Dark matter relic abundance from a critical-density instability

TL;DR

This paper explores a novel dark matter thermal history where collective many-body effects and a critical-density instability influence relic abundance, linking self-interactions and early-universe dynamics.

Contribution

It introduces a model where dark matter self-interactions cause a critical-density instability that determines relic abundance, differing from standard freeze-out scenarios.

Findings

Relic abundance is primarily set by the critical density n_c.

The model achieves observed relic densities with TeV-scale dark matter and sub-GeV mediators.

Self-interaction cross sections are compatible with small-scale structure observations.

Abstract

We study a nonstandard dark-matter thermal history in which strong self-interactions give rise to collective many-body effects at high number density, as in strongly interacting quantum media. At early times, dark matter occupies a correlated phase in which its coupling to a light mediator is dynamically screened, suppressing annihilation far below the perturbative rate. As the Universe expands and the number density decreases, this screened phase becomes unstable at a critical density n_c, triggering a rapid, far-from-equilibrium annihilation episode. We show that this annihilation burst fixes the final relic abundance, which is governed primarily by n_c rather than by the microscopic annihilation coupling. Using a minimal effective parametrization, we solve the resulting modified Boltzmann evolution and map the viable parameter space. For TeV-scale dark matter and sub-GeV mediators, we find relic abundances consistent with observations together with self-interaction cross sections relevant for small-scale structure, realizing a consistent and predictive nonstandard thermal history.