If you look farther and farther away, you also look farther and farther into the past. The farthest we can see back in time is 13.8 billion years: our estimate for the age of the Universe. Despite the uncertainties we have in our science, this figure is solidly known to uncertainties of ~1% or less. (Credit: NASA/ESA/STScI/A. Feild)

Ask Ethan: How do we know the universe is 13.8 billion years old?

It’s been precisely 13.8 billion years since the Big Bang occurred. Here’s how we know.

One of the most revelatory facts about the universe is that we actually know how old it is: 13.8 billion years old. If we could step back through time, we’d find that the universe as we know it was a very…