InterACTing dark radiation models after ACT

TL;DR

This paper evaluates how recent ACT DR6 measurements impact two interacting dark radiation models that could resolve the Hubble tension, finding that constraints tighten and limit their viability.

Contribution

It provides the first comprehensive analysis of ACT DR6 data on interacting dark radiation models previously shown to alleviate the Hubble tension.

Findings

Constraints on dark radiation density are significantly tightened by ACT DR6.

Most of the parameter space allowing higher H0 values is now excluded.

The generalized model with both free-streaming and self-interacting dark radiation remains consistent with ACT findings.

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.