[Paper Review] Atomic Dark Matter, Interacting Dark Radiation, and the Hubble Tension

Manuel A. Buen-Abad, Zackaria Chacko+4 more|arXiv (Cornell University)|Nov 12, 2024

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper proposes a new dark sector model, nuADaM (New Atomic Dark Matter), that resolves the Hubble tension by introducing a tightly coupled fluid of interacting dark radiation (DR) and dark matter (DM) subcomponents. The DM forms dark atoms that decouple from DR after dark recombination, allowing early-time acoustic oscillations while preserving self-interactions in DR, leading to a significantly better fit to cosmological data than ΛCDM or conventional atomic DM models.

ABSTRACT

We present a new class of interacting dark sector models that can address the Hubble tension. Interacting dark radiation (DR) has previously been put forward as a solution to the problem, but this proposal is disfavored by the high-$\ell$ cosmic microwave background (CMB) data. We modify this basic framework by introducing a subcomponent of dark matter (DM) that interacts strongly with the DR, so that together they constitute a tightly coupled fluid at early times. We show that if this subcomponent decouples from the interacting DR during the CMB epoch, the $\ell$ modes of the CMB that entered the horizon before decoupling are impacted differently from those that entered after, allowing a solution to the problem. We present a model that realizes this framework, which we dub "New Atomic Dark Matter", or nuADaM, in which the interacting dark matter (iDM) subcomponent is composed of dark atoms, and dark "neutrinos" with long-range interactions contribute to the DR, hence the name of the model. This iDM subcomponent is acoustic at early times but decouples from the DR following dark recombination. In contrast to conventional atomic dark matter (ADM) models, the dark photon is part of a richer DR sector, which ensures that it continues to be self-interacting even after recombination. We show that this model admits a fit to the available cosmological data that is significantly better than both $Λ$CDM and conventional ADM.

Motivation & Objective

• To address the Hubble tension, a persistent discrepancy between early-universe (Planck) and late-universe (SH0ES) measurements of H₀.
• To overcome the limitations of previous interacting dark radiation (DR) models, which are disfavored by high-ℓ CMB data due to excessive Silk damping.
• To develop a dark sector model where dark matter and DR form a tightly coupled fluid early on, but decouple during the CMB epoch, enabling differential impact on CMB modes.
• To construct a viable model—nuADaM—where dark atoms constitute the DM subcomponent and long-range interactions from dark neutrinos sustain DR self-interactions post-recombination.
• To demonstrate that this model provides a significantly better fit to combined cosmological datasets than ΛCDM or standard atomic dark matter (ADM) models.

Proposed method

• Introduce a subcomponent of dark matter (iDM) that strongly interacts with interacting dark radiation (DR), forming a tightly coupled fluid during the CMB epoch.
• Model the iDM as dark atoms formed via long-range interactions, ensuring acoustic behavior at early times.
• Implement a two-stage decoupling: the iDM subcomponent decouples from DR after dark recombination, breaking the tight coupling and altering the CMB power spectrum differently for modes entering the horizon before and after decoupling.
• Include dark 'neutrinos' with long-range interactions to form a self-interacting DR sector, preventing DR from becoming free-streaming after recombination.
• Use cosmological parameter inference with Planck CMB, BAO, and supernova data to test the model against ΛCDM and conventional ADM.
• Perform Bayesian model comparison using ΔBIC to quantify the improvement in fit, showing that nuADaM outperforms both ΛCDM and ADM.

Experimental results

Research questions

• RQ1 Can a dark sector model with a tightly coupled fluid of interacting dark radiation and dark matter subcomponents resolve the Hubble tension while remaining consistent with high-ℓ CMB data?
• RQ2 How does the decoupling of a dark matter subcomponent from interacting dark radiation during the CMB epoch affect the CMB power spectrum differently for early- and late-horizon-crossing modes?
• RQ3 Can a self-interacting DR sector be maintained after recombination if the dark matter component is atomic and decouples from DR?
• RQ4 Does a model with dark atoms and long-range interacting dark neutrinos provide a better fit to cosmological data than ΛCDM or conventional atomic dark matter models?
• RQ5 What is the quantitative improvement in Bayesian evidence for the nuADaM model compared to ΛCDM and ADM when fitted to combined Planck, BAO, and supernova datasets?

Key findings

• The nuADaM model achieves a significantly better fit to cosmological data than both ΛCDM and conventional atomic dark matter (ADM) models, as evidenced by Bayesian information criteria (ΔBIC).
• The model reduces the Hubble tension by decreasing the sound horizon at recombination through enhanced energy density during the CMB epoch, consistent with the required increase in H near recombination.
• The dark matter subcomponent (dark atoms) decouples from the interacting DR after dark recombination, allowing early acoustic oscillations while preserving DR self-interactions via long-range interactions from dark neutrinos.
• The model fits the Planck CMB high-ℓ data better than previous interacting DR models by avoiding excessive Silk damping, due to differential impact on CMB modes based on horizon entry time.
• Inferred parameters show that the dark matter fraction f_adm is consistent with a small but non-zero component, with m_e′/m_p′ ≈ 10^−3.8 to 10^−4.0, indicating a light dark sector.
• The model yields H₀ = 68.98⁻¹.³²⁺⁰.⁷⁶ km/s/Mpc (with dataset D) and H₀ = 72.49⁻⁰.⁸¹⁺⁰.⁷⁶ km/s/Mpc (with dataset DH), bringing it into better agreement with the SH0ES value of 73.04 ± 1.04 km/s/Mpc.