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.