Gravitational wave detector LIGO/Virgo has discovered a black hole 142 times the mass of the sun, the first time astronomers have observed a medium-mass black hole and the largest to be observed with gravitational waves.
The black hole is made up of two black holes 85 times the mass of the sun and 65 times the mass of the sun, which also challenges the current theory of black hole formation, which predicts that a black hole 85 times the mass of the sun is unlikely to exist.
The findings, published today in physical Review Letters and the Astrophysical Journal Letters, will help us better understand supermassive black holes at the center of some galaxies.
Picture: N. Fischer, H. Pfeiffer, A. Max Planck Institute for Gravitational Physics, Simulating eXtreme Spacetimes (SXS) Collaboration.
The seemingly empty universe is actually full of gravitational wave strings. Gravitational waves are produced by extreme astrophysical phenomena that stir up rings in space-time, just as bells in the universe transmit sound. This time, the researchers heard a loud bang.
The event is the largest black hole merger observed using gravitational waves, and the formation of 142 times the sun’s mass is also the first clearly detected medium-mass black hole (i.e., a black hole with a mass 100 to 1,000 times the mass of the sun). The black hole is made up of two black holes 85 times the mass of the sun and 65 times the mass of the sun, which also challenges the current theory of black hole formation: according to current theory, 85 times the mass of the sun’s black hole cannot exist.
This gravitational wave signal was detected on May 21, 2019 by the Laser Interferon Gravitational Wave Observatory (LIGO) in the United States and the Virgo interferometer in Italy, so the researchers named it GW190521.
Today, a team of LIGO and Virgo scientists published two papers in international journals reporting on the signal’s findings. One, published in physical Review Letters, details the discovery of gravitational wave signals, and the other, published in The Astrophysical Journal Letters, discusses the physical properties and astrophysical significance of signals.
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Previously, astrophysicists observed that all black holes could be divided into two categories: star-level black holes and supermassive black holes. Star-level black holes are several to dozens of times the mass of the sun and are generally thought to have formed after the death of large stars, while supermassive black holes are hundreds, thousands or even billions of times the mass of the sun, and there is a black hole at the center of the Milky Way. However, the final black hole that releases this gravitational wave signal is 142 times the mass of the sun, between a star-level black hole and a supermassive black hole.
According to stellar evolutionary physics, photons and gases in the core of a star produce outward pressure, balancing the gravity that drives the transverse matter inward, thus maintaining a stable state, such as the sun. And when a heavy element atom inside a star, such as iron, is fusioned, it cannot produce pressure that supports the outer layer. When the outward pressure is less than gravity, the star collapses under its own weight, causing a core-collapse supernova to explode, creating a black hole.
Photo credit: LIGO/Caltech/MIT/R. Hurt (IPAC)
This process could explain how stars 130 times the mass of the sun form black holes up to 65 times the mass of the sun. But for heavier stars, instability is also a factor to consider. When photons in a star’s nucleo are extremely energetic, they form electron and anti-electron pairs. These positive and negative electrons produce less pressure than photons, making the star more unstable and prone to gravitational collapse, which in turn produces a violent explosion large enough to destroy everything. Even larger stars, such as 200 times the mass of the sun, eventually collapse into black holes at least 120 times the mass of the sun. As a result, collapsed stars do not produce black holes with a mass between 65 and 120 times the mass of the sun, a range known as the pair instability mass gap.
But now, the heavier of the two black holes that send out the GW190521 gravitational wave signal is 85 times the mass of the sun, making it the first black hole so far to fall into this zone.
Nelson Christensen, a Virgo member and a researcher at france’s National Centre for Scientific Research (CNRS), said: “The fact that we have observed black holes with mass falling in this zone is enough for many astrophysicists to scratch their ears and try to study the origin of these black holes. “
One possibility is the hierarchical merger mentioned in the second article: before they approach each other and merge, the two original black holes are merged by two smaller black holes.
“This astrophysical event has raised more questions than it answers,” said Alan Weinstein, a professor of physics at the California Institute of Technology and a member of LIGO. “
This signal lasts extremely short and is less than 0.1 seconds long. The researchers speculate that the GW190521 gravitational wave signal came from a galaxy five gigasconds away from Earth, reflecting the fact that the universe is half its current state and has undergone a 7 billion-year journey to Reach Earth. It is also the most distant source of gravitational waves ever detected.
As for the source of the signals, scientists have speculated, based on advanced computational modeling tools, that GW190521 is most likely a signal generated by the merger of dual black holes of a particular nature. So far, almost all confirmed gravitational wave signals have come from 2-star mergers, including 2-black hole merges and 2-neutronic star merges.
The LIGO-Virgo team also measured the rotation of the two black holes separately, and found that as the black holes spinning closer to each other, their respective axis of rotation may deviate from the axis of the orbit. As the two behemoths continue to rotate close to each other, the misalbiting of the axes causes their orbits to wobble, or move.
The merger creates a larger black hole, about 142 times the mass of the sun, that releases a huge amount of energy in the form of gravitational waves to the universe that is eight times the mass of the sun. “It’s not the same signal that we usually detect chirping, ” says Christensen. “Compared to the gravitational waves first detected by LIGO in 2015,” the signal is like a loud bang, the most powerful source ligo and Virgo have ever found. “
The LIGO project is funded by the National Science Foundation (NSF) and consists of a pair of 4-kilometer-long interferometers. “LIGO surprised us again. Not just because it detects black holes of unexplained size, but because the technology it uses is not specifically designed to study galaxy mergers,” said Pedro Marronetti, director of the NSF Gravitational Physics Program. LIGO tells us that it can observe unexpected phenomena. “
When the LIGO and Virgo probes detect gravitational wave signals passing through the Earth, the automatic search program combs through the input data for signals of interest to the researchers. There are two methods of search: one is to use algorithms to find data patterns that may be generated by systems consisting of two dense stars, and the other is to look for all abnormal signals in a more common “burst searches”.
Salvatore Vitale, a LIGO member and assistant professor of physics at the Massachusetts Institute of Technology (MIT), likens algorithms that look for specific patterns to “passing through data with combs and always capturing something of a particular shape”, while burst search is a more general approach.
In the discovery of the GW190521 signal, it was the second method that singled out a clearer signal and found that it was unlikely that the gravitational waves came from sources other than 2-star mergers.
“The threshold for asserting that we find something new is high,” says Weinstein, “and we’ve always followed the Okam razor principle: the simpler the explanation, the better, and in terms of the GW190521 signal, the best explanation is a double black hole (merger). “
But what if this gravitational wave signal comes from a completely new object? It’s a fascinating prospect, and in the published article, scientists briefly consider other possible sources of gravitational waves. For example, it may have been emitted by a collapsing star in the Milky Way, or it may have come from strings produced in the early universe. However, these hypothes are not as consistent with the data as the double black hole merger hypothesis.
“Since LIGO was put into service, all credible observations have been collisions of black holes or neutre stars. ” “In this case, our analysis suggests that it may not have been a collision like this,” Weinstein said. Although this phenomenon is consistent with the unique mass of the twin black hole merger events, other explanations are not favourable, it still gives us confidence. It’s exciting. Because we are all looking forward to finding new things, eager to discover unexpected phenomena to challenge our existing knowledge. And this astronomical discovery does just that. “