wolram said:
What sound wave was this and how did the scientists deduce it?
To set the stage, I first have to write a few words about the early universe.
Early on, the universe was much smaller than it is now, and the stuff in it was much more tightly packed than it is now. The universe was a hot, dense mixture of dark matter, neutrinos,and a plasma of neutrons, protons electrons, and photons. The photons had enough energy that they prevented the formation of atoms. The photons scattered around again an again off the electric charges, never managing to propagate very far with respect to their local patch of matter. Effectively, matter and photons were locked together.
About 380,000 years after the Big Bang, the universe had expanded and cooled enough that most photons had insufficient energy to stop electrons from becoming bound to protons. Atoms formed and photons stopped scattering and started streaming freely through the universe. Although this didn't happen instantaneously, the transition was fairly fast.
Back to the time before the transition. The mixture that made up the stuff in the universe was an *almost* homogeneous mixture. Some places were , however, slightly more dense than others. These place that were over-dense in everything, dark matter, protons, neutrons, photons.
Consider one such over-dense region. The pressure of the photons bouncing around prevents the region prevents from collapsing gravitationally. In fact, in such a region the photon pressure can be so high that the photons drive a spherical wave (ripple) of normal matter outwards (more than just normal cosmic expansion) from the centre of the region. Dark matter stays at the center, since photons don't interact with dark matter. This spherical sound wave in the early universe is like a circular ripple in a pond that propagates out from where a pebble plopped in, but this wave is driven by photons.
When 380,000 year transition occurs, matter and photons become unlocked, i.e., photons stream away freely and the spherical matter wave, without its driving force, stops propagating. It doesn't die completely in amplitude, it just becomes locked into the expansion of the universe.
So now we have an over-dense dark matter at the centre (remember, the photons didn't drive the dark matter), and a (very slightly) over-dense region of normal matter where the spherical wave froze into the fabric of the universe. The dark matter at the centre gravitationally attracts normal matter and the spherical ripple of normal matter gravitationally attracts dark matter. The centre becomes a region of above average galaxy formation and the spherical ripple become a region of (slightly) above average galaxy formation.
What is the distance between these regions of above average galaxy formation? 380,000 years ago, the ripple froze in position with respect to its center, and the expansion of the universe has since expanded this distance to about 500 million light-years.
This story is repeated for many over-dense regions, and so should be noticeable statistically in galaxy surveys. Universe models without dark matter make different (acoustic oscillation) predictions than universes with dark matter.
I have mentioned dark matter, but I have yet to mention the connection to dark energy. Maybe in another post. Also, if anyone is interested, I think I can find links to explanations of the above.
