Ultralight Vector Dark Matter from non-slowroll Inflation

About

The longitudinal mode of a massive vector field, generated during inflation, offers a well motivated and phenomenologically rich candidate for dark matter. We show that a rapid variation in the mass of the vector boson, occurring during a brief phase of non-slowroll inflationary evolution, can naturally give rise to extremely small vector masses after inflation ends, corresponding to an ultralight dark matter candidate, with masses as low as m ≃ 10^{−19} eV or even smaller. This represents a refined and self-contained production mechanism: it does not rely on additional assumptions beyond the inflationary dynamics itself and the presence of a massive field. It also predicts, as a probe, a stochastic gravitational-wave background, generated at second order by non-adiabatic longitudinal vector fluctuations. This constitutes a distinctive observational signature of our framework. By leveraging a brief departure from slowroll dynamics, this framework also establishes a novel connection between ultralight vector dark matter and primordial black hole physics, suggesting a possible unified setting for mixed dark matter scenarios.

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