We present a new constraint on the effective number of relativistic species in the early universe, $N_{\mathrm{eff}}$, by combining recent primordial helium abundance measurements from the Large Binocular Telescope $Y_p$ Project with primordial deuterium abundance data, CMB observations from Planck, the Atacama Cosmology Telescope, and the South Pole Telescope, and baryon acoustic oscillation (BAO) data from the Dark Energy Spectroscopic Instrument, yielding $N_{\mathrm{eff}}=2.990\pm 0.070$ (68% CL). This is the tightest constraint on $N_{\mathrm{eff}}$ to date, and is in excellent agreement with the standard model prediction of $N_{\mathrm{eff}}=3.044$. Furthermore, we constrain excess contributions to $N_{\mathrm{eff}}$ beyond the three neutrino species, finding $\mathrm{\Delta }{N}_{\mathrm{eff}}<0.107$ (95% CL). This bound nearly approaches the minimum contribution to $\mathrm{\Delta }{N}_{\mathrm{eff}}$ from a light spin-$3/2$ particle that decoupled at any time after inflation ended. Our baseline analysis does not include large-scale Planck polarization information, enabling a fully consistent combination of state-of-the-art CMB and BAO measurements. As a byproduct, we show that current $N_{\mathrm{eff}}$ bounds are essentially insensitive to the inclusion or exclusion of optical depth constraints inferred from large-scale CMB polarization data, making $N_{\mathrm{eff}}$ 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.

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• Big bang nucleosynthesis
• Cosmic microwave background
• Cosmological parameters
• Neutrinos