Current theories say the universe’s continued expansion depends on how much physical matter exists in the universe, and we aren’t sure just how much stuff there is.
The amount of matter that would keep the universe stable is called omega.
Omega is a hypothetical, unknown quantity of matter, but scientists have determined what is likely to happen if there is more or less matter than the omega amount.
If there is less matter than omega, the universe will continue to expand and the matter will continue to cool as it grows farther and farther apart.
Stars will eventually die out, leaving all matter dark and cold, and always moving away from other matter.
This concept of a constantly expanding universe is known as the open universe model.
If there is more matter than the omega amount, the forces of gravity, every bit of matter exerts gravity on other matter, will sooner or later slow and stop the expansion.
Gravity will cause the matter to draw together and the universe will begin to contract.
At some point, the universe will then fall in upon itself.
This is called the closed universe model, nicknamed the big crunch.