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4.3 Gravitational waves

One topic that may merit more than a brief paragraph in a few years’ time is the study of cosmology using gravitational waves. In particular, a coalescing binary system consisting of two neutron stars produces gravitational waves, and under those circumstances the measurement of the amplitude and frequency of the waves determines the distance to the object independently of the stellar masses [138]. This was studied in more detail in [22] and extended to more massive black hole systems [5829]. More massive coalescing signals produce lower-frequency gravitational wave signals which can be detected with the proposed LISA space-based interferometer. The major difficulty is obtaining the redshift measurement to go with the distance estimate, since the galaxy in which the coalescence event has taken place must be identified. Given this, however, the precision of the H0 measurement is limited only by weak gravitational lensing along the line of sight, and even this is reducible by observations of multiple systems or detailed investigations of matter along the line of sight. H0 determinations to ∼ 2% should be possible in this way after the launch of the LISA satellite in 2015.


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