Exploring the discrepancy between Planck PR3 and ACT DR4

Abstract We explore the scales and the extent of disagreement between Planck PR3 and Atacama Cosmology Telescope (ACT) DR4 data. Planck and ACT data have substantial overlap in the temperature anisotropy data between scales corresponding to multipoles ℓ ≃ 600–2500 with complementing coverage of larger angular scales by Planck and smaller angular scales by ACT. Since the same cosmology should govern the anisotropy spectrum at all scales, we probe this disagreement in the primordial power spectrum. We use a parametric form of power law primordial spectrum that allows changes in the spectral tilt. We also reconstruct the primordial spectrum with a non-parametric method from both Planck and ACT temperature data. We find the disagreement exists within scales 0.08–0.16 Mpc -1 where ACT temperature data prefers a scale invariant/blue spectrum. At scales larger and smaller than this window, ACT data strongly prefers a red tilt, which is consistent with Planck . This change in the spectral tilt can be identified in the ACT data at 2 σ C.L. without using Planck data, indicating that the tension is driven by different preferences for tilts within the ACT data. The addition of Planck data up to intermediate scales ( ℓ ≤ 650) increases this significance to 3 σ . Given the large overlap between Planck and ACT within 0.08–0.16 Mpc -1 and considering the internal consistency between different Planck temperature and polarization spectra, the scope of new physics as a solution to the tension remains limited. Our results — a strong preference for an intermediate transition in spectral tilt and the variation of this preference in different data combinations — indicate that systematic effects can be misperceived as new physics emerging from different non-standard cosmological processes.