1. Particle Physics Seminar

    "Constraining Dark Matter-Baryon Scattering with Linear Cosmology"

    Presented by Kfir Blum, Institute of Advanced Study, Princeton

    Thursday, October 10, 2013, 3 pm
    Small Seminar Room, Bldg. 510

    Hosted by: Anze Slosar

    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.