How Long Has The Universe Been Accelerating?

Perhaps the biggest discovery about the Universe in the last generation came at the very end of the 20th century, when we uncovered one of the most disconcerting cosmic truths: the distant galaxies, as time goes on, aren’t just receding from us, they’re speeding up as they move away from us. The discovery of the accelerated expansion of the Universe, by the Supernova Cosmology Project and the High-z Supernova Search Team, was awarded the 2011 Nobel Prize in Physics, but is one of the most bizarre and unexplained phenomena in the Universe. The thing is, the Universe wasn’t always accelerating away from us like this. For billions of years, the expansion was slowing down, and to someone alive ten billion years ago, it might have looked like it could recollapse. Let’s take a look at what happened, and how we know.