How Delicate is Brane-Antibrane Inflation?

TL;DR

This paper thoroughly investigates the parameter space of a string theory-based brane-antibrane inflation model, demonstrating reduced fine-tuning requirements and predicting observable spectral index values near 0.99, enhancing its testability.

Contribution

It provides a systematic analysis of the model's parameters, showing that fine-tuning is significantly alleviated and identifying a natural parameter region with testable predictions.

Findings

Fine-tuning is reduced by 8 orders of magnitude near optimal parameters.

Larger spectral index values close to 0.99 are favored.

The model remains consistent with WMAP5 constraints.

Abstract

We systematically explore the parameter space of the state-of-the-art brane-antibrane inflation model (Baumann {\it et al}., arXiv:0706.0360, arXiv:0705.3837) which is one of the most rigorously derived from string theory, applying the cosmic background explorer normalization and constraint on the spectral index. We improve on previous treatments of uplifting by antibranes and show that the contributions from noninflationary throats play an important role in achieving a flat inflationary potential. To quantify the degree of fine-tuning needed by the model, we define an effective volume in the part of parameter space which is consistent with experimental constraints, and using Monte Carlo methods to search for a set of optimal parameters, we show that the degree of fine-tuning is alleviated by 8 orders of magnitude relative to a fiducial point which has previously been considered. In fact, close to the optimal parameter values, fine-tuning is no longer needed for any of the parameters. We show that in this natural region of the parameter space, larger values of $n_s$ close to 0.99 (still within 2$\sigma$ of the WMAP5 central value) are favored, giving a new aspect of testability to the model.