It's...basically a result of the cessation of entropy in the physical universe. It's the laws of thermodynamics, entropy will always increase as more energy is exerted within it. And because the universe is always using more energy than it's creating, eventually, after entropy has increased to a certain point, there won't be enough energy left to sustain things like planets and stars or even basic life form.

The universe will cool off to absolute zero because energy is equal, and nothing will be left...that's heat death.