Speaker
Description
The existence of an early matter-dominated epoch prior to the big bang nucleosynthesis may lead to a scenario where the thermal dark matter starts to cool faster than the plasma before the onset of reheating. In the standard radiation-dominated epoch, the thermal dark matter free-streams after it decouples both chemically and kinetically from the plasma. In presence of an early matter-dominated epoch, the chemical decoupling of the dark matter may be followed by a partial kinetic decoupling prior to the reheating, depending upon the contribution of different partial wave amplitudes in the elastic scattering of the dark matter. We show that the s-wave scattering is sufficient to keep the dark matter partially decouple from the plasma, provided the entropy injection during the reheating epoch depends on the bath temperature, while p-wave scattering will lead to a full decoupling in such cosmological backdrop. Decoupling during the early matter domination leads to an additional amount of cooling before the commencement of radiation dominated universe, which reduces the free-streaming horizon of dark matter compared to usual radiation-dominated cosmology. The enhanced matter perturbations for scales entering the horizon before, or during reheating, together with the reduced free-steaming horizon of dark matter increases the number density of sub-earth mass halos. As a result the annihilation signatures of the dark matter in the galactic centres may receive a significant boosts, providing an intrguing probe to differentiate non-standard cosmological epoch prior to BBN. Fo illustration we examine two simple dark matter models, one featuring a scalar dark matter with s-wave scattering and the other a fermion dark matter with p-wave scattering.
