# Does the edge of the Universe travel faster than the speed of light?

The Universe is expanding faster than the speed of light then does that mean matter at the edge of the universe is also traveling at the speed of light or are ether opposite sides of the universe are expanding over half the speed of light equaling more than the speed of light ? Looking from outside of the universe to the nearest edge would light proceed slower or faster combined with the speed of matter traveling on the edge of the universe?

• In standard Big Bang theory the universe has various horizons, but it has no edges or outside. Jul 17 '18 at 22:55

## 2 Answers

The universe has no edge, as far as we can tell.

Now, the observable universe does have an "edge" - it's the most distant places we can see. But that's just us and our limited instruments.

Due to the expansion of the universe, it is true that two objects, far enough from each other, could actually move faster than light relative to each other. That means light sent from one of them would never reach the other.

However, the speed of light, no matter how you measure it, is always the same. It does not matter how fast or slow its source was moving - when you measure the speed of that light, it's always, ALWAYS the same, for you, locally.

So how can that be?

Well, it's because the universe is expanding because space itself is growing. There's more space all the time in between distant objects. And the further apart two objects are, the faster the space between them is growing.

It's like a canvas with galaxies painted on it, but the canvas itself is growing. However, when light is traveling across the canvas, its speed is always the same relative to whatever galaxies it may find along the way.

• Actually, it is possible for two observers to recede from each other faster than c, and still have a light ray travel from one to another. All galaxies farther away than the Hubble sphere (at d ~ 14.4 Glyr) recede faster than c, but all galaxies inside the Event Horizon (at d ~ 17 Glyr) may emit light today and we may receive it one day. In the future, this "shell" of galaxies for which it's possible, but in the past it was bigger. For instance, the galaxy GN-z11 receded at ~4c when it emitted the light we see today.
– pela
Jul 20 '18 at 7:13

Nothing travels through space "faster than the speed of light". Distant galaxies can appear to be receding "faster than the speed of light" because the space between they and us is expanding.

There is no physics that prevents a distant object appearing to travel faster than the speed of light. The speed of light limit and constancy applies to local measurements. The speed of light is not a constant when it is measured "elsewhere" (by this, I mean somewhere different to where you are).

You cannot "look from outside the universe".

• The speed of light is not a constant when it is measured "elsewhere". I feel like this warrants further explanation. In what scenario is the speed of light actually not constant (discounting optical illusions, etc.)? Jul 18 '18 at 12:45
• @zephyr The speed of light near a black hole (for example), as estimated by a distant observer, is obtained from the gradient of a lightlike geodesic. It is $c (1 -r_s/r)$ and light apparently takes an infinite time to reach the event horizon. physics.stackexchange.com/questions/77227/… Jul 18 '18 at 13:01
• Okay, I see what you mean. I was under the impression you meant, if you went elsewhere in the universe and measured the speed of light locally there, you'd find a different value, not that your local measurement of light from elsewhere may not be $c$. Jul 18 '18 at 13:04