ACT DR6 & SPT-3G D1 CMB Data

• ACT DR6 and SPT-3G D1 CMB Data are advanced ground-based datasets providing high-resolution temperature and polarization maps that probe the cosmic microwave background.
• They employ state-of-the-art instrumental designs and rigorous noise modeling, achieving superior sensitivity and effective foreground separation compared to earlier surveys.
• Combined with Planck data, these datasets yield stringent constraints on ΛCDM parameters and offer robust platforms for testing extensions to the standard cosmological model.

The Advanced ACT DR6 and SPT-3G D1 CMB datasets represent the forefront of contemporary ground-based cosmic microwave background (CMB) observation, mapmaking, and cosmological inference. These public and collaboration-level data products, stemming from the Atacama Cosmology Telescope (ACT) Advanced DR6 and the South Pole Telescope third-generation Deep Field 1 (SPT-3G D1), are designed to probe the CMB’s temperature and polarization at high angular resolution and high sensitivity across large swathes of the sky at multiple frequency bands. They have rapidly become foundational for studies of the early universe, late-time structure, and extensions to $\Lambda$CDM, as well as for the joint analysis with other cosmological probes.

1. Instrumentation, Observing Strategies, and Data Products

The ACT DR6 dataset includes temperature and polarization maps over 19,000 deg$^2$ obtained from the Advanced ACTPol camera operating between 98, 150, and 220 GHz in the period 2017–2022. The median combined depth is 10 µK-arcmin, achieving white noise in polarization that is three times lower than Planck over much of the sky (Naess et al., 18 Mar 2025). The SPT-3G D1 dataset is derived from 2019–2020 observations covering 4% of the sky (essentially 1500 deg$^2$) primarily at 95, 150, and 220 GHz, using a $\sim$16,000 detector focal plane of polarization-sensitive transition-edge sensor (TES) bolometers with mapping depths reaching 4.9 µK-arcmin in polarization (Camphuis et al., 25 Jun 2025, Ge et al., 8 Nov 2024).

Both experiments use advanced off-axis Gregorian optical designs, large-format cryogenic detector arrays, multi-frequency channels, and frequency-domain multiplexed readout. SPT-3G leverages alumina refractive optics with broadband PTFE-based anti-reflection coatings, and its detector technology is optimized for photon-noise-limited performance with median TES saturation powers tailored to minimize bias and phonon noise (Bender et al., 2018, Anderson et al., 2019, Sobrin et al., 2021). Calibration, beam measurement, and consistent time-domain filtering approaches form part of their overall mapmaking pipelines. ACT DR6 data processing utilizes HEALPix, DUCC, and pixell for spherical harmonic transformations, custom apodizations, and pipeline modularization (Naess et al., 18 Mar 2025). Crucially, both collaborations engage in frequent cross-comparisons and calibration against Planck.

2. Power Spectrum Measurements and Foreground Modeling

Both ACT DR6 and SPT-3G D1 datasets provide high-fidelity measurements of the CMB anisotropy power spectra:

• ACT DR6 reports TT, TE, and EE spectra to arcminute scales using a pseudo-$C_\ell$ MASTER algorithm, correcting for instrumental beams, transfer functions, and sky masks. Power spectra are generated over 10,000 deg$^2$ (Louis et al., 18 Mar 2025). ACT's multi-frequency coverage provides strong leverage in foreground separation, especially for the thermal and kinematic Sunyaev-Zel’dovich (tSZ, kSZ) and galactic/extragalactic sources.
• SPT-3G D1 supplies TT/TE/EE spectra over $750 \le \ell < 3000$ (SPT-3G 2018: $750<\ell<3000$ in T, $300<\ell<3000$ in P; SPT-3G D1 2019–2020: much higher fidelity at $\ell=1800$–$4000$ in EE, $\ell=2200$–$4000$ in TE) (Camphuis et al., 25 Jun 2025, Balkenhol et al., 2022). Polarization spectra are especially competitive at high–$\ell$.

Foreground modeling is critical: the ACT DR6 parametric foreground model (thermal and kinetic SZ, cosmic infrared background, radio emission) is validated against Planck and SPT, tested with non-Gaussian sky simulations, and adopts a flexible form where amplitude and spectral parameters for each emission component are marginalized with cosmological parameters (Beringue et al., 6 Jun 2025). Drift in foreground parameters impacts cosmological inference by less than 0.5$\sigma$.

3. Covariance Structures and Noise Modeling

Precise cosmological parameter estimation from CMB power spectra necessitates rigorous noise and covariance modeling:

• ACT DR6 employs inhomogeneous, semi-analytic covariance matrices that extend the MASTER framework to incorporate survey non-uniformity and Fourier-space filtering effects. These matrices are tested against simulations and improved to sub-percent agreement using post-hoc correction (rotation into analytic eigenbases and Gaussian process smoothing) (Atkins et al., 10 Dec 2024).
• Map-based noise simulations for ACT adopt directional wavelet kernels to capture atmospheric and scanning-induced anisotropies, noting that analytic covariance underestimates diagonal noise by about 20%, a factor corrected through noise modeling (Atkins et al., 2023). SPT-3G independently incorporates similar 2D anisotropic noise models, with consistent thematics but different implementation details.

