What can CMB observations tell us about the neutrino distribution function?

Abstract

Cosmic Microwave Background (CMB) observations have been used extensively to constrain key properties of neutrinos, such as their mass. However, these inferences are typically dependent on assumptions about the cosmological model, and in particular upon the distribution function of neutrinos in the early Universe. In this paper, we aim to assess the full extent to which CMB experiments are sensitive to the shape of the neutrino distribution. We demonstrate that Planck and CMB-S4-like experiments have no prospects for detecting particular features in the distribution function. Consequently, we take a general approach and marginalise completely over the form of the neutrino distribution to derive constraints on the relativistic and non-relativistic neutrino energy densities, characterised by $N_{\mathrm{eff}}=3.0\pm 0.4$ and ${\rho }_{\nu ,0}^{\mathrm{NR}}<14\mathrm{eV}{\mathrm{cm}}^{-3}$ at 95% CL, respectively. The fact that these are the only neutrino properties that CMB data can constrain has important implications for neutrino mass limits from cosmology. Specifically, in contrast to the $\Lambda$CDM case where CMB and BAO data tightly constrain the sum of neutrinos masses to be $\sum m_{\nu }<0.12\mathrm{eV}$, we explicitly show that neutrino masses as large as $\sum m_{\nu }\sim 3\mathrm{eV}$ are perfectly consistent with this data. Importantly, for this to be the case, the neutrino number density should be suitably small such that the bound on ${\rho }_{\nu ,0}^{\mathrm{NR}}=\sum m_{\nu }{n}_{\nu ,0}$ is still satisfied. We conclude by giving an outlook on the opportunities that may arise from other complementary experimental probes, such as galaxy surveys, neutrino mass experiments and facilities designed to directly detect the cosmic neutrino background.

16 pages, 7 figures. Minor modifications, some references added. Matches published version in JCAP

References in corpus (17)

• The Clustering of the SDSS DR7 Main Galaxy Sample I: A 4 per cent Distance Measure at z=0.15
• The DESI Experiment Part I: Science,Targeting, and Survey Design
• Massive neutrinos and cosmology
• The Cosmic Linear Anisotropy Solving System (CLASS) I: Overview
• The effects of He I 10830 on helium abundance determinations
• A precision calculation of relic neutrino decoupling
• Combined analysis of the integrated Sachs-Wolfe effect and cosmological implications
• Determining the neutrino mass with Cyclotron Radiation Emission Spectroscopy - Project 8
• Neutrino physics from precision cosmology
• Primordial Deuterium after LUNA: concordances and error budget
• Structure formation with massive neutrinos: going beyond linear theory
• Determination of the primordial helium abundance based on NGC 346 an HII region of the Small Magellanic Cloud
• Improved constraints on lepton asymmetry from the cosmic microwave background
• Cosmological Constraints on Light (but Massive) Relics
• How to relax the cosmological neutrino mass bound
• Cosmic Neutrino Background Detection in Large-Neutrino-Mass Cosmologies
• Fermi-Boltzmann statistics of neutrinos and relativistic effective degrees of freedom

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• Unveiling Neutrino Halos with CMB Lensing
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• Dynamics of metastable Standard Model particles from long-lived particle decays in the MeV primordial plasma
• Cosmic Neutrino Decoupling and its Observable Imprints: Insights from Entropic-Dual Transport
• Primordial neutrinos fade to gray: constraints from cosmological observables
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• Neutrino decays as a natural explanation of the neutrino mass tension
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