Efficient self-resonance instability from axions

Abstract

It was recently shown that a coherent oscillation of an axion can cause an efficient parametric resonance, leading to a prominent emission of the gravitational waves (GWs). In this paper, conducting the Floquet analysis, we investigate the parametric resonance instability, which potentially triggers the emission of the GWs from axions. Such a resonance instability takes place, when the time evolution of the background field significantly deviates from the harmonic oscillation. Therefore, the resonance instability cannot be described by the Mathieu equation, whose stability/instability chart is well known. In this paper, introducing an explicitly calculable parameterq, which can be used to classify different types of the parametric resonance described by the general Hill's equation, we investigate the stability/instability chart for the general Hill's equation. This can also apply to the case where the background oscillation is anharmonic. We show that the flapping resonance instability, which takes place forq = O(1), typically leads to the most significant growth of the inhomogeneous modes among the self-resonance instability. We also investigate whether the flapping resonance takes place for the cosine potential or not.

Oscillons and dark matter

“…The reason is that the homogeneous configuration suffers from parametric resonant instability once the scalar starts to (homogeneously) oscillate: field fluctuations of certain wavelengths can grow exponentially and thus lead to the fragmentation of the scalar field. This mechanism in turn provides the right ground on which oscillons can form (as confirmed by numerical simulations, see [41,61]). We have studied the condition under which parametric resonance is efficient and we find that the localized overdensities are indeed generated for mildly large initial values of the scalar field, not far from F .…”

The First Three Seconds: a Review of Possible Expansion Histories of the Early Universe

“…Even in this simple case, the post-inflation epoch can host interesting nonlinear physics. In particular, for M M P , as the field explores the flatter-than-quadratic region of the potential (φ M with α < 2), broad self-resonance drives copious production of oscillons [117,118,119,120,121,122,123]. Oscillons are highly overdense, spatially localized, exceptionally long-lived field configurations [124,125,126,127,128,129,130] where the field amplitude in their cores does not redshift as the universe expands.…”

Analytic study of dark photon and gravitational wave production from axion

“…Oscillation of the axion results in an energy transfer to fields that are coupled to it via the mechanism of parametric resonance very similar to the mechanism of preheating after inflation [31,32]. The amplification of the fields coupled to the oscillating axion-like fields leads to some interesting features which were the subject of many recent investigations [33][34][35][36][37][38][39][40][41]. In this work, we consider a U(1) gauge boson in the dark sector coupled to the axion field.…”