Constraining Nonthermal Dark Matter's Impact on the Matter Power Spectrum

TL;DR

This paper studies how nonthermal dark matter produced during an early matter-dominated era affects structure formation, showing that relativistic velocities can erase small-scale perturbations and constrain dark matter properties.

Contribution

It provides a detailed analysis of the velocity distribution of nonthermal dark matter from early matter domination and its impact on the matter power spectrum and structure formation.

Findings

Relativistic dark matter particles erase small-scale perturbations.

Constraints on dark matter velocity at production based on reheat temperature.

Free streaming prevents formation of satellite galaxies and affects Lyman-alpha forest observations.

Abstract

The inclusion of a period of (effective) matter domination following inflation and prior to the onset of radiation domination has interesting and observable consequences for structure growth. During this early matter-dominated era (EMDE), the Universe was dominated by massive particles, or an oscillating scalar field, that decayed into Standard Model particles, thus reheating the Universe. 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 particles and Standard Model particles would be produced with relativistic velocities. We investigate the effects of the nonthermal production of dark matter particles with relativistic velocities on the matter power spectrum by determining the resulting velocity distribution function for the dark matter. We find that the vast majority of dark matter particles produced during the EMDE are still relativistic at reheating, so their free streaming erases the perturbations that grow during the EMDE. The free streaming of the dark matter particles can also prevent the formation of satellite galaxies around the Milky Way and the structures observed in the Lyman-$\alpha$ forest. For a given reheat temperature, these observations put an upper limit on the velocity of the dark matter particles at their creation. For example, for a reheat temperature of 10 MeV, dark matter must be produced with a Lorentz factor $\gamma \lesssim 550$.