Current cosmological constraints on the scalar spectral index of primordial fluctuations n _ { s } in the \Lambda CDM model have excluded the minimal scale-invariant Harrison-Zel ’ dovich model ( n _ { s } = 1 ; hereafter HZ ) at high significance , providing support for inflation .
In recent years , however , some tensions have emerged between different cosmological datasets that , if not due to systematics , could indicate the presence of new physics beyond the \Lambda CDM model .
In the light of these developments , we evaluate the Bayesian evidence against HZ in different data combinations and model extensions .
Considering only the Planck temperature data , we find inconclusive evidence against HZ when including variations in the neutrino number N _ { eff } and/or the Helium abundance Y _ { He } .
Adding the Planck polarization data , on the other hand , yields strong evidence against HZ in the extensions we considered .
Perhaps most interestingly , Planck temperature data combined with local measurements of the Hubble parameter \citep R16 , riess2018 give as the most probable model an HZ spectrum , with additional neutrinos .
However , with the inclusion of polarisation , standard \Lambda CDM is once again preferred , but the HZ model with extra neutrinos is not strongly disfavored .
The possibility of fully ruling out the HZ spectrum is therefore ultimately connected with the solution to current tensions between cosmological datasets .
If these tensions are confirmed by future data , then new physical mechanisms could be at work and an HZ spectrum could still offer a valid alternative .