Bose-Einstein condensation of CMB and damping of sound waves in the early Universe ("State of the Universe" seminar series)

In the early Universe, before recombination, we have tightly coupled
baryon-electron-photon plasma in the early Universe. The photons are
observed today in the microwave frequency range, the cosmic microwave
background (CMB).  Non-relativistic particles (baryons and electrons)
cool faster than radiation as the Universe expands. Compton
scattering, however, maintains photons and electrons at same
temperature by transferring energy from photons to electrons/baryons.
We also have standing sound waves in the plasma, excited by initial
spatial fluctuations in the energy density of matter/radiation. On
small scales, photons diffuse through the plasma, destroying the sound
waves of wavelength corresponding to the diffusion length. The energy
lost in the sound waves heats up the photons. I will discuss the
theoretical and observational consequences of this cooling and heating
of CMB and how it may provide a window into the high energy physics of
the early Universe.