Epoch of Reionization
The Epoch of Reionization (EoR) is a pivotal phase in cosmic history, spanning roughly 150 million to 1 billion years after the Big Bang (redshifts z ≈ 6–20), during which the first stars and quasars emitted enough ultraviolet radiation to ionize the neutral hydrogen that pervaded the universe after recombination. This process transformed the intergalactic medium from a neutral, opaque state back into the fully ionized, transparent plasma we observe today. Understanding the EoR is one of the central goals of modern observational cosmology, as it marks the emergence of the large-scale structure of the universe and the first generation of galaxies.
Observational Probes of the Epoch of Reionization
| Probe / Method | Wavelength | Redshift Range | What It Measures |
|---|---|---|---|
| 21-cm hydrogen line | Radio (~21 cm) | z = 6–20 | Neutral H distribution |
| Lyman-alpha forest | UV/Optical | z = 5–7 | IGM opacity to Lyman-alpha |
| Quasar spectra (Gunn-Peterson trough) | Optical | z > 6 | Neutral H fraction |
| CMB polarization (τ) | Microwave | Integrated | Optical depth to reionization |
| High-z galaxy surveys (JWST) | Infrared | z = 6–15 | Early galaxy UV luminosity |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Cosmic Dark Ages
The Cosmic Dark Ages refer to the period in the early universe from approximately 380,000 to 150 million years after the Big Bang, during which the universe was filled with neutral hydrogen gas and no stars or galaxies had yet formed to illuminate it. During this era, the universe was opaque to most forms of electromagnetic radiation except for the cosmic microwave background (CMB), which had just been released during recombination. This period ended when the first stars and galaxies formed, producing ultraviolet radiation that began reionizing the neutral hydrogen — an event known as the Epoch of Reionization.
Cosmic Microwave Background
The Cosmic Microwave Background (CMB) is the thermal electromagnetic radiation permeating the entire observable universe, representing the afterglow of light released approximately 380,000 years after the Big Bang when the universe cooled enough for protons and electrons to combine into neutral hydrogen atoms. It is observed today as a nearly uniform blackbody radiation at a temperature of approximately 2.725 K, with tiny temperature fluctuations of about 1 part in 100,000 that encode the seeds of large-scale cosmic structure. The CMB is considered one of the strongest pieces of evidence for the Big Bang model and provides precise measurements of cosmological parameters.
Large-Scale Structure of Universe
The Large-Scale Structure (LSS) of the universe refers to the web-like spatial distribution of matter — including galaxies, galaxy clusters, filaments, walls, and voids — on scales from tens to hundreds of megaparsecs. Driven by gravitational collapse of primordial density fluctuations seeded during inflation, this cosmic web is the largest organized structure in existence. Mapping and modeling the LSS provides critical tests of cosmological models, measurements of dark matter and dark energy densities, and constraints on the neutrino mass.
From Latin epocha (from Greek epokhē, meaning "a fixed point in time") and "reionization" — the prefix re- (Latin, meaning "again") combined with ionization, referring to the process of stripping electrons from atoms. The term reflects that hydrogen was first ionized in the hot early universe, neutralized during recombination, and then ionized again during this epoch.