We present a nearly model-independent estimate that yields the predictions of a class of simple inflationary and ekpyrotic or cyclic models for the spectral tilt of the primordial density inhomogeneities that enables us to compare the two scenarios. Remarkably, we find that the two produce an identical result, $n_s\approx 0.95$. For inflation, the same estimate predicts a ratio of tensor to scalar contributions to the low $l$ multipoles of the microwave background anisotropy of $T/\mathrm{S}\approx 20\%$; the tensor contribution is negligible for ekpyrotic or cyclic models, as shown in earlier papers.

• Received 3 February 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.161301