One Extension to Explain Them All, One Scale-Invariant Spectrum to Test Them All, and in One Model Bind Them

Abstract

The increasing precision of Cosmic Microwave Background (CMB) observations has unveiled significant tensions between different datasets, notably between Planck and the Atacama Cosmology Telescope (ACT), as well as with the late-Universe measurements of the Hubble constant. In this work, we explore a variety of $\Lambda$CDM extensions to assess their ability to reconcile these discrepancies. The statistical preference for these extensions remains moderate, and imposing $n_s=1$ often worsens model performance. Our findings highlight the limitations of incremental modifications to $\Lambda$CDM and suggest that either more complex new physics or, more likely, improved systematic understanding in the CMB sector may be required to fully address the observed tensions. While CMB experiments are often considered the gold standard of precision cosmology, our results reinforce that these measurements are not immune to systematic uncertainties, which may be underestimated in current analyses.