For example, one topic that may merit more than a footnote in the future 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 [193]. This was studied in more detail by [36] and extended to more massive black-hole
systems [90, 45]. More massive coalescing signals produce lower-frequency gravitational-wave signals, which can be
detected with the proposed LISA space-based interferometer (
http://lisa.nasa.gov/documentation.html). 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 
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. 
determinations
to 
2% should be possible, but depend on the launch of LISA or a similar mission. This is an event that
is probably decades away, although a pathfinder mission to test some of the technology is due for launch in
2015.
