Does inflation squeeze cosmological perturbations?

TL;DR

The paper critically examines the common belief that inflation squeezes and entangles cosmological perturbations, highlighting ambiguities in defining squeezed states in dynamic backgrounds and concluding that inflation does not necessarily squeeze modes.

Contribution

It challenges the standard interpretation of mode squeezing during inflation by analyzing ambiguities in defining squeezed states and particles in time-dependent backgrounds.

Findings

In generic backgrounds, the notion of squeezing is ambiguous.

In de Sitter-like spacetimes, the ambiguity is resolved.

Inflation does not necessarily squeeze modes, but perturbations remain quantum.

Abstract

There seems to exist agreement about the fact that inflation squeezes the quantum state of cosmological perturbations and entangles modes with wavenumbers $\vec k$ and $-\vec k$. Paradoxically, this result has been used to justify both the classicality as well as the quantumness of the primordial perturbations at the end of inflation. We reexamine this question and point out that the definition of two-mode squeezing of the modes $\vec k$ and $-\vec k$ used in previous work rests on choices that are only justified for systems with time-independent Hamiltonians and finitely many degrees of freedom. We argue that for quantum fields propagating on generic time-dependent Friedmann-Lema\^itre-Robertson-Walker backgrounds, the notion of squeezed states is subject to ambiguities, which go hand in hand with the ambiguity in the definition of particles. In other words, we argue that the question "does the cosmic expansion squeeze and entangle modes with wavenumbers $\vec k$ and $-\vec k$?" contains the same ambiguity as the question "does the cosmic expansion create particles?". When additional symmetries are present, like in the (quasi) de Sitter-like spacetimes used in inflationary models, one can resolve the ambiguities, and we find that the answer to the question in the title turns out to be in the negative. We further argue that this fact does not make the state of cosmological perturbations any less quantum, at least when deviations from Gaussianity can be neglected.