Cosmological Constraints on Secluded Dark Radiation

TL;DR

This work systematically investigates secluded dark radiation (DR) in cosmology, classifying DR by self-interaction strength into four regimes (free-streaming, coupled, decoupling, recoupling) and exploring both adiabatic and DR isocurvature initial conditions while varying neutrino energy density and DR density. By modifying CLASS and performing extensive MCMC analyses with Planck, BAO, SPT-3G, Pantheon+, and SH0ES data, the paper demonstrates that current data show no strong preference for new DR physics beyond ΛCDM but reveal notable degeneracies among DR and neutrino parameters, with massless DR often favored in some scenarios and DRID enabling higher $N_{ m eff}$. DR affects the CMB through changes in the expansion rate, sound horizon, damping scale, and lensing, with distinct signatures for each DR type; isocurvature DRID spectra tend to be blue-tilted and can modestly improve fits in certain combinations. Importantly, while DR interactions and DRID can modestly ease the $H_0$ tension, none of the secluded DR models fully resolves it, though the best-case scenario reduces the tension from $ m ~5.6σ$ to about $ m ~3.5σ$, highlighting a nuanced interplay between DR properties and cosmological observables.

Abstract

Dark radiation (DR) is ubiquitous in physics beyond the Standard Model (SM), and its interactions with the SM and dark matter (DM) lead to a variety of interesting effects on cosmological observables. However, even in scenarios where DR is 'secluded', i.e., only gravitationally interacting with SM and DM, it can leave discernible signatures. We present a comprehensive study of four different types of DR: free-streaming, self-interacting (coupled), decoupling, and recoupling DR, and vary initial conditions to include both adiabatic and isocurvature perturbations. In addition to these properties, we also vary neutrino energy density, DR energy density, and the SM neutrino masses to perform a general analysis and study degeneracies among neutrino and DR properties. We derive constraints using the cosmic microwave background, large-scale structure, and supernova datasets. We find no significant preference for physics beyond the $Λ$CDM model, but data exhibit interesting interplays between different physical quantities. When the neutrino energy density is allowed to vary, we find that the cosmological dataset prefers massless free-streaming DR over massive neutrinos, leading to a significant relaxation of the neutrino mass bound. Although we do not find any evidence of DR isocurvature, the data show support for a strong blue tilt of the isocurvature power spectrum. Our analysis also highlights the degeneracy of various DR parameters with the Hubble constant $H_0$ resulting in a mild relaxation of the $H_0$ tension.

Figures (13)

• Figure 1: The CMB temperature anisotropy power spectrum ($D^{\rm TT}_\ell\equiv T_0^2\ell(\ell+1)C_\ell^{\rm TT}/(2\pi)$) for different cases of DR scenarios: FDR, CDR, decoupling and recoupling DR. For each type of DR, we show the effects of adiabatic and isocurvature initial conditions, as well as massive neutrinos, comparing to the $\Lambda\rm CDM$ case. We also show data points from Planck 2018 Planck:2019nip and SPT-3G SPT-3G:2022hvq.
• Figure 2: The fractional difference of $C^{\rm TT}_\ell$ with respect to the adiabatic FDR case for different cases of DR scenarios with the same $\Delta N_{\rm eff}=0.4$. Therefore, the FDR line in the top left panel is flat. We show the effects of including isocurvature IC (top right and bottm left), as well as massive neutrinos (bottom right).
• Figure 3: The CMB lensing spectrum for different cases of DR scenarios: FDR, CDR, decoupling and recoupling DR. For each type of DR, we show the effects of adiabatic and isocurvature initial conditons, as well as massive neutrinos, comparing to the $\Lambda\rm CDM$ case. Data points are taken from Planck 2018 Planck:2018lbu.
• Figure 4: The fractional difference of $C^{\phi\phi}_L$ with respect to the adiabatic FDR case for different cases of DR scenarios with the same $\Delta N_{\rm eff}=0.4$. Here we show the effects of adiabatic and isocurvature IC, as well as massive neutrinos.
• Figure 5: Triangle plots for parameters for FDR (left) and CDR (right) with adiabatic perturbations. Constraints are noted at the upper corners of the 1D contours. The error bars are at $1\sigma$ and the upper limits are mentioned at $95\%$ confidence level (C.L.).