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

Expected gravitational-wave results from LISA 145problems. Consideration of such algorithms for use with LISA data is juststarting, but it currently appears unlikely that the difficulty of the search problemwill be a real limitation in the use of the LISA data, even for the case of highlyunequal mass binaries. For all other expected types of LISA sources, the searchproblem is much easier than for ground-based detectors because of the roughly10 4 times lower number of data points that have to be handled for a year ofobservations.The other important question is whether the ability to calculate theoreticaltemplates will have improved enough by the time LISA data is available to carryout a thorough test of general relativity. For comparable mass MBHs, there is avery long way to go to accomplish this. However, for the highly unequal masscase, the chances for fairly rapid progress seem much better. It is hoped thatnumerical methods can be developed that start from the test mass approximation,and converge moderately well. Still, since small changes in the initial conditionscan lead to very large changes in the motion a year later, it is important thatthis problem as well as the search problem for the highly unequal mass case bepursued vigorously in the next few years.For either detailed tests of general relativity with LISA or for studies ofastrophysical questions concerning MBHs, the first requirement is that somesignals involving MBHs be seen. While it seems likely that several of the types ofMBH sources discussed earlier will be observed, this certainly is not guaranteed.Thus, the situation is somewhat like that for ground-based observations, wherethe detection of signals within the next decade seems quite likely, but is notcertain. Still, a reasonable prediction is that both ground-based detectors in thenext decade and LISA not too much later will detect the desired types of signals,and extremely strong tests of general relativity will be among the most importantscientific results.Another fundamental physics test that LISA may contribute to concernsthe possible existence of a primordial gravitational-wave background [75, 76].Standard inflation theory predicts a nearly scale-invariant spectrum, in which thespectral amplitude for the gravitational waves falls off at about the −1.5 powerof the frequency. However, the observed COBE microwave background spectrumis believed to be determined mainly by the large-scale density fluctuations at thetime of decoupling, and with a scale invariant spectrum would predict a very lowamplitude in the frequency range of LISA and of ground-based detectors.There are theories that could give a non-scale-invariant spectrum anddetectable amplitudes for a cosmic background at LISA or ground-basedfrequencies. LISA could detect an isotropic cosmic background near 10 mHz if itsenergy density were roughly 10 −11 of the closure density. A candidate for giving adetectable background is a phase transition at about the electro-weak energy scale,but it would have to be strongly first order in order to give enough amplitude. Butthis is not currently thought to be likely. Another possibility is associated withsuggested effects of extra dimensions, which may have become very small at atime which would produce a peak in the gravitational-wave spectrum near the