Black Holes in the Cosmos (16th Madan Lal Mehta Lecture)

Supermassive black holes lurk in the very centers of galaxies. The Milky Way has a central black hole of four million solar masses. Today it is  quiescent. But millions of years ago it was active. Traces of exploded debris are seen around our galactic centre that arose in a violent explosion some tens of millions of years ago. Most galaxies have
massive central black holes, in some cases weighing billions of solar masses. These were the sites of the most energetic phenomena in the Universe, that astronomers recognise as quasars. Such immensely luminous objects in the nuclei of galaxies were active when the universe was young. Even dwarf galaxies have central massive black holes. How did supermassive black holes form? The ultimate window on building massive black holes is gravity waves, and I will describe experiments that are being planned to search for traces of the formation of such black holes.