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Authors: Tristan L. Smith, Elena Pierpaoli, Marc Kamionkowski (Caltech)

Date: Mon, 6 Mar 2006

Abstract: Primordial gravitational waves (GWs) with frequencies > 10ˆ{-15} Hz contribute to the radiation density of the Universe at the time of decoupling of the cosmic microwave background (CMB). The effects of this GW background on the CMB and matter power spectra are identical to those due to massless neutrinos, unless the initial density-perturbation amplitude for the gravitational-wave gas is non-adiabatic, 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 (~10ˆ{-15} Hz rather than the ~10ˆ{-10} Hz lower limit from BBN): at 95% confidence-level, Omega_gw hˆ2 < 6.9 x 10ˆ{-6} for homogeneous (i.e., non-adiabatic) initial conditions. Future CMB experiments, like Planck and CMBPol, should allow sensitivities to Omega_gw hˆ2 < 1.4 x 10ˆ{-6} and Omega_gw hˆ2 < 5 x 10ˆ{-7}, respectively.

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