Recombination and Decoupling: Origins of Cosmic Microwave Background

This lecture discusses the processes of recombination and decoupling of photons, which are fundamental to understanding the origin of the cosmic microwave background (CMB). The instructor begins by reviewing previous topics, including the Boltzmann equation and collision integrals, before introducing the concept of thermal equilibrium and the significance of the relaxation time approximation. The lecture emphasizes the transition from a highly ionized universe to a neutral state, where photons can travel freely. The instructor explains the terminology surrounding recombination, clarifying that it refers to the formation of neutral atoms from protons and electrons. The discussion includes the conditions under which the universe becomes transparent to photons, leading to the last scattering surface. The lecture also covers the role of neutrinos in this context, highlighting their weak interactions and thermalization in the early universe. The instructor concludes by discussing the implications of these processes for our understanding of cosmic evolution and the measurements of the CMB.