Professor, Department of Astronomy and Astrophysics
University of Chicago

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Gravitational Compression

Key Concepts

What is doing the pushing that compresses the fluid? Gravity.

The photon-baryon fluid is sitting in the gravitational potential wells that are the seeds of structure in the universe.  As gravity tries to compress the fluid, radiation pressure resists resulting in acoustic oscillations.  Because it acts to resist compression, we will represent the radiation pressure abstractly as springs.  Likewise we will represent the inertia of the fluid, or loosely speaking its mass (really energy density), as massive balls falling under gravity: