Cosmological Recombination: When the Universe Became Transparent
Map of the sky near the galactic south pole, as seen by the Planck satellite's bolometers.
380,000 years after the Big Bang, the Universe became visible as photons began to propagate freely: this is known as cosmological recombination.
Image source: ESA - Planck-HFI collaboration (new window).
What is Cosmological Recombination?
Recombination is the era when, about 380,000 years after the Big Bang (scientific model describing the origin and evolution of the Universe), the temperature of the Universe dropped below 3000 K. Free electrons were then able to bind to proton nuclei to form neutral hydrogen atoms. This process allowed photons, previously trapped in an opaque plasma, to travel freely: the Universe became transparent.
N.B.:
Neutral hydrogen refers to a hydrogen atom whose proton is bound to an electron, without being ionized. This gas, denoted H I, is the dominant constituent of the Universe after recombination.
Its abundance and distribution can be studied using the so-called "21 cm" emission, resulting from a hyperfine transition of the electron in the ground state.
Not to be confused with the Lyman-α line (121.6 nm), which corresponds to an electronic transition between levels \(n=2\) and \(n=1\).
A Turning Point for the Universe
This moment marked the release of the cosmic microwave background (fossil radiation witnessing the state of the primordial Universe) (CMB), first observed by Arno Penzias (1933-2024) and Robert Wilson (1936-) in 1965. Recombination was not instantaneous but gradual, spread over several tens of thousands of years.
Observational Consequences
Recombination now allows us to probe the young Universe. The small temperature fluctuations in the CMB, measured by the COBE, WMAP, and Planck satellites, are directly related to the anisotropies present at that time. They provide a wealth of information about the density of matter, the geometry of the Universe, and the initial conditions for galaxy formation.
N.B.:
The anisotropies of the cosmic microwave background refer to the tiny temperature variations (≈ 10-5) observed in the fossil radiation.
They reflect local differences in density and velocity in the primordial plasma before recombination. These inhomogeneities served as gravitational seeds for the formation of galaxies and large-scale structures. Their precise mapping constitutes one of the finest pieces of evidence for the Big Bang model.
| Event | Period (time after Big Bang) | Physical Consequence | Comment |
|---|---|---|---|
| Formation of hydrogen atoms | ≈ 380,000 years | Neutralization of plasma | Electrons bind to protons |
| Decoupling of photons | ≈ 380,000 - 400,000 years | Light free to travel | Origin of the cosmic microwave background |
| Observation of CMB by COBE | 1992 (COBE satellite) | Detection of anisotropies | Confirmation of the Big Bang theory |
| High-precision mapping | 2003 (WMAP), 2009-2013 (Planck) | Precise measurement of fluctuations | Most precise cosmological data to date |
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