A Step in understanding the Hubble tension

As cosmological data have improved, tensions have arisen. One such tension is the difference between the locally measured Hubble constant H0 and the value inferred from the cosmic microwave background (CMB). Interacting radiation has been suggested as a solution, but studies show that conventional models are precluded by high-if CMB polarization data. It seems at least plausible that a solution may be provided by related models that distinguish between high- and low-if multipoles. When interactions of stronglycoupled radiation are mediated by a force carrier that becomes nonrelativistic, the dark radiation undergoes a ???step??? in which its relative energy density increases as the mediator deposits its entropy into the lighter species. If this transition occurs while CMB-observable modes are inside the horizon, high- and low-if peaks are impacted differently, corresponding to modes that enter the horizon before or after the step. These dynamics are naturally packaged into the simplest supersymmetric theory, the Wess-Zumino model, with the mass of the scalar mediator near the eV scale. We investigate the cosmological signatures of such WessZumino dark radiation (WZDR) and find that it provides an improved fit to the CMB alone, favoring larger values of H0. If supernovae measurements from the SH0ES Collaboration are also included in the analysis, the inferred value of H0 is yet larger, but the preference for dark radiation and the location of the transition is left nearly unchanged. Utilizing a standardized set of measures, we compare to other models and find that WZDR is among the most successful at addressing the H0 tension and is the best of those with a Lagrangian formulation.

Automated summary

As cosmological data have improved, tensions have arisen, particularly regarding the difference between the locally measured Hubble constant H0 and the value inferred from the cosmic microwave background. Interacting radiation has been suggested as a solution, but conventional models are precluded by high polarization CMB data. It is possible that related models that distinguish between high- and low-multipole interactions may provide a solution.