A step in the right direction? Analyzing the Wess Zumino Dark Radiation solution to the Hubble tension

Abstract

The Wess Zumino Dark Radiation (WZDR) model first proposed in [1] shows great promise as a well-motivated simple explanation of the Hubble tension between local and CMB-based measurements, reducing the tension from 4.8σ to around 2.7σ. In this work we investigate the assumptions made in the original proposal and confront the model with additional independent data sets. We show that the original assumptions can have an impact on the overall results but are usually well motivated. If one assumes that the abundance of the WZDR is already produced during times relevant for big bang nucleosynthesis, the ability of the model to ease the Hubble tension is restricted. We further demonstrate that the preference for negative Ω k observed in Planck data remains at a similar level as for the ΛCDM model, while the AL tension is slightly increased. Furthermore, the tension between Planck data for ℓ < 800 and ℓ ≥ 800 is significantly reduced for the WZDR model.The Planck-independent data sets show slightly more permissive bounds on the Hubble parameter, allowing the tension to be further reduced from 2.7σ to 2.1σ (CMB-independent) or 1.9σ (ACT+WMAP). However, no combination shows a large preference for the presence of WZDR. We also investigate whether additional dark radiation-dark matter interactions can help in easing the S 8 tension as well. Assuming all of the dark matter to be interacting and a temperature-independent scattering rate, we find that the CMB data are too restrictive on this additional component as to allow a significant decrease in the clustering.