Abstract
Future detection/non-detection of tensor modes from inflation in cosmic microwave
background observations presents a unique way to test certain features of
string theory. The current limit on the ratio of tensor to scalar perturbations,
r = T/S, is 
; future detection may take place for 
–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
Kachru–Kallosh–Linde–Trivedi 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, 
TeV, this leads to an extremely strong bound 
. A discovery of tensor perturbations with 
would imply that the gravitinos in this class of models are superheavy, 
GeV. This would have important implications for particle phenomenology based on string
theory.