"The size of the largest of these ripples were set by the speed at which a soundwave could travel through space's plasma. This size came out to be around 500 million light-years and, once the plasma had cooled down, left behind vast 3D ripple imprints of sorts.
As the universe aged further, galaxies began to form at density peaks with tremendously wide bubble-like cocoons. That means observing these bubbles via the distributions of galaxies could ultimately help reveal the properties of BAOs. "
The plasma was so dense that it had sound waves?