Neutrinos and other light relics leave characteristic imprints in the cosmic microwave background anisotropies, making their observation a sensitive probe of the particle content and thermal history of the early universe. The energy density in these relativistic species is parameterized by their effective number $N_\mathrm{eff}$. Measuring this parameter at the percent level, which is a long-standing science goal of CMB-S4 and other experiments, would test a wide range of well-motivated physics within and beyond the Standard Model of particle physics. In this paper, we present Fisher-matrix forecasts of the projected sensitivity to $N_\mathrm{eff}$ of several CMB-S4 survey configurations considered during its extensive design phase. The conceptual design reaches $\sigma(N_\mathrm{eff})<0.03$ over its seven-year observing period, while the revised configuration achieves the same precision over a longer timescale. We complement these results with a cosmic-variance-limited survey over the same multipole range to quantify the room for improvement accessible with additional instrumental, observational, and theoretical efforts. Finally, we discuss the broad implications of precise $N_\mathrm{eff}$ measurements for the radiation sector, big bang nucleosynthesis, light thermal relics, and other early-universe physics. The forecasts presented in this work are performed with the publicly released DRAFT (Dark Radiation Anisotropy Flowdown Team) tool. It provides an end-to-end pipeline from simulated foreground maps and component separation to delensing and projected sensitivities for any cosmological parameter, and it can be directly applied to other cosmic microwave background survey designs.
C. Trendafilova, Srinivasan Raghunathan, B. Wallisch et al.· 0 citations
Neutrino oscillations establish that neutrinos are massive, providing the only laboratory detection of physics beyond the Standard Model. Direct kinematic experiments bound the electron-neutrino mass to $m_{\nu_e}<0.45$ eV (KATRIN, 90% CL), implying $\sum m_\nu \lesssim 1.3$ eV. Conversely, cosmology within $\Lambda$CDM is highly constraining: Planck CMB, CMB lensing, and DESI DR2 BAO yield $\sum m_\nu<0.056$ eV (95% CL), in 2-3$\sigma$ tension with the inverted-ordering floor (0.10 eV). However, this bound relies on $\Lambda$CDM, while data hint at an evolving dark energy. To determine the model dependence of cosmic neutrino mass bounds, we deconstruct each probe's sensitivity to late-time physics and pursue two robust routes to a $\sum m_\nu$ bound: (i) The existing dark-energy-marginalized route, retaining all data and marginalizing over $(w_0, w_a)$, is shown to also be immune to flexible binned and cubic $w(a)$ histories, yielding $\sum m_\nu<0.152$ eV, sharpening to $\sigma(\sum m_\nu) \approx 0.043$ eV with Simons Observatory lensing and Spec-S5 BAO. (ii) A new late-Universe-free route combines primary CMB, marginalizing over acoustic-peak smoothing via $A_{\rm lens}$, with the reconstructed lensing spectrum $C_L^{\kappa\kappa}$, removing late-time expansion dependence by construction. This yields $\sum m_\nu<0.41$ eV today, tightening to 0.31 eV (Simons Observatory) and 0.28 eV (cosmic-variance limit) across all tested dark-energy models. These relaxed bounds trade statistical power for model independence. Interestingly, they land in the sensitivity range targeted by next-generation laboratory experiments like Project 8 ($m_{\nu_e} \sim 0.1$ eV), motivating vital synergies between future cosmological and terrestrial neutrino measurements.
F. Qu, F. Ge, Hanyue Wang et al.· 0 citations
We use cookies to run the site and, with your consent, for analytics and to show ads.
See our Cookie Policy.
