# Testing String Theory with CMB

## TL;DR

Future CMB observations detecting tensor modes could challenge existing string theory inflation models, especially if tensor-to-scalar ratios exceed current theoretical bounds, implying superheavy gravitinos and impacting string phenomenology.

## Contribution

This paper analyzes how potential future detection of tensor modes in CMB can test and constrain string theory inflation models, particularly those based on KKLT constructions.

## Key findings

- Current models predict undetectably low tensor modes.
- Detection of r > 10^{-3} would imply superheavy gravitinos.
- A high tensor-to-scalar ratio would challenge existing string inflation models.

## Abstract

Future detection/non-detection of tensor modes from inflation in CMB observations presents a unique way to test certain features of string theory. Current limit on the ratio of tensor to scalar perturbations, r=T/S, is r < 0.3, future detection may take place for r > 10^{-2}-10^{-3}. At present all known string theory inflation models predict tensor modes well below the level of detection. Therefore a possible experimental discovery of tensor modes may present a challenge to string cosmology. The strongest bound on r in string inflation follows from the observation that in most of the models based on the KKLT construction, the value of the Hubble constant H during inflation must be smaller than the gravitino mass. For the gravitino mass in the usual range, m_{3/2} < O(1) TeV, this leads to an extremely strong bound r < 10^{-24}. A discovery of tensor perturbations with r > 10^{-3} would imply that the gravitinos in this class of models are superheavy, m_{3/2} > 10^{13} GeV. This would have important implications for particle phenomenology based on string theory.