Ultralight fermionic dark matter

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 similar to 500 GeV, to be N less than or similar to 10(62). This, in particular, implies that roughly degenerate fermionic dark matter must be heavier than similar to 10(-14) eV, which thus relaxes the Tremaine-Gunn bound by similar to 16 orders of magnitude. Slightly weaker constraints applying to masses up to similar to 100 TeV exist from cosmic ray measurements while various constraints on masses less than or similar to 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 N less than or similar to 10(32).

Automated summary

The conventional lore excludes fermionic dark matter with mass lighter than a few hundred electronvolts based on the Pauli exclusion principle. A new method is proposed in this paper which involves numerous quasi-degenerate species of fermions without couplings to the standard model to evade this bound. Gravitational interactions impose constraints from measurements at the LHC, cosmic rays, supernovae, and black hole spins and lifetimes, with a particular limit on the number of distinct species of particles being less than or around 10^62.