Testing ns=1 in light of the latest ACT and SPT data
Abstract
It is commonly recognized that the primordial scalar spectral index ns is approximately 0.96-0.975, depending on the dataset. However, this view is being completely altered by the early dark energy (EDE) resolutions of the Hubble tension, known as the most prominent tension the standard model is suffering from. In corresponding models with pre-recombination EDE, resolving the Hubble tension (i.e., achieving H0 73km/s/Mpc) must be accompanied by a shift of ns towards unity to maintain consistency with the cosmological data, which thus implies a scale invariant Harrison-Zel'dovich spectrum with ns=1 (|ns-1| O(0.001)). In this work, we strengthen and reconfirm this result with the latest ground-based CMB data from ACT DR6 and SPT-3G D1, the precise measurements at high multipoles beyond the Planck angular resolution and sensitivity. Our work again highlights the importance of re-examining our understanding on the very early Universe within the broader context of cosmological tensions.
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