Ultralight fermionic dark matter

Abstract

Conventional lore from Tremaine and Gunn excludes fermionic dark matter lighter than a few hundred eV, based on the Pauli exclusion principle. We highlight a simple way of evading this bound with a large number of species that leads to numerous nontrivial consequences. In this scenario there are many distinct species of fermions with quasidegenerate masses and no couplings to the standard model. Nonetheless, gravitational interactions lead to constraints from measurements at the LHC, of cosmic rays, of supernovae, and of black hole spins and lifetimes. We find that the LHC constrains the number of distinct species, bosons or fermions lighter than ∼500 GeV, to be N1062. This, in particular, implies that roughly degenerate fermionic dark matter must be heavier than ∼10-14 eV, which thus relaxes the Tremaine-Gunn bound by ∼16 orders of magnitude. Slightly weaker constraints applying to masses up to ∼100 TeV exist from cosmic ray measurements while various constraints on masses 10-10 eV apply from black hole observations. We consider a variety of phenomenological bounds on the number of species of particles. Finally, we note that there exist theoretical considerations regarding quantum gravity which could impose more severe constraints that may limit the number of physical states to N1032.