SPT-3G produces high-precision CMB lensing map, sharpens neutrino-mass and structure-growth limits

The SPT-3G Collaboration has posted a preprint describing a new map of cosmic microwave background, or CMB, lensing from South Pole Telescope observations taken in 2019 and 2020 across a 1,500-square-degree survey area. In the paper, “SPT-3G D1: Quadratic-Estimator CMB Lensing Reconstruction and Cosmology,” posted Aug. 31 on arXiv as arXiv:2608.31136 and dated Sept. 1 in PDF form, the authors say the reconstruction delivers the highest signal-to-noise per mode yet reported and measures the lensing amplitude at 2% precision in a result consistent with standard cosmology.

The analysis uses the SPT-3G Main field, called the D1 dataset, and reconstructs the CMB lensing potential with a quadratic estimator that combines temperature data with two kinds of polarization information, known as E modes and B modes. With nuisance parameters fixed to best-fit values, the paper reports a lensing amplitude consistent with unity relative to the standard ΛCDM cosmology favored by combined Planck, ACT DR6 and SPT-3G D1 CMB data. From the SPT-3G D1 lensing spectrum alone, the team finds a structure-growth parameter of 0.6046 plus or minus 0.0096; when combined with ACT DR6 and Planck PR4 CMB lensing, that becomes 0.6020 plus or minus 0.0084, which the paper describes as the tightest constraint of its kind from CMB lensing to date.

CMB lensing tracks how the universe’s oldest light has been subtly deflected by matter on its way to Earth, making it a powerful way to map the growth of cosmic structure over time. In the new preprint, the authors say the lensing map is dominated by polarization information on angular scales up to about L equals 600, a sign that the measurement is drawing much of its statistical power from polarization rather than temperature alone. SPT-3G is the third-generation camera on the 10-meter South Pole Telescope at the National Science Foundation’s Amundsen-Scott South Pole Station.

The paper also reports a notable combined cosmology result: when the new lensing dataset is folded into the authors’ CMB_SPA dataset and DESI DR2 baryon acoustic oscillation measurements, the analysis gives a summed neutrino mass limit of less than 0.072 electron volts at 95% confidence within ΛCDM. That figure is model- and dataset-dependent, not a standalone direct neutrino measurement, and it should be understood as coming from that specific combination of CMB and galaxy-survey data. While competitive, it is not necessarily the single tightest recent cosmological bound, because different analyses and data combinations can produce different limits.

Another combined result links the new lensing map with the Dark Energy Survey’s Year 3 “3x2pt” analysis, which combines several weak-lensing and galaxy-clustering measurements. There the paper reports S8 — another standard measure of the universe’s late-time clustering strength — of 0.811 plus or minus 0.011, which it describes as a 1.4% constraint. The preprint also says this newer data combination agrees better with DESI DR2 BAO than some earlier work, leading to a more relaxed neutrino-mass upper bound and reducing earlier, roughly 2-sigma hints of nonzero spatial curvature or evolving dark energy.

The work is a preprint and has not yet gone through peer review. The first author is Yuuki Omori, writing for the SPT-3G Collaboration. The paper says residual foregrounds and instrumental systematics become important at this level of precision and are handled through a simulation-based emulator and nuisance-parameter treatment. Associated SPT-3G D1 maps, bandpowers and likelihood products are listed through NASA’s LAMBDA archive.