Professor, Department of Astronomy and Astrophysics
University of Chicago

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Under Pressure

Key Concepts

The photon-baryon fluid, like a gas, has pressure.  In this case, the pressure is supplied by the radiation itself.

If you press on a gas it will compress, but gas pressure will fight you and eventually reverse the compression.  It will overshoot its original size and rarefy the medium until pressure from the surrounding gas reverses the motion again.  The result is an oscillating sequence of compressions and rarefactions.

By analogy to the process in air where a travelling compressional wave is perceived as sound, we call these oscillations in the photon-baryon fluid sound waves or acoustic oscillations.