
TL;DR
This paper develops a method to calculate the free-streaming length in mixed dark matter models, accounting for various particle types and decoupling scenarios, with implications for structure formation.
Contribution
It introduces a comprehensive framework to compute free-streaming lengths in mixed dark matter, including fermions, bosons, and WIMPs, considering their decoupling conditions and distribution functions.
Findings
Sterile neutrinos decouple out of equilibrium with free-streaming length around 7 kpc for keV masses.
WIMPs with masses above 100 GeV have negligible free-streaming lengths, behaving as cold dark matter.
Bose-Einstein condensates as dark matter have free-streaming lengths consistent with cold dark matter.
Abstract
Free streaming in a \emph{mixture} of collisionless non-relativistic dark matter (DM) particles is studied by implementing methods from the theory of multicomponent plasmas. The mixture includes Fermionic, condensed and non condensed Bosonic particles decoupling in equilibrium while relativistic, heavy non-relativistic thermal relics (WIMPs), and sterile neutrinos that decouple \emph{out of equilibrium} when they are relativistic. The free-streaming length is obtained from the marginal zero of the gravitational polarization function, which separates short wavelength Landau-damped from long wavelength Jeans-unstable \emph{collective} modes. At redshift we find ,where are the \emph{fractions} of the…
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