I've been reading and thinking about this subject for some time, but I can't seem to find a clear answer.

After reading Q14:

Aspects of the Higgs boson suggest that our Universe is only “marginally stable,” or is perhaps in a transition phase to a more stable state that would result in a universe with fundamentally different properties.

And this article:

With the discovery of the Higgs field, one question we can ask is whether the Universe as a quantum system it is in a stable, low-energy state, or in a metastable higher energy state?

According to the standard model of particle physics, the answer depends upon the masses of the top quark and the Higgs boson. The more massive the Higgs boson, the more likely the Universe is to be stable. The more massive the top quark, the more likely the Universe is to be metastable, or even unstable. From our current measurements of the top and Higgs masses, it seems that our Universe is metastable.

Maybe an answer already been found, but I just didn't find it, or am I not on the right level.

Is our universe stable or metastable?

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    $\begingroup$ Possibly for Physics SE. $\endgroup$ – Alchimista Jan 19 '19 at 9:15
  • $\begingroup$ I think you have answered the question yourself. With the data physicists currently have, it is unknown, but possibly metastable. I'd say so far, nearly 14 billion years, it's done okay $\endgroup$ – Rory Alsop Jan 19 '19 at 10:28
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    $\begingroup$ Physics Stack has basically the same question: physics.stackexchange.com/questions/31291/… The question is framed basically the same way you framed it. The answer, at least to me, is somewhat harder to follow. I don't think physicists have settled on an answer to this question yet. $\endgroup$ – userLTK Jan 19 '19 at 12:24
  • $\begingroup$ Can you share the source of your information ? $\endgroup$ – Layla Jan 20 '19 at 7:38

Yes ... and no.

Experimentally, there's no indication that the laws of physics are changing (c.f. conservation of energy: if the laws of physics change with time, energy would not be conserved via Noether's theorem). So if you're an empiricist, you'd conclude that the universe is stable (or at least its mean lifetime must be more than ~14 billion years, the age of the universe).

However, if we believe string theory (which has no compelling experimental evidence), then we expect roughly $10^{500}$ "vacuum states", which correspond to $10^{500}$ different possible universes, each with their own laws of physics. It's possible one of these other universes have a lower "total energy" than us (this word's meaning is vacuous since if one varies the laws of physics, the word 'energy' itself might lose meaning). This means it's in principle possible that our universe will quantum tunnel to a universe at lower energy, in which case our universe is metastable.

Of course, this leads to other problems. How would one prove the multiverse exists? If a generally accepted way exists, I've not heard of it. But if you can't prove the multiverse exists, how would you prove our universe is metastable? For that matter, how do we know the electron is stable? Why can't it have a mean lifetime of, say, $10^{100}$ years (so long that we don't observe it decaying)?

At this point, we're purely in the realm of speculation. We're talking about things so far above the experimental threshold that some people will just dismiss it as pointless. Suffice to say that if the universe isn't stable and quantum tunnels to a lower energy, we won't be able to see the tunneling coming, and we're all instantly dead anyway. Whether the question is worth thinking about further depends on the individual.

tl; dr:

  • Experimentally, universe is stable.
  • Theoretically, maybe it isn't, depending on what your theory is because we don't have a widely-accepted, experimentally-backed theory.
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    $\begingroup$ Why can't it have a lifetime of, say, 20 billion years Because unstable particles don't have a simple fixed lifetime, they have a mean lifetime. See en.wikipedia.org/wiki/Exponential_decay#Mean_lifetime and the section on half-life. Experiments trying to see if protons are unstable have constrained the proton half-life to be greater than $1.67\times 10^{34}$ years. $\endgroup$ – PM 2Ring Apr 22 '19 at 12:22
  • $\begingroup$ I think you should use the word "hypothesis" instead of "theory" since string hypothesis and multiverse hypothesis do not have any experimental basis, as you've said. It gives shame to actual theories of science to call string hypothesis a theory. $\endgroup$ – Daddy Kropotkin May 25 at 15:29
  • $\begingroup$ @DaddyKropotkin The accepted term for String Theory is just that, not String Hypothesis. $\endgroup$ – nick012000 May 26 at 3:59
  • $\begingroup$ There was never a conversation about it. That's what physicists who worked on string hypothesis called it. It is a hypothesis, not a scientific theory. We don't call MOND a theory...... but it actually has SOME experimental motivation en.wikipedia.org/wiki/Modified_Newtonian_dynamics These accepted naming conventions are not standardized. That's why I suggested it here for the sake of clarity regarding known laws of physics. $\endgroup$ – Daddy Kropotkin May 26 at 13:25

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