Dark photon dark matter from a rolling inflaton

TL;DR

This paper explores a novel mechanism where a rolling inflaton during inflation produces non-thermal dark photon dark matter, resulting in a clumpy distribution with potential observable small-scale structures.

Contribution

It introduces a detailed analysis of dark photon production via tachyonic instability at inflation's end, including relic abundance and power spectrum evolution, expanding understanding of dark matter origins.

Findings

Dark photons are produced with a peak at the Hubble scale at inflation's end.

The resulting dark matter distribution is clumpy with small-scale density fluctuations.

The peak in the power spectrum is preserved through cosmic evolution.

Abstract

We study in detail a recently proposed mechanism for producing non-thermal dark photon dark matter at the end of inflation in the mass range $\mu\,{\rm eV} \lesssim m \lesssim 10\,{\rm TeV}$. A tachyonic instability induced by a rolling inflaton leads to the coherent production of dark (abelian) gauge bosons with a peak in the power spectrum corresponding to the Hubble scale at the end of inflation. As the Universe expands after inflation the dark photons redshift and, at some point in their cosmic evolution, they obtain a mass. We focus in particular on the case where the dark photons are relativistic at the time their mass is generated and examine the associated cosmic evolution to compute the relic abundance today. We also examine the late time power spectrum demonstrating explicitly that it preserves the peak generated at the end of inflation. We show that the peak corresponds to small physical scales today, $\ell_{\rm today} \sim {\rm cm} - 100\,{\rm km}$, with large density fluctuations at $\ell_{\rm today}$ leading to a clumpy nature for the dark photon dark matter. We also discuss potential phenomenology and future directions, briefly commenting on the non-relativistic case.