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Comment by DougWebb | Hacker News Reader

Off the cuff thought: the overall universe is infinite and not expanding, and it's only the visible universe that's expanding into that infinite space.

Now try to wrap your mind around this: someone that's one light year to the left is going to see a slightly different visible universe, also expanding, into the same infinite space. But if we look in their direction, we see the edge of our visible universe expanding into the void, but from their point of view looking in the same direction our edge is one light year short of their edge. So what's our edge expanding into?

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I've always wondered; is there a "last" galaxy in any direction, such that for an observer in that galaxy, no further light or radiation can be detected from that direction? (outside that galaxy)

That, must be a terrifying place to live in......

The typical way to picture the universe is like the surface of an expanding balloon, a 2-manifold which happens to be embedded in 3-space. If you picture galaxies as spots on the balloon, there's no "last" galaxy, they're all roughly equidistant from their neighbors. Analogously, our Euclidean universe is thought of (in terms of noncompact spatial dimensions) as an expanding 3-manifold. There's no last galaxy there either. However, if you include time, then at some point in the distant future there will be a last galaxy because old stars will all burn out or be sucked into black holes. And further in the future protons will decay, so there will be no baryonic matter left, plus black holes will eventually evaporate. Much sooner than that though, the continued expansion of spacetime means that galaxies will in time disappear over the expanding cosmological horizon, and future lifeforms will know nothing of the universe outside their own aging galaxies. See: https://en.wikipedia.org/wiki/Future_of_an_expanding_univers...

> If you picture galaxies as spots on the balloon, there's no "last" galaxy

What about looking "up" and "down?" i.e. into the inside of balloon or away from its surface?

I have a hard time wrapping my head around the balloon surface analogy, because galaxies seem to be in all directions of each other..

The analogy fails there because our 3manifold space isn't necessarily "embedded" into a higher dimensional construct the way the surface of a sphere is embedded into our 3 dimensional universe. So there isn't necessarily a hyperdimensional up or down to consider.

Or, alternatively, if you think of time as a dimension, then we can call "up" the future and "down" the past. In that case, if you look down inside the past, you'll see that in that analogy the center of the universe corresponds with the big bang! And all the galaxies are equidistant from that point/moment in spacetime.

The surface of the ballon represents a 2d space, where you cannot look "up-down" in the 3rd dimension, just like in our 3d space you cannot look "up-down" in the 4th dimension.

I wonder if there would be a Restaurant at the End of the Universe

The last thing we can see with light in any direction is the background radiation. That's when light and matter separated.

That's why gravitational waves are such a big deal: they allow us to look further. (Not further than the limit imposed by the speed of light, though.)

I think they would have to be at the edge of their galaxy for this effect to take place ..

What about look up or down? If it's a flatter galaxy like our Milky Way spiral.

https://news.ycombinator.com/item?id=11594728

Obviously I have no evidence, but my suspicions are that its like the pac-man world, it just continuously loops on itself. Its just really big...

Too late to edit, but I must add that I'm not necessarily asking if there's an "edge" of the Universe.

I don't see how our situation is any less terrifying haha

It's certainly less terrifying than having to look at an infinite abyss.

I really enjoy your belief that you're somehow not always looking at an infinite abyss :P

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