A black hole is a region of spacetime where gravity is so extreme that nothing - including light and other electromagnetic radiation - can escape once it crosses the boundary known as the event horizon. Black holes form when mass is compressed beyond a critical density, causing spacetime to curve so severely that escape velocity exceeds the speed of light. They are predicted by general relativity and are now understood to be common astrophysical objects rather than theoretical curiosities. Black holes vary enormously in mass, from stellar-mass objects a few times the mass of the Sun to supermassive black holes billions of times more massive, found at the centers of most large galaxies.

A black hole is characterized by three externally measurable quantities: mass, electric charge, and angular momentum (spin). This is summarized in the “no-hair theorem,” which holds that all information about the matter that formed a black hole is encoded in these three parameters. The event horizon is the point of no return - not a physical surface but a boundary in spacetime beyond which the escape velocity exceeds the speed of light. Interior to the event horizon, general relativity predicts a singularity, a point (or ring, in rotating black holes) where density becomes infinite and the known laws of physics break down. The region just outside a rotating black hole where spacetime itself is dragged in the direction of rotation is called the ergosphere.

Black holes are categorized primarily by mass:

• Stellar-mass black holes (roughly 3 to ~100 solar masses) form from the gravitational collapse of massive stars at the end of their lives, typically in a supernova event.
• Intermediate-mass black holes (roughly 100 to 100,000 solar masses) are less well-established observationally and their formation pathways are debated.
• Supermassive black holes (millions to billions of solar masses) reside at the centers of most large galaxies, including the Milky Way, whose central black hole - Sagittarius A* - has a mass of approximately 4 million solar masses.
• Primordial black holes are hypothetical objects proposed to have formed in the early universe from density fluctuations, before any stars existed.

In 1974, Stephen Hawking proposed that black holes emit thermal radiation - now called Hawking radiation - due to quantum effects near the event horizon, rather than being perfectly non-radiating. This radiation causes black holes to slowly lose mass and, in principle, evaporate over extremely long timescales. Hawking radiation has not been directly observed, but it is widely accepted in theoretical physics.

Direct and indirect evidence for black holes has accumulated substantially since the late twentieth century. Stellar orbits around Sagittarius A* have been tracked for decades, providing strong evidence for a compact, massive, dark object at the Milky Way's center. X-ray binaries - systems where a black hole accretes matter from a companion star - have been identified throughout the galaxy. Gravitational wave observatories (LIGO and Virgo) detected the first confirmed merger of two stellar-mass black holes in 2015, opening a new observational channel. The Event Horizon Telescope collaboration released the first direct image of a black hole's shadow in 2019 (M87*) and followed with an image of Sagittarius A* in 2022.

Several fundamental questions about black holes remain unresolved. The information paradox - whether information about matter falling into a black hole is permanently destroyed, in apparent conflict with quantum mechanics - has been debated since the 1970s and is not settled. The nature of the singularity is widely regarded as a sign that general relativity is incomplete at extreme scales, requiring a theory of quantum gravity to resolve. The formation mechanisms of supermassive black holes, particularly those observed at high redshift in the early universe, are not fully understood. The existence and properties of intermediate-mass black holes remain an active area of investigation.

There is broad consensus across astrophysics and physics on the existence of black holes as described by general relativity, on the reality of stellar-mass and supermassive black holes as distinct observed populations, and on the detection of gravitational waves from black hole mergers. See Black Holes - Astrophysics Consensus and Black Holes - Physics Consensus. Questions involving quantum gravity, the information paradox, and the nature of singularities are not settled, and viewpoints on these are described below.

• Information is destroyed - Some physicists have held that black hole evaporation genuinely destroys information, accepting a break with unitarity in quantum mechanics. See Black Holes - Information Loss Viewpoint.
• Information is preserved - The dominant position in recent decades holds that information must be conserved, with various proposed mechanisms (holography, black hole complementarity, the Page curve) explaining how. See Black Holes - Information Preservation Viewpoint.
• Singularities are unphysical - Some researchers argue that true singularities do not occur in nature and that a complete quantum theory of gravity will replace them with a finite description. See Black Holes - No Singularity Viewpoint.
• Primordial black holes as dark matter - A minority viewpoint holds that primordial black holes could account for some or all of the universe's dark matter. See Black Holes - Primordial Black Hole Dark Matter Viewpoint.
• Firewall paradox - A proposal by Almheiri, Marolf, Polchinski, and Sully (2012 preprint, published 2013) argued that preserving unitarity requires a “firewall” at the event horizon that would destroy infalling observers, contradicting general relativity's equivalence principle. This remains contested. See Black Holes - Firewall Viewpoint.

• Black Holes - History
• Black Holes - Debate
• Black Holes - Astrophysics Consensus
• Black Holes - Physics Consensus
• General Relativity
• Quantum Gravity
• Hawking Radiation
• Dark Matter
• Gravitational Waves
• Information Paradox

1. Kerr, R. P. (1963). “Gravitational field of a spinning mass as an example of algebraically special metrics.” Physical Review Letters 11 (5): 237-238.
2. Hawking, S. W. (1974). “Black hole explosions?” Nature 248 (5443): 30-31.
3. Abbott, B. P., et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). “Observation of gravitational waves from a binary black hole merger.” Physical Review Letters 116 (6): 061102.
4. Event Horizon Telescope Collaboration (2019). “First M87 Event Horizon Telescope results. I. The shadow of the supermassive black hole.” The Astrophysical Journal Letters 875 (1): L1.
5. Event Horizon Telescope Collaboration (2022). “First Sagittarius A* Event Horizon Telescope results. I. The shadow of the supermassive black hole in the center of the Milky Way.” The Astrophysical Journal Letters 930 (2): L12.
6. Almheiri, A., Marolf, D., Polchinski, J., and Sully, J. (2013). “Black holes: complementarity or firewalls?” Journal of High Energy Physics 2013 (2): 62.
7. Penrose, R. (1965). “Gravitational collapse and space-time singularities.” Physical Review Letters 14 (3): 57-59.
8. Ghez, A. M., et al. (2008). “Measuring the mass of Sgr A* from stellar orbits.” The Astrophysical Journal 689 (2): 1044-1062.