List of Footnotes

1 In terms of event rates the current best estimates for neutron-star–binary merger rates, based on the known population of neutron-star–binary systems, gives a 95% confidence interval between 1 – 1000 × 10–6 per year per Milky Way Equivalent Galaxy (MWEG), where MWEG is equivalent to a volume that contains a blue light luminosity with L = 9× 109L⊙ (MWEG was used in the S1 and S2 LIGO search, but was then changed to the L10 unit, where L10 is given as 1010 times the blue-light luminosity of the sun, although there is only a 10% difference between the two), [3Jump To The Next Citation Point, 196, 197], with a peak in the distribution at 100 × 10–6 per year per MWEG – or ≈ 0.02 per year for initial LIGO at design sensitivity. The expected rate of black-hole binary systems, or black-hole–neutron-star systems is far harder to infer as none have been observed, but estimates can be made on the population for a wide variety of models and give a 95% confidence range of 0.05 – 100 × 10–6 per year per MWEG and 0.01 – 30 × 10–6 per year per MWEG respectively [3Jump To The Next Citation Point, 248, 249, 39Jump To The Next Citation Point]. As an example of how to convert from rates to event numbers, cumulative blue-light luminosities with respect to distance from the Earth in Mpcs, and the horizon distances of the LIGO detectors from S2 through to S4, can be seen in Figure 3 of [39Jump To The Next Citation Point].
2 The result published in [10Jump To The Next Citation Point] give an upper limit value of Ωgw < 23, but this is for a Hubble constant of 100 km s–1 Mpc–1, so for consistency with later results it has been converted to use a Hubble constant of 72 km s–1 Mpc–1 as in [21Jump To The Next Citation Point].
3 There is currently a plan that has been approved by the LIGO Laboratory and the NSF to potentially construct one of the Hanford detectors at a site in Australia [225], although this is reliant on construction and running costs being provided by the Australian government. Such an observatory in the southern hemisphere would greatly improve sky localisation of any transient sources and enhance electromagnetic follow-up observations (e.g., [91]).