# Joshua Eby (Kavli IPMU)

3:30pm Wednesday March 17, 2021

Webinar

**Axion Stars: Towards the Planck Scale**

*Axion stars are gravitationally-bound, ground-state configurations composed of very large numbers of axion particles. Their macroscopic properties are determined, in large part, by two fundamental energy scales: m << eV, the mass of the axion; and f >> GeV, the decay constant or symmetry-breaking scale. In this talk, I will review the well-known properties of axion stars, including the mass-radius relation, decay processes, and gravitational collapse instability, and importantly, I will show that these results are only valid when f << M_P ~ 10^{19} GeV, the Planck scale. In particular, I will show that as f approaches M_P, the usual separation of axion star configurations onto a stable dilute branch and an unstable dense branch breaks down, as the boundary between them becomes less pronounced; and furthermore, that states on the dilute branch which are ordinarily stable can in fact decay with very short lifetime when f > 10^{17} GeV. Finally, I will close with a discussion of possible detection of axions with very large decay constants, which is made difficult (but not impossible!) by the suppression of Standard Model couplings by 1/f.*

*Categories*