Starlight dotting the seemingly empty space is a beautiful and inspiring sight, one that can be an impressive backdrop in a movie or a setting in a novel. But what if along with the spectacular view comes a piece of information that the space is permeated with sound? Will you dismiss the scene as nonsensical, badly executed science-fiction, or will you be willing to suspend your disbelief for a while?
My guess is that even those comfortable with wormholes, aliens, and Greedo firing first, won’t easily accept sound traveling through vast distances of space. Looking at space beyond the “cozy” confines of our solar system, one usually perceives it as mostly empty and absolutely quiet. So, shouldn’t we be skeptical about sound waves stretching as far as half a billion light years (about a hundred and fifty million parsecs, or about two hundred times the distance between Milky Way and Andromeda)?
Although celestial sound might seem too weird for science-fiction, existence of sound waves dating from early universe was theoretically predicted in the late 1960s (by Peebles and Yu of Princeton University and by Zel’dovich and Sunyaev of the Moscow Institute of Applied Mathematics). In 2005, a group of researchers from the Sloan Digital Sky Survey, detected the acoustic peak in a luminous red galaxy sample. Since then, celestial sound, more commonly known as Baryon Acoustic Oscillations (or BAO), has been used as a standard ruler to study dark energy. For those curious, here is a very brief account of the origins of this primordial sound.
Many things have changed since the Big Bang, but not the fact that the universe we observe is expanding and cooling. Once upon a time (or some 13.7 billion year ago, a few minutes after the Big Bang), the universe was so much denser and hotter that massive particles moved with velocities close to the speed of light, collided with each other and darted away. Photons were also part of the mixture, frequently scattering from electrons, which combined and separated from protons. While the distances between particles grew as universe expanded, they collided frequently enough to maintain an equilibrium, so photons, electrons and protons behaved as a single (ionized) gas. This gas was distributed almost evenly (homogeneously and isotropically) in space, but there were small fluctuations in its density. Like with sound waves in the air, tiny density differences resulted in compression and rarefaction in the particles gas. Pressure exerted by the photons kept spots with surplus of massive particles from aggregating under the pull of gravity (like dark matter was doing meanwhile). Compression drove the over dense regions to expand, until they became under-dense, then gravity pulled them back. The result of this pull and push was sound waves propagating through the ionized gas. At the early stage, the speed of these sound waves was comparable to the speed light!
As universe continued cooling and expanding, the particles’ density gradually diluted. Massive particles like electrons and protons lost much of their kinetic energy. The speed of the sound waves also slowed. When the universe was about 400,000 years old, electrons and protons cooled enough to combine into stable hydrogen atoms. Having no electrons left to scatter from, photons decoupled from matter, and started their free stream through space. This radiation, known as Cosmic Microwave Background (CMB), permeates space and provides astronomers with a snapshot of tiny temperature (and consequently – density) differences in the universe at the times of photons’ last scattering (during the recombination between protons and electrons to hydrogen atoms).
So much had happened in universe in the intervening time (this is about 13.3 billion years), that one might wonder how these sound waves could ever be detected. Left without the motive force of photons’ pressure, baryons (most of the weight of ordinary matter comes from protons and neutrons, which are baryons) remained overdense in a shell of a radius about 500,000 light years (about 150Kpc). As the universe expanded about 1000-fold since the recombination, nowadays, each shell stretches to nearly one billion light-years across (radius of about 150MPc)! This mind-boggling distance is actually what prevented the imprint of the primordial sound waves from washing out. The processes that caused matter to clump together and eventually form stars, galaxies and other celestial structures, broadened the width of the shell, but not much else. Why so? Gravitational collapse and galaxy formation are relevant on a length scale of about 30 million light years (10 Mpc) and smaller. Acoustic imprints on that length scale were indeed either smeared or wiped out, but on BAO length scale of a half a billion light years, “local” phenomena had very small affect.
Note: This blog post was inspired by Raymond & Beverly Sackler Lecture in Astrophysics, given by Daniel Eisenstein a few days ago. This lecture, titled “Dark Energy and Cosmic Sound” was not only “free and open to the public,” but one of the liveliest cosmology talks I have attended.
For those who are intrigued to find out more, a non-technical article, titled “The Cosmic Symphony” by Wayne Hu and Martin White was published in Scientific American (2004). A pdf of the paper is available on professor Hu’s website. A comprehensive review of the observational probes of dark energy, including a chapter on the BAO method is available through the arXiv (here).
The man for wisdom’s various arts renown’d,
When Danielle, the protagonist of Initial Conditions, wants to work on the recently-discovered 
Hungarian stamp commemorating 100 years of general relativity. Image from 
