# Basic explanation of baryon acoustic oscillations; what if anything is actually oscillating?

Sci-News' Sloan Digital Sky Survey Collaboration Releases New 3D Map of Universe links to the two videos

and says

The team’s results will be published in a series of 20 papers in the Monthly Notices of the Royal Astronomical Society.

After 01:30 in the second video Jiamin Hou from the Max Planck Institute for Extraterrestrial Physics says:

However the red shift has an additional component that is due to the galaxy’s own velocity, and which moves in response to the gravitational attraction of the surrounding matter, and these two components can not be separated from each other. But the statistical analysis of the eBOSS allows us to distinguish the effects of velocity from that of expansion.

I've also looked at:

but I can't get a grasp of what the term "baryon acoustic oscillations" really means.

Question: Is it possible to write or quote a basic explanation of what these are? For example, does oscillation mean there are things moving back and forth due to some restoring force? Is there something analogous to a "sound wave" out there somewhere? Or is there just a plot of something that shows a wiggly line that goes up and down?

• are there other tags that should be included? – uhoh Jul 22 '20 at 17:15

tl;dr nothing is oscillating now, but it was in the early universe. Now we just see the frozen relics

This is just an attempt at a simplified explanation of the Wikipedia article linked in the question.

There were pressure waves in the very early universe. They came from a balance of gravity and radiation pressure (the matter at the time was hot enough to be opaque). Whereever there happened to be a locally higher density (of matter, radiation and dark matter) the excess pressure would cause an expansion and then gravity would cause a contraction sending waves out at (according to the article) half the speed of light.

As the universe expanded and cooled, matter eventually became transparent (it was hydrogen and helium gas) letting the radiation pressure escape and leaving higher densities both in the location of the original concentration and at the crest of the wave a certain distance away (but the same certain distance for the first waves from each concentration). Those high densities are where the galaxies formed, so what we look for today is an excess of galaxies at a specific separation and that's what we see.

• Okay this is helpful thank you! In hot opaque matter is radiation pressure sufficient for pressure waves to exist and gravity just modifies the waves' behavior, or do you need both for the waves to exits? – uhoh Jul 23 '20 at 3:44
• @uhoh My understanding is that gravity was important (but I'm no expert). Since dark matter is affected by gravity but not by radiation pressure, the behaviour of the waves depends on the ratio of dark matter to baryonic matter. Observations are consistent with the ratio determined in other ways which is one of the reasons for Dark Matter being broadly accepted as real. – Steve Linton Jul 23 '20 at 9:19
• That's really helpful, thank you! If you think it can, maybe consider incorporating that into the answer post itself? – uhoh Jul 23 '20 at 11:48
• @uhoh I'll need to look for a source. – Steve Linton Jul 23 '20 at 15:24
• I just saw An Alternative to Dark Matter Passes Critical Test and found it quite interesting and the explanations understandable, including the discussion on "one of the reasons for Dark Matter being broadly accepted as real". It links to several recent papers in arXiv. – uhoh Aug 2 '20 at 8:49