Global View of Axion Stars with (Nearly) Planck-Scale Decay Constants

TL;DR

This paper investigates the stability of axion stars with near-Planck-scale decay constants, revealing their rapid decay and highlighting the importance of gravity and relativity effects at high decay constants.

Contribution

It demonstrates that axion stars with nearly Planck-scale decay constants are unstable and cannot be simply extrapolated from smaller decay constant models, emphasizing the role of gravity and relativity.

Findings

Axion stars with large decay constants are unstable to decay.

Gravity and special relativity effects are significant at high decay constants.

Fast decay rates occur even for dilute axion star states at large $f$.

Abstract

We show that axion stars formed from axions with nearly Planck-scale decay constants $f$ are unstable to decay, and are unlikely to have phenomenological consequences. More generally, we show how results at smaller $f$ cannot be naively extrapolated to $f=\mathcal{O}(M_P)$ as, contrary to conventional wisdom, gravity and special relativity can both become relevant in the same regime. We clarify the rate of decay by reviewing and extending previous work on oscillons and axion stars, which imply a fast decay rate even for so-called dilute states at large $f$.