From the cosmic microwave background, one can extract the scalar spectral index $n_s$. It is measured to be smaller than 1 by several standard deviations. Wikipedia says that it is a parameter of the Λ-CDM model. This is puzzling me. I thought that Λ-CDM includes a cosmological constant (i.e. no inflaton field necessary), standard model matter, radiation and WIMP dark matter. How do these parameters determine $n_s$?
Note: I don't want to get into the argument "Oh, but to explain the cosmological constant, we need an inflaton field", I think that's not relevant to this question.
Edit: If the answer is "You're wrong, Λ-CDM is always with an inflaton and a constant Λ has been ruled out", that's a perfectly valid answer to me, if you can point at some resources.