Is Gravity Always Enough to Yield a Classical Universe?

TL;DR

This paper examines whether gravity alone can explain the classical appearance of the universe, exploring quantum-to-classical transition mechanisms and potential residual quantum features in cosmic structures.

Contribution

It critically analyzes standard assumptions about gravity-induced decoherence and introduces phase-space methods to detect possible surviving quantum signatures.

Findings

Non-linear cosmological perturbations can produce non-classical features.

Quantum signatures may persist in observable cosmic structures.

Phase-space analysis offers a concrete approach to identify quantum effects.

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.