Speaker
Description
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: $m \ll$ eV, the mass of the axion; and $f \gg$ GeV, the decay constant or symmetry-breaking scale. In this talk, I will show how the properties of axion stars derived in previous literature, including the mass-radius relation and stability conditions, cannot be extrapolated to large values of $f \sim M_P \sim 10^{19}$ 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, and that contrary to previous assumptions, states on the dilute branch can decay with very short lifetime when $f > 10^{17}$ GeV. Finally, I will discuss possible detection of axions with large decay constants, which is made difficult (but not impossible!) by the suppression of Standard Model couplings by $1/f$.