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Baryon acoustic oscillations: Mapping the universe with cosmic echoes

Writer's picture: Adam BeckerAdam Becker

Baryon acoustic oscillations – ripples in the plasma of the early universe – serve as a “standard ruler” for tracing its expansion history.


In space, nobody can hear you scream. But cosmologists have ways of “hearing” the sound of the universe anyhow. Through sound waves reconstructed mathematically from density fluctuations in the primordial plasma that followed the Big Bang, they can tell an interesting story about the universe’s earliest days.


Sound is a pressure wave. Stereo speakers pulsate, creating alternating areas of higher and lower air pressure, and that wave of air pressure vibrates our eardrums, which our brains interpret as music or speech or static. But in the vacuum of space, there’s no air (or any other substance) that can undergo changes in pressure, so there’s no sound.


The Big Bang, primordial plasma, and dark matter

This wasn’t always the case. Nearly 14 billion years ago, shortly after the Big Bang, the universe was filled with a primordial plasma. This ionized gas, made of positively charged protons, light nuclei (with varying positive charges), negatively charged electrons, and neutral photons, glowed as it slowly cooled over the course of the first few hundred millennia of the universe’s existence. About 380,000 years after the Big Bang, that plasma finally cooled enough to become a hot, neutral gas, turning the universe transparent and releasing light to freely stream across the cosmos, which can now be detected as the cosmic microwave background (CMB) radiation.


The cosmic microwave background
The cosmic microwave background, the oldest light in our universe (Credit: NASA, ESA, and the Planck Collaboration)

During those hundreds of thousands of years when the universe was filled with a wispy, bright plasma, there was sound in space, of a sort. This happened because the plasma wasn’t totally uniform – and because the plasma wasn’t the only thing in the universe at the time.


Thanks to tiny quantum variations left over from the Big Bang, the plasma was slightly denser in some regions and less dense in others. These differences were small – only about one part in a hundred thousand – but they mattered. And they weren’t only in the plasma. Theories suggest these tiny differences were also present in dark matter, which is thought to comprise about 80% of the matter in the universe, influencing the formation of large-scale structures.


Current thinking suggests that dark matter is transparent, does not interact with light or any other form of electromagnetism, and can pass through ordinary matter – but does interact with regular matter through gravity. Its mass attracts other mass, just like planets, stars, and the familiar objects of everyday life.


A cosmic bell: Sound waves in space

That’s exactly what happened in the era of the primordial plasma. Gravity acted on the little differences in density, both in the dark matter and the plasma itself. The denser spots in the dark matter got denser, as the rest of the dark matter slowly fell toward them. Because most of the existing matter, then and now, was dark matter, the plasma followed along, pulled in by gravity.


That plasma had an internal pressure, like a hot gas. And like a gas, the plasma’s pressure and temperature increased along with its density as more of the plasma fell into the over-dense regions of the early universe. The plasma’s pressure rose until it overcame the pull of gravity, pushing some of it back out of the over-dense regions. But once that happened, the pressure dropped again, allowing more plasma to fall back in – which then increased the pressure, pushing it out, and the cycle began again.


This cycle was slow, because the densities involved were very low and the distances involved were huge. But over hundreds of thousands of years, this in-and-out cycle led to bubbles of higher plasma pressure – sound waves in space. The over-dense regions of the plasma rang like a cosmic bell. These sound waves formed spheres around the over-dense regions of the primordial universe, like waves rippling out from a stone thrown in a pond.


Baryon acoustic oscillations: Ripples in the primordial plasma

Most of the mass in the plasma was in the protons and neutrons, which are each nearly 2,000 times more massive than an electron. Protons and neutrons both belong to a class of particles known as baryons, a name that appropriately derives from the Greek for “heavy.” Thus, the matter in the plasma – the ordinary matter that went on to form atoms and the familiar visible world around us today – is known as baryonic matter. And the ripples in the cosmic pond of plasma in the early universe are called baryon acoustic oscillations, or BAO.


Illustration of baryon acoustic oscillations among a distribution of galaxies.
Illustration of baryon acoustic oscillations among a distribution of galaxies (Credit: Gabriela Secara/Perimeter Institute)

The BAO resembled the rings rippling out from a handful of pebbles thrown into a pond, a cosmic cacophony of overlapping and interfering waves. But 380,000 years after the Big Bang, the surface of that pond froze: the plasma cooled into a neutral gas, its internal pressure dropping dramatically as the photons trapped inside were freed. When this happened, that mess of ripples froze in place, their observable imprint evolving only due to the expansion of the universe.


