title: New cosmic microwave background constraint to primordial gravitational waves.
abstract: Primordial gravitational waves (GWs) with frequencies > or approximately equal to 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 (approximately 10(-15) Hz rather than the approximately 10(-10) Hz from BBN): at 95% confidence level, omega(gw)h(2) <or approximately = 8.4 x 10(-6) for homogeneous (i.e., nonadiabatic) initial conditions. Future CMB experiments, like Planck and CMBPol, should allow sensitivities to omega(gw)h(2) <or approximately = 1.4 x 10(-6) and omega(gw)h(2) < or approximately 5 x 10(-7), respectively.
authors.affiliation: California Institute of Technology, Mail Code 130-33, Pasadena, 91125, USA.
authors.forename: Tristan L
authors.initials: TL
authors.lastname: Smith
authors.forename: Elena
authors.initials: E
authors.lastname: Pierpaoli
authors.forename: Marc
authors.initials: M
authors.lastname: Kamionkowski