Stephen Hawking is most famous for his theory about black holes. The theory hypothesizes that black holes release matter in the form of radiation. He is also renowned for his book "A Brief History of Time."
Hawking became fascinated with black holes early in his career. At the core of black holes were thought to be event horizons from which no form of matter could escape once it was within their gravitational pull. Hawking posited, however, that matter could escape from black holes in the form of radiation. This type of radiation became known as Hawking Radiation.
Hawking once again shook the scientific world when he attempted to redefine black holes by putting forth the theory that event horizons do not exist. Instead, Hawking believes there is an apparent horizon that can grow or shrink in size. He posits that if it shrinks to a certain size that allows the gravitational pull to realign, it will disappear, releasing everything that has been trapped inside. The released objects, however, would be unrecognizable as the pull of the black hole would have severely warped them. Hawking's newest theory regarding black holes is a direct response to the problem of black hole firewalls, which are thought to be a point on the event horizon at which everything that passes through is incinerated.