# Cosmic Microwave Background as a thermal gas of SU(2) photons: Implications for the high-z cosmological model and the value of $H_0$

## TL;DR

This paper proposes a novel high-redshift cosmological model replacing dark matter with an SU(2) photon gas, addressing the Hubble constant tension by revising early universe physics and analyzing CMB data.

## Contribution

It introduces an SU(2) gauge principle for CMB photons, eliminating dark matter and resolving the Hubble tension through a new all-redshift cosmological framework.

## Key findings

- The Hubble tension increases from 3σ to 5σ when considering precise CMB measurements.
- A new interpolation model based on axion condensate vortices connects high and low redshift cosmology.
- The model's angular scale of the sound horizon at decoupling aligns with observations.

## Abstract

Presently, we are facing a 3$\sigma$ tension in the most basic cosmological parameter -- the Hubble constant $H_0$. This tension arises when fitting the Lambda-cold-dark-matter model ($\Lambda$CDM) to the high-precision temperature-temperature (TT) power spectrum of the Cosmic Microwave Background (CMB) and to local cosmological observations. We propose a resolution of this problem by postulating that the thermal photon gas of the CMB obeys an SU(2) rather than U(1) gauge principle, suggesting a high-$z$ cosmological model which is void of dark matter. Observationally, we rely on precise low-frequency intensity measurements in the CMB spectrum and on a recent model independent (low-$z$) extraction of the relation between the comoving sound horizon $r_s$ at the end of the baryon drag epoch and $H_0$ ($r_s H_0 = \text{const}$). We point out that the commonly employed condition for baryon-velocity freeze-out is imprecise, judged by a careful inspection of the formal solution to the associated Euler equation. As a consequence, the above mentioned 3$\sigma$ tension actually transforms into a 5$\sigma$ discrepancy. To make contact with successful low-$z$ $\Lambda$CDM cosmology we propose an interpolation based on percolated/depercolated vortices of a Planck-scale axion condensate. For a first consistency test of such an all-$z$ model we compute the angular scale of the sound horizon at photon decoupling.