Eight hundred million light years from Earth, two massive cosmic objects merged after a long spiral dance, the violent burst of energy at the end literally shaking the fabric of space-time. One of these objects was a large black hole. The other was … well, something. It is unclear what: a very large neutron star on the hairy thin edge of becoming a black hole, or the smallest black hole ever seen.
Either way, this is very good.
The event itself is called GW190814 – GW for the gravitational wave, followed by the date: August 14, 2019. Gravitational waves are waves in space-time emitted when massive compact objects merge. This was predicted by Einstein’s theory of general relativity, but these waves were not directly detected until 2015, when a pair of fused black holes were observed more than a billion light-years away from Earth.
Since then, about a dozen confirmed gravitational wave emission events have been seen, including one of a pair of fused neutron stars, which was a big problem. But for that one exception, all of these have been from two black holes that merged after orbiting each other for eons. Not only that, but the two black holes have almost always been very similar in mass.
But that has now changed. An article analyzing GW190814 has just come out, and scientists discovered it has the most asymmetric mass ratio ever seen.
One of the objects was 23 times the mass of the Sun, so it was definitely a black hole. But the other was much lower in mass, between 2.5 – 2.67 Soles. That’s … peculiar. It is certainly possible Having a black hole with such a low mass, but it’s rare. However, it would be even stranger if it were a neutron star. The most massive ever found is 2.14 times the mass of the Sun, which is dangerously close to the upper limit a beast can have without collapsing into a black hole. So finding a neutron star with an even higher mass is highly unlikely! It may not be strictly impossible, but if I had to bet, I would bet it would be another black hole.
But that’s also great! It would be the lowest-mass black hole ever detected, which is quite a find. So right away this is a pretty important discovery for astrophysics.
The mass of the second black hole is placed directly into the so-called “mass gap”. Black holes more than about 5 times the mass of the Sun are seen quite frequently, but none (or at least very few) are between 2.5 to 5 solar masses. Theoretically speaking, black holes like this can exist, although they may not be as common. But it has been an open question if they really are very rare or if we are simply not very good at finding them. Our methods for finding black holes favor larger ones, so perhaps we are just missing them.
So if this second object is a black hole (again, bet like that), then this means that do we exist and we can Find them. It doesn’t necessarily answer the question, but it does raise hopes that as we find more fused black holes, those with less mass will be in the mix.
This is the most unequal mass fusion ever seen, and that brings another cool thing. Gravitational waves are emitted by each black hole as they oscillate around the other. If they have the same mass / size, then the waves we see are more or less the same in both (which is usually the case). But if their masses are unequal, as in this case, the smallest black hole forms a wide orbit around its center of mass, and the largest black hole forms a smaller orbit. That means that the waves are different and create what is called multipolar, like the harmonics in the frequencies. If you play a guitar string, it will vibrate at a certain frequency, but there will also be higher frequency vibrations, also called harmonics.* *.
With gravitational waves (and also other types of systems), these come in powers of two, so you get dipoles, quadrilaterals, octapes, etc. That’s good, because it breaks some symmetry seen in the signal that makes it difficult to decipher certain characteristics. For example, distance and orbital angle are tangled in the signal, but if you can see higher order things can be separated (which is partly how the 800 million light-year distance was found).
Still, this leaves the question of how this system was formed. You need very massive stars to explode and form black holes (probably 15 times the mass of the Sun or more), and generally very massive stars in binaries have similar masses. So this system was special somehow. I was surprised to read that it could have formed on the disk of matter by rotating around a supermassive black hole at the center of an active galaxy! Typically, two uneven (but still very) massive stars would have difficulty connecting, but in a dense disk of gas physics makes it easier; The gas acts as a drag on the stars, making it possible for them to become binary.
It is also possible that it was a triple system, with two stars orbiting each other, and a third further away. If they pass near the 23-solar-mass black hole, the third star could be ejected and the binary would orbit around the black hole. The two stars could merge, explode, and form the lowest-mass black hole. It’s a bit of a Wile E. Coyote scenario, but it’s possible.
I have to mention one more thing. These waves in the structure of space are little. I like it little little. LIGO and Virgo, the two observatories that detected this gravitational wave event, use lasers and mirrors to measure them; the setup can detect incredibly small changes in the length the laser travels during its multi-kilometer journey that bounces between the mirrors. As a wave passes through Earth, the distance between the mirrors changes by about 1/1000 of the diameter of a single proton.
A. Single. Proton.
This is why it took so long to detect the first event after they were predicted in the early 1900s. It is basically Star Trek technology However, you can “hear” the fusion of a pair of black holes (and even hear harmonies!) a great fraction of the way through the observable Universe.
Because that’s how great science is.
* *Gravitational waves can be converted into sound, and they sound like a screech: the frequency increases as black holes get closer, and the tone increases rapidly at the end. You can listen to the simulated merge sound for this binary with just the fundamental background hereand with the harmonic harmonics here.