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.
Highlights
The history of the Universe in the standard cosmological model is believed to proceed from an initial period of inflation (Guth 1981; Linde 1982; Albrecht and Steinhardt 1982), during which the Universe is in a quasi-de Sitter state with weakly broken scale invariance
We apply the squeezed state formalism to scalar field dark matter perturbations generated during inflation
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
Summary
The history of the Universe in the standard cosmological model is believed to proceed from an initial period of inflation (Guth 1981; Linde 1982; Albrecht and Steinhardt 1982), during which the Universe is in a quasi-de Sitter state with weakly broken scale invariance. The smaller scale modes (which are smaller than the cosmic de Broglie wavelength/Jeans scale), might retain memory of their quantum initial state It is in axion isocurvature fluctuations that enter the horizon prior to axion particle production where the initial quantum state could be encoded in the evolution in the form of the phase, which is normally neglected. In the present work we model this possibility and, in linear perturbation theory, evolve the axion field from the initial state during inflation, through reheating, and horizon re-entry during the hot big bang phase right up until the moment of axion DM production. Where we have assumed that the initial displacement of the axion field is less than the Peccei-Quinn symmetry breaking scale, φ < fa, such that the axion potential is approximately quadratic This choice is motivated purely by simplicity and our conclusions concerning the quantum state would not be strongly affected by using the full axion potential.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
