Constraints on cosmic strings from ultracompact minihalos
  Cosmic strings are expected to form loops. These can act as seeds for
accretion of dark matter, leading to the formation of ultracompact minihalos
(UCMHs). We perform a detailed study of the accretion of dark matter onto
cosmic string loops and compute the resulting mass distribution of UCMHs. We
then apply observational limits on the present-day abundance of UCMHs to derive
corresponding limits on the cosmic string tension $G\mu$. The bounds are
strongly dependent upon the assumed distribution of loop velocities and their
impacts on UCMH formation. Under the assumption that a loop can move up to a
thousand times its own radius and still form a UCMH, we find a limit of
$G\mu\le 5\times10^{-8}$. We show, in opposition to previous results, that
strong limits on the cosmic string tension are not obtainable from UCMHs when
more stringent (and realistic) requirements are placed on loop velocities.
