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The origins of massive black holes


Massive black holes (MBHs) inhabit galactic centres, and power luminous quasars and active galactic nuclei, shaping their cosmic environment with the energy they produce. The origins of MBHs remain a mystery, and the recent detection by LIGO/Virgo of a black hole of almost 150 solar masses has revitalized the questions of whether there is a continuum between ‘stellar’ and ‘massive’ black holes, and what the seeds of MBHs are. Seeds could have formed in the first galaxies or could be related to the collapse of horizon-sized regions in the early Universe. Understanding the origins of MBHs straddles fundamental physics, cosmology and astrophysics, and bridges the fields of gravitational-wave physics and traditional astronomy. With several existing and upcoming facilities in the next 10–15 years, we foresee the possibility of discovering the avenues of formation of MBHs. This Review links three main topics: the channels of black hole seed formation, the journey from seeds to MBHs, and the diagnostics on the origins of MBHs. We highlight and critically discuss current unsolved problems, touching on recent developments.

Key points

  • 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.

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Fig. 1: Growth of massive black holes (MBH) in galaxies.


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The authors thank F. Antonini, R. Brandenberger, G. Bertone, J. Gair, J. Greene, K. Inayoshi, M. Mapelli, P. Natarajana and P. Pani for comments on the manuscript, and T. Hartwig for estimating the number of Population III relics for this Review.

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Solar mass, a unit of mass that corresponds to 1.98847 × 1033 g.

High-contrast density perturbations

The contrast of a density perturbation corresponds to the ratio of the difference between said density and the mean background density, called overdensity, to the mean background density. A patch of the Universe that has a high density contrast has a chance of collapsing under its own gravity, and in the most extreme cases the collapse can lead to a black hole.


Metallicity is the sum of the mass fraction of all the elements present in the system heavier than hydrogen and helium. For metal-enriched systems, the Sun is often used as a unit of measure for metallicity, with Z = 0.012.


Comoving megaparsec (1 parsec, denoted pc, corresponds to 3.0857×1018 cm). Comoving distances — for which we prefactor a letter ‘c’ — are independent of cosmic expansion, whereas proper distances account for that, so that proper distances decrease at earlier cosmic times.


Short for ‘cosmological redshift’ in this Review, and used as indicator for distance and cosmic time. Given a cosmological model, there is a unique relation between the redshift of a source and its distance from us, as well as the age of the Universe at that redshift.


Parsec, a unit of length used in this Review that corresponds to 3.0857 × 1018 cm.

Dynamical encounter

Here we refer to the close interaction of a single object (either a star or a black hole) with a binary (either a star and a black hole, or a double black hole binary). In a close fly-by, the incoming object extracts gravitational energy from the binary, reducing its semi-major axis. In an exchange, the lightest member of the binary is kicked off by the incoming heavier object, and a new heavier binary forms.

Quantum-chromodynamic phase transition

As the temperature of the Universe decreases, free quarks become confined in hadrons (baryons and mesons, containing an odd and even number of quarks respectively). Examples of baryons are protons and neutrons; examples of mesons are pions and kaons.

Compact objects

These are relics of stars and comprise white dwarfs, neutron stars and stellar black holes.

Eddington luminosity

Maximal luminosity above which radiation pressure on electrons overcomes gravity on the infalling matter, under the assumption of spherical symmetry.

Active galactic nuclei (AGN)

AGN and quasars are sources powered by an accreting massive black hole. Quasars are the most luminous among AGN.


Physical processes in which the energy/momentum output of a system (or a fraction of the output) returns to or impacts the system’s input.

Radiative efficiency

ε is the efficiency at which gravitational energy is converted into radiation. It establishes the link between the accretion luminosity L and mass accretion rate \(\dot{M}\): \(L=\varepsilon \dot{M}{c}^{2}.\) In geometrically thin, optically thick accretion disks around black holes, ε ~ 0.06−0.32, depending on the spin, with ε ~ 0.1 used as reference value. ε can be lower depending on the geometry of the flow. \((1-\varepsilon )\dot{M}\) gives the mass growth rate of an MBH.

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Volonteri, M., Habouzit, M. & Colpi, M. The origins of massive black holes. Nat Rev Phys 3, 732–743 (2021).

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