What, no planet kablooey?

tlb wrote:Possibility 1: the expansion reverses due to gravity and the Universe collapses back to a point; possibly to undergo another Big Bang (this is not expected based on current understanding).

Possibility 2: the expansion accelerates as increasing distance weakens gravity ties, and each star system is left alone in the dark. With star formation no longer possible; the Universe dies as each remaining star burns out. A form of entropy death.

PS: I may have overstated the second possibility; because galaxies might still remain, but each galaxy will be alone. Same eventual result, but on a bigger scale.

tlb wrote:The only possibilities for Universe destruction (that I know about) lie in its expansion:

Possibility 1: the expansion reverses due to gravity and the Universe collapses back to a point; possibly to undergo another Big Bang (this is not expected based on current understanding).

Possibility 2: the expansion accelerates as increasing distance weakens gravity ties, and each star system is left alone in the dark. With star formation no longer possible; the Universe dies as each remaining star burns out. A form of entropy death.

PS: I may have overstated the second possibility; because galaxies might still remain, but each galaxy will be alone. Same eventual result, but on a bigger scale.

Loren Pechtel wrote:We can't be certain that the spacetime we see is truly the lowest energy state. If it's not...

tlb wrote:If it's not, then why did it wait until now to occur. If the odds in favor are 1 in 4 billion years (whatever that works out to as an actual probability), then there is a good chance that it is not energetically favored.

In any case, if it occurs, it is not the result of any human agency.

tlb wrote:If it's not, then why did it wait until now to occur. If the odds in favor are 1 in 4 billion years (whatever that works out to as an actual probability), then there is a good chance that it is not energetically favored.

In any case, if it occurs, it is not the result of any human agency.

ThinksMarkedly wrote:Our current best models say that it isn't the lowest possible energy overall, only the lowest energy locally. That is, all other energy levels starting from vacuum energy today is higher, but if you go high enough you may find a path to a lower value than our current vacuum. We don't know that those do exist, but our models can't rule them out either.

And yes, it could happen by human agency, if we create that lower energy level, then regular vacuum will promptly decay to it and propagate at the speed of light.

That is the vacuum decay I mentioned above.

tlb wrote:I realize that you both were talking about the same thing. Considering all the things that have happened in the Universe, from supernova to blackholes, I find it difficult to imagine that humans could do something so unprecedented. I prefer to think that if it could happen, then it already would have happened.

ThinksMarkedly wrote:Human agency has created things that nature can't. Take the super-heavy atoms, above Z=100. Nature creates atoms (much) heavier than iron via neutron bombardment during neutron star collisions or supernovae, but the weight of such nucleus is limited by the number of neutrons being captured by the nucleus versus its own spontaneous decay. At very high masses, such nuclei decay to lighter elements too quickly, more quickly than they can capture more neutrons. This is the explanation I've heard why no atoms in the purported "island of stability" around Z=126 have been found in the universe, if it exists.

And we can also create anti-hydrogen, but the universe is conspicuously lacking it. This leads to headlines in non-scientific publications like "scientists conclude universe shouldn't exist."

The Higgs potential determines whether the Universe is in one of two states: a true vacuum, or a false vacuum. A true vacuum is the stable, lowest-energy state, like sitting still on a valley floor. A false vacuum is like being nestled in a divot in the valley wall – a little push could easily send you tumbling. A universe in a false vacuum state is called “metastable”, because it’s not actively decaying (rolling), but it’s not exactly stable either.

There are two problems with living in a metastable universe. One is that if you create a high enough energy event, you can, in theory, push a tiny region of the universe from the false vacuum into the true vacuum, creating a bubble of true vacuum that will then expand in all directions at the speed of light. Such a bubble would be lethal.

The other problem is that quantum mechanics says that a particle can ‘tunnel’ through a barrier between one region and another, and this also applies to the vacuum state. So a universe that is sitting quite happily in the false vacuum could, via random quantum fluctuations, suddenly find part of itself in the true vacuum, causing disaster.

The possibility of vacuum decay has come up a lot lately because measurements of the mass of the Higgs boson seem to indicate the vacuum is metastable. But there are good reasons to think some new physics will intervene and save the day.

One reason is that the hypothesised inflationary epoch in the early Universe, when the Universe expanded rapidly in the first tiny fraction of a second, probably produced energies high enough to push the vacuum over the edge into the true vacuum. The fact that we’re still here indicates one of three things. Inflation occurred at energies too low to tip us over the edge, inflation did not take place at all, or the Universe is more stable than the calculations suggest.

If the Universe is indeed metastable, then, technically, the transition could occur through quantum processes at any time. But it probably won’t – the lifetime of a metastable universe is predicted to be much longer than the current age of the Universe.

So we don’t need to worry. But what would happen if the vacuum did decay?

The walls of the true vacuum bubble would expand in all directions at the speed of light. You wouldn’t see it coming. The walls can contain a huge amount of energy, so you might be incinerated as the bubble wall ploughed through you. Different vacuum states have different constants of nature, so the basic structure of matter might also be disastrously altered. But it could be even worse: in 1980, theoretical physicists Sidney Coleman and Frank De Luccia calculated for the first time that any bubble of true vacuum would immediately suffer total gravitational collapse.

They say: “This is disheartening. The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in a new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it.

tlb wrote:Are you really saying that nature CANNOT create these things or are you saying that they are so short-lived in nature, that there is no evidence of them?

The possibility of vacuum decay has come up a lot lately because measurements of the mass of the Higgs boson seem to indicate the vacuum is metastable. But there are good reasons to think some new physics will intervene and save the day.
...
If the Universe is indeed metastable, then, technically, the transition could occur through quantum processes at any time. But it probably won’t – the lifetime of a metastable universe is predicted to be much longer than the current age of the Universe.
...

I find that a good reason to expect that our understanding is incorrect.

ThinksMarkedly wrote:That's not supported by evidence, because we don't have such a model. You and I and all the scientists may agree that the theory is probably incorrect because it predicts this metastable universe (and we do know that QFT, QCD and others must be incomplete because they don't include gravity, despite being the most successful theory ever in the history of predictions), but we can't scientifically say it is incorrect.

ThinksMarkedly wrote:Human agency has created things that nature can't. Take the super-heavy atoms, above Z=100. Nature creates atoms (much) heavier than iron via neutron bombardment during neutron star collisions or supernovae, but the weight of such nucleus is limited by the number of neutrons being captured by the nucleus versus its own spontaneous decay. At very high masses, such nuclei decay to lighter elements too quickly, more quickly than they can capture more neutrons. This is the explanation I've heard why no atoms in the purported "island of stability" around Z=126 have been found in the universe, if it exists.

And we can also create anti-hydrogen, but the universe is conspicuously lacking it. This leads to headlines in non-scientific publications like "scientists conclude universe shouldn't exist."