How will the universe end? “Not with a bang but with a whimper,” wrote the American poet T.S. Eliot regarding the end of the world. But if you want a more definite response, you’ll find that physicists have spent countless hours turning this question over in their minds, and have neatly fit the most plausible hypotheses into a few categories.

“In textbooks and cosmology class, we learn there are three basic futures for the universe,” said Robert Caldwell, a cosmologist at Dartmouth University in Hanover, New Hampshire.

In one scenario, the cosmos could continue to expand forever, with all matter eventually disintegrating into energy in what’s known as a “heat death,” Caldwell said. Alternatively, gravity might cause the universe to re-collapse, creating a reverse Big Bang, called the Big Crunch (we’ll explain this later). Or, there is the possibility that dark energy will cause the universe’s expansion to accelerate faster and faster, evolving into a runaway process known as the Big Rip.

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