Effects of Relativistic Nonthermal Dark Matter on the Matter Power Spectrum

The thermal production of dark matter during the radiation-dominated era is a simple and amply motivated theory. However, as the parameter space for particles that can fit this model continues to narrow, many compelling alternatives have been proposed. Notable among these modifications is a period of (effective) matter domination prior to the onset of radiation domination, which has interesting and observable consequences for structure growth. During this early matter-dominated era (EMDE), the Universe was dominated by massive particles or oscillating scalar fields that decayed into Standard Model (SM) particles, thus reheating the Universe after inflation. This decay process could also be the primary source of dark matter. In the absence of fine-tuning between the masses of the parent and daughter particles, both dark matter and SM particles would be born relativistic. We investigate the effects of the nonthermal production of dark matter particles with relativistic velocities on the matter power spectrum. We determine whether enhanced structure growth during the EMDE can be preserved and under what conditions lower limits on small-scale power obtained from the Lyman-alpha forest can be satisfied.