Cosmologically Degenerate Fermions

TL;DR

This paper explores how light fermionic dark matter particles, due to Pauli degeneracy pressure in the early universe, can have significant momentum, leading to new bounds on their mass and density fraction from cosmological observations.

Contribution

It introduces a novel perspective on fermionic dark matter momentum distribution caused by degeneracy pressure, deriving new bounds on mass and density fraction from cosmological data.

Findings

Derived bounds on fermion mass and density fraction from cosmological measurements.

Identified that degeneracy pressure can give fermions high momenta independent of thermal energy.

Improved existing mass bounds for fermionic dark matter to 2 keV for f_psi=1.

Abstract

Dark matter (DM) with a mass below a few keV must have a phase space distribution that differs substantially from the Standard Model particle thermal phase space: otherwise, it will free stream out of cosmic structures as they form. We observe that fermionic DM psi in this mass range will have a non-negligible momentum in the early Universe, even in the total absence of thermal kinetic energy. This is because the fermions were inevitably more dense at higher redshifts, and thus experienced Pauli degeneracy pressure. They fill up the lowest-momentum states, such that a typical fermion gains a momentum ~ O(p_F) that can exceed its mass m_psi. We find a simple relation between m_psi, the current fraction f_psi of the cold DM energy density in light fermions, and the redshift at which they were relativistic. Considering the impacts of the transition between nonrelativistic and relativistic behavior as revealed by measurements of DNeff and the matter power spectrum, we derive qualitatively new bounds in the f_psi-m_psi plane. We also improve existing bounds for f_psi = 1 by an order of magnitude to m_psi=2 keV. We remark on implications for direct detection and suggest models of dark sectors that may give rise to cosmologically degenerate fermions.