# Evidence of CMB redshift

Are there any known spectral lines shifted by ~1100? If not, then how certain is mainstream that the CMB has a redshift of ~1100? All I see is a blackbody radiation curve void of spectral lines.

## 1 Answer

The redshift of the CMB is not measured, it is calculated.

The brief explanation is that as the universe expands and cools, it becomes energetically favourable to form bound atoms. The reduction in free electrons allows the universe to become transparent and photons escape as a blackbody radiation field and eventually form the CMB.

The temperature at recombination, $T$, can be calculated using well understood physics (see https://en.m.wikipedia.org/wiki/Recombination_(cosmology)) and if the current temperature $T_0$ is known, then the redshift is simply given by $z = T/T_0 -1$.

• What about the spectral lines OP asked for? Is it valid to say that there are none? Maybe also because the CMB's origin lies within a time frame where there were no atoms (but only plasma) to produce such spectral absorption lines? – Alfe Jul 30 '17 at 1:22
• @Alfe a blackbody spectrum is a continuum, with a peak at a wavelength that depends on temperature. No lines. – ProfRob Jul 31 '17 at 20:16
• A nice summary is also given at thecuriousastronomer.wordpress.com/2015/07/30/… (Also perhaps worth a look there: thecuriousastronomer.wordpress.com/2016/06/13/… ) – Jacob C. Sep 18 '18 at 19:26