<p>We observe the temperature of the CMB as a ~2.7 K blackbody, but that's the redshifted version we observe. The CMB is also know as the "surface of last scattering" at the point of  <a href="http://en.wikipedia.org/wiki/Recombination_(cosmology)">recombination</a> when nuclei and electrons combined to form neutral atoms the universe went from opaque to transparent.</p>

<p>This happens at a <a href="http://ned.ipac.caltech.edu/level5/Sept02/Kinney/Kinney3.html">temperature</a> of ~3000 K. From this we can estimate the redshift (z~1100) of the CMB which corresponds to an age given our cosmology.</p>

CC BY-SA 3.0
2014-09-28T17:59:28.723Z
2014-09-28T17:59:28.723Z
Aaron
<p>It is said that the cosmic microwave radiation (CMB) was formed when the universe was 379,000 years old. How is this calculated?</p>

<cosmology><big-bang-theory><cosmic-microwave-background>
Formation of Cosmic Microwave Background
aphys
# Formation of Cosmic Microwave Background

(Asked by: username_0 on 2014-09-27)

It is said that the cosmic microwave radiation (CMB) was formed when the universe was 379,000 years old. How is this calculated?

**Tags:** cosmology, big-bang-theory, cosmic-microwave-background

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## 1 answers

### Answer by username_1 on 2014-09-28. Score: 5 | Accepted answer

We observe the temperature of the CMB as a ~2.7 K blackbody, but that's the redshifted version we observe. The CMB is also know as the "surface of last scattering" at the point of [recombination](http://en.wikipedia.org/wiki/Recombination_(cosmology)) when nuclei and electrons combined to form neutral atoms the universe went from opaque to transparent.

This happens at a [temperature](http://ned.ipac.caltech.edu/level5/Sept02/Kinney/Kinney3.html) of ~3000 K. From this we can estimate the redshift (z~1100) of the CMB which corresponds to an age given our cosmology.


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