Sterile neutrinos with eV masses in cosmology -- how disfavoured exactly?

TL;DR

The paper investigates whether eV-mass sterile neutrinos, assumed fully thermalised before neutrino decoupling, can be reconciled with cosmological data. It analyzes precision CMB and LSS measurements within three frameworks: $ ull\Lambda$CDM with a sterile species, $ ull\Lambda$CDM+$\Delta N$ with extra relativistic degrees of freedom, and $w$CDM+$\Delta N$ with $w\neq -1$, using MCMC to infer parameter shifts and goodness-of-fit. It finds that in basic $ ull\Lambda$CDM, such sterile states worsen the hot-dark-matter bound and are disfavoured, while allowing additional radiation or a nonstandard $w$ can improve the fit; however, BBN strongly constrains the radiation content unless a neutrino chemical potential is invoked. The combined BBN+CMB+LSS analysis yields $N_{ m eff}=3.90^{+0.39}_{-0.56}$ at 95% and tight upper limits on the number of sterile species, implying that accommodating eV-mass sterile neutrinos requires substantial changes to other cosmological parameters (e.g., a higher $\omega_{\rm cdm}$) and potentially nontrivial lepton asymmetries. Future Planck measurements of $\Delta N_{ m ml}$ will be crucial to testing these scenarios.

Abstract

We study cosmological models that contain sterile neutrinos with eV-range masses as suggested by reactor and short-baseline oscillation data. We confront these models with both precision cosmological data (probing the CMB decoupling epoch) and light-element abundances (probing the BBN epoch). In the minimal LambdaCDM model, such sterile neutrinos are strongly disfavoured by current data because they contribute too much hot dark matter. However, if the cosmological framework is extended to include also additional relativistic degrees of freedom -- beyond the three standard neutrinos and the putative sterile neutrinos, then the hot dark matter constraint on the sterile states is considerably relaxed. A further improvement is achieved by allowing a dark energy equation of state parameter w<-1. While BBN strongly disfavours extra radiation beyond the assumed eV-mass sterile neutrino, this constraint can be circumvented by a small nu_e degeneracy. Any model containing eV-mass sterile neutrinos implies also strong modifications of other cosmological parameters. Notably, the inferred cold dark matter density can shift up by 20 to 75% relative to the standard LambdaCDM value.

Figures (6)

• Figure 1: 2D marginal 68%- and 95%-credible regions in the $(\Delta N_{\rm ml},\omega_{\rm cdm})$-plane for three $\Lambda$CDM+$\Delta N$ class models containing one thermalised sterile species of mass $m_{\rm s} = 0$ eV (solid), 1 eV (dashed), and 2 eV (dotted).
• Figure 2: 2D marginal 68%- and 95%-credible regions in the $(w,\omega_{\rm cdm})$-plane for three $w$CDM+$\Delta N$ class models containing one thermalised sterile species of mass $m_{\rm s} = 0$ eV (solid), 1 eV (dashed), and 2 eV (dotted).
• Figure 3: Primordial abundances of D, $^4$He and $^7$Li as functions of the cosmological parameters. Top left: dependence on $\omega_{\rm b}$ for $N_{\rm s} = \xi = 0$. Top right: dependence on $N_{\rm s}$ for $\omega_{\rm b} = 0.0225$ and $\xi = 0$. Bottom: dependence on $\xi$ for $\omega_{\rm b} = 0.0225$ and $N_{\rm s} = 0$.
• Figure 4: 2D marginal 90%- and 99%-credible regions in the $(\omega_{\rm b},N_{\rm s})$-plane, assuming the BBN+sterile scenario. Black lines denote results from elemental abundance measurements alone (solid lines assume $\tau_n = 878.5~{\rm s}$, dotted lines $885.7~{\rm s}$). The red lines include a CMB+LSS prior on $\omega_{\rm b}$. Top left: D data. Top right:$^4$He data. Bottom: D+$^4$He data.
• Figure 5: 1D posterior pdf for $N_{\rm s}$, marginalised over $\omega_{\rm b}$, for the BBN+sterile scenario and D+$^4$He data. Same colour/line coding as in figure \ref{['fig:bbn1']}.