In physics, whether the universe will expand forever or contract back into a big crunch depends on how space is curved overall. Relativity lets space curve but doesn’t define how it curves. In mathematics, a positively curved space will eventually stop expanding and contract in a big crunch but a negatively curve space will expand faster and faster forever, as there isn’t enough mass to stop it. A positive or flat curve was expected until cosmology found that the expansion of space is accelerating not slowing down (Cowen, 2013), so space is negatively curved.
Quantum realism expects our space as the inner surface of an expanding hyper-bubble to have the slight negative curve that cosmology found, but doesn’t conclude that it will expand forever. If our universe is an expanding bubble in a quantum bulk, there are probably others so they will eventually meet. What happens when one “pocket universe” as Guth calls them meets another?
The answer depends on whether they are matter or anti-matter. If our universe meets another matter universe, they will just merge into a bigger bubble. If this has already happened, our universe will be bigger than it could be by its own expansion, but there is also the Armageddon option, that it meets an anti-matter universe.
Gravity is all powerful in our universe because it only adds so nothing opposes it. One can block an electric field with an opposite field but nothing opposes gravity. It reigns supreme because our universe took the matter path but matter has an anti-matter opposite that could not only shield gravity but would also fall up on earth. If our matter universe meets an anti-matter universe, both will annihilate back into the quantum bulk, to return from whence they came.
If Armageddon has already begun, we won’t know right away because it will happen at light speed. Cosmology estimates that our galaxy is over 100,000 light years across and the observable universe is 90 billion light years across so it could take a while to shut-down. Will our telescopes see it coming? There would be no signs, as we see galaxies as they were millions of years ago. When our physical universe is packed away, it will be at the speed of light with no possible warning.