DR. RISHI KHATRI

In the early Universe, before recombination, we have tightly coupled baryon-electron-photon plasma. 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. The speaker 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.