The universe vibrates positively with gravitational waves.
Scientists working with LIGO / Virgo have just announced that they have discovered a total of 39 gravitational wave events in the first six months of their third observation, 26 of which have been previously announced, but 13 are new. Given that the first two observations give a total of 11 in total, this is a huge jump. And this is only the first half of the run.
This is a big deal. Gravitational waves are literally ripples in the fabric of space time, generated when extremely dense compact objects such as black holes and neutron stars spiral together and merge. The merge event shakes space-time, hard, generating thousands of times more energy than exploding starsbut everything goes in gravitational waves that move outward at the speed of light. They are ridiculously difficult to detect and take decades to create enough sensitive detectors to see.
This new puzzle includes some strange balls, including the merging of the black hole with the lowest mass and what could be a neutron star torn and eaten by a black hole. This also means that some cool science comes from having enough discoveries to start extrapolating trends.
So how does this work?
LIGO, the gravitational wave observatory for a laser interferometer, was the first to discover them in 201
As gravitational waves pass through the Earth, they literally shrink and expand space-time. The observatories are underground in long perpendicular tunnels. A few kilometers away are mirrors that separate and reflect powerful lasers. The beams are then combined together, and if a gravitational wave passes through the device, the distance between the mirrors changes, affecting the laser light. When combined, they create an interference pattern similar to that when you swing in a bathtub, making waves and sometimes combining to make even larger splashes.
Keep in mind that the amount that the waves change the distance in LIGO is incredibly small: about 1/1000 of the width of a proton! But this is enough to create a model in the laser light that can be seen.
The first two observations went well, revealing 11 positively identified events, including a fusion of black holes and even a pair of neutron stars colliding, exploding and creating what could be a black hole. But before the third run, the equipment was upgraded, including better lasers, better mirrors and better noise reduction techniques (a truck moving on a nearby highway is easily detected by LIGO).
They also created better “waveforms.” These are models made by mergers in detectors. As the two objects twist together, the gravitational waves they send increase in both amplitude (force) and frequency. This creates a pattern called “chirping” (because chirping, like chirping birds, is what you hear when sound waves increase in frequency and volume), and they are different for different mergers. So two small black holes that merge will generate a different waveform than two much more massive, or small and massive. A library of theoretical waveforms (created using the equations of general relativity) is used to match what is seen and determine what kind of event occurred.
The new mergers include several exceptional cases. One of them, called GW190924_021846 (for the Gravitational Wave event on September 24, 2019 and UTC time), is from the merger of the two confirmed black holes with the lowest mass ever; one is 6 times the mass of the Sun and the other is 9. The fusion discovered earlier may have had a lower mass component, but it is not clear that this is a black hole.
Another, GW190426_152155, may be from a black hole that breaks and eats a neutron star, but the signal is so weak that it’s hard for a scientist to be sure.
Now that so many mergers are being seen, some interesting statistics are beginning to emerge. For example, when looking at the two black holes that merge, there is a sharp drop in the number of mergers, where most of the pair is more massive than about 40 times the mass of the Sun. Sure, there are black holes more massive than that, but they look rarer (or at least don’t merge with other black holes as often). It is not clear why.
There is also great interest in finding black holes with a smaller mass. They form when massive stars explode and their nuclei collapse, forming black holes. The theoretical lower limit of such a black hole is about 3 times the mass of the Sun. What they found, however, was a reduction in the number of black holes by about 8 times the mass of the Sun and lower. There are those with a lower mass, but they are seen much less often in mergers. It is possible for the universe to make black holes from more massive stars, so the black hole itself is much more massive than the lower limit.
Interestingly, one of the documents that came with this edition was looking for gravitational waves coming from gamma-ray bursts, huge explosions that occur when black holes are first born. No one was seen, which is an important result. Maybe they are too far away or make the waves too weak to detect them. I hope someone happens close enough (though not too close!) To get a good discovery.
I love this! Gravitational wave astronomy is a whole new field (even younger than exoplanetary astronomy), and although the first fusion was discovered only three years ago, this discipline has come a long way. There are mergers of black holes that happen somewhere in the universe The whole time, generating background noise from gravitational waves, which one day we should be able to detect. Until then, we will have thousands of mergers discovered by all sorts of sites.
What strange and wonderful things will we learn by then?