Dynamics and Phenomenology of Ultralight Bosons Around Compact Objects

by Yifan Chen (Niels Bohr Institute)

Wednesday 12 Nov 2025, 14:00 → 15:00 Europe/Berlin

SR2

Ultralight bosons around compact objects can form gravitationally bound states, some of which grow exponentially by extracting rotational energy from black holes or through relaxation from ambient waves. These bound states can reach field amplitudes approaching the Planck scale, leading to phenomena analogous to early-universe cosmology and producing signals far stronger than those expected from local dark-matter detection. For instance, ultralight bosons coupled to photons can induce oscillations of the fine-structure constant, observable through spectroscopic measurements of S-stars orbiting the Galactic-center supermassive black hole. Around comparable-mass black-hole binaries, bosons can form molecular-like configurations that co-rotate with the binary, resulting in characteristic circularization effects on binary evolution. Finally, axions with sufficiently strong self-interactions can accumulate into spherical configurations with field amplitudes near the decay constant, exhibiting either Bosenova collapse or saturation depending on the growth rate and axion mass. The resulting emission flux displays discrete spectral features determined by the axion potential, making terrestrial detection of axion flux a possible probe of ultraviolet axion models.