Axion Dark Matter and Cosmological Parameters

TL;DR

This paper proposes that axion Bose-Einstein condensates could cool photons after BBN, resolving lithium abundance discrepancies and predicting a high effective neutrino number observable in the CMB.

Contribution

It introduces a novel mechanism where axion BECs facilitate photon cooling, linking dark matter properties to cosmological parameter measurements.

Findings

Photon cooling can reconcile lithium abundance with predictions.

Axion BECs may lead to a higher effective number of neutrinos.

Predicted effects are testable via cosmic microwave background observations.

Abstract

We observe that photon cooling after big bang nucleosynthesis (BBN) but before recombination can remove the conflict between the observed and theoretically predicted value of the primordial abundance of $^7$Li. Such cooling is ordinarily difficult to achieve. However, the recent realization that dark matter axions form a Bose-Einstein condensate (BEC) provides a possible mechanism, because the much colder axions may reach thermal contact with the photons. This proposal predicts a high effective number of neutrinos as measured by the cosmic microwave anisotropy spectrum.