Quantum Linear Scalar Fields with Time Dependent Potentials: Overview and Applications to Cosmology
Abstract
:1. Introduction
- (1)
- the vacuum state be invariant under the isometries of the spatial manifold ,
- (2)
- the dynamics dictated by the field Equation (1) be unitarily implementable,
2. The Classical Setting
2.1. Complex Structure: Definition and Some Key Results
2.2. The Scalar Field: Classical Theory
2.3. Complex Structures in FLRW Spacetimes
3. Quantization
3.1. Canonical Quantization on a Hilbert Space
- (i)
- A selection of basic (elementary, or fundamental) classical observables .
- (ii)
- The construction of an abstract quantum algebra of observables from , with the following two properties: (iia) for each basic observable there must be one, and only one, abstract quantum basic operator (observable) , and (iib) basic operators must satisfy the Dirac quantization condition, relating their commutators with the corresponding PBs.
- (iii)
- The specification of a Hilbert space and a representation of the abstract basic observables as self-adjoint operators on .
3.2. Linear Scalar Field Theory: Quantization
3.3. Bogoliubov Transformations and Unitary Implementability
3.4. The Scalar Field with Time Dependent Mass
4. Uniqueness of the Description for Quantum Gowdy Cosmologies
5. Scalar Fields in FLRW Spacetimes: Invariance, Unitarity, and Uniqueness
5.1. Closed FLRW Spacetimes
- (1)
- Let us recall that given a free scalar field propagating in a globally hyperbolic spacetime, a Hadamard representation of the CCRs can be specified [1] by looking for a vacuum state with a two-point function that has a short-distance behavior of the Hadamard type [93]. Although the Hadamard criterion does not suffice to pick out a unique preferred quantization in general, it has been shown that, for the case of free scalar fields in spacetimes with compact Cauchy slices, all Hadamard vacua belong to the same class of unitarily equivalent states [1]. Since this result applies to a free scalar field propagating in a closed FLRW spacetime, we have at our disposal two different criteria (the Hadamard approach, on the one hand, and the discussed requirements of invariance and unitarity, on the other hand) in order to select a unique preferred quantization of the scalar field. One may be wonder whether the unitary and the Hadamard quantizations are in conflict or not. The answer, as it is shown in Ref. [17], is that no conflict arises between the two approaches. In fact, since Hadamard states are unitarily equivalent to adiabatic vacuum states [94,95], one can proceed to translate the form of adiabatic states from the original -description to the scaled -description and then realize the equivalence of the resulting quantization with the -Fock representation [17]. That is, when the Hadamard quantization is reformulated in the scaled field description, the resulting representation of the CCRs is related to the -Fock representation by means of a unitary transformation. So, in the framework of the scaled -description, the Hadamard and the unitary quantizations allow for equivalent physical predictions. It is in this sense that one can assure that there is no tension between the invariant, unitary -Fock representation and the Hadamard quantization.
- (2)
- As we have mentioned in Section 3.1, the choice of fundamental classical variables involves an inherent ambiguity in the quantization of both mechanical and field systems. On account of this ambiguity, it is natural to ask whether a quantization with invariance and unitary dynamics can be achieved for a distinct pair of fundamental canonically conjugate variables, say , related to by a time dependent canonical transformation, compatible with the symmetries of the field equations and with all linear structures on phase space, namelyHere, F and G are restricted to be smooth real functions of time, with different from zero everywhere. Without loss of generality, one can set and . (In fact, the initial values and define an irrelevant time independent linear canonical transformation, which does not modify the spatial symmetries, nor the dynamics. The quantum representation for the transformed and the original fields is actually the same [22].)In this way, relationships (123) carry all the time dependence of the possible change of variables. By analyzing the new dynamics obtained with the transformation (123), it has been demonstrated in Refs. [14,15] that no transformation of this type (apart for the identity) can lead to a classical evolution that admits a unitary implementation with respect to any of the Fock representations defined by a invariant complex structure. Hence, the criteria of invariance and of unitarity fix not only the representation of the CCRs (up to unitary transformations) but, remarkably, the choice of field description as well. In this sense, the ambiguities in the quantization process are fully removed.
5.2. Flat FLRW Spacetimes
6. Uniqueness for Scalar Fields in de Sitter Spacetime
7. Uniqueness for Scalar Fields in Bianchi I Universes
- (1)
- There exists a subset of , differing from the latter set in a finite number of elements at most, such that
- (2)
- The subdominant terms and must satisfy for all the square summability condition
- (3)
- To satisfy restriction (159), the subdominant terms must be related by
8. Conclusions
- Generalizations to other dimensions: As we pointed out, the uniqueness of the -Fock representation extends to spacetimes with arbitrary compact Riemannian sections of dimension . The proof of this result is based on the behavior of the time evolution in the ultraviolet regime. Using this behavior, the satisfaction of the condition that guarantees the unitary implementability of the dynamics depends critically on the dimension of and, though the condition is fulfilled for [16], in general it is not satisfied in dimensions greater or equal than four. In such cases, an open issue is whether one can still find a different Fock representation that leads to a unitary evolution and analyze whether its equivalence class is singled out uniquely by our criteria of invariance and of unitarity.
- Other backgrounds: Other interesting backgrounds where the criteria of invariance and of unitarity can be tested are shear free anisotropic spacetimes, like Bianchi III cosmologies.
- Other fields: Even though the discussion has been focused primarily on the uniqueness of scalar and fermionic fields, there seems to be no obstacles (neither conceptual nor technical) to extend the analysis to other kind of fields, for example Maxwell fields, applying to them the proposed criteria to pick out a unique preferred Fock quantization.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Cortez, J.; Mena Marugán, G.A.; Velhinho, J. Quantum Linear Scalar Fields with Time Dependent Potentials: Overview and Applications to Cosmology. Mathematics 2020, 8, 115. https://doi.org/10.3390/math8010115
Cortez J, Mena Marugán GA, Velhinho J. Quantum Linear Scalar Fields with Time Dependent Potentials: Overview and Applications to Cosmology. Mathematics. 2020; 8(1):115. https://doi.org/10.3390/math8010115
Chicago/Turabian StyleCortez, Jerónimo, Guillermo A. Mena Marugán, and José Velhinho. 2020. "Quantum Linear Scalar Fields with Time Dependent Potentials: Overview and Applications to Cosmology" Mathematics 8, no. 1: 115. https://doi.org/10.3390/math8010115
APA StyleCortez, J., Mena Marugán, G. A., & Velhinho, J. (2020). Quantum Linear Scalar Fields with Time Dependent Potentials: Overview and Applications to Cosmology. Mathematics, 8(1), 115. https://doi.org/10.3390/math8010115