Lukewarm dark matter: Bose condensation of ultralight particles

TL;DR

This paper explores the thermal history and Bose-Einstein condensation of ultralight dark matter particles, showing they can form stable galactic halos and address small-scale structure issues without affecting Big Bang Nucleosynthesis.

Contribution

It provides a detailed analysis of the thermal evolution, condensation, and cosmological implications of ultralight scalar dark matter particles with masses around 10^{-23} eV.

Findings

Particles decouple before Standard Model annihilation, resulting in a temperature of about 0.9 K.

Ultralight particles can form stable galactic halos and suppress small-scale structures.

The model aligns with cosmological observations without fine-tuning.

Abstract

We discuss the thermal evolution and Bose-Einstein condensation of ultra-light dark matter particles at finite, realistic cosmological temperatures. We find that if these particles decouple from regular matter before Standard model particles annihilate, their temperature will be about 0.9 K. This temperature is substantially lower than the temperature of CMB neutrinos and thus Big Bang Nucleosynthesis remains unaffected. In addition the temperature is consistent with WMAP 7-year+BAO+H0 observations without fine-tuning. We focus on particles of mass of $m\sim 10^{-23}$ eV, which have Compton wavelengths of galactic scales. Agglomerations of these particles can form stable halos and naturally prohibit small scale structure. They avoid over-abundance of dwarf galaxies and may be favored by observations of dark matter distributions. We present numerical as well as approximate analytical solutions of the Friedmann-Klein-Gordon equations and study the cosmological evolution of this scalar field dark matter from the early universe to the era of matter domination. Today, the particles in the ground state mimic presureless matter, while the excited state particles are radiation like.