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 gravitational wave background. The production and cosmological evolution of these gravitational waves are implemented in class.vfdm, a modified version of class, from which we obtain the present-day spectrum.

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• Cosmology
• Gravitational waves
• Particle dark matter