Cosmology deals with the big questions of the universe, often the same questions that keep philosophers up at night. When did the universe begin? How did it start? Has the universe always been expanding? (For the record, the answers are: about 13.8 billion years ago, in a high-density state that rapidly expanded called the Big Bang and yes, but not always at the same speed.) But here’s a question they haven’t figured out yet: How’s it all going to end?
It’s a big question all right, but we’ve made surprising headway toward an answer. In the last years of the 20th century, the astrophysical community was stunned to learn that the universe was driving itself apart. For decades, scientists had known that distant galaxies all move away from us, with the farther ones moving the fastest. The only way this makes sense is if the universe itself is expanding. Given all the matter in the cosmos, the force of gravity should be slowing down that expansion. But when cosmologists calculated just how much it’s slowed down, they got a negative result — the expansion of the universe is speeding up!
Nobody knows what’s driving the acceleration, so cosmologists have dubbed that mystery dark energy. It is so dominant (about 69 percent the total content of the entire cosmos) that dark energy quickly became a part of any discussions about the final end of the universe. And while there are no definite answers yet, those discussions have come up with a few interesting possibilities.
