  The spectral index $n_s$ of scalar perturbation is the significant initial
condition set by inflation theory for our observable Universe. According to
Planck results, current constraint is $n_s = 0.965\pm 0.004$, while an exact
scale-invaiant Harrison-Zeldovich spectrum, i.e. $n_s=1$, has been ruled out at
$8.4\sigma$ significance level. However, it is well-known that the standard
$\Lambda$CDM model is suffering from the Hubble tension, which is at $\sim
5\sigma$ significance level. This inconsistency likely indicates that the
comoving sound horizon at last scattering surface is actually lower than
expected, which so seems to be calling for the return of $n_s=1$. Here, in
light of recent observations we find strong evidence for a $n_s=1$ Universe.
And we show that if so, it would be confirmed conclusively by CMB-S4
experiment.
