Ultralight Axions Versus Primordial Black Holes

TL;DR

This paper argues that ultralight axions, due to their high entropy, are more plausible dark matter candidates than primordial black holes, challenging previous entropy-based support for black holes as dark matter.

Contribution

It demonstrates that ultralight axions can have greater entropy than primordial black holes, suggesting they are more suitable dark matter candidates.

Findings

Ultralight axions can surpass black holes in entropy.

Entropy considerations favor axions over black holes as dark matter.

Axions in a broad mass range are viable dark matter constituents.

Abstract

We reconsider entropy arguments which have been previously argued to support the idea that the dark matter constituents are primordial black holes with many solar masses. It has recently been shown that QCD axions which solve the strong CP problem may have masses $m_a$ in the extended range $10^{-3}eV > m_a > 10^{-33} eV$. Ultralight axions provide so many degrees of freedom that their entropy can exceed that of primordial black holes. This suggests that ultralight axions are more suited than primordial black holes to be constituents of dark matter.