Brief Thermal History

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Key Concepts:

• Ionization/Recombination
• Tightly-coupled photon-baryon fluid
• Compression results in acoustic oscillations or sound waves

Let's review the basic elements of the thermal history of the CMB.

Ionization/Recombination: Going backwards in time, the CMB photons get hotter and hotter until they can ionize hydrogen.

Photon-Baryon Fluid: Thomson scattering and electromagnetic interactions glue the photons to the baryons.

Sound Waves: Radiation pressure resists any attempt to compress the fluid setting up acoustic oscillations.