Cosmological Dynamics of Multi-Axion Quintessence

One fundamental physics interpretation of Dark Energy Spectroscopic Instrument (DESI) results is that the observed accelerated expansion of the Universe is driven not by the cosmological constant, but by a slowly rolling scalar field, a natural model for which is an axion with a decay constant close to the Planck scale. In the "string axiverse’’ one expects not one, but many, axions and this may allow the ability to engineer large effective decay constants. We thus investigate dark energy dynamics in a toy model with two axions, and compute the prior probability for the dark energy equation of state parameters \((w_0,w_a)\) implied by priors on the fundamental parameters of the model conditioned weakly on the resulting cosmology. We find various interesting behaviours, for example where both fields can be in an oscillating regime while maintaining \(w_0<-1/3\), and models with opposite sign for \(w_a\) than in a single-field model. The prior probability in \((w_0,w_a)\) gains support away from the "thawing quintessence’’ behaviour preferred by DESI, and when the axions have cross-interactions this effect becomes stronger. When the axions interact, one in general still requires large decay constants, but if the axions are non-interacting the largest decay constant can be reduced. The novel equations of state we illustrate may also be of relevance to models of early dark energy or inflation/reheating with mutliple axions. Further exploration of dynamics in models with \(N\gg 1\) axions is warranted.