Title : Constraining Dark Matter-Baryon Scattering with
Linear Cosmology
Speaker: Kfir Blum, Institute of Advanced Study, Princeton
Abstract:
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We derive constraints on elastic scattering between baryons and dark
matter, using cosmological data including the cosmic microwave
background and the Lya forest. Elastic scattering allows the baryon
and dark matter systems to exchange momentum in the early Universe,
affecting the dynamics of linear density perturbations. We calculate
the momentum exchange for a wide range of velocity dependent cross
sections, improving on previous estimates where they exist. In the
early Universe, the rate of momentum exchange is controlled by the
thermal velocities of the particles, while bulk motion factors out in
linear theory. This allows us to compare directly the scattering mean
free path at high redshift to that in contemporary systems like our
own Milky Way Galaxy and dwarf galaxies, and derive model-independent
constraints on the mean free path today in those systems. In all
cases, cosmological constraints imply that the mean free path is at
least 2-4 orders of magnitude larger than the characteristic size of
the system.