<sod>The CMB gives a snapshot of the [[universe]] when, according to standard cosmology, the temperature dropped enough to allow [[electron]]s and [[proton]]s to form [[hydrogen]] atoms, thereby making the universe nearly transparent to radiation because light was no longer being [[Thomson scattering|scattered]] off free electrons. When it originated some 380,000 years after the Big Bang—this time is generally known as the "time of last scattering" or the period of [[recombination (cosmology)|recombination]] or [[Decoupling (cosmology)|decoupling]]—the temperature of the universe was about 3000&nbsp;K. This corresponds to an energy of about 0.25&nbsp;[[electronvolt|eV]], which is much less than the 13.6 eV ionization energy of hydrogen.<ref>{{cite journal|last=Brandenberger|first=Robert H.|date=1995|title=Formation of Structure in the Universe|bibcode=1995astro.ph..8159B|pages=8159|arxiv=astro-ph/9508159}}</ref><eod>
<sod>The CMB gives a snapshot of the [[universe]] when, according to standard cosmology, the temperature dropped enough to allow [[electron]]s and [[proton]]s to form [[hydrogen]] atoms, thereby making the universe nearly transparent to radiation because light was no longer being [[Thomson scattering|scattered]] off free electrons. When it originated some 380,000 years after the Big Bang—this time is generally known as the "time of last scattering" or the period of [[recombination (cosmology)|recombination]] or [[Decoupling (cosmology)|decoupling]]—the temperature of the universe was about 3000&nbsp;K. This corresponds to an energy of about 0.25&nbsp;[[electronvolt|eV]], which is much less than the 13.6 eV ionization energy of hydrogen.<ref>{{cite journal|last=Brandenberger|first=Robert H.|date=1995|title=Formation of Structure in the Universe|bibcode=1995astro.ph..8159B|pages=8159|arxiv=astro-ph/9508159}}</ref> As seen from the present day viewpoint of an expanded universe, this moment in time forms a spherical perimeter termed the "last scattering surface".<eod>