Is the Harrison-Zel'dovich spectrum coming back? ACT preference for $n_s \sim 1$ and its discordance with Planck

TL;DR

The paper discusses the ACT data's preference for a primordial spectrum index near 1, which conflicts with Planck results, highlighting a significant tension in cosmological measurements that remains unresolved despite additional data considerations.

Contribution

It presents an analysis of ACT Data Release 4 indicating a preference for an Harrison-Zel'dovich spectrum and discusses the tension with Planck data, exploring potential theoretical and observational solutions.

Findings

ACT data favors n_s ~ 1, consistent with Harrison-Zel'dovich spectrum.

Significant tension (99.3% CL) with Planck measurements of n_s.

Adding large scale structure or polarization data does not resolve the discrepancy.

Abstract

The Data Release 4 of the Atacama Cosmology Telescope (ACT) shows an agreement with an Harrison-Zel'dovich primordial spectrum ($n_s=1.009 \pm 0.015$), introducing a tension with a significance of $99.3\%$ 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.