Quantum-to-classical transition for fluctuations in the early Universe

TL;DR

This paper explains how quantum fluctuations in the early Universe transition to classical inhomogeneities without altering inflationary predictions, through squeezing of states and decoherence mechanisms.

Contribution

It demonstrates the quantum-to-classical transition of cosmological fluctuations via state squeezing and decoherence, preserving inflationary model predictions.

Findings

Quantum metric perturbations become highly squeezed during expansion.

Decoherence selects the field amplitude basis as the classical pointer basis.

Perturbations appear classical and stochastic today without changing inflationary outcomes.

Abstract

According to the inflationary scenario for the very early Universe, all inhomogeneities in the Universe are of genuine quantum origin. On the other hand, looking at these inhomogeneities and measuring them, clearly no specific quantum mechanical properties are observed. We show how the transition from their inherent quantum gravitational nature to classical behaviour comes about -- a transition whereby none of the successful quantitative predictions of the inflationary scenario for the present-day universe is changed. This is made possible by two properties. First, the quantum state for the spacetime metric perturbations produced by quantum gravitational effects in the early Universe becomes very special (highly squeezed) as a result of the expansion of the Universe (as long as the wavelength of the perturbations exceeds the Hubble radius). Second, decoherence through the environment distinguishes the field amplitude basis as being the pointer basis. This renders the perturbations presently indistinguishable from stochastic classical inhomogeneities.