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Comment by empath75 | Hacker News Reader

> Furthermore, since the lifetime is finite and the Universe infinite, there is likely a bubble of true vacuum already out there, far away. It is sobering to envision this bubble, with its wall of negative energy, barreling towards us at the speed of light. It seems the long-term future of our Universe is not going to be slow freezing due to cosmic acceleration but an abrupt collision with one of these bubble walls.

It seems like the more interesting calculation is how far away one of these bubbles is likely to be.

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I think the most solid argument in favor of the quantum multiverse is along the lines of "The universe is virtually certain to fall into a lower-energy quantum vacuum state. That we have not observed it to be the case means either that we have been astonishingly lucky, or that there is a selection process going on. There is such a selection process; we can only witness futures in which the entire past lightcone of the universe has not collapsed; despite the near-certainty that such has occurred there are still far more possible universes than that." In short, the quantum multiverse argument can be run backwards in time, too; a given quantum system will observe a superposition of all possible past states of the universe as it evolves forward in time, and we can only see the ones in which we still survive.

Now, I still don't believe in the quantum multiverse. We have not proved to my satisfaction that lower-energy quantum vacuum states are inevitable, on the grounds that we don't know enough yet to know why it hasn't happened. (i.e., there may be other effects keeping it from happening, or perhaps there are reasons why it actually can't happen such as the lower state being non-viable for some other reason, etc.) But in my opinion, this is still the most solid argument in its favor; if it can be established that our current universe with near-certainty should evolve into something that we can't live in or observe with a bubble expanding at the speed of light, it's going to take something equally potent to counteract that.

Or, in other words, under this theory, for every split second of your entire life, vastly, vastly more of your worldlines are getting lightspeed-annihilated than not. %99.999... "keep writing 9s as long as you like" of them. But the destruction is so total and instantaneous that you can never observe this fact, so you go on happily living in the 0.00...01% of the remainder, happy as a clam. Again, this isn't my own personal belief, but it's a theory.

> The universe is virtually certain to fall into a lower-energy quantum vacuum state. That we have not observed it to be the case means either that we have been astonishingly lucky, or that there is a selection process going on.

If that is the most solid argument in favor of the quantum multiverse, this paper is a disaster. See Eq. 6.28: the probability that we should have seen a bubble by now is < 10^-516.

When I quoted that argument, I was not saying it was 100% accurate; after all, I don't believe it myself. If further work comes along to prove that the collapse into the lower state is not highly probable, then the argument collapses. Again, not accepting the multiverse anyhow, that the best argument in its favor collapses would not particularly bother me.

Of course, it would not constitute evidence against, merely a lack of evidence for. Now, some people with a insufficiently deep understanding of probabilities and such will say those two things are completely different, but they aren't; lack of evidence for is indeed a form of evidence against, especially if one is actively searching for the evidence. It just isn't very strong, and it certainly isn't a proof.

'infinity' is more of a logical construction than a physical one.

Going with the paper's result: with 95% confidence, more than 10^58 light years. :)

I think that was about the entire Universe, not our local part of it.

The result reported by this paper is decay rate per unit volume (see Eq. 6.26). That tells you how long it takes for a bubble to nucleate. After that, it expands at the speed of light. How long does it take for a bubble expanding at the speed of light to destroy an infinite universe?

So it would be multiple bubbles spread throughout that would wipe out an infinite universe.

... which is expanding. So now you have a percolation problem: what grows faster, the bubbles or the universe? The latter is also thought to be expanding at an accelerating rate, which leads to funny scenarios like this: https://arxiv.org/abs/1503.08130

So "get out before the bubble pops" applies all the way up to cosmic scales?

It’s worth pointing out that we don’t know if we live in an infinite or finite universe, so that’s an assumption made by the authors.

I think your question actually raises a key point ... "one of these bubbles". How accurate is their prediction that these bubbles aren't common? And is there somewhere nearby that might be hiding one?

On their time-scale, Musk's goal of becoming an interplanetary species seems like a good near-term hedge. Being able to traverse our own galaxy would be a good second step but exponentially harder. People have discussed the almost impossible task of moving between galaxies (without something like a worm-hole).

So, assuming the multi-verse theory is correct, I guess we humans need to look into how to get to another universe before this big cataclysm. (It seems pretty easy on "Stranger Things")

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