Armageddon, the biblical end of days, describes how our world ends but in physics, how the universe ends depends on how space is curved. Relativity lets space curve but doesn’t specify its overall curve. A positively curved space will eventually stop expanding and contract in a big crunch, but a negatively curved space will expand faster and faster because its mass can’t stop it, ending in a big freeze. The latter was expected until cosmology found that space is expanding faster not slower, so space is negatively curved (Cowen, 2013).
In this model, our space is the inner surface of an expanding hyper-bubble (2.4.1), so it will have a negative curvature as cosmology found, but that doesn’t mean it will expand forever. If our universe is just a bubble in a quantum bulk, there are probably others, so they must eventually meet. What happens when one pocket universe, as Guth calls them, meets another?
That depends on whether they are made of matter or antimatter. If one matter universe meets another, they just merge into a bigger bubble, so if our universe has already done this, it will be bigger than its own expansion allows. The alternative, that our universe meets an antimatter universe, is the Armageddon option.
What happens when matter meets antimatter? Essentially, they destroy each other, so if a matter universe met an anti-matter universe, both would be destroyed to some degree. Antimatter is rare in our universe but is produced in tiny amounts by cosmic rays in thunderstorms. It soon vanishes when it meets matter, taking some of it with it, so if our bubble universe met its antimatter equal, both could annihilate back into the quantum bulk from whence they came.
This Armageddon would spread at light speed, but it could take a while, as our galaxy is an estimated 100,000 light years across, and the observable universe is 90 billion light years across. Even so, could our telescopes see it coming? Unfortunately no, as we see galaxies as they were millions of years ago. Our earth would just be there one moment and gone the next, so when this piece of the universal game is packed away, it will be at the speed of light, with no possible warning.