Harrison–Zeldovich spectrum

Summary

The Harrison–Zeldovich spectrum is the scale-invariant spectrum of primordial density fluctuations. In it the matter power spectrum of the initial fluctuations is proportional to $k$, so the spectral index is $n_s = 1$. Each scale then has the same density contrast when it enters the cosmological horizon. It was proposed by Edward Harrison and by Yakov Zeldovich, and independently by Jim Peebles and Jer Yu. It is the reference case for measured spectral indices.