Inflationary Gravitational Wave Background Research Articles

High-energy effects on the spectrum of the inflationary gravitational wave background in braneworld cosmology

We discuss the cosmological evolution of the inflationary gravitational wave background in the Randall-Sundrum single-brane model. In braneworld cosmology, in which the three-dimensional spacelike hypersurface that we live in is embedded in five-dimensional anti-de-Sitter (AdS{sub 5}) spacetime, the evolution of gravitational wave (GW) modes is affected by the nonstandard expansion of the universe and the excitation of the Kaluza-Klein modes. These are significant in the high-energy regime of the universe. We numerically evaluate these two effects by solving the evolution equation for GWs propagating through the AdS{sub 5} spacetime. Using a plausible initial condition from inflation, we find that the excitation of Kaluza-Klein modes can be characterized by a simple scaling relation above the critical frequency f{sub crit} determined from the length scale of the fifth dimension l. The remarkable point is that this relation generally holds as long as the matter content of the universe is described by the perfect fluid with the equation of state p=w{rho} for 0{ f{sub crit}. This indicates that in the radiation dominant case (w=1/3), the two high-energy effects accidentally compensate each othermore » and the spectrum becomes almost the same as the one predicted in the four-dimensional theory, i.e., {omega}{sub GW}{proportional_to}f{sup 0}.« less

Direct detection of the inflationary gravitational-wave background

Inflation generically predicts a stochastic background of gravitational waves over a broad range of frequencies, from those accessible with cosmic microwave background (CMB) measurements, to those accessible directly with gravitational-wave detectors, like NASA's Big-Bang Observer (BBO) or Japan's Deci-Hertz Interferometer Gravitational-wave Observer (DECIGO), both currently under study. Here we investigate the detectability of the inflationary gravitational-wave background at BBO/DECIGO frequencies. To do so, we survey a range of slow-roll inflationary models consistent with constraints from the CMB and large-scale structure (LSS). We go beyond the usual assumption of power-law power spectra, which may break down given the 16 orders of magnitude in frequency between the CMB and direct detection, and solve instead the inflationary dynamics for four classes of inflaton potentials. Direct detection is possible in a variety of inflationary models, although probably not in any in which the gravitational-wave signal does not appear in the CMB polarization. However, direct detection by BBO/DECIGO can help discriminate between inflationary models that have the same slow-roll parameters at CMB/LSS scales.

PRIMORDIAL GRAVITATIONAL WAVES AND INFLATION: POSSIBILITIES FOR DETECTION

The curl-modes of Cosmic Microwave Background (CMB) polarization probe horizon-scale primordial gravitational waves related to inflation. A significant source of confusion is expected from a lensing conversion of polarization related to density perturbations to the curl mode, during the propagation of photons through the large scale structure. Either high resolution CMB anisotropy observations or 21 cm fluctuations at redshifts 30 and higher can be used to delens polarization data and to separate gravitational-wave polarization signature from that of cosmic-shear related signal. Separations based on proposed lensing reconstruction techniques for reasonable future experiments allow the possibility to probe inflationary energy scales down to 1015GeV . Beyond CMB polarization, at frequencies between 0.01 Hz to 1 Hz, space-based laser interferometers can also be used to probe the inflationary gravitational wave background. The confusion here is related to the removal of merging neutron star binaries at cosmological distances. Given the low merger rate and the rapid evolution of the gravitational wave frequency across this band, reliable removal techniques can be constructed. We discuss issues related to joint constraints that can be placed on the inflationary models based on CMB polarization information and space-based interferometers such as the Big Bang Observer.