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What kinds of tools do astronomers use to calculate the age of the universe, and how can they determine the speed of its expansion?

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One thing that humans simply cannot stop doing is trying to figure out where we came from and why we exist. That’s, like, Our Big Thing.

That, and getting really inventive with fried foods, but that’s a subject for another channel. Philosophers and fry cooks are still working on those questions, but science can at least answer one thing: when the universe began. And right now, we’re pretty confident that it all happened around 13.8 billion years ago with the Big Bang.

Figuring that out hasn’t been easy, and our estimates could still get better, but thanks to some useful tools and a lot of math, we’re off to a really good start. To calculate the age of the universe, astronomers use two main tools, or pieces of evidence. The first is pretty intuitive: They look for old stuff.

For the most part, that means looking at really faraway stuff. See, the universe has been expanding ever since the Big Bang, so the very oldest objects have been hurtling away from the Earth for billions of years. And based on how far an object is, astronomers can get a rough idea of how old it is.

So far, using photos, like ones from the Hubble Space Telescope, researchers have been able to find clusters of stars as old as 13.2 billion years! But they don’t know the stars are old just by looking at them. To figure it out, they have to take into account some of the cool, weird ways that light behaves.

See, as the universe expands, it also stretches the light waves traveling through it. So if a star is moving away from Earth, its light will be stretched and have a longer wavelength by the time it gets to us. This is called redshift.

By seeing how stretched, or redshifted, a star’s light is, and doing some math, astronomers can get a rough idea of how far and how old a star is. But that isn’t the only tool they use, because even though we can detect some faraway objects, others are still really difficult to observe over those large distances. Mostly, this just tells us that the universe has to be at least 13.2 billion years old.

To refine the estimate, astronomers also use measurements about the expansion of the universe itself. We’ve known the universe was expanding since the late 1920s, but understanding how it’s expanding is what’s especially useful. Knowing how fast it’s happening and how that speed is changing really allows researchers to work backwards from right now, to find out exactly when the universe was a tiny seed of everything.

It’s basically like how a forensic scientist can study an explosion site and tell you when the bomb went off. Just with a much bigger bomb in this case. Two major discoveries have helped us understand this expansion.

The first was type Ia supernovas, which form from the explosion of a tiny white dwarf star and some other stellar companion. In 2011, a team of scientists won a Nobel prize for using them to prove that the universe’s expansion is getting faster. These supernovas are extremely bright, and their brightnesses are all pretty uniform, so one of them will look a lot like any other.

This makes them really good for calculating distances, or what astronomers call standard candles. Since we know what their brightnesses should be at any given distance, whether it’s a million or a billion light-years away, they’re easy to use in measurements. And after years of measurements, astronomers noticed that the redshifts for these supernovas was a lot smaller than they should’ve been for galaxies so far away.

That means that, sometime after the supernovas emitted their light, they actually got farther from Earth than expected. That could could only be explained by a universe that’s expanding faster as the years go on, although we aren’t positive what’s causing that to happen. But it has helped us understand when the Big Bang happened.

Before we knew that the expansion of the universe was accelerating, our calculations about its age could be pretty inaccurate. Astronomers used to assume a constant rate of expansion, so if you’re working backwards from our current rate, you’d get a universe that’s way too young. Or, if you picked a bad standard candle, you’d get inaccurate measurements, too.

For instance, Hubble’s original calculations from the 1920s used a type of star called a Cepheid variable as the standard candle, and that suggested the universe was about 2 billion years old. Which is definitely not right. Science is a process.

Still, type Ia supernovas aren’t the only way we’ve studied how the universe is expanding. The other way we’ve figured out the rate of expansion is with the Cosmic Microwave. Background, or CMB, which is the energetic glow left over from the Big Bang.

Well, it’s not wildly energetic, it comes in at 2.7 Kelvin, or about -270 degrees Celsius. But it’s not 2.7 Kelvin everywhere you look! And that’s super useful to us!

Those temperature variations can tell us about the movement of objects and the densities of gases in the universe, both of which are used in calculating the universe’s rate of expansion. And along with type Ia supernovas, these studies have allowed us to get a much more precise picture of how the universe has been growing since it began. That’s let us zero in on our current age estimate: 13.8 billion years.

And our observations are just going to get better from here on out! We think 13.8 billion is pretty solid, but don’t be surprised if you hear that number change a little bit as we make better observations. It just means we’re getting better at science.

And speaking of CMB, has a lesson in their astronomy unit that covers even more of what that background radiation has taught scientists about the universe. And it’s really fun, because each question makes you an active problem solver. So let’s see if we learned anything from everything I was just telling you about.

I’m going to take the quiz here in the studio, but also set up a link so that you can test your knowledge at home for free. You can find that at So like all Brilliant quizzes, this one opens with giving you more information about how to solve these problems. This quiz kind of starts with explaining the CMB a little bit more before we dive into answering questions about it, and one of the things that I love about Brilliant is that it’s not just text-based, that you’re working with, you have amazing images like this map.

Not only do they help me understand the problem that I’m trying to solve, but they give context to everything that they’re talking about in this quiz, and that helps me make more sense of it, but also retain that information over the long run and be excited about it. So check out the quiz for yourself, and let us know how you do in the comments below. This quiz is totally free for you to play with, so have at it.

And the first 200 to sign up at will get 20% off of their annual Premium subscription, and be supporting SciShow Space, so thank you! [♪ OUTRO].