The scalar "spectral index" describes how the density fluctuations vary with scale. As the size of these fluctuations depends upon the inflaton's motion when these quantum fluctuations are becoming super-horizon sized, different inflationary potentials predict different spectral indices. These depend upon the slow roll parameters, in particular the gradient and curvature of the potential. In models where the curvature is large and positive formula_11. On the other hand, models such as monomial potentials predict a red spectral index formula_12. Planck provides a value of formula_13 of 0.96.