Reionization

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Reionization is the last global transition in the history of the Universe, stretching from redshifts z ≈ 15 to z ≈ 6 (about 400–1000 million years after the Big Bang). Starting in a completely neutral medium left over from the recombination epoch (z ~ 1100), the process was driven by ultraviolet radiation from the first light sources—massive Population III stars, dwarf galaxies, and possibly accreting black holes. Ionized bubbles, growing and merging, filled almost the entire volume by z ≈ 6. The main evidence for reionization comes from polarization data of the cosmic microwave background (optical depth τ ≈ 0.054) and the nearly complete absorption of quasar radiation beyond redshift 6 (the Gunn–Peterson effect).

History

Modern ideas about reionization are based on the Big Bang theory (Georges Lemaître, 1927) and the discovery of the cosmic microwave background (Penzias and Wilson, 1965), confirming a hot beginning. In 2001, the Sloan Digital Sky Survey discovered the Gunn–Peterson effect: in the spectra of quasars with redshifts greater than 6, light was completely absorbed by neutral hydrogen. This indicated that reionization ended about 1 billion years after the Big Bang.

How it works

An ultraviolet photon with energy above 13.6 eV can knock an electron out of a hydrogen atom. The first stars emitted many such photons. Around each galaxy, a region of ionized gas formed—a growing bubble. Gradually, the bubbles expanded and merged until they filled the entire Universe, making it transparent to ultraviolet light.

💡 The James Webb Space Telescope can see the most distant galaxies whose light was emitted during the reionization epoch. It is now finding increasingly earlier objects, pushing back the boundary of the known 'dark ages'.

Cosmic reionization is the epochal transition of the intergalactic medium (IGM) from an almost completely neutral state to an ionized one. Ionizing radiation was supplied by the first galaxies and accreting black holes. Key photons are Lyman-continuum (energy > 13.6 eV). An important parameter is the fraction of such photons that can escape their parent galaxy (escape fraction). Reionization was inhomogeneous: in dense regions, it ended later. It is now believed that the main stage ended by z ≈ 6. Observations of the cosmic microwave background provide an independent estimate: Thomson scattering on free electrons creates a characteristic polarization signal and slightly suppresses anisotropy at small angular scales. The integrated optical depth τ = ∫ n_e σ_T dr is about 0.054.

History

The idea of secondary ionization of the Universe naturally follows from the hot Big Bang theory (G. Lemaître, 1927) and the discovery of the cosmic microwave background (A. Penzias and R. Wilson, 1965). A breakthrough observational result was obtained in 2001: in the spectra of distant quasars (z > 6) from the Sloan survey, the Gunn–Peterson effect was detected—almost complete absorption due to neutral hydrogen. The WMAP (2003) and Planck (2013) missions measured the Thomson optical depth, refining it from 0.17 to ~0.054. Hubble and later James Webb have been directly searching for reionization-era galaxies, gradually refining the picture.

How it works

At the heart is photoionization: H + γ → p + e. The ionization front is initially rarefied, but as density increases and recombinations become important, it becomes dense. Global dynamics are described by the equation for the ionized fraction Q: dQ/dt = (ṅ_ion / ⟨n_H⟩) – Q/t_rec, where ṅ_ion is the emission rate of ionizing photons, ⟨n_H⟩ is the mean hydrogen density, t_rec is the characteristic recombination time. The observed optical depth τ = c σ_T ∫ n_e(z) (1+z)⁻¹ H(z)⁻¹ dz determines the mean-weighted moment of reionization. Radio astronomy at the 21 cm line (projects LOFAR, HERA, Square Kilometre Array) promises to build a detailed 3D map of the process.

\tau = \int_{0}^{z_{rec}} \frac{c \sigma_T n_e(z)}{(1+z) H(z)} dz

τ — optical depth (dimensionless); c = 3×10⁸ m/s — speed of light; σ_T = 6.65×10⁻²⁹ m² — Thomson cross section; n_e(z) — free electron density as a function of redshift; H(z) = H₀√(Ω_m(1+z)³ + Ω_Λ) — Hubble parameter at redshift z, H₀ ≈ 70 km/s/Mpc; z_rec ≈ 1100 — redshift of recombination.

Open problems & fun facts

Identification of the main suppliers of ionizing photons: star-forming galaxies or accreting black holes.; Direct measurement of the Lyman-continuum escape fraction at z > 6.; Reconstruction of the spatiotemporal picture of reionization: when and how ionized bubbles merged.; The role of hypothetical first stars (Population III) and their initial mass function.; Contribution of dwarf galaxies too faint for direct observations.

💡 Reionization was the last global transformation of matter in the Universe. Since then, large-scale cosmic structure has not changed its phase state, and the Universe has only continued to expand, gradually accelerating.

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Reionization

History

How it works

History

How it works

History

How it works

Open problems & fun facts

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