Inflationary Predictions for Scalar and Tensor Fluctuations Reconsidered

We reconsider the predictions of inflation for the spectral index of scalar (energy density) fluctuations (n_s) and the tensor/scalar ratio (r) using a discrete, model-independent measure of the degree of fine-tuning required to obtain a given combination of (n_s, r). We find that, except for cases with numerous unnecessary degrees of fine-tuning, n_s is less than 0.98, measurably different from exact Harrison-Zel’dovich. Furthermore, if n_s≳0.95, in accord with current measurements, the tensor/scalar ratio satisfies r≳10^-2, a range that should be detectable in proposed cosmic microwave background polarization experiments and direct gravitational wave searches.