String theoretic axion coupling and the evolution of cosmic structures

We examine the effects of the axion coupling to R (R) over tilde on the evolution of cosmic structures. It is shown that the evolutions of the scalar- and vector-type perturbations are not affected by this axion coupling. However, the axion coupling causes an asymmetric evolution of the two polarization states of the tenser-type perturbation, which may lead to a sizable polarization asymmetry in the cosmological gravitational wave if inflation involves a period in which the axion coupling is important. The polarization asymmetry produced Juring inflation is conserved over the subsequent evolution as long as the scales remain in the large-scale limit, and thus this may lend to an observable trace in the cosmic microwave background radiation.