[neutralino]

Ok, so now for a more thorough post on the topic that I promised yesterday.

As I said above, there are various event horizons that arise in cosmology. The most "famous" event horizon (that is, the one that is most likely to be called an event horizon) is the one associated with a black hole. A black hole event horizon can be very loosely described as the "boundary" of a black hole-- anything (light, or any other particle) at a radius smaller than the event horizon will not be able to escape into the rest of the universe and all possible worldlines of the particle will end up at the singularity. For the most simple black hole (a spherically symmetric, non-rotating black hole) the event horizon coincides with the Schwarzschild radius [1], but for others it does not.

Another interesting horizon in cosmology is the particle horizon. This is defined as the maximum distance from us that light can have possible travelled since the beginning of the universe. Sometimes this is called the "observable universe."

A final horizon that is often discussed is normally just called the "event horizon." This is defined as the current boundary, beyond which objects' light will never reach us in the future.