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 ($\sim {10}^{-5}$) 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 $Q$.