Dynamical Selection of the Primordial Density Fluctuation Amplitude

TL;DR

This paper explores how certain ekpyrotic models derived from heterotic M-theory can naturally select the observed amplitude of primordial density fluctuations without fine-tuning, contrasting with inflationary models.

Contribution

It demonstrates a dynamical mechanism in ekpyrotic models that favors the observed fluctuation amplitude, reducing reliance on anthropic explanations.

Findings

Dynamical selection naturally favors observed Q value

Ekpyrotic models can address fine-tuning issues

Contrast with inflationary model predictions

Abstract

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.