Quantum Statistical Effects on Warm Dark Matter and the Mass Constraint from the Cosmic Structure at Small Scales

TL;DR

This paper explores how quantum statistical effects, such as degeneracy pressure and Bose-Einstein condensation, influence the small-scale structure constraints on Warm Dark Matter mass, revealing significant impacts for bosonic cases.

Contribution

It introduces a unified framework to analyze quantum effects in fermionic and bosonic WDM, highlighting their influence on mass bounds and the evolution of BEC fractions.

Findings

Degenerate fermion effects slightly lower WDM mass bounds.

High initial BEC fractions significantly lower mass bounds.

BEC fraction diminishes during cosmic evolution and can vanish if initially below ~64%."

Abstract

The suppression of the small-scale matter power spectrum is a distinct feature of Warm Dark Matter (WDM), which permits a constraint on the WDM mass from galaxy surveys. In the thermal relic WDM scenario, quantum statistical effects are not manifest. In a unified framework, we investigate the quantum statistical effects for a fermion case with a degenerate pressure and a boson case with a Bose-Einstein condensation (BEC). Compared to the thermal relic case, the degenerate fermion case only slightly lowers the mass bound, while the boson case with a high initial BEC fraction ($\gtrsim90\%$) significantly lowers it. On the other hand, the BEC fraction drops during the relativistic-to-nonrelativistic transition and completely disappears if the initial fraction is below $\sim64$\%. Given the rising interest in resolving the late-time galaxy-scale problems with boson condensation, a question is posed on how a high initial BEC fraction can be dynamically created so that a condensed DM component remains today.