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

Sources detectable from ground and from space 49This background would be easily detectable by LIGO II.There may also be a cosmological background from either topologicaldefects (e.g. cosmic strings) or from inflation (which amplifies initial quantumgravitational fluctuations as it does the scalar ones that lead to galaxy formation).Limits from COBE observations suggest that standard inflation could not producea background stronger than inflationgw ∼ 10 −14 today. This is too weak for anyof the planned detectors to reach, but it remains an important long-range goalfor the field. However, there could also be a component of background radiationthat depends on what happened before inflation: string cosmological models, forexample, predict spectra growing with frequency [22].First-generation interferometers are not likely to detect these backgrounds:they may not be able to go below the upper limit set by the requirementthat gravitational waves should not disturb cosmological nucleosynthesis, whichis gw = 10 −5 . (This limit does not apply to backgrounds generated afternucleosynthesis, like the r-mode background.) Bar detectors may do as well orbetter than the first generation of interferometers for a broad-spectrum primordialbackground: as we have noted earlier, their noise levels within their resonancebands are very low. However, their frequencies are not right for the r-modebackground.Second-generation interferometers may be able to reach to 10 −11 of closureor even lower, by cross-correlation of the output of the two detectors. However,they are unlikely to get to the inflation target of 10 −14 . LISA may be able to go aslowas10 −10 (if we have a confident understanding of the instrumental noise), butit is likely to detect only the confusion background of binaries, which is expectedto be much stronger than a cosmological background in the LISA band.4.1.6 The unexpectedAt some level, we are bound to see things we did not expect. LISA, with itshigh signal-to-noise ratios for predicted sources, is particularly well placed to dothis. Most of the universe is composed of dark matter whose existence we caninfer only from its gravitational effects. It would not be particularly surprising ifa component of this dark matter produced gravitational radiation in unexpectedways, such as from binaries of small exotic compact objects of stellar mass. Wewill have to wait to see!