event horizon An event horizon is a property of space-time, normally described by Albert Einstein's theory of general relativity. It separates the regions of space-time that we will be able to see (i.e. receive radiation from) from those that we never will. Event horizons arise in two main contexts, black holes and accelerating cosmological models. The black hole event horizon marks the edge of the region which, if entered, can never be left again. Its existence first became apparent in the static black hole space-times discovered by Karl Schwarzschild in 1916, but it exists also in more complicated black holes with charge or rotation.
In fact, it took around fifty years for the Schwarzschild space-time to be fully understood, through use of coordinate systems that simplified the description of the space-time. In cosmology, event horizons may or may not exist depending on the details of the model. None of the simple Friedmann cosmologies without a cosmological constant has an event horizon; even in the recollapsing case the entire Universe has become visible by the time of the big crunch. However, if the Universe is accelerating, objects far enough away from us will be receding so swiftly that their light can never reach us, even if the Universe becomes infinitely old.
This is sometimes known as the de Sitter horizon, as its simplest example is de Sitter space. A distinction between the black hole and cosmological settings is that for a black hole all observers outside the black hole infer the event horizon to be in the same place, whereas in cosmology each observer has their own distinct event horizon centred on their location. While in the classical theory of general relativity the event horizons are absolute, Stephen Hawking showed in 1975 that once quantum effects are included black holes are able to radiate energy and shrink.
The precise physical picture of what goes on is, as usual with quantum mechanics, rather unclear, but should perhaps be thought of as negative energy particles (in the sense of their negative gravitational potential energy exceeding their positive mass–energy) entering the event horizon, rather than anything leaving it. Related to this is the black hole information paradox, whereby information entering a black hole appears to be lost even though quantum mechanics insists that information is preserved. Recently, superstring theory has led to some progress in understanding this issue.
The quantum creation of inflationary perturbations can be regarded as analogous to black hole evaporation, with the cosmological event horizon taking the role of the black hole event horizon. The existence of cosmological event horizons is sometimes said to cause problems for superstring theory, which presently is only formulated properly if interacting strings can end up infinitely separated. Whether this is a genuine problem or a shortcoming of present theoretical understanding remains to be seen.
The event horizon is sometimes confused with a related concept, the particle horizon. The particle horizon is the limit to the region of the Universe that we can see at the present time, whereas the event horizon limits the regions we will be able to see at any point in our future, even if that future is infinitely long.
How to cite this entry: "event horizon" The Oxford Companion to Cosmology. Andrew Liddle and Jon Loveday. Oxford University Press 2008. Oxford Reference Online. Oxford University Press. 3 January 2012
