This project continues our work to characterise the behaviour of light fields in strong gravity environments, in particular focussing on the potential for gravitational wave signatures from dark matter, exotic compact objects and the possible formation of primordial black holes.

A key highlight this year was an investigation of superradiant clouds around black holes with spatially varying masses. Superradiance is a process by which light fields can extract energy from spinning black holes, leading to the build up of a “cloud” if the particle has a Compton wavelength comparable to the black hole’s Schwarzschild radius. One interesting possibility is that superradiance may occur for photons in a diffuse plasma, where they gain a small effective mass. We found either a constant asymptotic mass or a shell-like plasma structure is required for superradiant growth to occur. We studied thin disks and found a leakage of the superradiant cloud that suppresses its growth, concluding that thick disks are more likely to support superradiance.

Time evolution of the field’s energy density. We show snapshots at three different time steps: (1) when the field is in the transient phase (i.e. nonsuperradiant modes in the initial data are decaying into the BH or radiating away); (2) when the superradiant mode is just becoming dominant; and (3) when the cloud has been growing in the superradiant phase for some time. The values within the BH horizon are set to zero to mask the excision region. (See for more details.)

Categories: 2021 Highlights