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Published July 14, 2006 | Published
Journal Article Open

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.

Additional Information

© 2006 The American Physical Society (Received 6 March 2006; revised 26 May 2006; published 14 July 2006) T.L.S. was supported by the NSF. E.P. was supported by NSF Grant No. AST-0340648, also supported by NASA Grant No. NAG5-11489. This work was supported in part by DOE Grant No. DE-FG03-92-ER40701 and NASA Grant No. NNG05GF69G. This work was supported in part by the NSF through the TeraGrid resources provided by NCSA and SDSC under TeraGrid Grant No. AST050005T.

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