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The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would pr
by antognini 11y ago
The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would produce NS-NS mergers. The BH-BH merger rate is almost totally unconstrained, although it is generally thought to be less than the NS-NS merger rate. So the fact that a BH-BH merger was the first detection, and the fact that it was detected so soon after the sensitivity increases is evidence that the BH-BH merger rate is probably somewhat higher than expected. But we won't know for sure until LIGO detects more events and the rate can be better constrained. Sometimes you do just get lucky.
I should add that there are lots of selection biases and educated guesses in all of this, too. The signal from BH-BH mergers is louder and easier to detect from larger distances. At the same time, NSs are probably more common than BHs, but it's not really clear whether there are more NS-NS binaries than BH-BH binaries because NSs receive kicks from the supernova when they are born but BHs (probably) do not. This may have the effect of blowing apart many nascent NS-NS binaries but leaving the BH-BH binaries intact.
- tomp 11y agoIf this events are so rare (that we don't even know how rare they are), how is it possible that they achieved the required certainty (5 sigma)? I guess you could count one looong wave as a series of one-time events/measurements, but it could as well be a loooong interference.
- cdelsolar 11y agoBoth detectors detected the same event at slightly different times
- 1024core 11y agoAbout 7ms apart, which ties in well with the distance between the two detectors.
- davrosthedalek 11y agoIt's not a counting experiment, which makes the calculation of a false positive rate somewhat harder. The key for LIGO is certainly that they saw the signal coincident at two stations, far apart.
- antognini 11y agoThe detection uncertainty is a separate matter from the predicted rate. Sure, if you had a strong prior that GWs should be detected once in a billion years, then you would want a better detection. But as it is the priors on the detection are pretty weak and this is totally consistent with what is expected.
- dragonwriter 11y agoOthers have answered other aspects of this, but as I understand it, it is not the case that we don't know how rare they (BH-BH events) are because they are so rare, we don't know how rare they are, because we don't have a really good model for them. So, we don't know how often we'd expect to detect them, once we had a detector.
- jcoffland 11y agoI believe the issue is that we don't know of they are rare at all. Perhaps they occur quite frequently. That is the big question.
- drx 11y agoFrom the paper: "To account for the search background noise varying across the target signal space, candidate and background events are divided into three search classes based on template length. The right panel of Fig. 4 shows the background for the search class of GW150914. The GW150914 detection- statistic value of ρˆ_c = 23.6 is larger than any background event, so only an upper bound can be placed on its false alarm rate. Across the three search classes this bound is 1 in 203 000 years. This translates to a false alarm probability < 2 × 10^−7, corresponding to 5.1σ. A second, independent matched-filter analysis that uses a different method for estimating the significance of its events [85,86], also detected GW150914 with identical signal parameters and consistent significance" (https://dcc.ligo.org/LIGO-P150914/public https://dcc.ligo.org/LIGO-P150914/public). Take a look at Figure 4 as well. In case you'd like to dig deeper, the 85 and 86 mentioned are: [85] K. Cannon et al., Astrophys. J. 748, 136 (2012). [86] S. Privitera, S. R. P. Mohapatra, P. Ajith, K. Cannon, N. Fotopoulos, M. A. Frei, C. Hanna, A. J. Weinstein, and J. T. Whelan, Phys. Rev. D 89, 024003 (2014),
- 21 11y agoThis is about detection. To put it another way, you need a single black swan to prove that black swans exists (to whatever sigma).
- rdancer 11y agoBut gravitational waves are a phenomenon without equivalent. Nothing like that had been observed before. (Am I wrong?) W̶h̶e̶n̶ ̶t̶h̶e̶ ̶L̶H̶C̶ ̶d̶e̶t̶e̶c̶t̶e̶d̶ ̶H̶i̶g̶g̶s̶,̶ ̶f̶o̶r̶ ̶e̶x̶a̶m̶p̶l̶e̶,̶ ̶t̶h̶e̶y̶ ̶h̶a̶d̶ ̶b̶e̶e̶n̶ ̶f̶i̶n̶d̶i̶n̶g̶ ̶p̶a̶r̶t̶i̶c̶l̶e̶s̶ ̶f̶o̶r̶ ̶m̶i̶l̶l̶i̶o̶n̶s̶ ̶o̶f̶ ̶m̶a̶n̶-̶y̶e̶a̶r̶s̶,̶ ̶a̶n̶d̶ ̶ s̶o̶ ̶i̶t̶ ̶s̶t̶a̶n̶d̶s̶ ̶ t̶o̶ ̶r̶e̶a̶s̶o̶n̶ ̶t̶h̶a̶t̶ ̶t̶h̶e̶y̶ ̶o̶n̶l̶y̶ ̶n̶e̶e̶d̶e̶d̶ ̶o̶n̶e̶ ̶e̶v̶e̶n̶t̶ ̶t̶o̶ ̶b̶e̶ ̶c̶e̶r̶t̶a̶i̶n̶ ̶t̶h̶e̶y̶'̶v̶e̶ ̶f̶o̶u̶n̶d̶ ̶ i̶t̶.̶ Why are the researchers certain that their equipment and methodology results can be trusted to such high degree, if there ever has been one positive measurement?
