Squeezing the Axion

TL;DR

This paper applies the squeezed state formalism to scalar field dark matter during inflation, showing that the quantum state remains highly squeezed after horizon re-entry, supporting the classical stochastic description of dark matter perturbations.

Contribution

It extends the squeezed state formalism to scalar dark matter, demonstrating the persistence of squeezing and the classical nature of dark matter perturbations across cosmic history.

Findings

Scalar dark matter perturbations become highly squeezed during inflation.

The quantum state remains squeezed after horizon re-entry during the hot big bang.

Large-scale modes are expected to decohere due to gravitational interactions.

Abstract

We apply the squeezed state formalism to scalar field dark matter (e.g. axion) perturbations generated during inflation. As for the inflationary perturbations, the scalar field state becomes highly squeezed as modes exit the horizon. For as long as $H>m_\phi$ (with $H$ the Hubble rate and $m_\phi$ the scalar mass) the scalar field field does not interact during reheating, and we follow its evolution exactly as modes re-enter the horizon. We find that the quantum state remains squeezed after horizon re-entry during the hot big bang. This demonstrates a fact well-known in the theory of inflation: cosmological observables for scalar dark matter are accurately modelled by a classical stochastic field with a fixed phase. Our calculation covers all modes smaller than the present-day cosmic de Broglie wavelength. Larger scale modes mix gravitationally with the environment when $H<m_\phi$, and are thus expected to decohere.