(ed.). Gravitational waves (IOP, 2001)(422s).

References 239of [71]). However, for frequencies below a few mHz, one expects a stochasticbackground due to a large number of galactic white dwarf binaries. The estimateof this background depends on the rate of white dwarf mergers, which is uncertain.With rates of order 4 × 10 −3 per year (which should be a secure upper limit [67]),the background can be as large as h 2 0 gw ∼ 10 −8 at f = 10 −3 Hz. This number isactually quite uncertain, and in [71], it is used another plausible rate, which givesfor instance h 2 0 gw ∼ 10 −11 –10 −10 at f = 10 −3 Hz. Above a frequency of theorder of a few times 10 −2 Hz, most signals from galactic binaries can be resolvedindividually and no continuous background of galactic origin is presently knownat the level of sensitivity of LISA.It should be observed that, even if an astrophysical background is present,and masks a relic background, not all hopes are lost. If we understand wellenough the astrophysical background, we can subtract it, and the relic backgroundwould still be observable if it is much larger than the uncertainty that we haveon the astrophysical background. In fact, LISA should be able to subtract thebackground due to white dwarf binaries, since there is a large number of binariesclose enough to be individually resolvable [71]. This should allow us to predictwith some accuracy the space density of white dwarf binaries in other parts of theGalaxy, and therefore to compute the stochastic background that they produce.Furthermore, any background of galactic origin is likely to be concentrated nearthe galactic plane, and this is another handle for its identification and subtraction.The situation is more uncertain for the contribution of extragalactic binaries,which again can be relevant at LISA frequencies. The uncertainty in the mergingrate is such that it cannot be predicted reliably, but it is believed to be lower thanthe galactic background [67]. In this case the only handle for the subtractionwould be the form of the spectrum. In fact, even if the strength is quite uncertain,the form of the spectrum may be quite well known [71].References[1] Kolb E and Turner M 1990 The Early Universe (New York: Addison-Wesley)[2] Kosowsky A, Kamionkowski M and Turner M 1994 Phys. Rev. D 49 2837[3] Fixsen D J et al 1996 Astrophys. J. 473 576[4] Allen B 1996 Relativistic gravitation and gravitational radiation Proc. Les HouchesSchool on Astrophysical Sources of Gravitational Waves ed J Marck and J Lasota(Cambridge: Cambridge University Press)Allen B 1996 Preprint gr-qc/9604033[5] Maggiore M 1998 High-energy physics with gravitational-wave experiments grqc/9803028[6] Birrel N and Davies P C W 1982 Quantum Fields in Curved Space (Cambridge:Cambridge University Press)[7] Allen B and Romano J 1999 Phys. Rev. D 59 102001[8] Michelson P 1987 Mon. Not. R. Astron. Soc. 227 933[9] Christensen N 1992 Phys. Rev. D 46 5250[10] Flanagan E 1993 Phys. Rev. D 48 2389