Fermionic Dark Matter and Self Interactions

Motivated by some tensions on small scale observables with the existing $\Lambda$CDM cosmological paradigm, several extensions to CDM were proposed that modify predictions on subhalo scales and lower. We study the combination of two well known extensions, both the inclusion of thermal velocity dispersion in the species (WDM) as well as the presence of elastic collisions between DM particles (SIDM). Assuming fermionic DM, we consider the effects of these combined models across several stages of the universe, focusing on the evolution of perturbations and the dynamics of thermalized halos. We reach a number of exciting conclusions and show that the combined model can both alleviate these small scale tensions as well as solve some problems proper to light DM models. We propose interesting observational ramifications of these models, and show that further constraining of the parameter space can be obtained via 21-cm and stellar streams analysis, and discuss their impact on the small scale behavior of DM models.