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 $n_s\gtrsim 0.95$, in accord with current measurements, the tensor/scalar ratio satisfies $r\gtrsim {10}^{-2}$, a range that should be detectable in proposed cosmic microwave background polarization experiments and direct gravitational wave searches.