| Astrophysics (Index) | About |
The particle horizon (aka the cosmological horizon, comoving horizon, or cosmic light horizon) is the boundary in the universe defined as the furthest distance from which light could travel through the age of the universe to you, taking into account the speed of light and the expansion of the universe. Particles reaching us (whose motion is bounded by the speed of light) must have come from a portion of the universe within this boundary, which is termed the observable universe. The distance through which such light would have traveled is called the horizon distance.