Low-frequency gravitational waves from cosmological compact binaries

We consider gravitational waves emitted by various populations of compact b inaries at cosmological distances. We use population synthesis models to ch aracterize the properties of double neutron stars, double black holes and d ouble white dwarf binaries, and white dwarf-neutron star, white dwarf-black hole and black hole-neutron star systems. We use the observationally determined cosmic star formation history to reco nstruct the redshift distribution of these sources and their merging rate e volution. The gravitational signals emitted by each source during its early spirallin g in phase add randomly to produce a stochastic background in the low-frequ ency band with spectral strain amplitude between similar to 10(-18) and sim ilar to 5x10(-17) Hz(-1/2) at frequencies in the interval similar to 5x10(- 6)-5x10(-5) Hz. The overall signal, which at frequencies above 10(-4) Hz is largely dominat ed by double white dwarf systems, might be detectable with LISA in the freq uency range 1-10 mHz and acts like a confusion-limited noise component, whi ch might limit the LISA sensitivity at frequencies above 1 mHz.