Halo Substructure Boosts to the Signatures of Dark Matter Annihilation
  The presence of dark matter substructure will boost the signatures of dark
matter annihilation. We review recent progress on estimates of this subhalo
boost factor---a ratio of the luminosity from annihilation in the subhalos to
that originating the smooth component---based on both numerical $N$-body
simulations and semi-analytic modelings. Since subhalos of all the scales,
ranging from the Earth mass (as expected, e.g., the supersymmetric neutralino,
a prime candidate for cold dark matter) to galaxies or larger, give substantial
contribution to the annihilation rate, it is essential to understand subhalo
properties over a large dynamic range of more than twenty orders of magnitude
in masses. Even though numerical simulations give the most accurate assessment
in resolved regimes, extrapolating the subhalo properties down in sub-grid
scales comes with great uncertainties---a straightforward extrapolation yields
a very large amount of the subhalo boost factor of $\gtrsim$100 for galaxy-size
halos. Physically motivated theoretical models based on analytic prescriptions
such as the extended Press-Schechter formalism and tidal stripping modeling,
which are well tested against the simulation results, predict a more modest
boost of order unity for the galaxy-size halos. Giving an accurate assessment
of the boost factor is essential for indirect dark matter searches and thus,
having models calibrated at large ranges of host masses and redshifts, is
strongly urged upon.
