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Comment by pmoriarty | Hacker News Reader

It would happen if there's a big crunch.

Also, a lot of astronomy goes on the assumption that the laws of physics are the same far away as they are near by, and that they don't change over time. It's possible that if either of these two assumptions are incorrect that anything goes.

I'm also reminded of Arthur C. Clarke's famous dictum that, "when a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."

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But the expansion of the universe is accelerating which tells us we are not in a big crunch scenario. And we have probed the laws of physics over very long distances and far into the past by looking at very distant objects and have to date not found any strong evidence for varying laws of physics over space or time. There are for example some claims of variations in the fine structure constant but as far as I can tell those claimed variations may still be explained by experimental errors and are therefore not widely accepted.

If space is curved rather than flat we could still have a big crunch on the far side.

I think consensus is that it is flat and we keep finding more and more evidence suggesting so.

I heard that "if you go straight long enough, you end up where you were", as analogous to walking on the surface of a globe. Are you saying that evidence against that has surfaced?

The current assumption of the cosmological standard model is spatial flatness, which is compatible with the observations. In principle, space could still curve back on itself on a large enough scale, but assuming there's no big crunch coming, you'd have to go straight for a longer-than-infinite duration to come back to the place you started from.

Longer than infinite?

What does a big crunch have to do with it? Maybe I don't know what a big crunch is... I currently think the big crunch is the idea that gravity will eventually coalesce all matter into a single point.

Longer than infinite?

In an expanding universe, objects can be separated by a cosmic horizon. Nevertheless, from the comoving perspective, they may very well still move towards each other - but without ever meeting up. However, if you extended conformal time beyond infinity, they would. That's of course not physically possible, so that particular comment was tongue in cheek.

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