DETECTABILITY OF INFLATIONARY GRAVITATIONAL-WAVES WITH MICROWAVE BACKGROUND POLARIZATION

Inflation predicts specific relations between the amplitudes and spect ral indices of the primordial spectrum of density (scalar metric) pert urbations and gravitational waves (tensor metric perturbations). Detec tion of a stochastic gravitational-wave background is essential for id entifying this unmistakable signature. Polarization of the cosmic micr owave background can isolate these tensor modes in a model-independent way and thereby circumvent the cosmic-variance limit to detecting a s mall tensor signal with only a temperature map. Here we assess the det ectability of a gravity-wave background with a temperature and polariz ation map. For detector sensitivities better than 10-20 mu K root sec, the sensitivity to a tensor signal is always dominated by the polariz ation map. With a detector sensitivity of order 1 mu K root sec, polar ization could improve on a temperature-map sensitivity to tensor modes by two to three orders of magnitude. Even a small amount of reionizat ion substantially enhances tensor-mode detectability. We also argue th at the sensitivity of the Planck Surveyor to tensor modes is significa ntly improved with polarization, even taking into account the resultin g degradation of the intensity determination in the high-frequency cha nnels.