InterACTing dark radiation models after ACT

Abstract

In this work we assess the implications of the Atacama Cosmology Telescope DR6 measurements for two interacting dark radiation scenarios previously shown to mitigate the Hubble tension. The first model, Wess-Zumino dark radiation (WZDR), features a mass threshold in the dark sector that induces a step-like reduction in the dark radiation abundance as the dark temperature evolves. The second model, new atomic dark matter (nuADaM), introduces dark radiation that remains coupled to a subcomponent of dark matter until shortly before matter-radiation equality. Earlier analyses using Planck data demonstrated that these interactions significantly relax constraints on the dark radiation density and allow values of $H_0$ consistent with local distance-ladder determinations. Incorporating ACT DR6, which extends CMB measurements deep into the high-$\ell$ damping tail, we find that constraints on the additional radiation component tighten substantially in both scenarios, closing most of the parameter space that previously enabled higher values of $H_0$. We further analyze a generalized model including both free-streaming and self-interacting dark radiation, and show that the resulting constraints are consistent with ACT's findings for the limiting cases of purely free-streaming or purely self-interacting radiation. Overall, ACT DR6 significantly restricts interacting dark radiation as a solution to the Hubble tension.

Figures (7)

• Figure 1: CMB residuals of SIDR compared to $\Lambda{\mathrm{CDM}}$, using the best fit points from a fit to Planck+SH0ES from Ref Aloni:2021eaq.
• Figure 2: Left figure:$67\%$ and $95\%$ CL for $H_0$ and $N_\text{idr}$ for WZDR (purple) and nuADaM fit to P-ACT-LB + $S_8$. Right figure:$67\%$ and $95\%$ CL for $H_0$ and $N_\text{idr}$ for WZDR (purple) and nuADaM fit to P-ACT-LB + $S_8$ + SH0ES. The gray band correspond to the $1\sigma$ and $2 \sigma$ regions for $H_0$ from the SH0ES collaboration Riess:2021jrx.
• Figure 3: CMB residuals of WZDR and NuADaM compared to $\Lambda{\mathrm{CDM}}$, using the best fit points in Table \ref{['tab:best-fit']}.
• Figure 4: WZDR (left) and nuADaM (right) fit to the full $\ell_\mathrm{max}=6500$ range of ACT DR6 in P-ACT-LBS and the truncated P-ACT-LBS$|_{\ell<3000}$.
• Figure 5: Generalized dark radiation model containing a free-streaming component (which include the SM neutrinos) parametrized by $N_{\mathrm{eff}}$ and a self-interacting componenet parametrized by $N_{\rm idr}$. The total amount of energy density in radiation is parametrized by $N_{\rm tot} = N_{\rm eff} + N_{\rm idr}$.