We present constraints on extensions to the $\mathrm{\Lambda }\mathrm{CDM}$ cosmological model from measurements of the $E$-mode polarization autopower spectrum and the temperature-$E$-mode cross-power spectrum of the cosmic microwave background (CMB) made using 2018 SPT-3G data. The extensions considered vary the primordial helium abundance, the effective number of relativistic degrees of freedom, the sum of neutrino masses, the relativistic energy density and mass of a sterile neutrino, and the mean spatial curvature. We do not find clear evidence for any of these extensions, from either the SPT-3G 2018 dataset alone or in combination with baryon acoustic oscillation and Planck data. None of these model extensions significantly relax the tension between Hubble-constant, $H_0$, constraints from the CMB and from distance-ladder measurements using Cepheids and supernovae. The addition of the SPT-3G 2018 data to Planck reduces the square-root of the determinants of the parameter covariance matrices by factors of 1.3–2.0 across these models, signaling a substantial reduction in the allowed parameter volume. We also explore CMB-based constraints on $H_0$ from combined SPT, Planck, and ACT DR4 datasets. While individual experiments see some indications of different $H_0$ values between the $TT$, $TE$, and $EE$ spectra, the combined $H_0$ constraints are consistent between the three spectra. For the full combined datasets, we report $H_0=67.49\pm 0.53\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$, which is the tightest constraint on $H_0$ from CMB power spectra to date and in $4.1\sigma$ tension with the most precise distance-ladder-based measurement of $H_0$. The SPT-3G survey is planned to continue through at least 2023, with existing maps of combined 2019 and 2020 data already having $\sim 3.5\times \mathrm{lower}$ noise than the maps used in this analysis.

• Received 29 March 2021
• Accepted 18 August 2021

DOI:https://doi.org/10.1103/PhysRevD.104.083509