[tashmahalic]

Gravity travels at the speed of light (the speed of causality). If the sun suddenly disappeared, the earth would continue orbiting around where it used to be, for about 8 minutes.

Nothing can travel faster than light through space, in a vacuum. However, if you pick two points in space that are far enough apart (e.g. at opposite sides of the observable universe), these points will be moving apart faster than light, because space itself is expanding.

The expansion of space isn’t coming from a single point outward, like an explosion. It’s expanding by the same amount at every point in the universe. People analogize this in lower dimensions to stretching fabric or blowing up a balloon.

[masked_titan]

How do we know that the space is expanding if there is nothing to compare it to?

[tashmahalic]

The farther away a galaxy is, the more red-shifted the light from that galaxy is, when it reaches us.

Also, really far-away galaxies are receding from us faster than light.

And, the existence of the cosmological event horizon seems to support this too, I think. Things beyond that horizon will emit light in our direction, but that light will never reach us. In fact, without this horizon, I believe that the night sky would be much brighter.

I find it kind of sad that, as time passes, more of the universe becomes unobservable. Eventually the only stars visible from our POV will be those in our own galaxy (and galaxies that ours has merged with, in the interim).

More info at https://en.wikipedia.org/wiki/Expansion_of_the_universe