Demagnified GWs from Cosmological Double Neutron Stars and GW Foreground Cleaning Around 1Hz

Gravitational waves from cosmological double neutron star binaries can be significantly demagnified by strong gravitational lensing effect, and the proposed future missions such as BBO or DECIGO might miss some of the demagnified GW signals below a detection threshold. The undetectable binaries would form a GW foreground which might hamper detection of a very weak primordial GW signal. We discuss the outlook of this potential problem, using a simple model based on the singular-isothermal-sphere lens profile. Fortunately, it is expected that, for presumable merger rate of NS+NSs, the residual foreground would be below the detection limit Omega_{GW} ~ 10^{-16} realized with BBO/DECIGO by correlation analysis.