If you're getting 10 events/second with a device like this, you probably overpaid for sensitivity and if you're getting 1 event per century you're probably not going to be able to maintain the operating expenses to still be running when the detectable event occurs (and, as critically, none of the people involved will be able to get the data they need in the time they need it to get their PhD's, assistant profesorships, or tenured positions, so you can't get the labor force you need for your experiment to work on it, which is really what sets the acceptable duration of most experiments in practice).
It looks like the original estimates were pretty good, so events are coming in at about the rate the experimenters hoped they would see them.
There are estimates. But two of the three LIGO detections are of black holes that are more massive than we had expected to exist (~ a few tens of solar masses). Previously we had convincing examples of black holes with <~ 10 solar masses and others with >~ 1e6 solar masses. But since we didn't have any convincing observational detections of BHs with ~20-40 solar masses, it's safe to say that the volume density is poorly constrained for that mass range. At the high end we have a reasonable estimate of the volume density, because we think all galaxies with spheroid components have a black hole and that the black hole's mass is linked to the spheroid.
> I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs?
It depends on what you mean by "primordial". Micro-lensing experiments (when a star is brightly made brighter by the gravitational focusing of light from an object passing between us and the star), mostly looking towards the LMC/SMC [e.g., 0] have tried to address this. My recollection is that there aren't enough stellar mass black holes around to account for all of dark matter. Assuming Hawking radiation exists, low-mass primordial black holes should have evaporated by now, leaving only the more massive ones. There's a range in between the two, but I'm not sure if you can fit enough of them in a galaxy to account for dark matter while still being consistent with the sensitivity of the microlensing surveys.
[0] https://en.wikipedia.org/wiki/Optical_Gravitational_Lensing_...
> The truth or falsity of this would seem to have a big effect on rates.
Possibly. Though in order to emit GWs, pairs of black holes have to become bound to each other. If black holes make up the dark matter halos, they probably have large velocities relative to each other, which would limit their ability to form bound pairs (though it is possible with 3-body interactions). I am not aware of estimates of the BH pair-formation rate in halos _if_ DM haloes are in fact made of black holes. But the event rate probably can't be extraordinarily high, otherwise we might expect to see dark matter halos becoming less massive as the Universe ages. Though there are many confounding factors that might hide such a signal.
Aside from issues processing and disentangling the overlapping events in a situation with that high of an event rate, more events would not be bad, so I'm not sure I'd call it "overpaying". Imagine the kind of population demographics that could be built up if we were detecting that many events.
Agreed. I misunderstood your meaning then; I'd interpreted your wording to mean that "overpaid" was still within the bounds of reasonable expectations for funding. "Overpaid" didn't imply "too expensive to build", to me.
We will likely learn a great many things over the coming decades with this new way of looking at the universe, many of which we couldn't have even guessed we would learn!
Just detecting the initial wave was one of the most important measurements in the history of physics. Now that we know that gravitational waves exist, and can give us great insight into the mysteries of the universe, there will be more efforts to detect waves at smaller amplitudes and different frequencies. That is one reason the eLISA project is going on [1].
1. https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...
On the instrumental side, we've only just reached the sensitivity levels to make any detections in the first place, so it's not surprising that we're not getting a huge number of events (otherwise the previous generation of detectors would've seen something). In addition, each individual observatory has its own "antenna pattern", making us less less sensitive to certain sky locations. This will improve as VIRGO, KAGRA, and LIGO-India come online in the future.
Earth bound instruments will no doubt get better, but to get a real jump in quality, you need instruments in the stillness of space: http://www.einstein-online.info/spotlights/eLISA
I am just waiting for the $10,000 shielding for high end speakers to keep gravity waves from interfering with the acoustic purity of the sound they produce. :-)
There's also a more paper on this issue which you can find here: https://arxiv.org/pdf/1704.04628.pdf