Vector dark matter production at the end of inflation

TL;DR

This paper explores a novel mechanism for producing dark matter from transverse vector modes generated by a tachyonic instability during inflation, potentially explaining the observed dark matter density across a wide mass range.

Contribution

It introduces a new production process for transverse vector dark matter via a coupling with the inflaton, expanding the viable mass and inflation scale ranges beyond previous models.

Findings

Transverse vector modes can produce sufficient dark matter density.

The mechanism allows for larger vector masses and lower inflation scales.

Dark electromagnetic fields redshift appropriately to match dark matter observations.

Abstract

It has been shown that the longitudinal mode of a massive vector boson can be produced by inflationary fluctuations and account for the dark matter content of the Universe. In this work we examine the possibility of instead producing the transverse mode via the coupling $\phi F \tilde F$ between the inflaton and the vector field strength. Such a coupling leads to a tachyonic instability and exponential production of one transverse polarization of the vector field, reaching its maximum near the end of inflation. At production the mass is negligible and the vectors add up coherently to form a dark electromagnetic field. As the Universe expands, the energy density of the dark electromagnetic field then redshifts like radiation until its wavelength stretches to beyond its Compton wavelength. After this point the vectors become non-relativistic and their energy density redshifts like matter. We show that these polarized transverse vectors can account for the observed dark matter relic density in the mass range $\mu$eV to hundreds of GeV. We also find that the tachyonic production mechanism of the transverse mode can accommodate larger vector masses and lower Hubble scales of inflation compared to the production mechanism for the longitudinal mode via inflationary fluctuations.