Abstract
Primordial gravitational waves (GWs) with frequencies 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 ( rather than the from BBN): at 95% confidence level, for homogeneous (i.e., nonadiabatic) initial conditions. Future CMB experiments, like Planck and CMBPol, should allow sensitivities to and , respectively.
- Received 6 March 2006
DOI:https://doi.org/10.1103/PhysRevLett.97.021301
©2006 American Physical Society