Multi-species Dark Matter with Warmth and Randomness

Abstract

We present a general analytic framework for the evolution of cosmic structure in multi-species dark matter models that simultaneously incorporates finite velocity dispersion and Poisson fluctuations. Our approach accommodates arbitrary numbers of dark matter components with distinct mass fractions, velocity distributions, and number densities -- ranging from cold particles to warm species and sparse populations such as primordial black holes or solitons. The framework is based on solving a truncated BBGKY hierarchy, whose solution is obtained by solving Volterra integral equations. We provide an efficient algorithm to solve for the total, as well as inter- and intra-species power spectra. Worked examples with two-component mixtures illustrate how isocurvature (initially Poisson) and adiabatic spectra evolve differently depending on the properties of the warm or sparse fraction. This evolution is controlled by the free-streaming and Jeans scales, and the results match analytic estimates and $N$-body simulations.