Or equivalently, how long was the recombination period? Was it a fraction of a second? Millions of years?

  • $\begingroup$ according to wikipeida en.wikipedia.org/wiki/… 380,000 years after the Big Bang $\endgroup$
    – Jannick
    Jun 12, 2018 at 9:35
  • $\begingroup$ That's the date. I'm asking about the duration. $\endgroup$
    – Asmani
    Jun 12, 2018 at 9:36

2 Answers 2


The answer depends on the adopted baryon/photon ratio, the time-dependence of temperature in the expanding universe and what you define to be the ionisation fractions at the beginning and end of the (re)combination epoch.

As the universe cools, first the helium ions recombine, followed by the hydrogen ions (protons).

The time to go from a hydrogen ionisation fraction of 90% to say 10% is around 70,000 years, with a central epoch of around 300,000 years, for the usual $\Lambda$CDM model and a baryon to photon ratio of $6\times 10^{-10}$.


See Wikipedia: "Cosmic Dark Age" and "Recombination, photon decoupling, and the cosmic microwave background (CMB)" or "A more detailed summary":

"Recombination, photon decoupling, and the cosmic microwave background (CMB)":

"This period, known as the Dark Ages, began around 377,000 years after the Big Bang. During the Dark Ages, the temperature of the universe cooled from some 4000 K down to about 60 K, and only two sources of photons existed: the photons released during recombination/decoupling (as neutral hydrogen atoms formed), which we can still detect today as the cosmic microwave background (CMB), and photons occasionally released by neutral hydrogen atoms, known as the 21 cm spin line of neutral hydrogen.


Structures may have begun to emerge from around 150 million years, and stars and early galaxies gradually emerged from around 400 to 700 million years. As they emerged, the Dark Ages gradually ended. Because this process was gradual, the Dark Ages only fully ended around 1 billion (1000 million) years, as the universe took its present appearance.".

A non-Wikipedia explanation: "Lecture 31: The Cosmic Microwave Background Radiation" or Hyperphysic's WMAP webpage "Age of the Universe ".


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