Baryon acoustic oscillations

Distribution of galaxies observed by SDSS: Each dot is a galaxy (image source). More distant, older galaxies are on the top of the image, with closer, more recent galaxies on the bottom, showing the development of structure over time, from smooth to textured.

Distribution of galaxies observed by SDSS: Each dot is a galaxy (image source). More distant, older galaxies are on the top of the image, with closer, more recent galaxies on the bottom, showing the development of structure over time, from smooth to textured.

In Eve’s Diary, a short story by Mark Twain, Eve writes, “This majestic new world is marvelously near to being perfect, notwithstanding the shortness of the time, but there are too many stars in some places and not enough in others.” If you can get a good view of the sky, far from city lights, you’ll see that stars are grouped in clumps: random but not uniformly random.

Part of this is due to gravity. Stars that are close to one another gravitationally attract and tend to form tight clusters with gaps between the clusters. The same is true of whole galaxies — nearby galaxies tend to amalgamate into superclusters.

The other part of the explanation has to do with the distribution of matter in the early universe. The early universe was nearly uniform, but not perfectly so. Tiny quantum fluctuations, stretched to cosmic proportions by the expansion of space, provided the initial seeds that helped matter start coalescing into galaxies, stars, planets and us.

This phenomenon is known to astronomers as baryon acoustic oscillations (BAO). Baryons are particles of ordinary matter (as opposed to dark matter) — “baryon” is a general term for particles such as protons and neutrons, which provide most of the mass of normal atoms. The “acoustic oscillations” part refers to the fact that fluctuations in the early universe were actually sound waves. Before the universe was cool enough to transition from a glowing plasma into a transparent gas (the first 380,000 years), the light from the Big Bang exerted pressure on charged particles in the plasma, and the waves of high- and low-pressure were analogous to sound in air.

When matter became transparent, the light passed through it unimpeded, and this light is today visible as cosmic microwave background (CMB). Matter, on the other hand, was suddenly released from outward pressure and became subject only to gravity. The CMB is effectively a snapshot of the lumpiness of the universe when it was 380,000 years old, and the BAO is the same distribution amplified by gravity over the last 13.8 billion years.

Although the CMB and BAO have been separate for most of the history of the universe, the imprint of CMB-scale fluctuations has been discovered in the distribution of galaxies today. The characteristic wavelength between crests of galactic superclusters and troughs of intergalactic voids is about 500 million light-years, which corresponds to 73 octaves below middle C. This is exactly what would be expected from propagating the crests and troughs of the CMB forward by 13.8 billion years.

Jim Pivarski

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