1

The observable universe is limited by a cosmic horizon. Galaxies beyond the horizon move away from us faster than light, so we cannot see them. If we could see a planet close to our horizon, we would see time there strongly dilated and moving slower than ours conceptually coming to a halt at the horizon, as we see it.

Imagine a planet beyond our horizon moving faster than light away from us. We cannot see this planet, as it is shielded from us by the horizon. However, can we theoretically describe how exactly time moves there? If from our viewpoint time stops at the horizon, then what happens to time beyond the horizon?

None of the obvious logical possibilities feel right: time is frozen, starts moving, moves in reverse, becomes a spacelike coordinate. What is the proper answer?

safesphere
  • 13,017

1 Answers1

1

The answer is surprisingly boring as in the FLRW metric all comoving observers share the same time coordinate. That is, there is a universal time (comoving time) that records the time since the Big Bang, and all comoving observers share this time coordinate. So time is not dilated for distant observers and it doesn't do anything odd as we consider observers the far side of the cosmic horizon.

When you talk ask what happens if we see a planet close to our horizon you need to be clear what you are asking. If you're asking about the light falling on our CCDs and forming an image there then the simple answer is that we would see nothing as the light hasn't had a chance to reach us yet. If you are using the verb to see in the sense of to observe, i.e. the assignment of spacetime events, then my first paragraph applies and observers close to or beyond the horizon share the same time coordinate as us.

John Rennie
  • 367,598