Probing the inflationary moduli space with gravitational waves

We investigate the spectrum of gravitational waves arising from primordial inflation in the presence of a string-theoretical higher-curvature correction, specifically, the Gauss-Bonnet coupling term for the inflaton (modulus) field. We show that if the modulus field exhibits a wall-crossinglike behavior in the moduli space, there can be a period of Gauss-Bonnet coupling-term domination during the usual slow-roll. This phenomenon is potentially detectable as the gravitational wave spectrum exhibits a characteristic peak caused by the brief domination of the Gauss-Bonnet coupling term. We explore the possibility of measuring such gravitational waves with pulsar timing array experiments such as NANOGrav, and future space-borne interferometers such as LISA, DECIGO, and Taiji.