TY - JOUR

T1 - Emergence of classical behavior in the early Universe

AU - Ashtekar, Abhay

AU - Corichi, Alejandro

AU - Kesavan, Aruna

N1 - Funding Information:
A. A. thanks Jerome Martin for a discussion and Lajos Diósi for correspondence. This work was supported by the NSF Grants No. PHY-1505411 and No. PHY-1806356 and the Eberly Chair funds of Penn State; DGAPA-UNAM IN114620 and CONACyT 0177840 grants, and an Edward A. and Rosemary A. Mebus Graduate Fellowship in Physics and the Frymoyer Honors Fellowship at Penn State, to A. K.
Publisher Copyright:
© 2020 American Physical Society.

PY - 2020/7/15

Y1 - 2020/7/15

N2 - We investigate three issues that have been discussed in the context of inflation: Fading of the importance of quantum noncommutativity; the phenomenon of quantum squeezing; and the ability to approximate the quantum state by a distribution function on the classical phase space. In the standard treatments, these features arise from properties of mode functions of quantum fields in (near) de Sitter space-time. Therefore, the three notions are often assumed to be essentially equivalent, representing different facets of the same phenomenon. We analyze them in general Friedmann-Lemaître- Robertson-Walker space-times, through the lens of geometric structures on the classical phase space. The analysis shows that: (i) inflation does not play an essential role; classical behavior can emerge much more generally; (ii) the three notions are conceptually distinct; classicality can emerge in one sense but not in another; and, (iii) the third notion is realized in a surprisingly strong sense; there is exact equality between completely general n-point functions in the classical theory and those in the quantum theory, provided the quantum operators are Weyl ordered. These features arise already for linear cosmological perturbations by themselves: considerations such as mode-mode coupling, decoherence, and measurement theory - although important in their own right - are not needed for emergence of classical behavior in any of the three senses discussed. Generality of the results stems from the fact that they can be traced back to geometrical structures on the classical phase space, available in a wide class of systems. Therefore, this approach may also be useful in other contexts.

AB - We investigate three issues that have been discussed in the context of inflation: Fading of the importance of quantum noncommutativity; the phenomenon of quantum squeezing; and the ability to approximate the quantum state by a distribution function on the classical phase space. In the standard treatments, these features arise from properties of mode functions of quantum fields in (near) de Sitter space-time. Therefore, the three notions are often assumed to be essentially equivalent, representing different facets of the same phenomenon. We analyze them in general Friedmann-Lemaître- Robertson-Walker space-times, through the lens of geometric structures on the classical phase space. The analysis shows that: (i) inflation does not play an essential role; classical behavior can emerge much more generally; (ii) the three notions are conceptually distinct; classicality can emerge in one sense but not in another; and, (iii) the third notion is realized in a surprisingly strong sense; there is exact equality between completely general n-point functions in the classical theory and those in the quantum theory, provided the quantum operators are Weyl ordered. These features arise already for linear cosmological perturbations by themselves: considerations such as mode-mode coupling, decoherence, and measurement theory - although important in their own right - are not needed for emergence of classical behavior in any of the three senses discussed. Generality of the results stems from the fact that they can be traced back to geometrical structures on the classical phase space, available in a wide class of systems. Therefore, this approach may also be useful in other contexts.

UR - http://www.scopus.com/inward/record.url?scp=85088628763&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85088628763&partnerID=8YFLogxK

U2 - 10.1103/PhysRevD.102.023512

DO - 10.1103/PhysRevD.102.023512

M3 - Article

AN - SCOPUS:85088628763

VL - 102

JO - Physical Review D

JF - Physical Review D

SN - 2470-0010

IS - 2

M1 - 023512

ER -