We present a measurement of gravitational lensing over $1500{\mathrm{deg}}^2$ of the Southern sky using SPT-3G temperature data at 95 GHz and 150 GHz taken in 2018. The lensing amplitude relative to a fiducial Planck 2018 Lambda cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) cosmology is found to be $1.020\pm 0.060$, excluding instrumental and astrophysical systematic uncertainties. We conduct extensive systematic and null tests to check the robustness of the lensing measurements, and report a minimum-variance combined lensing power spectrum over angular multipoles of $50<L<2000$, which we use to constrain cosmological models. When analyzed alone and jointly with primary cosmic microwave background (CMB) spectra within the $\mathrm{\Lambda }\mathrm{CDM}$ model, our lensing amplitude measurements are consistent with measurements from SPT-SZ, SPTpol, ACT, and Planck. Incorporating loose priors on the baryon density and other parameters including uncertainties on a foreground bias template, we obtain a $1\sigma$ constraint on ${\sigma }_8{\mathrm{\Omega }}_{\mathrm{m}}^{0.25}=0.595\pm 0.026$ using the SPT-3G 2018 lensing data alone, where ${\sigma }_8$ is a common measure of the amplitude of structure today and ${\mathrm{\Omega }}_{\mathrm{m}}$ is the matter density parameter. Combining SPT-3G 2018 lensing measurements with baryon acoustic oscillation (BAO) data, we derive parameter constraints of ${\sigma }_8=0.810\pm 0.033$, $S_8\equiv {\sigma }_8({\mathrm{\Omega }}_{\mathrm{m}}/0.3{)}^{0.5}=0.836\pm 0.039$, and Hubble constant $H_0={68.8}_{-1.6}^{+1.3}\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$. Our preferred $S_8$ value is higher by 1.6 to $1.8\sigma$ compared to cosmic shear measurements from DES-Y3, HSC-Y3, and KiDS-1000 at lower redshift and smaller scales. We combine our lensing data with CMB anisotropy measurements from both SPT-3G and Planck to constrain extensions of $\mathrm{\Lambda }\mathrm{CDM}$. Using CMB anisotropy and lensing measurements from SPT-3G only, we provide independent constraints on the spatial curvature of ${\mathrm{\Omega }}_K=0.014_{-0.026}^{+0.023}$ (95% C.L.) and the dark energy density of ${\mathrm{\Omega }}_{\mathrm{\Lambda }}=0.722_{-0.026}^{+0.031}$ (68% C.L.). When combining SPT-3G lensing data with SPT-3G CMB anisotropy and BAO data, we find an upper limit on the sum of the neutrino masses of $\sum m_{\nu }<0.30\mathrm{eV}$ (95% C.L.). Due to the different combination of angular scales and sky area, this lensing analysis provides an independent check on lensing measurements by ACT and Planck.

• Received 22 August 2023
• Accepted 31 October 2023

DOI:https://doi.org/10.1103/PhysRevD.108.122005