system
Given the following post, respond as if you're commenting on Reddit:
human
This is really hard for me to word right. We only know the "observable universe" because light passed the 13.8 billion LY mark hasn't reached us yet. Is there new light showing up all the time?

I hope that makes sense. If I were to look into a live image of the farthest known universe, would I see new light popping up?
gpt
The Universe is either infinite or so damn big that it might as well be infinite as far as we're concerned. So over time we keep receiving light from new parts of the Universe, places which are so far away that their earliest light is just barely reaching us.

By the way, this will not go on ad infinitum. Because of the fact that the expansion of the Universe is *accelerating*, there are certain parts of the Universe so distant that their light will *never* reach us. This is true of anything farther away from us today than about 63.4 billion light years. As time goes on and on, we'll see the earliest light from objects which are closer and closer to that distance, called the *horizon*. But we can never see beyond it.

Why is this? It has to do with the fact that the expansion started off decelerating and is now accelerating. Roughly speaking, we'll see a galaxy (or some point in space) if it is receding from us at a speed below the speed of light. (This is a rough heuristic, so it's not exact, but it's close.) In a decelerating phase, a galaxy's velocity away from us slows down over time. That's what it means for the expansion of the Universe to be decelerating - things like galaxies are expanding away from us, but they do so at a smaller and smaller rate. So over time, galaxies above that speed-of-light threshold came below it, and as a result they came within our view - we could start to see their history.

But a few billion years ago, the expansion stopped decelerating and started to accelerate. This means that galaxies now move faster and faster away from us over time. So galaxies which we can see, which are receding from us more slowly than light, will eventually be moving away from us faster than light, and after that time (roughly) we won't be able to see them, because they'll be moving away from us more quickly than the light they emit can reach us. This means that past a certain distance, the horizon, we will never be able to see anything.