Dynamical selection of the primordial density fluctuation amplitude

In inflationary models, the predicted amplitude of primordial density perturbations Q is much larger than the observed value (∼10^-5) for natural choices of parameters. To explain the requisite exponential fine-tuning, anthropic selection is often invoked, especially in cases where microphysics is expected to produce a complex energy landscape. By contrast, we find examples of ekpyrotic models based on heterotic M theory for which dynamical selection naturally favors the observed value of Q.