New Cosmic Microwave Background Constraint to Primordial Gravitational Waves

Primordial gravitational waves (GWs) with frequencies >~10^-15 Hz contribute to the radiation density of the Universe at the time of decoupling of the cosmic microwave background (CMB). This affects the CMB and matter power spectra in a manner identical to massless neutrinos, unless the initial density perturbation for the GWs is nonadiabatic, as may occur if such GWs are produced during inflation or some post-inflation phase transition. In either case, current observations provide a constraint to the GW amplitude that competes with that from big-bang nucleosynthesis (BBN), although it extends to much lower frequencies (~10^-15 Hz rather than the ~10^-10 Hz from BBN): at 95% confidence level, Omegagwh2<~8.4×10^-6 for homogeneous (i.e., nonadiabatic) initial conditions. Future CMB experiments, like Planck and CMBPol, should allow sensitivities to Omegagwh2<~1.4×10^-6 and Omegagwh2<~5×10^-7, respectively.