Gravitational Production of Dark(est) Matter
Dr. Nathan Herring
University of Pittsburgh
I will present the results of our study of non-adiabatic cosmological production of dark matter. By stipulating that the dark matter be described by a spectator field in its Bunch-Davies vacuum state during inflation and concentrating on modes outside the particle horizon at the onset of radiation domination, the particle production for scalar dark matter, considering both minimal and conformal coupling to gravity, and fermionic dark matter is analytically computed. In all cases, the distribution of produced particles is peaked at low comoving momentum, self-consistent with the consideration of superhorzion modes. We obtain the full energy momentum tensor, show explicitly its equivalence with the fluid-kinetic one in the adiabatic regime, and extract the abundance, equation of state and free streaming length (cutoff in the matter power spectrum) for the dark matter. For both fermions and minimally coupled scalars, this production mechanism yields a cold dark matter particle consistent with astronomical observations, without any coupling to Standard Model species, and with solely gravitational interactions. Thus these models represent theories of the \emph{darkest} of dark matter. We argue that the abundance from non-adiabatic production yields a lower bound on generic scalar (ULDM) and axion-like particles (ALP) that must be included in any assessment of (ULDM)/(ALP) as a dark matter candidate. For fermions, we highlight how this production surprisingly leads to a nearly thermal distribution with an emergent temperature; a result which warrants further analysis.