Testing String Theory with CMB

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, m3/2 <∼ 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, m3/2 >∼ 1013 GeV. This would have important implications for particle phenomenology based on string theory.