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6.1 Gravitational wave observatories

Some time in the next decade, a new opportunity for testing relativistic gravity will be realized, when a worldwide network of kilometer-scale, laser interferometric gravitational wave observatories in the U.S. (LIGO project), Europe (VIRGO and GEO600 projects), and Japan (TAMA300 project) begins regular detection and analysis of gravitational wave signals from astrophysical sources. These broad-band antennas will have the capability of detecting and measuring the gravitational waveforms from astronomical sources in a frequency band between about 10 Hz (the seismic noise cutoff) and 500 Hz (the photon counting noise cutoff), with a maximum sensitivity to strain at around 100 Hz of − 22 h ∼ Δl∕l ∼ 10 (rms), for the kilometer-scale LIGO/VIRGO projects. The most promising source for detection and study of the gravitational wave signal is the “inspiralling compact binary” – a binary system of neutron stars or black holes (or one of each) in the final minutes of a death spiral leading to a violent merger. Such is the fate, for example, of the Hulse–Taylor binary pulsar B1913+16 in about 300 Myr, or the “double pulsar” J0737-3039 in about 85 Myr. Given the expected sensitivity of the “advanced LIGO” (around 2010), which could see such sources out to many hundreds of megaparsecs, it has been estimated that from 40 to several hundred annual inspiral events could be detectable. Other sources, such as supernova core collapse events, instabilities in rapidly rotating newborn neutron stars, signals from non-axisymmetric pulsars, and a stochastic background of waves, may be detectable (for reviews, see [1256]; for updates on the status of various projects, see [11445]).

A similar network of cryogenic resonant-mass gravitational antennas have been in operation for many years, albeit at lower levels of sensitivity (h ∼ 10–19). While modest improvements in sensitivity may be expected in the future, these resonant detectors are not expected to be competitive with the large interferometers, unless new designs involving masses of spherical, or nearly spherical shape come to fruition. These systems are primarily sensitive to waves in relatively narrow bands about frequencies in the hundreds to thousands of Hz range [20612332217], although future improvements in sensitivity and increases in bandwidth may be possible [61].

In addition, plans are being developed for an orbiting laser interferometer space antenna (LISA for short). Such a system, consisting of three spacecraft orbiting the sun in a triangular formation separated from each other by five million kilometers, would be sensitive primarily in the very low frequency band between 10–4 and 10–1 Hz, with peak strain sensitivity of order h ∼ 10–23 [90].

In addition to opening a new astronomical window, the detailed observation of gravitational waves by such observatories may provide the means to test general relativistic predictions for the polarization and speed of the waves, for gravitational radiation damping and for strong-field gravity.

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