Resilience and implications of adiabatic CMB cooling

TL;DR

This study tests the standard adiabatic evolution of the CMB temperature across redshifts using SZ and molecular data, finding strong support for the standard model and tight constraints on deviations.

Contribution

It provides the first comprehensive, model-independent analysis of the CMB temperature evolution up to z~6, confirming the standard adiabatic scaling with improved precision.

Findings

CMB temperature evolution consistent with standard model

No significant deviation from adiabatic scaling ($eta \, \approx \, 0$)

Tight constraints on the temperature at present-day ($T_0$) and deviations ($\beta$)

Abstract

We investigate potential deviations from the standard adiabatic evolution of the cosmic microwave background (CMB) temperature, $T_{\rm CMB}(z)$, using the latest Sunyaev-Zeldovich (SZ) effect measurements and molecular line excitation data, covering a combined redshift range of $0 < z \lesssim 6$. We follow different approaches. First, we reconstruct the redshift evolution of $T_{\rm CMB}(z)$ in a model-independent way using Gaussian Process regression. The tightest constraints come from SZ measurements at $z < 1$, while molecular line data at $z > 3$ 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 $T_0 = 2.744 \pm 0.019$ K. Next, we test for deviations from the standard scaling by adopting the parameterisation $T_{\rm CMB}(z) = T_0(1+z)^{1-\beta}$, where $\beta$ quantifies departures from adiabaticity, with $\beta = 0$ corresponding to the standard scenario. In this framework, we use Gaussian Process reconstruction to test the consistency of $\beta = 0$ across the full redshift range and perform $\chi^2$ minimisation techniques to determine the best-fit values of $T_0$ and $\beta$. In both cases, we find good consistency with the standard temperature-redshift relation. The $\chi^2$-minimisation analysis yields best-fit values of $\beta = -0.0106 \pm 0.0124$ and $T_0 = 2.7276 \pm 0.0095$ K, in excellent agreement with both $\beta = 0$ and independent direct measurements of $T_0$ 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.