Recombination -

380,000 Years

Astronomy & Cosmology -

The Early Universe

(or "What Banged in the Big Bang?")

The universe is filled with an opaque plasma of ionized hydrogen and helium.  That is, the positively charged atomic nuclei and negatively charged electrons cannot combine to form atoms as they are bombarded with energetic photons.  Once the temperature of the Universe cools down to around 9,000°K, the hydrogen and helium ions can capture electrons creating neutral atoms in a process called recombination.  Because the atoms are neutral, photons can no longer interact strongly with the nuclei, they become decoupled and begin to move freely, resulting in a transparent Universe.  The expansion of the fabric of space since then has progressively lowered the wavelength of this photon background light to the microwave background radiation we see today at a temperature of approximately 2.7°K.

Starting from this recombination process, the Universe enters the Dark Ages.  The Universe is transparent, but there is not much to see!  After recombination, the Universe would be relatively homogenous with small deviations.  The slightly denser areas would start to collapse under gravity, and after about 100 million years, the first population III stars and the quasars start to form.  This marks the start of the end of the Dark Ages, and the beginning of reionization.