Resilience and implications of adiabatic CMB cooling

We investigate potential deviations from the standard adiabatic evolution of the cosmic microwave background (CMB) temperature, TCMB(z), using the latest Sunyaev-Zeldovich (SZ) effect measurements and molecular line excitation data, covering a combined redshift range of 03 yield broader uncertainties. By combining both datasets, we find good consistency with the standard evolution across the full analysed redshift range, inferring a present-day CMB monopole temperature of T0=2.744±0.019 K. Next, we test for deviations from the standard scaling by adopting the parametrisation TCMB(z)=T0(1+z)1−β, where β quantifies departures from adiabaticity, with β=0 corresponding to the standard scenario. In this framework, we use Gaussian Process reconstruction to test the consistency of β=0 across the full redshift range and perform χ2 minimisation techniques to determine the best-fit values of T0 and β. In both cases, we find good consistency with the standard temperature-redshift relation. The χ2-minimisation analysis yields best-fit values of β=−0.0106±0.0124 and T0=2.7276±0.0095 K, in excellent agreement with both β=0 and independent direct measurements of T0 from FIRAS and ARCADE. We discuss the implications of our findings, which offer strong empirical support for the standard cosmological prediction and place tight constraints on a wide range of alternative scenarios of interest in the context of cosmological tensions and fundamental physics.