A theory that the universe is actually finite due to having a nontrivial topology (i.e. like a mobius strip).
An analogy of how you can know that something is finite, given by Janna Levin, is Pacman. The player exists on a 2D plane and when reaching one end of the screen, wraps around to the other side. We can make sense of that by visualizing in 3D (i.e. a higher dimension). However, the math does not require it, just our perceptions to understand what we see–Pacman need not exist in a higher dimension.
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In a thought experiment from The Beginning of Infinity, the author introduces a universe traveling device. During a set interval, you hold the button you go from universe 1 to universe 2 for one minute then on to universe 3 for 30 seconds and so on until you release it and are taken back to universe 1. By the time two minutes is up, you will have traversed the infinite set of universes. If the instrument could take readings along the way, you have a way to measure infinity.

A universe that collapses to a single point due to baryon annihilation required to produce energy needed for colonization. The universe is not infinite and even though we observe the universe expansion accelerating, that doesn’t mean it will continue to accelerate forever. This is like the counterpoint of the Big Bang—the “Big Crunch”.

An Object Can Not Go the Speed of Light Because it Would Have Infinite Mass
The equivalence of mass and energy means that an object (having mass) can not reach the speed of light. As it approaches the speed of light, the mass of the object would become infinite thereby taking an infinite amount of energy. Therefore, only something without any intrinsic mass (like light or a wave) can go the speed of light.

Theoretical cosmologist who researches black holes and gravitational waves. In particular, the universe is finite in size.