Recombination

Recombination is the cosmological epoch, roughly 380,000 years after the Big Bang, when the expanding universe cooled enough for free electrons and protons to combine into neutral hydrogen atoms. Before recombination, the universe was a hot plasma in which abundant free electrons scattered photons constantly, rendering it opaque — a glowing fog. As the temperature dropped below about 3000 K (a redshift of z ≈ 1100), the photons no longer carried enough energy to keep hydrogen ionized, the free electrons were captured into atoms, and the universe abruptly became transparent.

The term is something of a misnomer: the electrons and protons were never combined before, so 'combination' would be more accurate, but the name has stuck from atomic physics. The transition was not instantaneous but occurred over a span of redshift, because the high photon-to-baryon ratio kept a small ionized fraction around until the temperature fell well below the naive ionization energy of hydrogen.

Recombination defines the surface of last scattering — the moment, the same everywhere, when each cosmic microwave background photon scattered for the final time before streaming freely to us. It marks the boundary of the observable universe in light: we can see the CMB emitted at recombination, but nothing earlier, because the pre-recombination universe was opaque. Information from before this epoch can reach us only through neutrinos or gravitational waves, which decoupled far earlier.

The physics of recombination was worked out in detail by Yakov Zel'dovich and collaborators and independently by Jim Peebles in 1968, refining the simple Saha-equilibrium picture to account for the non-equilibrium kinetics of hydrogen formation in an expanding universe.