Our entire cosmic history is theoretically well-understood, but only because we understand the theory of gravitation that underlies it, and because we know the Universe’s present expansion rate and energy composition. Light will always continue to propagate through this expanding Universe, and we will continue to receive that light arbitrarily far into the future, but it will be limited in time as far as what reaches us. We still have unanswered questions about our cosmic origins, but the age of the Universe is known. (Credit: Nicole Rager Fuller/National Science Foundation)

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

We confidently state that the Universe is known to be 13.8 billion years old, with an uncertainty of just 1%. Here’s how we know.

How old is the Universe? For generations, people argued over whether the Universe had always existed, whether it had a beginning, or whether it was cyclical: with neither a beginning nor an end. But starting in the 20th and continuing into the 21st century, we not only drew a scientific conclusion to that question — the Universe (as we recognize it) began with a hot Big Bang — but we were able to pinpoint precisely when that beginning occurred.

We now state, with confidence, that the Universe is 13.8 billion years old. But how confident can we really be in that answer? That’s what Adimchi Onyenadum wants to know, asking:

“How did we arrive at the conclusion that the age of the Universe is 13.8 billion years?”

It’s a very bold claim, but one astronomers are more confident in than you might realize. Here’s how we did it.

The open star cluster NGC 290, imaged by Hubble. These stars, imaged here, can only have the properties, elements, and planets (and potentially chances-for-life) that they do because of all the stars that died before their creation. This is a relatively young open cluster, as evidenced by the high-mass, bright blue stars that dominate its appearance. Open star clusters, however, never live nearly as long as the age of the Universe. (Credit: ESA and NASA; Acknowledgment: E. Olszewski (University of Arizona))