Low-frequency gravitational waves from cosmological compact binaries

We consider gravitational waves emitted by various populations of compact binaries at cosmological distances. We use population synthesis models to characterize the properties of double neutron stars, double black holes and double 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 reconstruct the redshift distribution of these sources and their merging rate evolution. The gravitational signals emitted by each source during its early spiralling in phase add randomly to produce a stochastic background in the low-frequency band with spectral strain amplitude between ~10-18 and ~5×10-17Hz-1/2 at frequencies in the interval ~5×10-6-5×10-5Hz. The overall signal, which at frequencies above 10-4Hz is largely dominated by double white dwarf systems, might be detectable with LISA in the frequency range 1-10mHz and acts like a confusion-limited noise component, which might limit the LISA sensitivity at frequencies above 1mHz.