I started this post with the idea of summarising all of the new science which has resulted from observations made up to the end of the third observing run of the advanced LIGO and Virgo detectors. It turns out there’s rather a lot, so this looks like it’ll be the first of several! I’ve tried to include links to as many papers as possible.


The foundational analyses for the third observing run were reported in two catalogues, GWTC-2 (later updated by GWTC-2.1) and then GWTC-3. These catalogue papers summarise the analysis of ninety-one signals from the first three observing runs, with the bulk of new events coming from the third observing run. Ninety of these are regarded a confident gravitational wave detections, and one is a marginal candidate, lying on the threshold of what the collaborations deem a confident detection. These catalogues provide details of the properties of the astrophysical systems which created the detected gravitational waves, but don’t generally discuss what can be gleaned from this new knowledge.

In addition to the LVK catalogue, groups outside our collaboration also performed similar analyses:

Exceptional Events

Six events in the third observing run were deemed to be sufficiently novel or unusual to be the subject of individual publications.

GW190425 was observed to be produced by the merger of two compact objects with a total mass of 3.4 solar masses. This is generally considered too light to be a system containing a black hole, but a little too heavy for what we had generally expected from a pair of neutron stars. The working assumption is nevertheless that GW190425 was produced by two neutron stars colliding, but we can’t rule out the system having contained one or more unusually light black holes.

GW190412 was the first gravitational wave event which was observed to come from a “lop-sided” binary. The heavier black hole in the system had a mass around 30 solar masses, over three times heavier than its companion which weighed-in at around 8 solar masses. One of the interesting features of this sort of system is that they were predicted to emit gravitational wave energy at higher harmonic frequencies. This is something which we were able to detect in our analyses for the first time in the third observing run, and we were indeed able to measure this additional energy for the first time with this event.

GW190521 was an especially mysterious signal. We determined that it had been produced by a pair of colliding black holes with a total mass around 150-times greater than the Sun, with the individual components coming-in around 85 and 66 solar masses (we can blame some rounding-up here for why those numbers don’t all add-up!). This was problematic, because we don’t know of a physical process which can directly create an 85 solar-mass black hole. This lies in what we call the “mass gap”. Nuclear physics tells us that stars which have the right mass to collapse to a black hole of this size do not in fact collapse to a black hole, but instead undergo a violent explosion which doesn’t leave any form of compact remnant. The only way that we do currently know of producing a black hole of this size is through the merger of two existing black holes, making GW190521 the first time that we think we’ve seen what we call a hierarchical merger, where at least one of the black holes involved is the result of a previous merger. Perhaps more excitingly, the lighter, 66 solar mass black hole also sits in the mass gap. We’re not able to measure the masses precisely, however, so there is a reasonable chance it lies just below the mass gap too. This does mean that there’s a high probability that both of the black holes in this merger came from previous mergers.

GW190814 was a strange compact binary system, where the heavier component looked very definitely as if it was a black hole with a mass around 23 times that of the sun, but the secondary had a mass a little over two and a half times that of the sun. This would be either the lightest black hole we’ve ever observed or the heaviest neutron star. Either way, this was the most enigmatic of the O3 events.

GW200105 and GW200115 complete the gallery of exceptional events for O3. These events are the first examples of merging systems which contain both a neutron star and a black hole (conveniently known as neutron-star black-hole mergers, or NSBHs). These are a rather odd category of events, and they produce rather unusual signals which don’t quite look like a black hole merger, and don’t quite look like a neutron star merger (perhaps unsurprisingly!). They also represent a new discovery for gravitational wave astronomy. Binary neutron stars and binary black holes have been observed previously using electromagnetic radiation, either directly (for binary pulsars, for example), or indirectly (e.g. x-ray binaries). GW200105 was the first ever observation of both kind of compact remnant in the same system.

New science

Trying to summarise all of the new science which has been derived from the new observations would be rather difficult; the GWTC-3 catalogue paper has over 1,200 citations at time of writing!

However, I’ll try and cover some of the big outputs of my collaboration here briefly, and perhaps follow-up with some other results later.

Cosmic expansion

One of the most attractive features of gravitational waves is our ability to use them to infer the rate of cosmic expansion. To measure this we need to be able to measure two quantities: the so-called luminosity distance, which is a measure of how far photons have actually travelled from their source to reach the observer, and the recession velocity of the source from the observer. Using electromagnetic techniques it’s fairly easy to measure the latter by observing changes in spectra caused by redshift. The former is much harder to do, especially for distant sources. However, we can measure this quantity directly from gravitational wave signals. This means that if we observe a gravitational wave, and can observe its source with a telescope we can measure both of these quantities, and make inferences about the expansion of the Universe.

Things are not quite so simple as that, of course. First we can’t measure the distance very well for most gravitational wave events, so we have very broad uncertainties on any distance we measure. Second, we mostly observe events which can’t be observed in electromagnetic radiation: black hole mergers. We have, however measured one event with both techniques: GW170817.

In O3 we used a new technique which tries to statistically determine the likely host galaxy of our observations to partially overcome this problem, and as we observe more BBH events we can get a more refined measurement of the expansion rate using this technique. The full O3 results suggest this is 68km/s/Mpc which is currently consistent with other techniques’ measurements.

Testing General Relativity

General Relativity is the theory of gravity which was proposed by Albert Einstein in 1915, and remains the theory which best explains the behaviour of gravity throughout the Universe. The detection of the first gravitational wave, GW150914, was a major observational test of the theory, which matched the observational data perfectly. However, there is plenty of room for deviation between observation and theory, and as we observe more unusual astrophysical systems there are more avenues for these deviations to appear along.

The large number of events observed in O3 allowed much more sensitive tests than before, and no evidence was found for physics beyond General Relativity, and it retains its position as one of the most successful theories in modern physics. The tests also provided a more precise measurement for the mass of the graviton, the particle which is hypothesised to mediate the gravitational force in quantum field theory.

Astrophysical populations

I like to describe one of the major uses of gravitational wave observations to be a form of cosmological archaeology. Black holes are the end product of a star’s death, and what we’re doing is observing objects colliding in the stellar graveyard. Very metal. I think.

We can work backwards from the properties of the black holes we observe to the stars which originally produced them. It’s still early days for this area of research, but O3 shows growing evidence that the Universe contains an unexpectedly large number of black holes around ten times heavier than the Sun, and 35 times heavier.

This is definitely one of the areas of GW science which will be very exciting as we make more observations in O4 and beyond, as it benefits greatly from an increased number of observations to make inferences and discoveries.

Lensed signals

One of the more curious properties of gravity, which is the result of the deformation of the geometry of the Universe itself, is that light travelling through the Universe can be lensed by heavy objects, which bend the spacetime the light travels through.

This is also true of gravitational waves, raising the distinct possibility that we might observe lensed gravitational waves. This is most likely to manifest itself as multiple apparent observations of very similar looking gravitational waves, coming from a similar location on the sky but at different times. Each observation would be a different “image” of the same system.

So far we’ve not found any compelling evidence that any of the gravitational waves detected so far were lensed, but this is another topic which will only benefit from an increased number of detections.

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