Measurement of Gravitational Lensing of the Cosmic Microwave Background Using SPT-3G 2018 Data

We present a measurement of gravitational lensing over 1500 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 (.CDM) cosmology is found to be 1.020 +/- 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.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)Omega(0.25)(m) = 0.595 +/- 0.026 using the SPT-3G 2018 lensing data alone, where sigma(8) is a common measure of the amplitude of structure today and Omega(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 +/- 0.033, S-8 = sigma(8)(Omega(m)/0.3)(0.5) = 0.836 +/- 0.039, and Hubble constant H-0 = 68.8(-1.6)(+1.3) km s(-1) 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.CDM. Using CMB anisotropy and lensing measurements from SPT-3G only, we provide independent constraints on the spatial curvature of Omega(K) = 0.014(-0.026)(+0.023) (95% C.L.) and the dark energy density of Omega(Lambda) = 0.722(-0.026)(+0.031) (68% C.L.). When combining SPT-3G lensing data with SPT-3G CMB anisotropy P and BAO data, we find an upper limit on the sum of the neutrino masses of Sigma m(nu) < 0.30 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.