# What LIGO should detect that it has not yet?

What would the lack of detection of such phenomena tell us about the Universe?

What should be the most frequent signals detected, theoretically speaking? I'm a lay person in the area, so you might have to give a very didactic answer...

• Statistics. Enough events so that we can get good estimates of their frequency and the distribution of the masses of the objects involved. That in turn will let us build and check models of the evolution of things (stars) that give rise to these objects, and so on. – tfb Jun 2 '17 at 23:23
• Well, I know that with 3 (?) events it is hard to say much, but I expect somebody to come up with curves saying something about the distributions, detection window, and the most "banal" source of waves (as it might seem to a lay person that detections appear "common" in a sense). I must not be the first to ask this scientifically by a long shot, and I honestly want to know what would be the most ground breaking thing that could happen from the point of view of the strange physics that could come up from such information. – Vendetta Jun 2 '17 at 23:34
• "most interesting" is horribly opinion-based. Can you try to ask this question in a more objective way? – ACuriousMind Jun 3 '17 at 14:07
• I changed the focus of the question. – Vendetta Jun 6 '17 at 20:51
• Related: physics.stackexchange.com/q/336899/2451 and links therein. – Qmechanic Jun 6 '17 at 21:21

## 1 Answer

Good question. There are various categories of important findings.

One is related to the astronomy and astrophysics of black holes, and probably related phenomena. That includes how are those intermediate mass black holes formed (smaller ones of a few solar masses are understood, and supermassive ones partly, but not these 10-40 solar mass black holes), whether they are the source of gamma ray bursts and could that be black holes disappearing after Hawking radiation. Also finding any discrepancy with General Relativity such as whether black holes have any structure other than mass, change and rotation, or other possible violations of General Releativity (like dispersion of gravitational waves, which also was not seen in this last detection). Also neutron star structure and whether any are quark stars, supernova collapse dynamics, and others. Ths statistics are important not only to verify the observations, but more importantly to see if the numbers of black holes in the universe is what is expected, or if not if it indicates some cosmology factor we have not accounted for. See others at https://www.ligo.caltech.edu/page/science-impact

There is more also as we build others around the earth, and use them to correlate with LIGO to locate the exact sources, and thus marry the observations with radio and gamma ray observations, thus seeing the complete evolution. Also, as we build bigger ones, meaning interferometers in space with legs of 1000 Kms or more, we'll start seeing also the gravitational waves from supermassive black holes in the center of galaxies, or elsewhere, see their evolution and thus get some closure on galaxy formation. Even more, we might detect things that physics suspects might have occurred early in the universe (the first microseconds till about 380,000 years after the Big Bang. We could see the early effect of primordial Black Holes, and if they exist a first observation of cosmic strings and cosmic anomalies in the universe. This would have a large impact on particle physics, and for quantum gravity, indicating what may be valid for energies and sizes way beyond what we can do in the LHC, or any super LHCs, or anything we can see in earth bound gravitational interferometers. We might also find something about the sources of dark energy and dark matter.

Anything that proves more or disproves any part of General Relativity would be huge. Also, anything that gives us a hint of what very strong gravity and quantum gravity does would be a step to a still nonexistent a accepted theory of quantum gravity

See Scientific American more than a year old for some of these also, at https://www.scientificamerican.com/article/the-future-of-gravitational-wave-astronomy/. There should be more 'what's the future' articles coming out after this last detection.

• Just read the Scientific American article. Later I'll read the Caltech link. As far as I could understand this is the beginning of the "spectroscopy of gravitational waves". Kinda cool. – Vendetta Jun 12 '17 at 15:38
• You got it right. – Bob Bee Jun 13 '17 at 0:20
• I-is it too early to revisit this question? – Vendetta Oct 25 '17 at 15:17
• Well, yes , actually one very big thing. It's the multi messenger astronomy, combining GW detections and electromagnetic ones from mergers which produce both. In August 17, 2017, two neutron stars were detected merging, and both GW and EM waves detected. Big news. I did not realize at first that was after this posting, but definitely was. Not many questions on this site on it, but it confirmed that GW and EM waves travel at the same speed of light, to about a part in $10^{15}$. Google kilonova and post a question. The post below is wrong, and correctly voted down, proved in this new detection – Bob Bee Oct 26 '17 at 2:47
• Yeah, I knew about the detection of the two neutron stars (which is why I asked for a more recent comment). So it has been like 5 detections? I just hear about the big stuff - and I admit all detections might be big, still - so I'm not sure. I don't remember the rate they expected the detections to take, but it seems to be going well for an experiment so difficult to make. – Vendetta Oct 26 '17 at 13:41