# What will we learn from further LIGO events?

LIGO announced a third gravitational wave detection today. The news outlets didn't comment on significant differences between this one and previous detections. Did this third event teach us anything new about the cosmos?

More generally, what do we expect to learn from further gravitational wave detections by LIGO, now that we have confirmed that gravitational waves exist and conform to our previous expectations?

There are many things.

For instance the rate of black hole mergers as a function of look back time; the mass spectrum of the binary black holes; the spin of the black holes and whether they are always aligned or misaligned with the orbital angular momentum. All of these things give clues as to when and how these black holes formed, which may in turn be a step towards understanding how supermassive black holes are built.

To get any answers to these questions needs a population of events and an instrument with well calibrated sensitivity and noise properties. However, specifically this third event did provide strong (though not conclusive) evidence of misaligned spin and orbital angular momenta, which suggests the black holes didn't form in a binary system, but may have become a pair through some later interaction, perhaps in a dense cluster.

EDIT: An addition to this answer that I learned of recently, is that massive stellar evolutin models actually predict an upper limit to the mass of a black hole. The limit is caused by the probability of a pair instability supernova that ignites oxygeon in the core of more massive stars, resulting in total detonation and leaving no black hole. This upper limit could be established empirically by observing a large population of merging black hole binry systems. The result in turn constrains the uncertain physics inside massive stellar evolution models, particularly the highly uncertain carbon + alpha nuclear reaction rates.

• Can you comment on what kinds of selection bias might influence these observations? You need lots of events, for sure, but are there reasons to worry about events that we'd miss seeing for one reason or another? – Emilio Pisanty Jun 1 '17 at 20:13
• @emiliopisanty Hmm, well there are biases against finding low-mass and distant systems. These would be pretty well controlled for with a good knowledge of the sensitivity as a function of frequency. There is an orientation bias, but if we believe the universe is isotropic, that should be easily dealt with. – Rob Jeffries Jun 1 '17 at 20:21
• One can also wonder about a systematic bias concerning how binary system form. Addressing that calls for as many electromagnetic observations as possible, so that we can compare a population of observable black holes with the population of coalescing binaries. – dmckee Jun 1 '17 at 22:20

One of the outstanding issues in the theory of galactic evolution is the origin of the super-massive black holes at the center of many galaxies.

There are competing theories about how they formed from smaller holes, and knowing the distribution of masses of currently extant black hole might shed light on how the very large ones evolved, but only if a statistically significant number of observations are available.

• Overall one would expect at least some insight from seeing a population of events - how similar are they, how are they different, what is the rate, etc. We can now see something we could not before. So, what do we see? – Jon Custer Jun 1 '17 at 19:49

As pointed out in this article, there is a window for primordial black holes to form the bulk of the dark matter, the mass range is from about $20 M_{\odot}$ to $100 M_{\odot}$. These black holes would then have been formed at the end of the inflationary epoch. Further detections of black hole mergers in this mass range can confirm this possibility, if the event rate is too small then that would imply that primordial black holes could at most account or only a fraction of the dark matter.

• Continuing microlensing studies will slowly penetrate this window, but it's a long haul because the necessary density is fairly small so it is certainly possible that gravitational wave observation could confirm or deny the notion sooner. – dmckee Jun 1 '17 at 23:45
• I thought MACHOs as the bulk of dark matter was already ruled out, years ago. – JDługosz Jun 2 '17 at 10:52
• @JDługosz They were over a wide mass range from much less than one solar mass up to several solar masses. The scenario with a large population of black holes of many solar masses was simply considered unlikely. Still is as far as I am aware, but it isn't actually impossible given the data. – dmckee Jun 2 '17 at 17:00

This BBC article mentions that the signal showed that the two black holes rotated on distinct axes, suggesting that they did not form together (as their original stars, at least) in a single multiple star system, but rather formed a system later on. That such a thing is possible opens up a lot of possibilities for black hole mergers. It also provides another test of Einstein's theories, and the prediction that gravity waves are dispersionless. As the article says:

Also possible now are new investigations of the properties of black holes. The scientists can tell from the nature of the 4 January signal that the spins of the objects were not fully aligned when they came together.

This suggests they were not created from a pair of previously orbiting stars that exploded and then collapsed into black holes. Rather, their origin was more probably as stars that led independent lives and only at some end stage fell in as a duo.

"In that first case, we would expect that the spins would stay aligned," said Laura Cadonati, the collaboration's deputy spokesperson. "So, we have found a new tile to put in the puzzle of understanding formation mechanisms."

In addition, gravitational wave astronomy permits new tests of Einstein's theories. Because of the greater distance to this merger (twice the distance to the 2015 events), researchers could more easily look for an effect called "dispersion".

For light, this describes how electromagnetic radiation of different frequencies will travel at different speeds through a physical medium - to produce a rainbow in a glass prism, for example.

Einstein's general theory of relativity forbids any dispersion from happening in gravitational waves as they move out from their source through space towards Earth.

"Our measurements are really very sensitive to minute differences in the speeds of different frequencies but we did not discover any dispersion, once again failing to prove that Einstein was wrong," explained Bangalore Sathyaprakash, a LIGO team member from Penn State, US, and Cardiff University, UK.

The journal article itself can be found here.

For some casual comments by some distinguished LIGO members about the implications of this third observation (GW170104), see the new and quite heavily viewed Veritasium video NEW Gravitational Wave Discovery!

The caption says:

Scientists have JUST published this new observation. On January 4th, 2017 they detected the merger of two black holes 3 billion light-years away. This marks the furthest detection they've been able to make and increases confidence that these events will be seen with increasing frequency as the LIGO interferometers become more sensitive to low amplitude gravitational waves (as sources of noise are eliminated).

I have tried to transcribe a few of their comments below, but one should watch or listen to the entire video.

Starting at 02:05

Prof. David Reitze: This one really says ‘OK we now know that we’re going to be seeing a lot of these things.’

Prof. Rana Adhikari: It’s a relief to have another signal; to know that the Universe is not just populated by all tiny tiny black holes or no black holes.

Prof. David Reitze: If we improve our sensitivity by say a factor of 2 or 3, the rates will go up form seeing one every month or every two months, to seeing one every day or every week.

Prof. Rana Adhikari: I would say it’s very surprising now that our first three signals came from binary black hole mergers, which were pretty much an unexpected source as of mid-2015.

Prof. David Reitze: The working theory now is that the black holes we’re seeing now are primordial, that they weren’t formed through conventional supernova explosions, they were formed during the big bang themselves.

Prof. Rana Adhikari: It’s sort-of scratching at the door of the biggest mystery we have today in cosmology…

• Veritasium is excellent. I was just watching that vid to see if I could add anything here and saw your answer. Prof. Rana may be one of the most important people to get an opinion from on this subject. – Möoz Jun 5 '17 at 22:19

## protected by Qmechanic♦Jun 2 '17 at 4:41

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