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arXiv · 2508.16455

Ultralight dark matter from non-slowroll inflation

Martina La Rosa·Gianmassimo Tasinato
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Abstract

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. This mechanism predicts a stochastic gravitational-wave background, generated at second order by non-adiabatic longitudinal vector fluctuations and amplified at very low frequencies, yielding a distinctive observational signature of the scenario. By leveraging a brief departure from slowroll dynamics during inflation - commonly invoked in scenarios that produce primordial black holes - our framework 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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Martina La Rosa, Gianmassimo Tasinato. 2025-08-22. Ultralight dark matter from non-slowroll inflation. https://arxiv.org/abs/2508.16455

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