Affiliation: College of Arts and Sciences, Department of Physics and Astronomy
An early matter-dominated era (EMDE) is a brief period between the end of inflation and reheating where a matter-like energy source dominates the cosmic stage. During the EMDE, sub-horizon matter density perturbations grow linearly with the scale factor, as compared to logarithmically during the radiation-dominated (RD) era. Perturbation modes outside the comoving Hubble horizon at reheating, with R > R[H] ≡ 1/(a[RH]H[RH]) and thus k < k[RH], are unaffected by the EMDE. Free-streaming of the dark matter particle will eliminate structure formation on scales below the free-streaming cut-off scale R[cut] = 1/k[cut]. Modes in the wavenumber range k[RH] < k < k[cut] will be enhanced by the EMDE, and thus we are interested in the ratio k[cut]/k[RH] as a function of the reheat temperature T[RH], temperature at kinetic decoupling in a RD universe T[kdS], and dark matter mass m[x]. A larger ratio corresponds to a larger range of modes enhanced by the EMDE. We find that for a ratio T[kdS]=T[RH] ≈ 6, a ratio k[cut]/k[RH] = 20 is attainable. The enhanced matter power spectrum is used as initial conditions for the GADGET-2 N-body simulation code to study the formation and survival fraction of dark matter microhalos below the reheat mass. The microhalos that survive until today are bound into galaxy-mass host halos as substructure. This high-density substructure
causes a boost in the dark matter annihilation rate. Our simulation results demonstrate that slightly fewer microhalos survive than previously expected, resulting in a somewhat lower annihilation boost factor than that of analytical predictions.