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The currently detectable interval of black hole mergers is on the order of seconds or fractions of seconds. So each black hole merger, no matter how long the end to end process, is only detectable by LIGO for this tiny interval of time. How often are such intervals thought to happen in the observable unverse?

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marked as duplicate by Rob Jeffries, Danu, John Rennie, Kyle Kanos, ACuriousMind Mar 20 '16 at 11:30

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    $\begingroup$ I think this is a duplicate, but I my retract that if you clarify your question. The strain signal falls as $1/r$, so the entire volume of the observable universe is not the appropriate volume to consider when thinking about how many LIGO-detectable events there are. $\endgroup$ – Rob Jeffries Mar 20 '16 at 9:24
  • $\begingroup$ Also: physics.stackexchange.com/questions/236028/… $\endgroup$ – Rob Jeffries Mar 20 '16 at 9:26
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That is a non-trivial question... and one of those that LIGO is meant to answer by observation. Here is what they said in a publication: "The rate of binary black-hole mergers inferred from the observation of GW150914 is consistent with the higher end of rate predictions $>\approx 1Gpc^{−3}yr^{−1}$ from both types of formation models.", see "ASTROPHYSICAL IMPLICATIONS OF THE BINARY BLACK-HOLE MERGER GW150914", B. P. ABBOTT et al.. The volume of the comoving space in the observable universe is reported as approx. $1.224×10^4Gpc^3$ by Wikipedia (and one should check that against primary sources): https://en.wikipedia.org/wiki/Observable_universe. I doubt that multiplying the two numbers makes for good physics, but I might be wrong.

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