How We Know The Universe is Ancient

**SPEAKER_1** (0:00)
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**SPEAKER_2** (0:30)
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**SPEAKER_3** (0:39)
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**Matt O'Dowd** (1:00)
The universe is precisely 13.8 billion years old, or so our best scientific methods tell us.
But how do you learn the age of the universe when there's no trace left of its beginnings?
We see the same constellations on the night sky as did our astronomer ancestors. Familiar star maps are recorded in cave paintings tens of thousands of years old. We could be forgiven for assuming that the universe above is fixed and unchanging.
But the Earth was also thought to be timeless, until we learned to see billions of years of change in its geological layers. Well, the universe also changes and also had a beginning. In our recent episode, we learned how we calculate the age of the Earth based on radioactive decay in its most ancient rocks.
But there are no rocks from the beginning of the universe. There aren't even any photons from the time right after the Big Bang.
So today, we go deeper into deep time to understand how we can possibly have learned the birthday of the universe. Actually, you know what? Forget for a moment knowing the age of the universe. Our knowledge that the universe even had a beginning is relatively recent. Let's talk about that first. The discovery of the beginning of the universe corresponds to our discovery that there even is a universe outside the Milky Way galaxy. This is something we explored in our stellar series, but it's worth the review. At the beginning of the 20th century, astronomers were arguing about the nature of these faint fuzzy patches of light on the sky, known as spiral nebulae. Were they blobs of gas in the Milky Way, or vast, distant groups of stars? Other Milky Ways. Or as Immanuel Kant called them, island universes. We now call them galaxies. It all changed in the 1920s. First, Vesto Slipher found that the spiral nebulae are moving away from us at incredible speeds, based on their Doppler shift, the lengthening of the wavelengths of their light due to their motion. Then, Edwin Hubble figured out the distances to these objects by watching their stars pulse. He located a star type called a Cepheid variable, whose pulsation rate is proportional to its brightness, encapsulated in a simple period-luminosity relation. By measuring the pulsation rate, he could calculate its true brightness, undimmed by distance. He figured out that in order for these stars to appear so faint in his telescope, they had to be many millions of light years away. Combining Slyther's velocities, or redshifts, with Hubble's distances, we learned that essentially all galaxies are racing away from us, and the further the galaxy, the faster it seemed to be retreating. This was a shocking discovery. Until then...

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