Probing AGN physics and the Hubble constant with intensity interferometry

Pioneered in the 1950s by Hanbury Brown and Twiss, intensity interferometry refers to the correlation of light intensities incident on two telescopes. As its name suggests, it relies only on photon counting, allowing for interferometry with arbitrarily long baselines in optical wavelengths. Its chief drawback is the need for very bright sources and is thus restricted to date to the study of nearby stellar morphologies. I will show how recent advances in photodetector technology and spectroscopy will allow us to target the fainter morphologies of bright Active Galactic Nuclei (AGNs) and resolve them from the ground, utilizing cheap arrays of large mirrors. This will enable the study of accretion disk profiles and provide enough significance to distinguish even the innermost parts of the disk. Furthermore, I will explain how combining intensity interferometry on the Broad Line Region of AGNs with the long established technique of Reverberation Mapping can establish a novel geometric method to measure the Hubble constant to a few percent.