Reproductive Freeze-In of Self-Interacting Dark Matter

Understand

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.