Sound Waves from the Beginning of Time

SPEAKER_1 (0:01)

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SPEAKER_2 (0:26)

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Matt O'Dowd (0:56)

We want to thank Google's Science Journal app for supporting PBS Digital Studios. Invisible to the naked eye, our night sky is scattered with the hundreds of billions of galaxies that fill the known universe. Like the stars, these galaxies form constellations, hidden patterns that echo the reverberations of matter and light from an epoch long before galaxies ever formed. These are the baryon acoustic oscillations, and they may hold the key to understanding the nature of dark energy.

The field of cosmology, the study of the universe on its larger scales, was once the least precise in all of astrophysics. The impossibly vast distances made accurate measurements near impossible. But as our telescopes and our techniques improved over the last few decades, things are now very different. We live in the era of precision cosmology, in which we know to study in detail the properties that govern the very birth, evolution and the end of our universe. We talked about one of those properties in a recent episode, the Hubble Constant, and about a growing conflict in its measured value, which hints at strange new physics. Today we'll talk about another measurement that may help resolve this crisis, the baryon acoustic oscillations. They are the fossils of the first sound waves in the universe, imprinted on the distribution of galaxies on the sky. And in these patterns, we could read the expansion history of the universe. Let's start at the beginning. For the first few hundred thousand years in the life of our universe, all of space was filled with hydrogen and helium in plasma form. Protons and the lightest of nuclei forged in the first minutes after the big bang, and still so hot that no atoms could form, and electrons buzzed free of their nuclei.

These particles of matter are our baryons.

There was also light. In fact, around a billion photons for every electron. But no photon is safe from a free electron. Unbound electrons present a huge target to scatter any wavelength of light, and scatter they did, constantly. A photon could barely travel any distance before colliding with an electron. The electrons, in turn, exerted their electromagnetic pull on the nuclei. We say that in this state, light was coupled with matter. And baryons and photons formed a single strange fluid, a baryon-photon plasma. There are three profound differences between the behavior of matter in this state compared to the gentle gas nebulae of the modern universe. First, the baryon-photon plasma was opaque. There exists no lines of sight to anything during that time. And we'll come back to that later.

Second, light was able to exert an enormous pressure on this plasma. As we'll see, that will lead to the production of colossal sound waves.

And third, those sound waves traveled fast. Rapid interaction between the charged particles of the plasma via the trapped photons meant that ripples in the plasma traveled at over half the speed of light. Mixing this soup of baryons and photons was dark matter.

In fact, dark matter outweighs baryons by a factor of five, which means it was by far the dominant gravitational influence in the early universe, as it still is. But unlike baryons, dark matter does not interact with light at all. Light exerts no pressure on dark matter. OK, so the universe is filled with this hot ocean of baryons, photons, and dark matter. Now, in order for it to do anything interesting, we need one final ingredient, density fluctuations. A teensy bit more matter here, a teensy bit less there. These fluctuations probably were the remnants of random quantum fluctuations from when the universe.

SPEAKER_1 (4:57)

This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required compatibility and availability varies 18 plus.