Black holes are possibly the most intriguing objects in the Universe. They deform the space-time around them and prevent everything, including light, from escaping. They come in different sizes, or better, masses and they can be quite elusive. But after decades of hard work astronomers have recently succeeded at detecting gravitational waves from colliding black holes, opening a new window on the cosmos and a new era for research.
Observational evidence has been gathered for stellar mass black holes, with masses of about 10 solar masses, which are produced by the core collapse of massive stars and appear as X-ray sources when they accrete from their companion in binary systems, as well as for supermassive black holes, with masses of millions to billions of solar masses, which are found at the centres of galaxies and power the tremendous luminosities of active galactic nuclei. Both classes of black holes are ubiquitous in galaxies, including the Milky Way.
Black holes of intermediate mass have been theoretically predicted, but so far only tentative observational evidence exists, in the form of either ultra-luminous X-ray sources or a cusp in the density profiles of dense star clusters. Particularly intriguing is the possibility that black holes form binaries which shrink in separation until the two components collide and merge into a single black hole, giving off energy in the form of gravitational waves. These can travel unaffected through the cosmos, mere ripples in the fabric of space time.
In September 2015, the Advanced LIGO interferometer made the first breakthrough detection of gravitational waves from the merger of two black holes. This detection validated the predictions from Einstein’s theory of general relativity, and opened new perspectives for the study of compact object binaries, providing direct observational evidence for black holes, black hole binaries and their mergers.
Stellar origin black holes
The life of a massive star terminates with a spectacular explosion, a supernova, and the formation of a compact-object, either a neutron star or a black hole. The Advanced LIGO and VIRGO observatories have detected the gravitational waves generated by binary systems composed of two coalescing compact-objects.
The first gravitational wave detections provided direct evidence for the existence of binary black hole systems that inspiral and merge within the age of the Universe. In the coming years, as the current catalog of gravitational wave sources will continue to grow, our knowledge and understanding of how these binaries form and evolve will dramatically improve, shedding new light on stellar evolution and dynamical processes around us.
Supermassive black holes
Supermassive black holes are ubiquitously found at the centre of galaxies and are expected to form binaries when their host galaxies collide. These binaries shrink due to encounters with stars and eventually coalesce in a powerful emission of low frequency gravitational waves. Two different observatories will shape the future of GW detections in the near future.
The Pulsar Timing Array (EPTA, PPTA, IPTA) aims to use accurate timing data from millisecond pulsars to detect gravitational waves from the most massive black hole binaries. The space interferometer LISA instead will use the same detection principle as LIGO to detect GWs in space from low mass supermassive black hole binaries. These will provide crucial information on the formation and evolution of galaxies and black holes in a cosmological context.