm behaviour of the expectation values of the canonical quantities corresponds to the evolution of the classical variables. The solutions provided in an explicit example can be continued beyond the singularity at t = 0 , passing a finite minimal extension of the universe. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle
A Proposal of a Regular Black Hole Satisfying the Weak Energy Condition
Received: 29 November 2017 / Revised: 28 December 2017 / Accepted: 30 December 2017 / Published: 4 January 2018
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Abstract
We discuss a black hole generated by some matter fluid, whose stress-energy tensor is known. We show that it is regular and that it satisfies the Weak Energy Condition (WEC) and the null energy condition (NEC). Finally, we look for its asymptotic behavior. [...] Read more.
We discuss a black hole generated by some matter fluid, whose stress-energy tensor is known. We show that it is regular and that it satisfies the Weak Energy Condition (WEC) and the null energy condition (NEC). Finally, we look for its asymptotic behavior. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessFeature PaperArticle
Electromagnetic Casimir Effect in AdS Spacetime
Galaxies 2017, 5(4), 102; https://doi.org/10.3390/galaxies5040102
Received: 21 November 2017 / Revised: 10 December 2017 / Accepted: 12 December 2017 / Published: 19 December 2017
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Abstract
We investigate the vacuum expectation value (VEV) of the energy-momentum tensor for the electromagnetic field in anti-de Sitter (AdS) spacetime in the presence of a boundary parallel to the AdS horizon. On the boundary, the field obeys the generalized perfect conductor boundary condition. [...] Read more.
We investigate the vacuum expectation value (VEV) of the energy-momentum tensor for the electromagnetic field in anti-de Sitter (AdS) spacetime in the presence of a boundary parallel to the AdS horizon. On the boundary, the field obeys the generalized perfect conductor boundary condition. The VEV of the energy-momentum tensor is decomposed into the boundary-free and boundary-induced contributions. In this way, for points away from the boundary, the renormalization is reduced to that for AdS spacetime without the boundary. The boundary-induced energy density is negative everywhere, and the normal stress is positive in the region between the boundary and the AdS boundary and is negative in the region between the boundary and the AdS horizon. Near both the AdS boundary and horizon, the boundary-induced VEV decays exponentially as a function of the corresponding proper distance. Applications are given for even and odd vector fields in Randall–Sundrum model with a single brane. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle
Bell Inequality and Its Application to Cosmology
Received: 9 November 2017 / Revised: 9 December 2017 / Accepted: 11 December 2017 / Published: 12 December 2017
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Abstract
One of the cornerstones of inflationary cosmology is that primordial density fluctuations have a quantum mechanical origin. However, most physicists consider that such quantum mechanical effects disappear in CMB data due to decoherence. In this conference report, we show that the violation of [...] Read more.
One of the cornerstones of inflationary cosmology is that primordial density fluctuations have a quantum mechanical origin. However, most physicists consider that such quantum mechanical effects disappear in CMB data due to decoherence. In this conference report, we show that the violation of Bell inequalities in an initial state of our universe increases exponentially with the number of modes to measure in inflation. This indicates that some evidence that our universe has a quantum mechanical origin may survive in CMB data, even if quantum entanglement decays exponentially afterward due to decoherence. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle
Cosmological Effects of Quantum Vacuum Condensates
Received: 22 October 2017 / Revised: 5 December 2017 / Accepted: 6 December 2017 / Published: 12 December 2017
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Abstract
In quantum field theory, many phenomena are characterized by a condensed structure of their vacua. Such a structure is responsible of a non trivial vacuum energy. Here we analyze disparate systems and we show that the thermal vacuum state for hot plasmas, the [...] Read more.
In quantum field theory, many phenomena are characterized by a condensed structure of their vacua. Such a structure is responsible of a non trivial vacuum energy. Here we analyze disparate systems and we show that the thermal vacuum state for hot plasmas, the vacuum for boson field in curved space and the vacuum for mixed neutrinos have the state equation of dark matter, w = 0 , and values of the energy density which are in agreement with the one estimated for dark matter. Moreover, we show that the vacuum of axions mixed with photons has the state equation of the cosmological constant w = 1 and a value of the energy density compatible with the one of dark energy. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessArticle
Exploring String Axions with Gravitational Waves
Received: 9 November 2017 / Revised: 4 December 2017 / Accepted: 7 December 2017 / Published: 11 December 2017
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Abstract
We explore the string axion dark matter with gravitational waves in Chern–Simons gravity. We show that the parametric resonance of gravitational waves occurs due to the axion coherent oscillation. Remarkably, the circular polarization of gravitational waves is induced by the parity violating Chern–Simons [...] Read more.
