Reproductive Freeze-In of Self-Interacting Dark Matter

TL;DR

This paper introduces a new mechanism for dark matter production called reproductive freeze-in, involving self-interactions that prevent full thermal equilibrium and lead to unique cosmological signatures.

Contribution

It proposes a novel dark matter production process where self-interactions prevent equilibrium, resulting in distinctive thermal history and potential observable effects.

Findings

Self-interacting dark matter never reaches full thermal equilibrium.

DM kinetic energy converts to mass during the process.

Potential implications for galaxy dynamics and cosmological tensions.

Abstract

We present a mechanism for dark matter (DM) production involving a self-interacting sector that at early times is ultra-relativistic but far-underpopulated relative to thermal equilibrium (such initial conditions often arise, e.g., from inflaton decay). Although elastic scatterings can establish kinetic equilibrium we show that for a broad variety of self-interactions full equilibrium is never established despite the DM yield significantly evolving due to $2\to k$ ($k>2$) processes (the DM carries no conserved quantum number nor asymmetry). During the active phase of the process, the DM to Standard Model temperature ratio falls rapidly, with DM kinetic energy being converted to DM mass, the inverse of the recently-discussed `cannibal DM mechanism'. Potential observables and applications include self-interacting DM signatures in galaxies and clusters, dark acoustic oscillations, the alteration of free-streaming constraints, and possible easing of $\sigma_8$ and Hubble tensions.