Dark Matter Recoupling

TL;DR

This paper explores a novel scenario where dark matter weakly interacts early on but recouples to dark radiation at late times, potentially affecting cosmological observations and allowing for a fraction of dark matter to be strongly interacting today.

Contribution

It systematically studies the phenomenology of late-time dark matter recoupling to dark radiation, a scenario not previously analyzed in detail.

Findings

Dark matter can recouple to dark radiation at late times, affecting cosmological signals.

Current data constrains the interaction to be weak if all dark matter is coupled.

A small fraction (around 4%) of dark matter could still be strongly interacting today.

Abstract

In the late Universe, and on cosmological scales, dark matter is conventionally assumed to be collisionless, as a consequence of the strong existing bounds on dark matter interactions at the Cosmic Microwave Background last-scattering surface. Challenging this lore, here we show that dark matter interactions can be naturally weak at early times, but then grow to observationally relevant strengths at very late times, even significantly after reionization. This is realized if dark matter recouples to a dark radiation species in the range of redshifts probed by the current generation of galaxy surveys. We systematically study, for the first time, the phenomenology of this dark matter recoupling scenario. A combination of Cosmic Microwave Background and Baryon Acoustic Oscillation data show that the interaction needs to be weak at present, if the entirety of dark matter couples to dark radiation. From a complementary perspective, a 4% fraction of dark matter could still be strongly interacting today. Implications for a microscopic model realizing the recoupling dynamics are discussed.