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. (NICOLE RAGER FULLER / NATIONAL SCIENCE FOUNDATION)

This Is How Astronomers Know The Age Of The Universe (And You Can, Too)

The hot Big Bang occurred 13.8 billion years ago, and there’s no other possible answer consistent with what we know today.

Conceptually, it might seem like the simplest idea in existence to determine the age of the Universe. Once you figure out that the Universe is expanding, all you need to do is measure the expansion rate today and use the laws of physics to determine how the expansion rate must have changed over time. Instead of extrapolating forward to determine the fate of the Universe, you do the calculating backwards instead, and go all the way back until you achieve the conditions of the hot Big Bang itself.

This obvious method not only works, but it remains the best way we have to calculate the Universe’s age even today. Yet it’s very easy to go awry, as there are many simplifying assumptions you can make that will give you an easy answer that isn’t necessarily correct, including errors that even a Nobel Laureate made earlier this year. Here’s how you, too, can figure out the age of the Universe.

Standard candles (L) and standard rulers (R) are two different techniques astronomers use to measure the expansion of space at various times/distances in the past. Based on how quantities like luminosity or angular size change with distance, we can infer the expansion history of the Universe. Using the candle method is part of the distance ladder, yielding 73 km/s/Mpc. Using the ruler is part of the early signal method, yielding 67 km/s/Mpc. (NASA / JPL-CALTECH)