Harrison-Zel’dovich primordial spectrum is consistent with observations

Inflation predicts primordial scalar perturbations with a nearly scale-invariant spectrum and a spectral index approximately unity [the Harrison--Zel'dovich (HZ) spectrum]. The first important step for inflationary cosmology is to check the consistency of the HZ primordial spectrum with current observations. Recent analyses have claimed that a HZ primordial spectrum is excluded at more than 99% c.l. Here we show that the HZ spectrum is only marginally disfavored if one considers a more general reionization scenario. Data from the Planck mission will settle the issue.