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 simple way of evading this bound with large number species that leads to numerous nontrivial consequences. In scenario there are many distinct fermions quasidegenerate masses no couplings standard model. Nonetheless, gravitational interactions lead constraints measurements at LHC, cosmic rays, supernovae, black hole spins lifetimes. find LHC constrains species, bosons or $\ensuremath{\sim}500\text{ }\text{ }\mathrm{GeV}$, be $N\ensuremath{\lesssim}{10}^{62}$. This, in particular, implies roughly degenerate must heavier $\ensuremath{\sim}{10}^{\ensuremath{-}14}\text{ }\mathrm{eV}$, which thus relaxes Tremaine-Gunn by $\ensuremath{\sim}16$ orders magnitude. Slightly weaker applying up $\ensuremath{\sim}100\text{ }\mathrm{TeV}$ exist ray while various $\ensuremath{\lesssim}{10}^{\ensuremath{-}10}\text{ }\mathrm{eV}$ apply observations. consider variety phenomenological bounds particles. Finally, we note theoretical considerations regarding quantum gravity could impose more severe may limit physical states $N\ensuremath{\lesssim}{10}^{32}$.