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We revisit the possibility of constraining the properties of dark matter (DM)by studying the epoch of cosmic reionization. Previous studies have shown thatDM annihilation was unlikely to have provided a large fraction of the photonsthat ionized the universe, but instead played a subdominant role relative tostars and quasars. The DM, however, begins to efficiently annihilate with theformation of primordial microhalos at $z\sim100-200$, much earlier than theformation of the first stars. Therefore, if DM annihilation ionized theuniverse at even the percent level over the interval $z \sim 20-100$, it canleave a significant imprint on the global optical depth, $\tau$. Moreover, weshow that cosmic microwave background (CMB) polarization data and future 21 cmmeasurements will enable us to more directly probe the DM contribution to theoptical depth. In order to compute the annihilation rate throughout the epochof reionization, we adopt the latest results from structure formation studiesand explore the impact of various free parameters on our results. We show thatfuture measurements could make it possible to place constraints on the darkmatter's annihilation cross section that are at a level comparable to thoseobtained from the observations of dwarf galaxies, cosmic ray measurements, andstudies of recombination.

The Effects of Dark Matter Annihilation on Cosmic Reionization