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
(MWEG was used in the S1 and S2 LIGO search, but was then changed to the
unit,
where
is given as 1010 times the blue-light luminosity of the sun, although there is only a 10% difference
between the two), [3
, 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 [3
, 248, 249, 39
]. 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 [39
].