As the universe evolved, gravity continued to pull matter from less dense regions into more dense regions. Some of those more dense regions were the frozen ripples left behind by the BAO. As the matter of the universe slowly fell together over the following billions of years, the matter in the more dense regions of the universe accumulated and collapsed to form the galaxies, stars, and planets (and people!) we see around us today.


Galaxy surveys and the imprint of baryon acoustic oscillations

Surprisingly, even after billions of years, cosmologists can still detect the BAO. While the original ripples were a mess of waves, and the intervening eons have further distorted that mess, there’s still a signal to be seen through judicious use of cosmic statistics.


The BAO had a characteristic size – the main front of the ripples was a particular fixed distance from the center of the primordial over-densities that produced them. Those primordial over-densities produced galaxies. But the BAO did too: those ripples were, ultimately, shells of over-dense material in the early universe, and so there are stars and galaxies that formed due to the BAO themselves.


Therefore, not only did galaxies form at the sites of the initial over-densities, but they also preferentially formed at the characteristic BAO distance too. That makes it possible to measure the BAO distance in the universe today.


The Sloan Digital Sky Survey (SDSS) created a 3D map of the distribution of galaxies, with Earth at the center and each point representing a galaxy, typically containing about 100 billion stars.
The Sloan Digital Sky Survey (SDSS) created a 3D map of the distribution of galaxies, with Earth at the center and each point representing a galaxy, typically containing about 100 billion stars. (Credit: M. Blanton/SDSS)

First, create an enormous map of galaxies – what cosmologists call a galaxy survey. Using the data from that galaxy survey, chart out the probability of finding galaxies at various distances from each other. If you’re looking at one galaxy, what are the odds that there’s another galaxy a million light-years away? Ten million? A hundred million?


A plot of that probability would reveal a fairly smooth curve dropping as the distance gets bigger, but with a little bump at around 500 million light-years – the imprint of the BAO, frozen into the arrangement of galaxies across the universe. And that’s exactly what cosmologists found in 2005, after surveys of hundreds of thousands of galaxies by the Sloan Digital Sky Survey and Two-Degree Field Galaxy Redshift Survey.


Tracing the expansion history of the universe

Cosmologists went looking for that spike because they knew it should be there, but also because they knew it could tell them a lot about the universe. One of the most difficult problems in cosmology is discerning how far away things in the sky actually are, especially when they’re millions or billions of light-years away. Such distance measurements allow cosmologists to determine how quickly the universe was expanding at different times in its history. That history is one of the most important facts about the composition and structure of the universe, and says a lot about the origin and fate of the universe too.


Obtaining such distance measurements requires some kind of standardized object, with fixed properties that can be seen over great distances, like a particular type of star or supernova: because it has a known intrinsic brightness, cosmologists can compare its apparent brightness from here on Earth with its true brightness to calculate the distance to the object. It’s like seeing a streetlamp from far away on a dark night: you know it’s far away because you know how bright streetlamps generally are, and so the brightness of the lamp tells you the distance to it. These kinds of objects are called “standard candles,” and much of the history of cosmology is the search for new kinds of standard candles and new ways to use existing ones.


The BAO offers a complementary tool – an object not of fixed brightness but of fixed size that functions as a kind of “standard ruler.” Instead of a distant streetlamp, the BAO are like seeing a distant person: you know how big people are, in general, and that means you can tell how far away a distant person is by comparing their apparent size to their true size.


By tracking the BAO “bump” across the history of the universe, the large-scale structure of the cosmos itself – galaxies and galaxy clusters – can be used to trace out the expansion history of the universe. As larger and larger galaxy surveys have been conducted over the past few decades, cosmologists have used the BAO to get a finer and finer portrait of the universe, all using frozen sound from the beginning of time, imprinted into the deepest reaches of the sky.


Adam Becker is a science journalist with a PhD in astrophysics. He has written for The New York Times, the BBC, NPR, Scientific American, New Scientist, Quanta, and other publications. He is the author of two books, What Is Real? and the forthcoming More Everything Forever. He lives in California.

iStock-1357123095.jpg
iStock-1357123095.jpg

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