Dr. Hamsa Padmanabhan

Mapping the baryonic content of the Universe — the component directly observable to us — promises the richest available cosmological dataset in the coming years, facilitating the most precise constraints on fundamental physics. After summarising the main features of an innovative approach that allows us to fully utilize our current knowledge of astrophysics in order to develop cosmological forecasts from baryonic data, I will describe how this can be used to address the feedback and evolution scenarios of supermassive black holes (SMBHs) in the early Universe. Galaxy mergers at high redshifts trigger the activity of their central SMBHs, eventually leading to their coalescence – and a potential source of low-frequency gravitational waves (GWs). We can constrain several properties of the host dark matter haloes of dual and binary SMBHs (such as their occupation fractions at high redshift) with the forthcoming LISA observatory and the SKA Pulsar Timing Array (PTA). Also, the recent statistics of dual active galactic nuclei (AGN) in the James Webb Space Telescope (JWST) data provide evidence for the members of a binary BH ‘shining’ at the same time, rather than independently - which is consistent with gas-rich mergers triggering concurrent AGN activity. This conclusion is supported by the recent NANOGrav PTA measurements, whose upper limits on the GW strain lie below those expected from extrapolating the dual AGN fraction. I will discuss future prospects.