With the advent of detectors of gravitational waves (GW), which are currently under construction, a totally new possibility to study different astronomical objects opens up (Thorne 1987). In addition to "classic" sources of GW like merging compact binaries or rapidly rotating hot neutron stars etc., which will be studied at frequencies Hz, there should exist a specific cosmological background (noise) covering a very wide frequency band from to Hz. The cosmological background should bear imprints of the physical processes in the very early Universe (Grishchuk 1988, 1997 and references therein). The primordial GW background, which originates from vacuum fluctuations parametrically amplified by the very expansion of the Universe, has a power-law spectrum increasing toward lower frequencies (Rubakov et al. 1982, Grishchuk 1988, 1997), so the prospects for its detection appear to be the most favorable at the low frequency band ( Hz) which will be covered by LISA space interferometer (Larson et al. 1999).
At some frequencies, however, unresolved binary stars within our own Galaxy or beyond provide an important contribution in the LISA frequency band (Bender and Hils 1997, Postnov and Prokhorov 1998, Kosenko and Postnov 1998). As shown in these papers, the stochastic signal from unresolved merging white dwarf binaries dominates the LISA sensitivity curve up to Hz. At higher frequencies extragalactic merging white dwarf binaries contribute at a level roughly 10 times smaller, which is below the planned LISA sensitivity at these frequencies. This fact makes possible the detection of the primordial GW backgrounds by LISA (Grishchuk 1997).
However, some inherent uncertainties (e.g. in the galactic rate of binary white dwarf mergings) are present in the calculations so the astrophysical backgrounds can turn out to be higher than expected. Since astrophysical GW backgrounds are considered as an additional noise contributing to the intrinsic noise of the detector, as much as possible should be known in advance of their properties at all frequencies.
What are the specific features of astrophysical GW backgrounds? Clearly, those related to the galactic sources should follow the distribution of stars inside the Milky Way (Hils et al. 1990, Lipunov et al. 1995). As all the detectors (ground-based or space-born) should rotate with respect to the galactic plane, the signal modulation has been used as an advantage to detect them (Giazotto et al. 1997, Giampieri and Polnarev 1997). As for the backgrounds of extragalactic origin, only amplitudes at different frequencies from various sources have been computed so far (Kosenko and Postnov 1998, Ferrari et al. 1999a,b).
The purpose of the present paper is to study angular properties of the GW noise produced by extragalactic astrophysical sources at the degree scales. As most of these sources must reside in galaxies (only a tiny fraction of binaries or GW-emitting neutron stars is expected to be in the intergalactic space), the GW background should have distinctive angular correlation properties reflecting the large scale structure (LSS) of the Universe. This is exactly what we observe in electromagnetic radiation as fluctuations of, for example, IR background observed by COBE (Kashlinsky et al. 1999).
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000