The Big Bang’s accelerating expansion Some 13.8 billion years ago, our universe was born in big bang , and it’s been expanding ever since. Until a few decades ago, it looked like that expansion would eventually end. Astronomers’ measurements suggested there was enough matter in the universe to overcome expansion and reverse the process, triggering a so-called Big crunch . In this scenario, the cosmos would collapse back into an infinitely dense singularity like the one it emerged from. Perhaps this process could even spark another Big Bang, the thinking went. We’d be gone, but the Big Bang/Big Crunch cycle could infinitely repeat. In the years since then, the discovery of dark energy has robbed us of a shot at this eternal rebirth. In 1998, two separate teams of astronomers announced that they’d measured special exploding stars in the distant universe, called a type la supernova , which serv...