An indirect limit on the amplitude of primordial Gravitational Wave Background from CMB-Galaxy Cross Correlation

While large scale cosmic microwave background (CMB) anisotropies involve a combination of the scalar and tensor fluctuations, the scalar amplitude can be independently determined through the CMB-galaxy cross-correlation. Using recently measured cross-correlation amplitudes, arising from the cross-correlation between galaxies and the Integrated Sachs Wolfe effect in CMB anisotropies, we obtain a constraint r < 0.5 at 68% confidence level on the tensor-to-scalar fluctuation amplitude ratio. The data also allow us to exclude gravity waves at a level of a few percent, relative to the density field, in a low - Lambda dominated universe(Omega_Lambda~0.5). In future, joining cross-correlation ISW measurements, which captures cosmological parameter information, with independent determinations of the matter density and CMB anisotropy power spectrum, may constrain the tensor-to-scalar ratio to a level above 0.05. This value is the ultimate limit on tensor-to-scalar ratio from temperature anisotropy maps when all other cosmological parameters except for the tensor amplitude are known and the combination with CMB-galaxy correlation allows this limit to be reached easily by accounting for degeneracies in certain cosmological parameters.