The origins of massive black holes
Résumé
The discoveries of quasars at cosmic distances and of giant dark massive objects in today’s galaxies provide evidence of the ubiquity of massive black holes (MBHs).Understanding the origins of MBHs goes hand in hand with understanding the origins of the structures inside the cosmic web. MBHs are not born ‘massive’ but must have grown by several orders of magnitude from ‘seed black holes’. Gas accretion and black hole mergers are the drivers of their growth inside galaxies, but there are several bottlenecks in this journey.The origins of MBHs may be from exotic mechanisms or may well lie in known physics — particle, plasma and condensed matter physics, gravity and dynamics — extrapolated to untested regimes.Studying the origins of MBHs is a multi-scale problem: from the Schwarzschild radius to cosmological scales, from subsecond events to the age of the Universe.Paths to seed formation and growth are not mutually exclusive. Constraints will therefore come from a combination of observables: masses, spins, distances, spectra and light curves of populations of black holes. These indirect constraints can confirm that a given path exists but cannot rule out the existence of other paths. A combination of electromagnetic and gravitational-wave observations is the foreseen best strategy to constrain the origins of MBHs.The discoveries of quasars at cosmic distances and of giant dark massive objects in today’s galaxies provide evidence of the ubiquity of massive black holes (MBHs).Understanding the origins of MBHs goes hand in hand with understanding the origins of the structures inside the cosmic web. MBHs are not born ‘massive’ but must have grown by several orders of magnitude from ‘seed black holes’. Gas accretion and black hole mergers are the drivers of their growth inside galaxies, but there are several bottlenecks in this journey.The origins of MBHs may be from exotic mechanisms or may well lie in known physics — particle, plasma and condensed matter physics, gravity and dynamics — extrapolated to untested regimes.Studying the origins of MBHs is a multi-scale problem: from the Schwarzschild radius to cosmological scales, from subsecond events to the age of the Universe.Paths to seed formation and growth are not mutually exclusive. Constraints will therefore come from a combination of observables: masses, spins, distances, spectra and light curves of populations of black holes. These indirect constraints can confirm that a given path exists but cannot rule out the existence of other paths. A combination of electromagnetic and gravitational-wave observations is the foreseen best strategy to constrain the origins of MBHs.