Traces of early matter domination: dark matter cooling boosts sub-earth halo population.

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 the 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. The entropy injection during the reheating epoch impacts the chemical decoupling of the dark matter setting the present day relic abundance. On the other hand, the enhanced matter perturbations for scales entering the horizon before reheating, together with the reduced free-steaming horizon of the dark matter due to the extra cooling, 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 boost, providing an intriguing probe to differentiate dark matter models in non-standard cosmological epochs prior to the BBN. 

The talk will be based on: 2408.08360 and 2204.03670