A 2% determination of $N_{\rm eff}$ from primordial element abundance, cosmic microwave background, and baryon acoustic oscillation measurements

TL;DR

This paper provides the most precise measurement to date of the effective number of relativistic species in the early universe, combining multiple cosmological observations to test the standard model and constrain new physics.

Contribution

It presents a new, highly precise constraint on $N_{\rm eff}$ by combining primordial element abundances, CMB, and BAO data, nearly reaching the minimum contribution from certain light particles.

Findings

$N_{\rm eff}=2.990\pm0.070$, consistent with standard model

Excludes significant excess contributions to $N_{\rm eff}$ beyond three neutrinos

Constraints are robust against polarization data inclusion

Abstract

We present a new constraint on the effective number of relativistic species in the early universe, $N_{\rm eff}$, by combining recent primordial helium abundance measurements from the Large Binocular Telescope $Y_p$ Project with primordial deuterium abundance data, cosmic microwave background (CMB) observations from $\it{Planck}$, the Atacama Cosmology Telescope, and the South Pole Telescope, and baryon acoustic oscillation (BAO) data from the Dark Energy Spectroscopic Instrument, yielding $N_{\rm eff}=2.990\pm0.070$ (68% C.L.). This is the tightest constraint on $N_{\rm eff}$ to date, and is in excellent agreement with the standard model prediction of $N_{\rm eff}=3.044$. Furthermore, we constrain excess contributions to $N_{\rm eff}$ beyond the three neutrino species, finding $\Delta N_{\rm eff}<0.107$ (95% C.L.). This bound nearly approaches the minimum contribution to $\Delta N_{\rm eff}$ from a light spin-3/2 particle that decoupled at any time after inflation ended. Our baseline analysis does not include large-scale $\it{Planck}$ polarization information, enabling a fully consistent combination of state-of-the-art CMB and BAO measurements. As a byproduct, we show that current $N_{\rm eff}$ bounds are essentially insensitive to the inclusion or exclusion of optical depth constraints inferred from large-scale CMB polarization data, making $N_{\rm eff}$ highly robust in this regard. Our constraints place stringent limits on light particles in the early Universe and on a broad range of models aimed at increasing the CMB-inferred value of the Hubble constant.