Testing string theory by CMB

In this talk we propose to use the future detection/non-detection of tensor modes from inflation in CMB observations as a way to test certain features of string theory. Current limit on the ratio of tensor to scalar perturbations, r=T/S, is r < approx. 0.3, future detection may take place for r > or approx. 10-2 - 10-3. I will argue that 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. (author)