Probing the properties of relic neutrinos using the cosmic microwave background, the Hubble Space Telescope and galaxy clusters

TL;DR

This work tests relic neutrino properties within an extended ΛCDM framework by jointly analyzing Planck 2015 CMB data, BAO, HST $H_0$, and galaxy cluster abundance via $S_8$. It compares two models: Model I with $N_eff$, $\sum m_ν$, $c^2_{eff}$, $c^2_{vis}$ and $ξ_ν$, and Model II with $c^2_{eff}=c^2_{vis}=1/3$, constraining the CNB parameters using a CLASS/MontePython MCMC approach. The results show a mild excess in $ΔN_eff$ (roughly $0.36$–$0.61$ depending on GC inclusion) and small deviations in $c^2_{vis}$, while $ξ_ν$ remains consistent with zero; the inclusion of GC data significantly relaxes the $∑ m_ν$ bounds and nudges $H_0$ toward $70$, with persistent $\sigma_8$ tension. Overall, the data allow limited extra radiation compatible with dark radiation scenarios, but GC systematics may account for part of the signal, and no strong evidence for leptonic asymmetry is found.

Abstract

We investigate the observational constraints on the cosmic neutrino background (CNB) given by the extended $Λ$CDM scenario ($Λ$CDM $+ N_{\rm eff} + \sum m_ν + c^2_{\rm eff} + c^2_{\rm vis} + ξ_ν$) using the latest observational data from \textit{Planck} CMB (temperature power spectrum, low-polarisation and lensing reconstruction), baryon acoustic oscillations (BAOs) , the new recent local value of the Hubble constant from\textit{ Hubble Space Telescope} (\textit{HST}) and information of the abundance of galaxy clusters (GCs). We study the constraints on the CNB background using CMB + BAO + \textit{HST} data with and without the GC data. We find $ΔN_{\rm eff} = 0.614 \pm 0.26$ at 68 per cent confidence level when the GC data are added in the analysis. We do not find significant deviation for sound speed in the CNB rest frame. We also analyze the particular case $Λ$CDM $+ N_{\rm eff} + \sum m_ν + ξ_ν$ with the observational data. Within this scenario, we find $ΔN_{\rm eff} = 0.60 \pm 0.28$ at 68 per cent confidence level. In both the scenarios, no mean deviations are found for the degeneracy parameter.

Figures (5)

• Figure 1: One-dimensional marginalized distribution and 68 and 95 per cent CLs regions for some selected parameters of the Model I.
• Figure 2: One-dimensional marginalized distribution and 68 per cent CL and 95 per cent CL regions for some selected parameters of the Model II.
• Figure 3: The likelihoods of the parameter $H_0$ for Model I (left panel) and Model II (right panel), in red (CMB + BAO + HST) and green (CMB + BAO + HST + GC).
• Figure 4: Neutrino density perturbations as a function of scale factor for three fixed scales, $k =$ 0.001 Mpc$^{-1}$, 0.01 Mpc$^{-1}$ and 0.1 Mpc$^{-1}$. In drawing the graphs we have taken the best fit values from our analysis. The continuous and dashed line represent the models I and II from CMB + BAO + HST + GC, respectively.
• Figure 5: CMB temperature power spectrum difference among the models investigated here and the six parameter $\Lambda$CDM model, i.e., $\Delta C_l/C_l = (C^{\Lambda CDM \, \, extended}_l - C^{\Lambda CDM \, \, Planck \, \, 2015}_l)/C^{\Lambda CDM \, \, Planck \, \, 2015}_l$. The black continuous and dotted lines represent the Model II from CMB + BAO + HST and CMB + BAO + HST + GC, respectively. The orange continuous and dotted lines represent the Model I from CMB + BAO + HSTand CMB + BAO + HST + GC, respectively. In drawing the graphs we have taken the best fit values from our analysis and Planck collaboration paper.