(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.
Small variations in density in the early Universe, known as baryon acoustic oscillations, grew to produce the distribution of galaxies we see today. [Credit: Chris Blake/Sam Moorfield]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 ...