(Re)constructing Accurate Axion Oscillations

Abstract: The cosmological evolution of ultralight axions typically involves rapid oscillations on the timescale of the inverse mass, making it challenging to resolve the dynamics at both the background and perturbation levels. In this study, we numerically implement a novel approach to this problem based on an effective field theory (EFT) -- that is, for the first time, capable of reconstructing the relativistic oscillations intrinsic to the axion field. Compared to other available techniques, the reconstruction of the rapid oscillations is unique to the EFT approach, which describes the axion effective field through a wavefunction representation with relativistic corrections, rather than through effective fluid variables. From a computational standpoint, our approach archives a high level of accuracy -- up to a subpercent agreement with the exact solution -- while remaining relatively fast compared to alternative methods. As such, it opens up new possibilities for high-precision predictions for axion searches with future cosmological experiments.