Does inflation squeeze cosmological perturbations?

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-Lemaître-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.

arXiv comment19 pages + 8 pages as appendices

References in corpus (33)

• Separability Criterion for Density Matrices
• Peres-Horodecki separability criterion for continuous variable systems
• Separability criterion and inseparable mixed states with positive partial transposition
• Semiclassicality and Decoherence of Cosmological Perturbations
• Notes on Some Entanglement Properties of Quantum Field Theory
• Inflation and squeezed quantum states
• Quantum tensor product structures are observable-induced
• Quantum-to-classical transition for fluctuations in the early Universe
• Quantum-to-classical Transition of Cosmological Perturbations for Non-vacuum Initial States
• Entanglement entropy in de Sitter space
• Quantum Discord of Cosmic Inflation: Can we Show that CMB Anisotropies are of Quantum-Mechanical Origin?
• Shortcomings in the Understanding of Why Cosmological Perturbations Look Classical
• Asymptotics with a positive cosmological constant: I. Basic framework
• Quantum field theory on a cosmological, quantum space-time
• A model with cosmological Bell inequalities
• CSL Wave Function Collapse Model as a Mechanism for the Emergence of Cosmological Asymmetries in Inflation
• Signals of a Quantum Universe
• Entanglement Entropy of Cosmological Perturbations
• The Coherence of Primordial Fluctuations Produced During Inflation
• A cosmological open quantum system
• Canonical transformations and squeezing formalism in cosmology
• Unitarity and ultraviolet regularity in cosmology
• Revising the observable consequences of slow-roll inflation
• Real-space entanglement in the Cosmic Microwave Background
• Critical review of prevailing explanations for the emergence of classicality in cosmology
• The Power of Locality: Primordial Non-Gaussianity at the Map Level
• Emergence of classical behavior in the early universe
• Real-space Bell inequalities in de Sitter
• No intrinsic decoherence of inflationary cosmological perturbations
• Emergence of inflationary perturbations in the CSL model
• Reassessing the link between B-modes and inflation
• Expectation of primordial gravity waves generated during inflation
• Discussions about the landscape of possibilities for treatments of cosmic inflation involving continuous spontaneous localization models

Cited by in corpus (3)

• Comparing quantumness criteria
• Probing the initial state of inflation: analytical structure of cosmological correlators
• Primordial cosmic complexity and effects of reheating