Cosmological Gravitational Waves from Ultralight Vector Dark Matter

TL;DR

This paper calculates the stochastic gravitational wave background generated by ultralight vector dark matter, revealing how scalar perturbations source tensor modes due to background anisotropy.

Contribution

It derives the perturbation equations in an anisotropic background and implements the GW production in a modified CLASS code for cosmological predictions.

Findings

Scalar perturbations source tensor modes due to anisotropic background.

The GW spectrum is computed and characterized for ultralight vector dark matter.

The work links dark matter properties to observable gravitational wave signals.

Abstract

We compute the abundance of cosmological gravitational waves produced during the evolution of an ultralight vector (spin-1) dark matter field. A homogeneous background vector field breaks spatial isotropy, requiring a Bianchi I geometry and inducing a mixing between the scalar, vector, and tensor perturbation sectors. We derive the perturbation equations in this background and show that, as a consequence of this mixing, scalar perturbations act as a source of tensor modes, generating a stochastic GW background. The production and cosmological evolution of these gravitational waves are implemented in a modified version of CLASS, from which we obtain the present-day spectrum.