Cosmic microwave background temperature ($T_{\gamma }(z)=T_{\gamma ,0}(1+z)$)

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.