It is commonly recognized that the primordial scalar spectral index $n_s$ 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 $\mathrm{\Lambda }$ cold dark matter model is suffering from. In corresponding models with prerecombination EDE, resolving the Hubble tension (i.e., achieving $H_0\sim 73\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$) must be accompanied by a shift of $n_s$ toward unity to maintain consistency with the cosmological data, which thus implies a scale invariant Harrison-Zel’dovich spectrum with $n_s=1$ $(|n_s-1|\simeq \mathcal{O}(0.001))$. In this work, we strengthen and reconfirm this result with the latest ground-based cosmic microwave background data from ACT DR6 and SPT-3G D1, the currently most precise measurements at high multipoles beyond the Planck angular resolution and sensitivity. Our work again highlights the importance of reexamining our understanding of the very early Universe within the broader context of cosmological tensions.

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• Cosmology
• Inflation