Researchers have picked up no fewer than 35 signals, including two very special ones.

The famous physicist Albert Einstein already predicted the existence of gravitational waves (see box) some 100 years ago. But the first were only discovered in 2015. In that year, gravitational waves resulting from two merging black holes were detected. Things have been going fast since then; in the meantime, dozens of signals have been picked up. And now there are a few more. Because in a new study researchers announce that they have detected as many as 35 new gravitational waves.

More about gravitational waves
Gravitational waves are ripples in space-time that occur when two massive objects orbit or merge very closely around each other. Einstein already predicted the existence of gravitational waves at the beginning of the twentieth century. But it wasn’t until 2015, researchers managed to actually detect them. In the course of 2016 and 2017, a disturbance in the gravitational field will be detected several times as a result of merging black holes. In late October 2017, researchers then spotted gravitational waves created by the fusing of two neutron stars. And this year, for the first time, gravitational waves from a colliding black hole and a neutron star have also been picked up.

The researchers took a close look at the tsunami of detected gravitational waves. Because what exactly was behind these ripples in space-time? The researchers find that 33 signals likely resulted from mergers of black holes of various shapes and sizes.

Gravitational waves arise from merging black holes of various shapes and sizes.

But, the other two detections are even more special.

Black hole and neutron star merger

According to the researchers, one of the other two detected gravitational waves probably originated from a collision between a black hole and a neutron star, a much rarer event. The black hole was quite massive (about 33 times the mass of our sun) and may have collided with a very low-mass neutron star (about 1.17 times the mass of our sun). And that’s special. Because this neutron star has one of the lowest masses ever detected. “Our understanding of mergers between lower-mass objects has been vague,” said researcher Zoheyr Doctor. “But now that we have hard detections, we’re starting to fill in that lower end of the mass spectrum as well.”

More about black holes and neutron stars

Black holes and neutron stars are two of the most extreme objects ever observed in the Universe. How do they arise? When the most massive stars die, they collapse under their own gravity, leaving behind black holes. When stars that are slightly less massive die, they explode in a supernova. What remains are the dense and dead remnants of stars called neutron stars. Typical neutron stars have a mass one and a half times that of the Sun, but all that mass is contained in an extremely dense star. A teaspoon of neutron star weighs about as much as all of humanity!

In the latter case, a black hole (about 24 times the mass of our sun) collided with either a very light black hole or a very massive neutron star about 2.8 times the mass of our sun. Scientists suspect that it is most likely a black hole, but they are not entirely sure.

Increasing Detections

Taking the new detections into account, researchers have now detected a total of 90 gravitational waves. The fact that researchers are increasingly encountering gravitational waves in recent years can easily be explained. The waves reaching the earth are barely perceptible, making it very difficult to detect them in the beginning. But meanwhile, the tools that scientists can pick up on them are getting better and better. It means that scientists are now detecting such vibrations of the universe sometimes even several times a day.

“As our international network of interferometers becomes more sensitive and observes longer, we detect more gravitational waves,” says Doctor. “We can learn a lot from these very short, weak signals. We are starting to see rarer events and unlocking the diversity of mergers between black holes and neutron stars. Ultimately, we want to know how these exotic objects arise and collide with each other. And the new detections will help us do that.”

Diversity

So, as more detections are added to the catalog of gravitational waves, researchers are learning more and more about these extraordinary cosmic phenomena. And that means that we are actually only now beginning to see the wonderful diversity of black holes and neutron stars. The new detections prove that they are available in many sizes and combinations. By calculating the masses of the merging objects, astrophysicists can then better understand how stars live and die, and exactly why they collapse into black holes or neutron stars when they die. Ultimately, the researchers hope to unravel how stars — the building blocks of our universe — evolve.

The LIGO observatory and gravity detector Virgo is currently undergoing major updates. This will prime the powerful detectors for a new series of gravitational wave searches expected to start in late 2022. And it promises to be an exciting hunt. “Our improved detectors will be able to pick up quieter signals, including those from black holes and neutron stars that fused even further away and billions of years ago,” said study researcher Maya Fishbach. “I can’t wait to discover what else is out there.”