On the viability of the QCD axion - dark energy transition

TL;DR

This paper explores a scenario where a pseudo-Nambu Goldstone boson (pNGB) coupled with the QCD axion could explain the transition from dark matter to dark energy in the universe, involving energy transfer mechanisms.

Contribution

It proposes a novel mechanism where coupling a pNGB to the QCD axion enables energy transfer, potentially accounting for both dark matter and dark energy.

Findings

pNGB alone cannot produce observed dark energy levels

Coupling with QCD axion allows energy transfer to match observations

Scenario provides a unified explanation for dark matter and dark energy

Abstract

This conference thesis summarizes my presentation at Tartu Tuorla Cosmology meeting based on a collaborative work with E.Nardi and C.Smarra \cite{Muursepp:2024mbb}. It is well known that a pseudo-Nambu Goldstone boson (pNGB) coupled to a confining gauge group obtains a non-zero contribution to its mass through instanton effects. At non-zero temperature, this manifests in temperature-dependent mass $m^{2}(T) \propto T^{-n}$. If these particles come to dominate the energy density of the Universe in the non-relativistic regime they would accelerate the expansion of the Universe for $n>2$, thus providing a dark energy (DE) component. In this work, we outline a scenario in which a pNGB $\phi_{b}$ presently undergoing confinement could realize such a scenario. Using energetic considerations we find that $\phi_{b}$ alone is not enough to produce the experimentally observed amount of DE. However, coupling $\phi_{b}$ to the QCD axion $\phi_a$ allows the transfer of energy from the QCD axion to the pNGB via non-adiabatic level-crossing thus reproducing the observed amount of dark matter (DM) and DE.