supermassive-black-holes

Active galactic nuclei are galaxy cores lit up by supermassive black holes pulling in gas. Quasars, Seyfert galaxies, radio galaxies, and blazars are largely one machine seen from different angles and at different power.

Black holes form through several distinct pathways: massive stars collapsing after fusion ends, neutron star mergers that exceed the mass limit, direct collapse of gas clouds in the early universe, and possibly primordial black holes from density fluctuations just after the Big Bang. Supermassive black holes remain partly mysterious in origin, but likely grew through a mix of heavy seeds, mergers, and accretion.

A network of millisecond pulsars scattered across the Milky Way works as a galaxy-sized gravitational wave detector. In 2023, several independent collaborations reported the first evidence of a nanohertz gravitational wave background. Here is how pulsar timing arrays work and what they are finding.