blackhole

A black hole is a region of spacetime from which nothing can escape, not even light, due to an extremely strong gravitational field. The boundary surrounding this region is called the event horizon. Within the horizon, general relativity predicts a region where spacetime curvature becomes extreme and a singularity may form, where density is thought to diverge. Black holes can be characterized by mass, electric charge, and angular momentum, but the external spacetime is largely described by only mass and spin (the no-hair theorem). Rotating black holes (Kerr black holes) possess an ergosphere, where frame-dragging effects allow extraction of energy.

Black holes form by gravitational collapse of massive stars or through accretion of mass onto a pre-existing compact object. They are also believed to exist in several classes: stellar-mass black holes (a few solar masses), supermassive black holes (millions to billions of solar masses) at galaxy centers, and possibly intermediate-mass black holes (hundreds to thousands of solar masses) and primordial black holes formed in the early universe.

Observational evidence includes X-ray binaries, where a star orbits an unseen compact companion; the motion of stars near galactic centers (notably Sgr A* in the Milky Way); variability and spectra from accretion disks; relativistic jets; indirect gravitational lensing effects; and gravitational waves from black hole mergers detected by LIGO and Virgo. In 2019, the Event Horizon Telescope produced the first direct image of a black hole's shadow in M87*.

In theoretical physics, Hawking radiation predicts black holes can emit particles due to quantum effects, causing slow evaporation for very small black holes. For astrophysical ones the effect is negligible. The information loss paradox remains a topic of ongoing research.