Cosmic dark radiation and neutrinos

TL;DR

This work surveys cosmic dark radiation and neutrino physics in light of Planck data, focusing on the effective number of relativistic species $N_{ m eff}$ as a diagnostic for extra light particles beyond the Standard Model. Using CosmoMC/CAMB with Planck+WP+highL and external data (e.g., $H_0$ priors and BOSS DR9), it explores extensions to $\Lambda$CDM that include $N_{ m eff}$, $\sum m_\nu$, $Y_{\rm he}$, and neutrino perturbation parameters $c_{\rm eff}^2$ and $c_{\rm vis}^2$, as well as the lensing amplitude $A_L$. The results show that $N_{ m eff}$ is consistent with the Standard Model under some priors but can exhibit evidence for extra radiation when including $H_0$ or if the helium fraction is fixed, e.g. $N_{ m eff} \approx 3.6 \pm 0.3$ with $H_0$ priors, while BBN consistency drives it closer to 3.046. The perturbation parameters appear as $c_{\rm eff}^2 \approx 0.31$ and $c_{\rm vis}^2 \approx 0.56$, indicating nonstandard clustering; combining datasets can tighten bounds on $\sum m_\nu$ to sub-eV levels, with upper limits around $0.5$–$0.7$ eV depending on the data combination, highlighting the model-dependent nature of dark radiation constraints and motivating future, more precise probes of light relics.

Abstract

New measurements of the cosmic microwave background (CMB) by the Planck mission have greatly increased our knowledge about the Universe. Dark radiation, a weakly interacting component of radiation, is one of the important ingredients in our cosmological model which is testable by Planck and other observational probes. At the moment the possible existence of dark radiation is an unsolved question. For instance, the discrepancy between the value of the Hubble constant, H_0, inferred from the Planck data and local measurements of H_0 can to some extent be alleviated by enlarging the minimal LambdaCDM model to include additional relativistic degrees of freedom. From a fundamental physics point of view dark radiation is no less interesting. Indeed, it could well be one of the most accessible windows to physics beyond the standard model. An example of this is that sterile neutrinos, hinted at in terrestrial oscillation experiments, might also be a source of dark radiation, and cosmological observations can therefore be used to test specific particle physics models. Here we review the most recent cosmological results including a complete investigation of the dark radiation sector in order to provide an overview of models that are still compatible with new cosmological observations. Furthermore we update the cosmological constraints on neutrino physics and dark radiation properties focussing on tensions between data sets and degeneracies among parameters that can degrade our information or mimic the existence of extra species.

Figures (9)

• Figure 1: ISW contribution to the CMB temperature power spectrum. The raise at $\ell<30$ is due to the late Integrated Sachs Wolfe, while the peak around $\ell\sim200$ is the early Integrated Sachs Wolfe effect. The cosmological model is the $\Lambda$CDM with $N_{\textrm{eff}}$ equals to 3 (black solid line), 5 (red dashed line) and 7 (green dot-dashed line).
• Figure 2: CMB temperature power spectrum. The model and the legend are the same as in Figure \ref{['fig:isw']}, the grey error bars correspond to Planck data. Top panel: the total CMB temperature power spectrum. Middle panel: the $\ell$ axis has been rescaled by a factor $\theta_{\rm s}(N_{\textrm{eff}})/\theta_{\rm s}(N_{\textrm{eff}}=3)$. Bottom panel: the ISW contribution has been subtracted.
• Figure 3: $68\%$ and $95\%$ c.l. 2D marginalized posterior in the plane $N_{\textrm{eff}} - H_0$.
• Figure 4: $68\%$ and $95\%$ c.l. 2D marginalized posterior in the plane $N_{\textrm{eff}} - H_0$.
• Figure 5: $68\%$ and $95\%$ c.l. 2D marginalized posterior in the plane $N_{\textrm{eff}} - Y_{\textrm{he}}$.