A large group of astronomers announced that they had discovered 35 new compact object mergings, from black holes or neutron stars. This adds to the 90 previously detected in the past few decades. These explosions are some of the most powerful and energetic the Universe can produce, yet they emit virtually no light.
The three observatories, LIGO, Virgo and KAGRA Collaborations around the globe, were able to detect the mergers.
There were some funny ones in this new passel of mergings. One black hole ate a neutron-star, but not any neutron star. It was the one with the lowest mass. One created an intermediate mass black hole that was more than 100 times larger than the Sun. One more could be either from the lowest mass black holes or the highest mass neutron stars.
These observatories do not look for light. They look for gravitational wave: This is the literal shaking of spacetime's fabric as two very large but small objects spiral in towards each other before merging. Einstein's General Relativity states that any object can accelerate in spacetime and create these waves. These waves are a little like ripples in a bedsheet, depending on how you shake them. These waves are dependent on the object's mass and how hard it has been accelerated.
Illustration of two black holes orbiting one another. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)
This is where black holes are found. Although the waves created by two objects orbiting each other at first are very small, it takes energy to create them. They move closer together because they are stealing energy from their orbits. They orbit faster if they are closer, and generate more gravitational wave energy, which takes more energy. It's called a runaway effect. Although it's slow at first, the black holes are whirling around each others at a speed close to light. This is due to two large objects being accelerated very hard. After billions of years, the waves are strong and sharp. In the last few seconds, the black holes come together and coalesce, unleashing an enormous amount of gravitational energy.
These waves then travel at the speed of light and cross the Universe. They cause spacetime to expand or contract as they pass through Earth, but it is enough to measure with your instruments.
This is where the US LIGO, European Virgo and Japanese KAGRA (KAmioka GRAvitational wave detector), collaborations come in. The array includes mirrors and lasers that are separated by many kilometers. A gravitational wave can change the distance between mirrors, usually by about one thousandth of the diameter of a single proton. This can be detected by tiny changes in how the laser photons interact.
The patterns of the waves reveal what the merging objects looked like: Their mass, their orbits, and the total mass of any final object (usually a larger black hole) that was left.
It is amazing how much energy there is. This is an example of the energy. The gravitational waves were converted into the Sun's extra mass of 5 times. This amount of energy can melt your brain. It is roughly the same amount as the energy emitted for the same time by all stars in the Universe.
The black hole merger was the most powerful event in the Universe, but it was completely invisible. These mergers emit no light. All of it goes into shaking spacetime. It's amazing.
Illustration of the merging of a neutron star and a black hole. Credit: Carl Knox (OzGrav).
They also saw GW191219 which was a 31-solar-mass black hole that merged with an object of only 1.17 times Sun's weight. They found two waves that were consistent with the second being a neutron-star, which would make it the lowest-mass star ever observed. They form when a massive star explodes. They are also possible, but they are very difficult to make.
There were two other black hole/neutron-star mergers that I saw (which I also wrote about at the same time). However, re-analysis has shown that one of them (GW200105), may have been a false alarm. It's not clear.
GW200210 is a third interesting example. It's a merger of GW200210's 24 solar-mass with an object with a mass that's only 2.83x the Sun's. This is the very tipping point for the mass limit of a neutron Star, but it's very low for black holes. It could be either and I don't know which. The uncertainty in mass of the second object is approximately half that of the Sun, so if it is heavier than thought, it's most likely a black hole. If it's lighter, it's a neutron-star. This is all that can be said at the moment.
As of November 2021, all 90 gravitational waves detections have been made. Each vertical line represents a compact object merging (black holes are orange, neutron star red), with the two components below and the final product at its top. The object's mass in solar masses is represented by the vertical axis. (The Sun = 1 solar mass). Photo: Credit: LIGO-Virgo / Aaron Geller / Northwestern
These observatories are subject to "observing runs", where they are activated and listen for signals for several months. The observatories are then shut down for repairs, upgrades, mirror cleaning, etc. This third run, called O3, had a dramatically increased detection rate than the previous two because of all the upgrades. The current O3 run is in maintenance mode. O4 should begin in the latter part of 2022. O4 will likely see even more, considering the progress made since the beginning. The first gravitational wave event occurred on September 15, 2015.
This kind of astronomy has great importance. Although we know much about the formation of stars and neutron stars, this method is a great way to discover more, and even what happens afterwards. This is how black holes expand in size, something that we don't know much about. In galaxies less than a billion years old, we see black holes that are supermassive. They have a mass billion times greater than the Sun. How did this black hole grow so quickly? Are there mergers? Why is one like it so large, and another like the Milky Way's central dark hole -- which is only 4,000,000 solar masses -- so small.
Gravitational wave Astronomy opened up a new way of looking at the Universe. There are many answers to big questions that can be found now. We're just getting started.