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Adiabatic expansion preserves the blackbody form while each photon's energy redshifts as $a^{-1}$. Thus the cosmic microwave background temperature obeys $T(z)=T_0(1+z)$, giving

Instrument overview of FIRAS, describing its differential Michelson interferometer, internal reference and external calibrated blackbody. It reports the absolute cosmic microwave background temperature inferred from the spectral comparison.