Volume 474 -
2nd Training School and General Meeting of the COST Action COSMIC WISPers (CA21106) (COSMICWISPers2024) -
Wg2

Can the QCD axion feed a dark energy component?

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Abstract

This conference thesis summarizes my presentation at the second general meeting of the COST action COSMIC WISPers meeting based on collaborative work with E.Nardi and C.Smarra. Coupling a pseudo-Nambu Goldstone boson (pNGB) to the gauge bosons of a non-abelian gauge group may result in a non-zero contribution to the pNGB mass through instanton effects. Before confinement, this mass is temperature-dependent $m^{2}(T) \propto T^{-n}$. Thus if the pNGB particles come to dominate the energy density of the Universe in the non-relativistic regime they would accelerate the expansion of the Universe for $n>2$, 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 to convert a small fraction of the QCD axion number density into the number density of $\phi_b$ end thus enabling to explain both the observed amount of dark matter and dark energy.

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