## Cosmological Horizons

See Cosmological Horizons - further reading for a list of books related to this subject |

A recent paper covering this area in detail is Expanding Confusion: common misconceptions of cosmological horizons |

### Two non-horizons

The second thing to note is that light has a finite speed, and so when we observe distant parts of the universe, we are observing them when they were considerably younger than we are now. We are not seeing them 'now' (indeed 'now' isn't really defined ). This may seem to be labouring the obvious, but you have to be careful not to deduce that if we can't see a galaxy when it of the same age as us, then it must be behind some sort of horizon.

### Superluminality

See also Stretchy Space? for more discussion of how to interpret superluminal velocities |

What is going on here can best be seen by considering the Milne universe, which is a description of a universe without gravity but using the framework of General relativity. Here one can look at things in terms of GR or in the familiar terms of Special Relativity. However, the coordinate systems are different for the two cases. In the SR coordinates everything is travelling slower than light as we know it must. But anything with a redshift greater than a certain value will be travelling faster than light in the GR coordinates.

### Particle Horizons

*particle horizon*. This tells you how much of the universe you can have received information from at a given time. Note that graphs showing particle horizons are generally confusing, as they often show a background of the other parts of the universe evolving with time, implying that at a certain stage of their evolution they cross the particle horizon. This is not the case - whatever enters our past light cone has just experienced the big bang.

The other thing to note is that for particle horizons to occur the rate of expansion of the universe (the derivative of the scale factor with time) must be infinite at time zero. This is the case in all models of the actual universe, according to GR (Although you might not think so, looking at graphs of the expansion of the universe against time). However, in the Milne universe, where the rate is constant, there are no particle horizons.

### Goodbye Cruel World

*cosmological event horizon*. All we would see is an increasingly redshifted and time-dilated version of events in the galaxy, never quite reaching the time of crossing the horizon.

In special relativity there is a similar effect, which says that if you have a head start and can accelerate constantly then you can stay ahead of a light beam, despite the fact that you will never actually reach the speed of light. This means that you would never receive any information from Earth after a certain time. Unlike the cosmological case the situation is not symmetric, so Earth could still receive signals from you.

### On a clear day you can see forever...

What about black hole event horizons? You may be interested in Black Holes - do they exist? |