# How does cosmic horizon work?

This is about the horizon which divides us from stuff that is too far to see because it's moving away from us faster than the speed of light.

If point $A$ and $B$ are so far away that $B$ is a bit outside $A$'s horizon, what if I look at the point $C$ that's midway between them? It will get light from $B$, because there's nothing from stopping that happening. However, any light at $C$ must be moving with speed $c$ with respect to $A$ while $C$ itself is moving slower. Therefore, any light that is at $C$ should also reach $A$ if the direction is right (and it is, if it came from $B$).

Where is the flaw?

## 1 Answer

Why should light that can reach C also have time to reach A?

It does not, because B is so far away that light from there did not have time to reach us.
No problem that light from C had time to reach B and A - but that is unrelated.

You placed C near enough that it's light can reach us - we see C how it looked very long ago, We will see B if we wait another long time while light that already reached C travels towards us between C and A.

(The actual flaw un your argument is probably related to not taking the timing into account in some way.)