Cosmological Dynamics of Multi-Axion Quintessence

Abstract

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. In the ``string axiverse'' one expects not one, but many, axions. We thus investigate dark energy dynamics in a models with two, three, and four 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 multiple 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. Similarly, adding more axion fields and interactions among them leads to the prior support in the baryon acoustic oscillation distance measures moving further from the DESI data. Thus, interacting multifield models will not generically be preferred over simpler models unless very exotic dark energy evolution happens to be observed in future. The novel equations of state we illustrate may, however, be of relevance to models of early dark energy or inflation/reheating with multiple axions, and further study of multifield dynamics is warranted.