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Vacuum decay, or vacuum metastability event is the possibility in simple terms that the universe itself is not in in its ground state. If that's true, it might spontaneously change to its real ground state. Doing so will change fundamental things like the strength of electromagnetism, the weight of particles and so on. It would literally destroy everything in the universe, and we couldn't exist in what's coming after.
Good news, we're confident, that's probably not going to happen.
Would that be an instantaneous thing?
As far as we're concerned, yes. It literally would travel at the speed of light. But since the light from the momentarily-ago-normal universe would be traveling just ahead of it... Everything would look normal until it collapsed
The German Duck People made a video about it:
"German Duck People" is hilarious. Either the name of a cult or a band.
Or maybe Kurtzgesagt.
That narrator is so calming when he talks about ways we can all spontaneously die off.