### particle horizon

**(cosmological horizon, comoving horizon, cosmic light horizon)**
(furthest point from which light can reach us)

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.
As such, it constitutes a similar limit on any particle
whose motion is bounded by the speed of light.
The **observable universe** is all the universe within
this boundary.
The distance through which such light would have traveled
is called the **horizon distance**.

(*cosmology,relativity*)
http://en.wikipedia.org/wiki/Particle_horizon

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