Non-adiabatic cosmological production of ultra-light Dark Matter

TL;DR

This paper investigates the non-adiabatic production of ultra-light dark matter during inflation, showing it can produce cold dark matter with specific properties and establishing a lower bound on ULDM abundance.

Contribution

It provides a detailed analysis of non-adiabatic cosmological production of ULDM, deriving its distribution, abundance, and properties, and highlights its significance as a lower bound for ULDM dark matter.

Findings

ULDM produced non-adiabatically peaks at low momentum

The resulting ULDM is cold with a free streaming length of about 70 pc

The abundance saturates the dark matter density for a specific mass around 1.5e-5 eV

Abstract

We study the non-adiabatic cosmological production of ultra light dark matter (ULDM) under a minimal set of assumptions: a free ultra light real scalar as a spectator field in its Bunch-Davies vacuum state during inflation and instantaneous reheating into a radiation dominated era. For (ULDM) fields minimally coupled to gravity, non-adiabatic particle production yields a \emph{distribution function} peaked at \emph{low} comoving momentum $\mathcal{N}_k \propto 1/k^3$. The infrared behavior is a remnant of the infrared enhancement of light minimally coupled fields during inflation. We obtain the full energy momentum tensor, show explicity its equivalence with the fluid-kinetic one in the adiabatic regime, and extract the abundance, equation of state and free streaming length (cutoff in the matter power spectrum). Taking the upper bound on the scale of inflation from Planck, the (UDLM) saturates the dark matter abundance for $m \simeq 1.5\,\times 10^{-5}\mathrm{eV}$ with an equation of state parameter $w \simeq 10^{-14}$ and a free streaming length $\lambda_{fs} \simeq 70\,\mathrm{pc}$. Thus this cosmologically produced (ULDM) yields a \emph{cold} dark matter particle. We argue that the abundance from non-adiabatic production yields a \emph{lower bound} on generic (ULDM) and axion-like particles that must be included in any assessment of (ULDM) as a dark matter candidate.