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 $H_0$ 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-$\ell$ CMB polarization data. It seems at least plausible
that a solution may be provided by related models that distinguish between
high- and low-$\ell$ multipoles. When interactions of strongly-coupled
radiation are mediated by a force-carrier that becomes non-relativistic, 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-$\ell$ 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 "Wess-Zumino Dark Radiation" (WZDR) and find
that it provides an improved fit to the CMB alone, favoring larger values of
$H_0$. If supernovae measurements from the SH0ES collaboration are also
included in the analysis, the inferred value of $H_0$ 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 $H_0$ tension
and the best of those with a Lagrangian formulation.
