= Cosmic microwave background temperature
{title2=$T_\gamma(z)=T_{\gamma,0}(1+z)$}
The <blackbody> temperature of freely propagating <cosmic microwave background> photons scales inversely with the <scale factor>: cosmological redshifting changes every <frequency> by the same factor and preserves the <Planck spectrum> shape. Its present value is approximately $2.73\,\mathrm K$. Measuring that absolute spectral temperature is distinct from measuring <temperature fluctuations> about it.
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