4. Cosmological Parameter Constraints and Comparison

Both datasets, individually and in combination with Planck, set tight constraints on $\Lambda$CDM and its extensions:

Data Set(s)	$H_0$ [km/s/Mpc]	$\sigma_8$	$S_8$	$A_\text{lens}$	$N_{\rm eff}$
SPT-3G D1 (alone)	$66.66\pm0.60$	$0.8137\pm0.0038$	–	–	–
ACT DR6+Planck+DESI DR2	$68.43\pm0.27$	$0.813\pm0.005$	$0.765\pm0.032$ (w/ Planck, LG)	$1.056^{+0.071}_{-0.069}$	$2.86\pm0.13$
SPT-3G D1+ACT+Planck (CMB)	$67.24\pm0.35$	$0.8137\pm0.0038$	$0.804\pm0.013$ (high-$z$ lens)	–	see above
SPT-3G 2018 TT/TE/EE	$68.3\pm1.5$	–	$0.797\pm0.042$	$0.87\pm0.11$	$3.55\pm0.58$

These results demonstrate:

• Consistency: Both ACT DR6 and SPT-3G D1 results agree with Planck for all $\Lambda$CDM parameters within uncertainties (Camphuis et al., 25 Jun 2025, Louis et al., 18 Mar 2025, Balkenhol et al., 2022). No strong evidence for excess lensing power is observed in the ACT DR6 or SPT-3G D1 datasets compared to Planck (Louis et al., 18 Mar 2025).
• Tensions: A $6.2\sigma$ Hubble tension is present between SPT-3G D1 and local SH0ES measurements; $S_8$ values are slightly below Planck, but statistical consistency is preserved (Camphuis et al., 25 Jun 2025, Ge et al., 8 Nov 2024). Inclusion of DESI BAO data introduces mild ($2.8\sigma$) discrepancies in curvature, CMB lensing amplitude, or dark energy equation of state—potentially indicating the need for extended models (such as variable $e^-$ mass or modified recombination) although the evidence remains mild.
• Extensions: Both DR6 and D1 provide strong constraints limiting $N_\text{eff}$, $\sum m_\nu$, primordial helium, and primordial oscillatory features (Calabrese et al., 18 Mar 2025, Peng et al., 23 Jul 2025). For example, $N_{\rm eff} = 2.86 \pm 0.13$ shows no support for additional light relics.

5. Lensing, Structure Growth, and Cross-Correlations

The high resolution and low noise of ACT DR6 and SPT-3G D1 data underpin secondary science cases:

• CMB Lensing: Both experiments reconstruct the lensing power spectrum—SPT-3G using polarization-only data (e.g., via the MUSE algorithm) and ACT via temperature and polarization (Ge et al., 8 Nov 2024, Villagra et al., 11 Jul 2025).
• Structure Growth Probes: Cross-correlations between CMB lensing and low/high-$z$ galaxy or quasar samples (unWISE, Quaia, DESI Legacy Imaging, BOSS BAO) yield measurements of $S_8$ and $\sigma_8$ at intermediate and high redshift (Farren et al., 2023, Qu et al., 14 Oct 2024, Villagra et al., 11 Jul 2025), e.g., $\sigma_8 = 0.804 \pm 0.013$ at high $z$.
• Component Separation: ACT DR6 and Planck combination in the needlet domain achieves improved Compton-$y$ maps and CMB separation at arcminute scales, with greatly reduced noise and foreground contamination compared to Planck alone (Coulton et al., 2023).

6. Implications for Physics Beyond $\Lambda$CDM and Model Extensions

ACT DR6 and SPT-3G D1 analyses restrict a large number of beyond-$\Lambda$CDM scenarios:

• Primordial Power Spectrum Features: Stringent upper bounds on oscillating features: $A_\text{log,lin} \lesssim 0.029$ at 95% confidence when combining Planck, ACT, and SPT D1—eliminating previous hints of features and strongly favoring simple inflation (Peng et al., 23 Jul 2025).
• Dark Sector Interactions: Models such as WZDR+, which introduce dark radiation and dark matter interactions with a transition redshift, are tested against ACT/DR6, SPT-3G, and DES data. WZDR+ can broaden and lower the $S_8$ posterior compared to $\Lambda$CDM, partially relieving the $S_8$ tension, but fail to fully resolve $H_0$ discrepancies (Zhou et al., 10 Sep 2024).
• Neutrino Properties and Extended Energy Injection: Constraints on $\sum m_\nu<0.082$ eV at 95% CL, and tight limits on variations in $n_s$, running, and the amplitude of lensing $A_\text{lens}$, mean little scope remains for strongly interacting sectors or early dark energy as remedies to cosmological tensions (Calabrese et al., 18 Mar 2025).

7. Future Prospects and Synthesis

The ACT DR6 and SPT-3G D1 datasets demonstrate that ground-based CMB experiments have reached comparable constraining power to Planck on critical cosmological parameters. Key advances include:

• High Multipole Precision: SPT-3G offers superior EE and TE precision at $\ell>2000$. Combined ground-based TT/TE/EE+Lens data now match or surpass Planck for $H_0$ and $\sigma_8$ (Camphuis et al., 25 Jun 2025).
• Robustness to Systematics: Analytic and simulation-based error modeling, validated sky simulations, and carefully tested foreground models ensure cosmological constraints are insensitive to technical modeling choices at the $\leq0.5\sigma$ level (Beringue et al., 6 Jun 2025, Atkins et al., 10 Dec 2024).
• Multiproble Baselines for Next-Generation Surveys: The strategies, analysis infrastructure, and data products set technical and methodological benchmarks for future efforts such as CMB-S4 and Simons Observatory (Atkins et al., 10 Dec 2024, Sobrin et al., 2021).

The combination of ACT DR6, SPT-3G D1, and Planck provides the tightest available CMB constraints on fundamental cosmology, supports the baseline $\Lambda$CDM model within current uncertainties, and offers a robust data-driven platform for continued exploration of early universe physics, structure growth, and dark sector phenomenology.