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So I was watching a documentary on television about this and a number of theories that are proposed as to what the fate of the Universe will be:

  1. "Big Crunch"
  2. "Big Freeze"
  3. "Big Rip"

I understand these, but there was also another scenario which I don't understand:

  1. A bubble forms in the Universe and expands at the speed of light. It annihilates everything in the Universe that it comes into contact with, until eventually at a very, very distant time the Universe ceases to exist. Does anyone have any clue as to what they were talking about (because I don't)?
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I think I know what they mean. They're talking about a false vacuum scenario.

A vacuum state is a state of lowest energy. It's thought that the vacuum of our universe is in a lowest-energy state and is stable, and so nothing special will happen to spacetime. However, if our universe is actually a false vacuum, then it could merely be metastable, and some tiny perturbation could cause it to fall into a lower energy state - a true vacuum, or else a false vacuum of lower energy. The "bubble" that would appear would be a region of this lower-energy vacuum that would expand across the universe without stopping.

Scientists aren't sure if our universe is a false vacuum. Here's a chart of the masses of the Higgs boson and the top quark: Higgs boson vs. top quark

The latest measurements suggest that their masses lie in a metastable region of the graph (which could be bad), although it's towards the stable end of the region (which is good). This vacuum catastrophe could still happen, but the odds aren't in its favor. I also suggest reading some of the excellent papers in the references section of the Wikipedia article. They're quite comprehensive.

Better explanation:

The universe can be thought of as space containing a variety of quantum fields. A quantum field might be best thought of as something that has a value at every point in space. Put together a bunch of these fields and you can describe particles, particle interaction, and all the matter and energy in a given region of space - in fact, in the whole universe!

Picture a region of space with absolutely nothing in it. Nothing. (I'm ignoring vacuum energy, even though I really should discuss it) That's a true vacuum. Remove all the fields, particles, and other interesting stuff from our universe and that's what you'll get. Nothing.

The false vacuum scenario happens if there isn't actually "nothing" but "something". If our "vacuum" has some extra energy that it shouldn't (again, I'm not talking about normal vacuum energy). In other words, there's something where there should be nothing.

In this end-of-the-universe scenario, the universe goes from having this something to having nothing - from a false vacuum to a true vacuum. The region of space actually having nothing starts at a certain region and expands outward at the speed of light. That's what the scientists were talking about.

For anyone who wants a further explanation and/or could tell how many mistakes I made there: Yes, I'm aware that there were mistakes in that incomplete explanation. I didn't properly explain vacuum energy or an energy state, nor did I discuss stability or even properly touch on quantum fields. But quite frankly, I don't think this explanation needs that baggage. Does it add to the richness of the concept? Yes. But is it confusing? Also yes. I don't think working that in would be productive.

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  • $\begingroup$ Is anyone who is not a physicist capable of understanding this because I can't really say that I do ? I read the links you provided and it said that "a bubble could form at anytime and anywhere " I don't think that I am going to be too worried over this, because if a bubble hasn't formed near the Earth in the 3.5 to 4.5 billion years of its existence, then I hardly think it will do so within our very brief life here on Earth. Is this just speculation or is it an accepted theory among scientists as being a fact ? $\endgroup$
    – Peter U
    Jan 24, 2015 at 23:22
  • $\begingroup$ @PeterU I apologize for the unclear explanation; I tried my best. Anyway, you're definitely right about the odds of such a thing being very slim. Regarding the last sentence - it's a viable idea that has some credence to it, but there isn't much to support it. It's closer to speculation that an accepted theory. I'll try to edit my answer to make it a bit clearer. $\endgroup$
    – HDE 226868 ♦
    Jan 24, 2015 at 23:51
  • $\begingroup$ @PeterU Does this help a little? $\endgroup$
    – HDE 226868 ♦
    Jan 25, 2015 at 0:15
  • $\begingroup$ It is not your explanation that is difficult to understand but rather this idea of a "bubble" forming and annihilating everything that it encounters . What happens to the matter, how does it disappear ? What did this true vacuum do to it ? The Universe itself formed this way as well didn't it ? It just appeared out of nothing and started to expand. This idea is almost too much for our mind to comprehend that something just appears out of nothing . $\endgroup$
    – Peter U
    Jan 25, 2015 at 1:04
  • $\begingroup$ @PeterU I don't know it well enough to explain it properly. Try this post on Physics: physics.stackexchange.com/questions/770/… $\endgroup$
    – HDE 226868 ♦
    Jan 25, 2015 at 1:12
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Perhaps this analogy is apt, or perhaps not...

Sometimes when you go out to the car on a cold winter morning, the windscreen looks clear, but there is actually a supercooled film of water on it. As soon as you start up the engine, the vibration disturbs the unstable equilibrium, the water freezes on the windscreen, the phase transition occurring at points and spreading. At the same time latent heat of crystallisation is released.

I think the idea is meant to be that our universe could be in that supercooled state and that there is actually a more energetically favourable phase that the (false) "vacuum" could be in, but the transition to that new phase would release sufficient energy to drive a new bubble of cosmic inflation.

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