Black holes are objects so dense that not even light can escape their gravity and since nothing can travel faster than light, nothing can escape from inside a black hole. The expression black hole was coined by John Wheeler.
Theoretically, black holes can have different size and mass, from microscopic to giant black holes in the center of a galaxy called supermassive black holes. This black holes containing millions to billions of solar masses.
A common black hole formed when a massive star dies, through a process called gravitational collapse.
The existence of black holes in the universe is well supported by astronomical observation, particularly from studying supernova and X-ray emission from active galactic nuclei.
The "surface" of a black hole is the so-called event horizon an imaginary surface surrounding the mass of the black hole. At that point is the escape velocity equal to the speed of light.
Objects in a gravitational field experience a slowing down of time, called time dilation. This phenomenon taken into account in the GPS system. Near the event horizon, the time dilation increases rapidly.
At the center of the black hole, well inside the event horizon, general relativity predicts a singularity, a place where the curvature of spacetime becomes infinite and gravitational forces become infinitely strong.
To avoid offenses against the Second Law of Thermodynamics some scientist thought that black holes must emit radiation, but not classically emit radiation. However, in 1974, Hawking applied quantum field theory to the curved spacetime around the event horizon and discovered that black holes can emit thermal radiation, known as Hawking radiation. This means that black holes are not completely black, the effect implies that the mass of a black hole slowly evaporates with time.
Several alternate models, which behave like a black hole but avoid the singularity, are considered. The most prominent theory is the Gravastar.
July 2004 - Stephen Hawking proposed that quantum perturbations of the event horizon could allow information to escape from a black hole, where it can influence subsequent Hawking radiation. But this theory has not yet been confirm.
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