Reionization returns some neutral cosmic gas to an ionized state after cosmological recombination. The resulting free electrons increase the cosmological optical depth through Thomson scattering and create a second contribution to the cosmological visibility function.
Ignoring velocities and evolving potentials, a thin reionization screen transmits the primary Cosmic microwave background anisotropy with amplitude and adds a direction-averaged source. A plane wave propagating a distance from recombination to the screen has monopole factor . This suppresses the rescattered source at , while coherent large-scale modes are unchanged to leading order.
The primary small-scale Cosmic microwave background power spectrum measures the amplitude , because reionization damps each temperature amplitude once. The unaffected large-scale temperature modes are few and limited by cosmic variance. Polarization and gravitational lensing provide additional information that partly breaks this degeneracy.
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Reionization is a key phase in the history of the universe, referring to the process through which the predominantly neutral hydrogen gas that filled the cosmos after the Big Bang becomes ionized, meaning that electrons are separated from protons. This transformation occurred roughly 400 million to 1 billion years after the Big Bang, marking the end of the "cosmic dark ages" and the beginning of the era where stars and galaxies started to form and light up the universe.