Squeezing the Axion

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