Confronting brane inflation with Planck and pre-Planck data

TL;DR

This paper evaluates brane inflation models against Planck and pre-Planck data, highlighting their compatibility constraints and the need for fine-tuning to match observed spectral index and non-Gaussianity limits.

Contribution

It provides a detailed comparison of brane inflation models with observational data, identifying parameter constraints and the extent of fine-tuning required for viability.

Findings

KKLMMT model survives at 2σ only with very small β

IR DBI model requires fine-tuning to satisfy non-Gaussianity constraints

Planck data favors a spectral index less than unity with some running

Abstract

In this paper, we compare brane inflation models with the Planck data and the pre-Planck data (which combines WMAP, ACT, SPT, BAO and H0 data). The Planck data prefer a spectral index less than unity at more than 5\sigma confidence level, and a running of the spectral index at around 2\sigma confidence level. We find that the KKLMMT model can survive at the level of 2\sigma only if the parameter $\beta$ (the conformal coupling between the Hubble parameter and the inflaton) is less than $\mathcal{O}(10^{-3})$, which indicates a certain level of fine-tuning. The IR DBI model can provide a slightly larger negative running of spectral index and red tilt, but in order to be consistent with the non-Gaussianity constraints from Planck, its parameter also needs fine-tuning at some level.