We explore the string axion dark matter with gravitational waves in Chern–Simons gravity. We show that the parametric resonance of gravitational waves occurs due to the axion coherent oscillation. Remarkably, the circular polarization of gravitational waves is induced by the parity violating Chern–Simons coupling. In fact, the gravitational waves should be enhanced ten times every 10 8 pc propagation in the presence of the axion dark matter with mass 10 10 eV provided the coupling constant = 10 8 km . Hence, after 10 kpc propagation, the amplitude of gravitational waves is enhanced significantly and the polarization of gravitational waves becomes circular. However, we have never observed these signatures. This implies that the Chern–Simons coupling constant and/or the abundance of string axions should be constrained much stronger than the current limits. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperArticle
Reheating via Gravitational Particle Production in Simple Models of Quintessence or ΛCDM Inflation
Received: 11 October 2017 / Revised: 9 November 2017 / Accepted: 10 November 2017 / Published: 14 November 2017
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Abstract
We have tested some simple ΛCDM (the same test is also valid for quintessence) inflation models, imposing that they match with the recent observational data provided by the BICEP and Planck’s team and leading to a reheating temperature, which is obtained via [...] Read more.
We have tested some simple Λ CDM (the same test is also valid for quintessence) inflation models, imposing that they match with the recent observational data provided by the BICEP and Planck’s team and leading to a reheating temperature, which is obtained via gravitational particle production after inflation, supporting the nucleosynthesis success. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

Other

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Open AccessFeature PaperConference Report
Galactic Structures from Gravitational Radii
Received: 5 December 2017 / Revised: 29 December 2017 / Accepted: 2 January 2018 / Published: 7 February 2018
Cited by 1 | PDF Full-text (403 KB) | HTML Full-text | XML Full-text
Abstract
We demonstrate that the existence of a Noether symmetry in f(R) theories of gravity gives rise to an additional gravitational radius, besides the standard Schwarzschild one, determining the dynamics at galactic scales. By this feature, it is possible to explain [...] Read more.
We demonstrate that the existence of a Noether symmetry in f ( R ) theories of gravity gives rise to an additional gravitational radius, besides the standard Schwarzschild one, determining the dynamics at galactic scales. By this feature, it is possible to explain the baryonic Tully-Fisher relation and the rotation curve of gas-rich galaxies without the dark matter hypothesis. Furthermore, under the same standard, the Fundamental Plane of elliptical galaxies can be addressed. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperConference Report
The Third Quantization: To Tunnel or Not to Tunnel?
Received: 30 November 2017 / Revised: 18 January 2018 / Accepted: 19 January 2018 / Published: 2 February 2018
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Abstract
Within the framework of the third quantization, we consider the possibility that an initially recollapsing baby universe can enter a stage of near de Sitter inflation by tunnelling through a Euclidean wormhole that connects the recollapsing and inflationary geometries. We present the solutions [...] Read more.
Within the framework of the third quantization, we consider the possibility that an initially recollapsing baby universe can enter a stage of near de Sitter inflation by tunnelling through a Euclidean wormhole that connects the recollapsing and inflationary geometries. We present the solutions for the evolution of the scale factor in the Lorentzian and Euclidean regions as well as the probability that the baby universe indeed crosses the wormhole when it reaches its maximum size. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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Open AccessFeature PaperConference Report
Quantum-Gravitational Effects on Primordial Power Spectra in Slow-Roll Inflationary Models
Received: 30 November 2017 / Revised: 3 January 2018 / Accepted: 3 January 2018 / Published: 8 January 2018
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Abstract
We review the computation of the power spectra of inflationary gauge-invariant perturbations in the context of canonical quantum gravity for generic slow-roll models. A semiclassical approximation, based on an expansion in inverse powers of the Planck mass, is applied to the complete Wheeler–DeWitt [...] Read more.
We review the computation of the power spectra of inflationary gauge-invariant perturbations in the context of canonical quantum gravity for generic slow-roll models. A semiclassical approximation, based on an expansion in inverse powers of the Planck mass, is applied to the complete Wheeler–DeWitt equation describing a perturbed inflationary universe. This expansion leads to a hierarchy of equations at consecutive orders of the approximation and allows us to write down a corrected Schrödinger equation that encodes information about quantum-gravitational effects. The analytical dependence of the correction to the power spectrum on the wavenumber is obtained. Nonetheless, some numerical work is needed in order to obtain its precise value. Finally, it is shown that the correction turns out to be positive, which leads to an enhancement of the power spectrum especially prominent for large scales. We will also discuss whether this correction leads to a measurable effect in the cosmic microwave background anisotropies. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
Open AccessFeature PaperConference Report
The Mimetic Born-Infeld Gravity: The Primordial Cosmos and Spherically Symmetric Solutions
Received: 31 October 2017 / Revised: 14 November 2017 / Accepted: 15 November 2017 / Published: 27 November 2017
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Abstract
The Eddington-inspired-Born-Infeld (EiBI) model is reformulated within the mimetic approach. In the presence of a mimetic field, the model contains non-trivial vacuum solutions. We study a realistic primordial vacuum universe and we prove the existence of regular solutions. Besides, the linear instabilities in [...] Read more.
The Eddington-inspired-Born-Infeld (EiBI) model is reformulated within the mimetic approach. In the presence of a mimetic field, the model contains non-trivial vacuum solutions. We study a realistic primordial vacuum universe and we prove the existence of regular solutions. Besides, the linear instabilities in the EiBI model are found to be avoidable for some bouncing solutions. For a vacuum, static and spherically symmetric geometry, a new branch of solutions in which the black hole singularity that is replaced with a lightlike singularity is found. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)
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