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. 
A. Cooray (UC Irvine), P.S. Corasaniti (ISCAP, New York), T.   Giannantonio (Rome U.), A. Melchiorri (Rome U.)