Self-Interacting Dark Matter Scattering Rates Through Cosmic Time

TL;DR

This paper estimates dark matter scattering rates over cosmic time, highlighting differences between velocity-independent and velocity-dependent cross-sections, and discusses implications for structure formation and simulation accuracy.

Contribution

It provides the first detailed analysis of dark matter scattering rates throughout cosmic history, emphasizing the importance of velocity dependence and small-scale structures.

Findings

Scattering peaks at z~6 for velocity-independent models.

Half of scatterings occur after z~1 in massive structures.

Early interactions could mimic warm dark matter effects.

Abstract

We estimate the rate of dark matter scattering in collapsed structures throughout the history of the Universe. If the scattering cross-section is velocity-independent, then the canonical picture is correct that scatterings occur mainly at late times. The scattering rate peaks slightly at redshift z~6, and remains significant today. Half the scatterings occur after z~1, in structures more massive than 10^12 M_sun. Within a factor of two, these numbers are robust to changes in the assumed astrophysics, and the scatterings would be captured in cosmological simulations. However, for particle physics models with a velocity-dependent cross-section (as for Yukawa potential interactions via a massive mediator), the scattering rate peaks before z~20, in objects with mass less than 10^4 M_sun. These precise values are sensitive to the redshift-dependent mass-concentration relation and the small-scale cutoff in the matter power spectrum. In extreme cases, the qualitative effect of early interactions may be reminiscent of warm dark matter and strongly affect the subsequent growth of structure. However, these scatterings are being missed in existing cosmological simulations with limited mass resolution.