Backreaction of axion coherent oscillations

TL;DR

This paper studies how coherent oscillations of ultra-light axions influence their wave function evolution and gravitational potential, revealing effects that could impact cosmological structure formation.

Contribution

It demonstrates that oscillations induce higher harmonics and modify effective self-interactions, affecting the dynamics of axion condensates in cosmological contexts.

Findings

Oscillations produce higher harmonics in the wave function.

Oscillations in gravitational potential generate an attractive force.

Effects are significant at scales smaller than the axion's Compton wavelength.

Abstract

We investigate how coherent oscillations backreact on the evolution of the condensate wave function of ultra-light axions in the non-relativistic regime appropriate to cosmic structure formation. The coherent oscillations induce higher harmonics beyond the fundamental mode considered so far when a self-interaction is present, and imprint oscillations in the gravitational potential. We emphasize that the effective self-interaction felt by the slowly-varying envelop of the wave function always differs from the bare Lagrangian interaction potential. We also point out that, in the hydrodynamical formulation of the Gross-Pitaevskii equation, oscillations in the gravitational potential result in an attractive force that counteracts the effect of the quantum pressure arising from the strong delocalization of the particles. Since these effects become significant on physical scales less than the (large) Compton wavelength of the particle, they are presumably not very relevant on the mildly nonlinear scales traced by intergalactic neutral hydrogen for axion masses consistent with the bounds from the Lyman-$\alpha$ forest. However, they might affect the formation of virialized cosmological structures and their stability.