- pja 11y agoThe Higgs detection was not a single event but resulted from the statistical analysis of many events.
- rdancer 11y agoThank you for correcting me!
- cdelsolar 11y agoBecause of the shape of the event, detection in two places, and more importantly, it matching the signature of the theoretical event extremely closely (especially the ringing at the end)
- yaks_hairbrush 11y agoThis comment is making the page formatting gross. Those special characters with the strike-throughs make the entire page over-wide, thus requiring horizontal scrolling to read comments.
- deleted 11y ago[deleted]
- mikeash 11y agoIs there any idea how far away these black holes were? It would be interesting to know the volume of space it can potentially detect evens.
- drx 11y agoTFA says "they had heard and recorded the sound of two black holes colliding a billion light-years away" and "1.2 billion years ago". And from the paper: "The source lies at a luminosity distance of 410+160-180 Mpcc corresponding to a redshift z=0.09+0.03-0.04.". (https://dcc.ligo.org/LIGO-P150914/public https://dcc.ligo.org/LIGO-P150914/public) Which corresponds to 1.337+0.522-0.587 billion ly (or between 750.2 million and 1.859 billion ly).
- mikeash 11y agoOops, missed that. Thanks! Looks like there are roughly three million galaxies within a billion light years. Seems like lots of space for black hole pairs to live in. I suppose over the coming years, these gravity wave observatories will nail down just how common they are.
- xenophonf 11y agoThat's around 1/60th the diameter of the observable universe!
- gradi3nt 11y agoSo that's wayyyyyyy outside our galaxy? Any idea how many galaxies fit into a 1 billion ly sphere around the milky way? I'm guessing a shit ton, which makes the detection of a bh merger seem more realistic to me. That's some serious range!
- jballanc 11y agoIt was mentioned during the press conference today that gravitational waves are not affected by interstellar/intergalactic dust the same way light is. In theory, once our detectors are good enough we should be able to use gravitational wave astronomy to peer all the way back to the big bang!
- rdancer 11y agoWhat does that mean in plain English?
- lvs 11y agoEstimates about the frequency of observable events are not yet very good.
- zoke 11y agoI recall reading some years ago that gravitational wave would be used to prove multiverse theory. How would that scale compared to bh-bh or ns-ns mergers? Also, have read today that this discovery backs inflationary theory, how so? It seems highly unlikely that they could say a specific bh-bh merger was the cause. It seems implied they are triangulating the source, with two detectors?
- semi-extrinsic 11y agoAFAIK no multiverse theory has yet been put forth that is experimentally testable (even in theory given infinite time, energy etc.) So it's not a proper (falsifiable) scientific theory at present, merely a (in my opinion wild) conjecture.
- amelius 11y agoHow about a theory where spacetime touches upon another spacetime and interacts with it, perhaps through gravity? Or if spacetime folds back onto itself?
- semi-extrinsic 11y agoYou are suggesting possible future events that would provide evidence for a multiverse. That does not make something a falsifiable theory. Analogy: it could happen that tomorrow Jesus Christ descends from the heavens and brings the day of reckoning. That would prove Christianity to be true, but the fact that this could happen does not make Christianity a falsifiable theory. A falsifiable theory is a theory that predicts something that we can (in theory) measure today (possibly requiring infinite resources etc). Your second example (which is not a multiverse theory at all) is actually a good example of a falsifiable theory. People have calculated [1] that if spacetime was folded back onto itself at even just a single point, it would leave a distinct signature in the cosmic microwave background. We do not observe this signature, so we are pretty sure spacetime does not fold back onto itself. [1] http://arxiv.org/abs/1108.2842 http://arxiv.org/abs/1108.2842
- theptip 11y agoDo you know the current best estimate for BH-BH and NS-NS mergers? (Or at least, what they were before this experiment moved the needle). Interested to know what they were shooting for when they spun this experiment up.
- cyphar 11y agoWell, the number of solar-mass black holes in our galaxy is about 10^8. Since black holes form from stars, you can assume the probablity of having binaries is probably related to the probablity of having binary systems in stars, which is high. And the distance to the event is several megaparsecs (much bigger than our galaxy). The fact that they detected two 30 solar mass black holes coalescing 2 days after their sensitivity upgrades says that they almost certainly have had other, less pretty, detections in the few months they've been running their detectors for. Or they should go buy some lottery tickets.