Enrico Nardi, "Can the QCD axion feed a dark energy component?"

Wednesday 13 Nov 2024, 16:00 → 17:00 Europe/Rome

0M04

A pseudo Nambu-Goldstone boson (PNGB) coupled to a confining gauge group via an anomalous term is characterised, during the confining phase transition, by a temperature dependent mass $m^2(T) \propto T^{−n}$. For $n>2$, a non-relativistic population of such particles dominating the cosmological energy density would act as dark energy (DE), accelerating the expansion. We study the possibility that a PNGB $\varphi_b$  coupled to a hidden gauge group that is presently undergoing confinement could realise this scenario. To obtain the observed amount of DE, the number density of $\varphi_b$ must be boosted by some mechanism. Assuming that the QCD axion $\varphi_a$  constitutes the dark matter (DM), a non-adiabatic level crossing between $\varphi_a$  and $\varphi_b$  shortly before matter-DE equality can convert a small fraction of DM into DE, providing such a mechanism  and explaining the coincidence puzzle.