Density profiles of ultracompact minihalos: Implications for
  constraining the primordial power spectrum
  Enhanced density fluctuations on small scales would lead to the formation of
numerous dark matter minihalos, so limits on the minihalo abundance can place
upper bounds on the small-scale primordial power spectrum. In particular, the
ultracompact minihalo (UCMH), a dark matter structure hypothesized to possess a
$\rho\propto r^{-9/4}$ density profile due to its formation at $z\geq 1000$,
has been used to establish an upper bound on the primordial power spectrum at
scales smaller than 2 Mpc. The extreme slope of this density profile amplifies
the observational signals of UCMHs. However, we recently showed via N-body
simulations that the $\rho\propto r^{-9/4}$ density profile does not develop in
realistic formation scenarios, throwing UCMH-derived power spectrum constraints
into question. Instead, minihalos develop shallower inner profiles with
power-law indices between $-3/2$ and $-1$. In this paper, we expand on that
result and discuss its implications. Using a model that is calibrated to
simulation results and predicts halo structures in spiked power spectra based
on their formation times, we calculate new upper bounds on the primordial power
spectrum based on limits on the dark matter annihilation rate within the
Galaxy. We find that despite assuming shallower profiles, this minihalo model
actually yields stronger constraints than the previous UCMH picture owing to
its inclusion of all minihalos instead of only the earliest-forming ones.
