article · Journal of Cosmology and Astroparticle Physics
New cosmic microwave background measurements from the Atacama Cosmology Telescope Data Release 6 have been used to test the standard cosmological model and various proposed extensions. By combining these observations with datasets from the Planck satellite, baryon acoustic oscillation surveys, and supernovae, the analysis evaluates several alternative physics scenarios. The results confirm key standard model assumptions, including the adiabatic nature and near-scale invariance of primordial perturbations, as well as standard neutrino behaviours with an upper limit on total neutrino mass. The data show no evidence of new light relativistic species, dark radiation interactions, or early-universe variations in fundamental physical constants. Furthermore, the findings align closely with general relativity and a standard cosmological constant. Alternative models proposed to alter the inferred expansion rate or matter clustering are not favoured, reinforcing the standard cosmological paradigm across multiple observational datasets.
Testing the foundations of the standard cosmological model helps physicists determine whether current theories of the universe require major revisions or new physics. By combining high-precision telescope data across independent surveys, researchers can rigorously assess proposed extensions, such as novel forms of radiation or evolving physical constants, clarifying which theoretical explanations remain viable.
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Abstract We use new cosmic microwave background (CMB) primary temperature and polarization anisotropy measurements from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) to test foundational assumptions of the standard cosmological model, ΛCDM, and set constraints on extensions to it. We derive constraints from the ACT DR6 power spectra alone, as well as in combination with legacy data from the Planck mission. To break geometric degeneracies, we include ACT and Planck CMB lensing data and baryon acoustic oscillation data from DESI Year-1. To test the dependence of our results on non-ACT data, we also explore combinations replacing Planck with WMAP and DESI with BOSS, and further add supernovae measurements from Pantheon+ for models that affect the late-time expansion history. We verify the near-scale-invariance (running of the spectral index dn s / d ln k = 0.0062 ± 0.0052) and adiabaticity of the primordial perturbations. Neutrino properties are consistent with Standard Model predictions: we find no evidence for new light, relativistic species that are free-streaming ( N eff = 2.86 ± 0.13, which combined with astrophysical measurements of primordial helium and deuterium abundances becomes N eff = 2.89 ± 0.11), for non-zero neutrino masses (∑ m ν < 0.089 eV at 95% CL), or for neutrino self-interactions. We also find no evidence for self-interacting dark radiation ( N idr < 0.134), or for early-universe variation of fundamental constants, including the fine-structure constant ( α EM / α EM,0 = 1.0043 ± 0.0017) and the electron mass ( m e / m e,0 = 1.0063 ± 0.0056). Our data are consistent with standard big bang nucleosynthesis (we find Y p = 0.2312 ± 0.0092), the COBE/FIRAS -inferred CMB temperature (we find T CMB = 2.698 ± 0.016 K), a dark matter component that is collisionless and with only a small fraction allowed as axion-like particles, a cosmological constant ( w = -0.986 ± 0.025), and the late-time growth rate predicted by general relativity ( γ = 0.663 ± 0.052). We find no statistically significant preference for a departure from the baseline ΛCDM model. In fits to models invoking early dark energy, primordial magnetic fields, or an arbitrary modified recombination history, we find H 0 = 69.9 +0.8 -1.5 , 69.1 ± 0.5, or 69.6 ± 1.0 km/s/Mpc, respectively; using BOSS instead of DESI BAO data reduces the central values of these constraints by 1–1.5 km/s/Mpc while only slightly increasing the error bars. In general, models introduced to increase the Hubble constant or to decrease the amplitude of density fluctuations inferred from the primary CMB are not favored over ΛCDM by our data.
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DOI: 10.1088/1475-7516/2025/11/063
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