Parametric resonance production of ultralight vector dark matter

Vector bosons heavier than 10^-22 eV can be viable dark matter candidates with distinctive experimental signatures. Ultralight dark matter generally requires a nonthermal origin to achieve the observed density, while still behaving like a pressureless fluid at late times. We show that such a production mechanism naturally occurs for vectors whose mass originates from a dark Higgs. If the dark Higgs has a large field value after inflation, the energy in the Higgs field can be efficiently transferred to vectors through parametric resonance. Computing the resulting abundance and spectra requires careful treatment of the transverse and longitudinal components, whose dynamics are governed by distinct equations of motion. We study these in detail and find that the mass of the vector may be as low as 10^-18 eV, while making up the majority of the dark matter abundance. This opens up a wide mass range of vector dark matter as cosmologically viable, and further motivates the experimental searches for such particles.