Astrophysical Sources of Stochastic Gravitational Radiation in the Universe

Stochastic gravitational waves (GW) associated with unresolved astrophysical sources at frequency bands of the ongoing GW interferometers LIGO/VIRGO and LISA are studied. We show that GW noise from rotating galactic neutron stars with low magnetic fields may reach the advanced LIGO sensitivity level at frequency $f\sim 100$ Hz. Within LISA frequency band (10^{-4}-10^{-1} Hz), the GW background from galactic binary stars is shown to mainly contribute up to a frequency of 3.10^{-2} Hz, depending on the galactic rate of binary white dwarf mergers. To be detectable by LISA, relic GW backgrounds should be as high as \Omega_{GW}h_{100}^2>10^{-8} at 10^{-2} Hz.