00 · IN THREE MINUTES
The answer in three steps
- 1The early universe was a hot ionized plasma that scattered light repeatedly.
- 2Expansion cooled it until neutral atoms could form in large numbers.
- 3With far fewer free electrons, photons decoupled and began the long journey we observe as the cosmic microwave background.
01 · PLASMA MADE THE UNIVERSE FOGGY
Plasma made the universe foggy
Free electrons scatter photons efficiently. In the early dense plasma, light repeatedly changed direction and could not travel freely across cosmic distances.
02 · COOLING CHANGED THE CHARGE STATE
Cooling changed the charge state
Expansion lowered the temperature enough for electrons to remain bound mainly to hydrogen and helium nuclei. Cosmologists call this even though it was the first lasting combination.
Plasma made the universe foggy
Free electrons scatter photons efficiently. In the early dense plasma, light repeatedly changed direction and could not travel freely across cosmic distances.
ionized plasmaCooling changed the charge state
Expansion lowered the temperature enough for electrons to remain bound mainly to hydrogen and helium nuclei. Cosmologists call this recombination even though it was the first lasting combination.
atom formationLast scattering was a transition
Transparency did not switch on everywhere at one instant. As the free-electron fraction fell, the mean distance a photon could travel grew rapidly, producing a finite last-scattering surface.
photon decouplingThe afterglow stretched into microwaves
Those photons were emitted at much shorter wavelengths. Cosmic expansion stretched them by more than a thousandfold, so the relic radiation now has a near-perfect 2.7-kelvin microwave spectrum.
2.7 K today03 · LAST SCATTERING WAS A TRANSITION
Last scattering was a transition
Transparency did not switch on everywhere at one instant. As the free-electron fraction fell, the mean distance a photon could travel grew rapidly, producing a finite last-scattering surface.
04 · THE AFTERGLOW STRETCHED INTO MICROWAVES
The afterglow stretched into microwaves
Those photons were emitted at much shorter wavelengths. Cosmic expansion stretched them by more than a thousandfold, so the relic radiation now has a near-perfect 2.7-kelvin microwave spectrum.
05 · TRANSPARENCY PRECEDED THE FIRST STARS
Transparency preceded the first stars
A neutral cosmic dark age followed. Much later, the first luminous sources reionized intergalactic gas; recombination and reionization are different transitions.
06 · SOURCES AND EVIDENCE
Sources and evidence
The explanation follows the evidence chain below. Links open the original source.
- 01The History of the UniverseINSTITUTIONAL EXPLAINER ↗
Supports a specific claim or evidence boundary used in this article.
- 02Planck 2018 results. VI. Cosmological parametersPRIMARY STUDY ↗
Supports a specific claim or evidence boundary used in this article.
- 03The Cosmic Microwave BackgroundOFFICIAL DATA / TECHNICAL REPORT ↗
Supports a specific claim or evidence boundary used in this article.