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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.

After subtracting the mean, the motion-induced dipole and galactic foregrounds, intrinsic Cosmic microwave background anisotropy appears as angular hot and cold patches, typically at the $10^{-5}$ fractional scale. These spatial temperature fluctuations encode early density and velocity structure. The displayed patch pattern is illustrative, with no claim to reproduce an observed sky map.