Detecting a gravitational-wave background with next-generation space interferometers

Abstract

Future missions of gravitational-wave astronomy will be operated by space-based interferometers, covering a very wide range of frequencies. Search for stochastic gravitational-wave backgrounds (GWBs) is one of the main targets for such missions, and we here discuss the prospects for direct measurement of isotropic and anisotropic components of (primordial) GWBs around the frequency 0.1--10 Hz. After extending the theoretical basis for correlation analysis, we evaluate the sensitivity and the signal-to-noise ratio for the proposed future space interferometer missions, like Big-Bang Observer (BBO), Deci-Hertz Interferometer Gravitational-wave Observer (DECIGO), and the recently proposed Fabry-Perot type DECIGO. The astrophysical foregrounds which are expected at low frequency may be a big obstacle and may significantly reduce the signal-to-noise ratio of GWBs. As a result, the minimum detectable amplitude may reach ${h}^{2}{\ensuremath{\Omega}}_{\mathrm{gw}}={10}^{\ensuremath{-}15}\ensuremath{\sim}{10}^{\ensuremath{-}16}$, as long as foreground point sources are properly subtracted. Based on correlation analysis, we also discuss measurement of anisotropies of GWBs. As an example, the sensitivity level required for detecting the dipole moment of GWB induced by the proper motion of our local system is closely examined.