Novel standard candle: Collapsing axion stars

Abstract

The Hubble constant, $H_0$, is a crucial parameter in cosmology. However, various cosmic observations have produced differing posterior values for $H_0$, resulting in what is referred to as the $H_0$ tension. To resolve this discrepancy, utilizing other cosmological probes to constrain $H_0$ is advantageous. In the quest to identify dark matter candidates, the QCD axion and axionlike particles, collectively referred to as axions, have become leading contenders. These elusive particles can coalesce into dense structures known as axion stars via Bose-Einstein condensation. When these axion stars exceed a critical mass, typically through accretion or merging, they experience a self-induced collapse. This process results in short radio bursts, assuming a decay constant $f_a\lesssim10^{13}{\rm{GeV}}$, with the frequency depending on the axion mass and the luminosity determined by both the axion mass and decay constant. Therefore, we propose that collapsing axion stars could serve as a novel standard candle to constrain $H_0$. Even more interesting is that the radio bursts emitted by collapsing axion stars with specific parameters match the characteristics of observed non-repeating fast radio bursts (FRBs). Thus, FRBs generated by collapsing axion stars have the potential to be used as standard candles to constrain $H_0$.