Comment by btilly | Hacker News Reader

Comment by btilly | Hacker News Reader

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According to current cosmological thought, the laws of physics at that shell are very close to if not identical to the laws of physics here. And yet, that shell is retreating from us faster than the speed of light. The shell is ever expanding, but we shall never see what that current shell will look like when it is 10 billion years.

How can that be, you ask? It is quite simple. The speed of light is a limit on how fast light can travel through the universe. But the universe is expanding. Light trying to get to us is like a bug crawling over an expanding balloon. If the balloon is expanding faster than the bug is crawling, it can crawl forever but never reach the point it is trying to crawl to.

So there is a fundamental epistemological limit - we can't actually know that our current models are correct. But within existing theory, the location of "the farthest we can see" is not particularly meaningful physically.

> within existing theory, the location of "the farthest we can see" is not particularly meaningful physically

Except that you can always put a finite upper bound on that distance. That is what matters with regards to the topic under discussion.

Which means that it is exactly as meaningful as the event horizon of a black hole. The last point where the external observer (ie us) can see what happens. But in a different coordinate system, not particularly special at all.

Yes, that's right. Different observers have different light cones. Yours is a little different from mine. But that is missing the point, which is that every observer has a light cone beyond which they cannot see -- not even by collaborating with other observers with different light cones.

It's also worth noting that, in a sense, the shell is actually shrinking. Every moment, more of our observable universe disappears "beyond the veil", so-to-speak, as the universe on our side of the edge transitions from near-light-speed to beyond light-speed outside the shell. (I know that's a sloppy way to use the speed of light, but it works for me.)

Small nitpick, that shell---the Hubble sphere---is shrinking due the accelerating expansion of space. Constant expansion cooresponds to a constant Hubble radius, and decelerating expansion with an expanding radius.

Good point, thank you.

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