Recombination

Recombination refers to the event that occurred approximately 380,000 years after the Big Bang, when theuniverse had cooled sufficiently to allow free electrons to bind to atomic nuclei and form the first neutral atoms. This event transformed the universe from an opaque plasma into a transparent medium, releasing the photons that today make up the cosmic microwave background.

A Cooling Universe

Since the Big Bang, the expanding universe has been continuously cooling. During the first 380,000 years, the temperature was too high for neutral atoms to exist: thethermal energy immediately dissociated any electron that attempted to bind to a nucleus. The universe was a hot, dense plasma, completely opaque to light.

The Formation of the First Atoms

When the temperature dropped below about 3,000 K, the electrons no longer had enough thermal energy to resist the forces of electronic attraction. They gradually combined with protons to form neutral hydrogen atoms (about 75%) and helium nuclei to formhelium (about 25%). With no free electrons to scatter light, the universe suddenly became transparent.

Connection to the cosmic microwave background

The photons emitted at the time of recombination constitute the cosmic microwave background. These are the oldest photons we can observe. They bear the imprint of the conditions of the universe at 380,000 years: temperature, density, and primordial fluctuations. Analysis of this radiation is one of the richest sources of information about the earliest moments of the universe.

Did you know?

Despite its name, recombination is actually a first combination: electrons and nuclei had never bound together before in the history of the universe. The term has remained in use due to historical convention, although it is technically inaccurate.

Frequently Asked Questions

What is the difference between recombination and decoupling?

Recombination refers to the formation of the first neutral atoms. Decoupling refers to the moment when photons cease to interact with matter and propagate freely. The two are closely related and occur almost simultaneously, but they describe complementary aspects of the same event.

Was all the hydrogen formed during recombination?

Not entirely. Recombination is a gradual process that did not occur instantaneously. A fraction of the hydrogen remained ionized for some time. The reionization era, which occurred much later (after 150 millionyears), then reionized part of this neutral hydrogen under the influence of the first stars and galaxies.

Was the universe illuminated after recombination?

No. After recombination, the universe entered the Dark Ages: there were no stars yet, and the only light available was the cooling cosmic microwave background. The first stars formed about 200 million years after the Big Bang, marking the end of the Dark Ages.