Tests of the CMB temperature-redshift relation, CMB spectral distortions and why adiabatic photon production is hard

Understand

In the expanding Universe, the average temperature of the cosmic microwave background (CMB) is expected to depend like TCMB~(1+z) on redshift z.

• Adiabatic photon production (or destruction) or deviations from isotropy and homogeneity could modify this scaling and several observational tests have been carried out in response.
• Here, we explain why `adiabatic' conditions are extremely difficult to establish in the redshift range targeted by these tests.
• Thus, instead of leading to a simple rescaling of the CMB temperature, a spectral distortion should be produced, which can be constrained using COBE/FIRAS.