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Is Gravity Always Enough to Yield a Classical Universe?

Aurora Ireland

Abstract

The origin of cosmic structure is widely regarded as quantum, yet the Universe today appears classical. Standard lore attributes this to a "quantum-to-classical" transition on super-horizon scales during inflation. Gravity plays a central role: super-horizon dynamics squeeze quantum states, while the cosmological horizon enforces a system-environment split, leading to decoherence. But are these mechanisms always sufficient? We revisit this question, identifying assumptions and limitations in conventional arguments. We highlight recent work showing that beyond slow roll, non-linear dynamics of cosmological perturbations can generate non-classical features that may survive in observables. This raises the tantalizing possibility that quantum signatures may persist in cosmic structure. We propose a phase-space analysis based on the Wigner function as a concrete route to identifying and probing such signatures.

Is Gravity Always Enough to Yield a Classical Universe?

Abstract

The origin of cosmic structure is widely regarded as quantum, yet the Universe today appears classical. Standard lore attributes this to a "quantum-to-classical" transition on super-horizon scales during inflation. Gravity plays a central role: super-horizon dynamics squeeze quantum states, while the cosmological horizon enforces a system-environment split, leading to decoherence. But are these mechanisms always sufficient? We revisit this question, identifying assumptions and limitations in conventional arguments. We highlight recent work showing that beyond slow roll, non-linear dynamics of cosmological perturbations can generate non-classical features that may survive in observables. This raises the tantalizing possibility that quantum signatures may persist in cosmic structure. We propose a phase-space analysis based on the Wigner function as a concrete route to identifying and probing such signatures.

Paper Structure

This paper contains 11 sections, 6 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Classicalization at the closed-system level
  3. Classicalization at the open-system level
  4. Observational prospects
  5. Acknowledgements

Figures (1)

  • Figure 1: Schematic density plot of the Wigner function $W(\mathcal{R}, \pi_\mathcal{R})$ in slow roll (left) and ultra-slow roll (right) inflationary backgrounds. The former is a globally positive Gaussian while the latter displays interference fringes and regions of negativity.