Is the Harrison-Zel'dovich spectrum coming back? ACT preference for ns ∼ 1 and its discordance with Planck

The Data Release 4 of the Atacama Cosmology Telescope (ACT) shows an agreement with an Harrison-Zel'dovich primordial spectrum (n(s) = 1.009 +/- 0.015), introducing a tension with a significance of 99.3 per cent Confidence Level (CL) with the results from the Planck satellite. The discrepancy on the value of the scalar spectral index is neither alleviated with the addition of large scale structure information nor with the low multipole polarization data. We discuss possible avenues to alleviate the tension relying on either neglecting polarization measurements from ACT or in extending different sectors of the theory.

Automated summary

The Data Release 4 of the Atacama Cosmology Telescope (ACT) shows an agreement with a Harrison-Zel'dovich primordial spectrum, introducing a tension with the results from the Planck satellite. This discrepancy on the value of the scalar spectral index is not alleviated with the addition of large scale structure information or low multipole polarization data. Possible avenues to alleviate the tension are discussed, including neglecting polarization measurements from ACT or extending different sectors of the theory.