Thermal production of cold "hot dark matter" around eV

TL;DR

This paper shows that eV-mass bosonic dark matter can be thermally produced as cold dark matter through Bose-enhanced reactions, challenging previous assumptions that such light dark matter would be hot and incompatible with structure formation.

Contribution

It demonstrates that Bose-enhanced reactions allow for cold thermal production of eV-mass bosonic dark matter, revising previous views on hot dark matter scenarios.

Findings

Bose-enhancement can produce cold eV-mass bosonic DM from hot plasma.

Thermal production of bosonic DM is model-independent, applicable to axions and hidden photons.

Evidences suggest eV bosonic DM may be compatible with structure formation constraints.

Abstract

A very simple production mechanism of feebly interacting dark matter (DM) that rarely annihilates is thermal production, which predicts the DM mass around eV. This has been widely known as the hot DM scenario. Despite there are several observational hints from background lights suggesting a DM in this mass range, the hot DM scenario has been considered strongly in tension with the structure formation of our Universe because the free-streaming length of the DM produced from thermal reactions was thought to be too long. In this paper, I show that the previous conclusions are not always true depending on the reaction for bosonic DM because of the Bose-enhanced reaction at very low momentum. By using the simple $1\leftrightarrow 2$ decay/inverse decay process to produce the DM, I demonstrate that the eV range bosonic DM can be thermally produced $coldly$ from a hot plasma by performing a model-independent analysis applicable to axion, hidden photon, and other bosonic DM candidates. Therefore, the bosonic DM in the eV mass range may still be special and theoretically well-motivated.