Is there evidence for a hotter Universe?

TL;DR

This paper investigates whether the current temperature of the universe's CMB could be different enough to resolve the Hubble tension, finding no significant deviation from established measurements and suggesting new physics or better error understanding is needed.

Contribution

It provides an independent analysis of CMB temperature measurements to test the hypothesis that a hotter universe could solve the H0 tension.

Findings

Good agreement with FIRAS temperature measurement

Discrepancy of >1.9σ from the temperature needed to resolve H0 tension

Supports the need for new physics or better systematic error understanding

Abstract

The measurement of present-day temperature of the Cosmic Microwave Background (CMB), $T_0 = 2.72548 \pm 0.00057$ K (1$\sigma$), made by the Far-InfraRed Absolute Spectrophotometer (FIRAS), is one of the most precise measurements ever made in Cosmology. On the other hand, estimates of the Hubble Constant, $H_0$, obtained from measurements of the CMB temperature fluctuations assuming the standard $\Lambda$CDM model exhibit a large ($4.1\sigma$) tension when compared with low-redshift, model-independent observations. Recently, some authors argued that a slightly change in $T_0$ could alleviate or solve the $H_0$-tension problem. Here, we investigate evidence for a hotter or colder universe by performing an independent analysis from currently available temperature-redshift $T(z)$ measurements. Our analysis (parametric and non-parametric) shows a good agreement with the FIRAS measurement and a discrepancy of $\gtrsim 1.9\sigma$ from the $T_0$ values required to solve the $H_0$ tension. This result reinforces the idea that a solution of the $H_0$-tension problem in fact requires either a better understanding of the systematic errors on the $H_0$ measurements or new physics.