Will the universe expand forever? For physicists, this depends on how space overall is curved. General relativity lets space curve locally but doesn’t say how is space curved in general. The mathematics suggests that a positively curved universe will eventually stop expanding and shrink back in a big crunch, but a negatively curved universe will expand faster and faster forever, as there isn’t enough mass to stop it. A positive curvature was expected but cosmology measures suggest that the expansion of space is accelerating rather than 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 that it will therefore expand forever is not implied. If our universe is a bubble expanding in a quantum bulk, there are probably others so they will eventually meet. What then happens when one bubble universe meets another?
The answer depends on whether the universes took the matter or anti-matter path. If our matter universe meets another matter universe, they will simply merge into an even bigger bubble. If this has already happened to our universe, it 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 can oppose it. One can block an electric field with an opposite field but nothing opposes gravity, so it reigns supreme. This is because our universe took the matter path. Yet matter has an anti-matter opposite that could not only shield gravity but would also fall up on earth. Our universe has no anti-matter because it took the matter path but if it meets an anti-matter universe, both will annihilate back into the quantum bulk. In quantum realism, sooner or later, our universe will return from whence it came, and will do so with a bang not a whimper.
Yet even if Armageddon has already begun, we wouldn’t know right away, as the wave of destruction will travel at light speed. Cosmology estimates our galaxy is at least 100,000 light years across and the observable Universe is over 90 billion light years across so the shut-down could take a while. Will our telescopes see it coming? There could be no warning, as we only see galaxies as they were millions of years ago. When our physical universe is packed away, to return from whence it came, it will happen at the speed of light with no possible warning at all.