(The second installment in the irregular series “Alphabet of Cosmology“, in which I introduce a concept or project in cosmology that’s a little out of the ordinary.)
Sound waves in the early cosmos
Gather ’round, children, and I will tell you a story about the olden days—before there were any stars, galaxies, or even atoms. This elder Universe was hot, dense, and opaque. Constant collisions with photons kept any electrons from joining protons to form stable atoms, at least for long. However, this chaos wasn’t the only thing going on. Gravitational attraction between particles of matter—especially dark matter, which was not subject to the collisions with photons—led to clustering in small regions of space, where the density of mass grew a little higher than the surroundings.The push from photon collisions—known as radiation pressure—and the pull of gravity acted in opposite directions on the ordinary matter, which cosmologists perversely refer to as baryonic matter. That set up sound waves in the baryonic matter, exactly like sound in air, which is small fluctuations in pressure and density. The baryonic sound waves weren’t coherent sounds like human voices or music, but they were relatively huge, helping to shape the environment of the early Universe.
Cosmologists call these sound waves baryon acoustic oscillations (BAO). What’s interesting is that these waves had a characteristic size, set again by the balance between the forces of gravity and radiation pressure. As the Universe expanded, it cooled and grew less dense. About 400,000 years after the Big Bang, the density and temperature had fallen to the point where radiation decoupled from the baryons, making the Universe transparent and leaving matter to cluster by gravitation alone.
However, the sign of BAO was left in the distribution of atoms: regions where the density was still higher than average. Combined with the growing clusters of dark matter (which again weren’t affected by radiation pressure, and therefore could slowly build up by the action of gravity alone), these lumpy regions set the natural size for the distribution of galaxies that formed later. We can see the traces of BAO in the cosmic microwave background—the radiation left over from decoupling—but the really exciting aspects come from observing galaxies.
The face of BAOIn today’s Universe, galaxies aren’t spread uniformly around: they tend to cluster together and lie along filaments or large “walls”, leaving huge voids. Those are the modern remnants of BAO, so by studying the particular distribution of galaxies, we can perform archeology of the Universe before galaxies even formed.
That’s the mission of the Baryon Oscillation Spectroscopic Survey (BOSS), which I visited in October as part of my research for my book-in-progress, Back Roads, Dark Skies. BOSS looks for quasars, which are black holes actively spitting out huge amounts of light, and luminous red galaxies (LRGs), which as the name suggests are particularly bright and reddish from a spectral point of view. These objects are useful because they are bright, but also because astronomers can measure how far they are away from us, providing a three-dimensional reconstruction of their distribution in space. During its run (which is still ongoing), BOSS will map about 1.5 million galaxies out to a significant distance, and up to 160,000 quasars at even greater distances.BOSS uses the 2.5 meter Sloan telescope at the Apache Point Observatory in New Mexico, which I must say is the ugliest telescope I’ve ever seen. Unlike many telescopes, the instrument was built without a dome: it stands on a pier jutting from the side of the mountain. When it’s not in use, the telescope is covered by a metal shed. To protect it from the wind, the whole telescope is swathed in metal baffles, now battered and pitted from long use, so it looks more like a piece from a farming museum than a scientific instrument.
However, ugly signifies nothing: function is all. The Sloan telescope, both through its work on BOSS and the earlier Sloan Digital Sky Survey (SDSS), is the most important telescope in cosmology you (probably) haven’t heard of. By mapping the positions and distances to huge numbers of galaxies, the telescope has enabled cosmologists to get a clear picture of the evolution of structure in the Universe.
BAO happened long ago, but the past affects the present in cosmology, as it does with human history. By tracing the positions of modern galaxies, we can reconstruct a view of the Universe long before galaxies even formed.