Ultimate fate of the universe

In a Heat death, “As expansion continues, the universe becomes larger, colder, and more dilute; in time, all structures eventually decompose to subatomic particles and photons.”

In the Big Rip, "Expansion of space accelerates and at some point becomes so extreme that even subatomic particles and the fabric of spacetime are pulled apart and unable to exist."

In the Big Crunch, "Expansion eventually slows and halts, then reverses as all matter accelerates towards its common centre. Not now considered likely."

Vacuum instability is a possibility that means the "collapse of the quantum fields that underpin all forces, particles and structures, to a different from."