system
Given the following post, respond as if you're commenting on Reddit:
human
Will the Universe last long enough for light to travel all the way across it?

The Universe began ~14 billion years ago, and in that time it has expanded. It's constantly getting bigger (as far as I know, this will stop eventually, right?). Currently, the observable portion of the Universe has a diameter of 93 billion light years. So the Universe itself must have a much larger diameter. My question is this: Could a light source shine at one end of the Universe, and have that light get to the other side, before the Universe snuffs itself out?
gpt
We have no way of knowing how big the Universe "actually" is (defined in some suitable way) or if it's even finite in size at all. The expansion of the Universe is separate from its size. The expansion just means that stationary points are getting further apart from each other (more specifically, the *way* we measure distances is changing over time), as if you took a rubber sheet and stretched it out. The stretching of the sheet is independent of how big the sheet is - it can just as well work whether the sheet is a 4' x 4' square or infinite in size.

The answer to the more general question of "can light go from point A to point B before the end of the Universe" (i.e., in finite time) depends intricately on the nature of the expansion. For most (although not all) conceivable types of expansion, it comes down to whether the expansion is accelerating or decelerating. Light can travel between any two points, eventually, in a decelerating universe, while in an accelerating universe there will always be places that light cannot reach. What's the answer in real life? Our Universe is believed to have gone through at least three different types of expansion: one dominated by the energy from electromagnetic radiation, one dominated by the gravity from dark matter, and at least one dominated by a cosmological constant or "dark energy." The first two (radiation- and matter-dominated) are decelerating universes, while dark energy causes the Universe to accelerate. So it seems that our future, which as far as we know holds little other than further acceleration, will be of the type in which light *can't* be sent between any two points. Light can go infinitely far, of course - it's always travelling at the speed of light - but there will always be places which are, in a sense, expanding away "too quickly" for the light to reach it.