Was the Early Universe Quantum? Falsifying Classical Stochastic Inflation

TL;DR

This paper proposes a new method to test whether primordial fluctuations in the early universe are quantum or classical by using inequality constraints on classical models and analyzing cosmological data, bypassing Bell tests.

Contribution

It introduces a falsification approach with inequality constraints to distinguish quantum from classical primordial perturbations using cosmological observations.

Findings

Classical stochastic models satisfy specific inequality constraints.

Violations of these inequalities indicate quantum origins of fluctuations.

Large-scale structure and 21 cm surveys can test these inequalities.

Abstract

Inflationary cosmology successfully accounts for the observed properties of primordial fluctuations using quantum field theory in an expanding background. However, the quantum nature of these fluctuations has not been experimentally established, since classical stochastic models could reproduce the observed two-point statistics by construction. Existing approaches to testing primordial quantumness focus primarily on Bell inequalities, which provide a sharp conceptual criterion but are difficult to implement with cosmological observables. In this work we adopt a falsification-based approach. We define a precise classical hypothesis for the origin of primordial perturbations (local stochastic fields admitting a positive probability distribution) and identify inequality constraints that must be satisfied within this class. We show how violations of these classicality inequalities can be probed using realistic cosmological observables, without invoking Bell tests or non-commuting measurement settings. We further identify symmetry-protected spectator sectors in which quantum coherence is parametrically preserved during inflation, allowing violations of observable magnitude to survive decoherence. Our results show that large-scale structure and future 21 cm surveys provide a viable and quantitative route to falsifying classical stochastic descriptions of primordial fluctuations.