Using gravitational wave detectors, astronomers have discovered a black hole 142 times the mass of the sun, the largest black hole merger event ever observed. Astrophysicists are very concerned about this gravitational wave event because it challenges the current theory of black hole formation. The universe is filled with the reverberation of the ripples of time and space – gravitational waves. Now, a new sound has been added to the “cosmic Symphony” we hear. < / P > < p > since 2015, astrophysicists have been using gravitational wave detectors to “listen” to chirp like signals in the universe, and analyze large-scale collision events that emit these signals by decoding these tiny ripples in space-time. Now, according to a new study by the laser interference gravitational wave observatory (LIGO) and the European Virgo interferometer (Virgo), scientists have detected a new gravitational wave signal from a fast and violent “explosion”. The study of the event may help us to solve more cosmic mysteries. < / P > < p > “this is another first discovery,” said Gabriela Gonzalez, a physicist at Louisiana State University and a team member of the new study. “We’ll never get tired of being number one.” After more than a year of research, this strange signal, known as gw190521, convinced scientists that they had discovered the largest black hole merger event to date. The 142 times solar mass black hole formed by this event was also the first medium mass black hole to be clearly detected. < / P > < p > according to the gravitational wave signal detected on May 21, 2019, astronomers speculate that the signal is generated when a medium mass black hole with 85 times the mass of the sun collides with a stellar black hole of 66 times the mass of the sun, resulting in a medium mass black hole of 142 times the mass of the sun. < p > < p > when scientists carefully combed the observations of gravitational wave detectors, they quickly discovered this unique signal. Astronomer Zsuzsanna Marka of Columbia University, co-author of the new study, clearly remembers that the probe received the signal on May 21, 2019. She is one of the few astrophysicists to connect mobile phones to cosmic exploration in real time, and is alerted whenever gravitational wave detectors “hear” possible signals in the universe. < p > < p > after receiving the alert, Marca began to check for neutrino bursts in these events. But that night, she realized that it was probably a very special probe. “I couldn’t help noticing the huge mass,” Marca said. She remembers thinking, “that’s great. It’s very significant. It’s really one of the massive events we’d like to see, and it’s incredible, but it’s not clear if there’s such a massive black hole. ” < / P > < p > black holes vary in size. According to the data of NASA, the mass of stellar black hole (a kind of black hole formed by the gravitational collapse of a massive star) is 10 to 25 times of that of the sun; the mass of a supermassive black hole is hundreds of thousands to millions of times that of the sun. It is generally believed that there will be supermassive black holes in the centers of galaxies (including the galactic center). Astronomers speculate that there may be some kind of black hole in between, that is, a medium mass black hole. It is estimated that the mass of a medium mass black hole is about 100 to 1000 times that of the sun. < / P > < p > this image shows black hole collisions and neutron star collisions detected so far. The event producing gw190521 is located at the top of the center, which is the largest black hole mass of all collisions. < / P > < p > unlike most small black holes, medium mass black holes are not formed by the explosion of dying stars. Compared with the stellar black hole formed by the gravitational collapse of a single star, the mass of the medium mass black hole is obviously too large; the star will always lose some material in the process of explosion, but when the star reaches a certain volume, no matter how much it grows, it will form a black hole with the highest mass of about 65 times that of the sun when it explodes. According to the LIGO team, larger stars lose more material when they explode, eventually forming black holes of the same size. < p > < p > this process can explain how a star with 130 times the mass of the sun can form a black hole with a maximum mass of 65 times, while for a star with a larger mass (130 to 250 times the mass of the sun), instability is more likely to occur, and the star will be completely destroyed, leaving no black hole or any other debris. As a result, astronomers believe that star collapse will not produce black holes with masses between 65 and 120 times the mass of the sun, a range known as the “pair instability gap.”. < / P > < p > until recently, medium mass black holes were only mysterious objects in theory, which were very elusive even by black hole standards. Using the early detection results of LIGO, astronomers have observed stellar black holes; on the other hand, the event horizon telescope has also captured images of supermassive black holes in the center of M87 galaxy, but for medium mass black holes, it is not easy to detect them. < / P > < p > this new explosion is the first evidence that a medium mass black hole has been detected. Astronomers’ calculations show that the gravitational wave signal is produced by a collision between a medium mass black hole 85 times the mass of the sun and a stellar black hole of 66 times the mass of the sun. “LIGO has again surprised us that it not only detects black holes of unexplained size, but also uses techniques that are not specifically designed for stellar mergers,” Pedro marronetti, director of the National Science Foundation’s gravitational physics program, said in a statement The LIGO project is funded by the National Science Foundation of the United States. < / P > < p > “this is very important because it demonstrates the ability of the LIGO detector, which detects signals from completely unpredictable astrophysical events,” maronetti said. “This detection shows that LIGO can also observe objects that we don’t expect.” < / P > < p > as usual, when it comes to gravitational waves, astronomers have to build hypotheses around a small amount of information deciphered in the detection results. They named the gravitational wave signal gw190521 and found that its duration is much shorter than other signals detected by LIGO, which is only one tenth of a second, and its frequency is much lower than that generated by previous black hole merging events. Astronomers can also track the signal to specific areas of the sky. < / P > < p > based on this information, astrophysicists calculated the distance at which the collision occurred – about 7 billion light years. They were also able to calculate the mass of the two colliding objects, 85 and 66 times the mass of the sun, and the mass of the object after the collision was about 142 times the mass of the sun (some of the mass was lost in the form of gravitational wave energy during the collision). < / P > < p > due to the limitation of the size of black holes produced by dying stars, these initial masses suggest that at least the larger black holes – and possibly the smaller ones – may be the result of the collision of two black holes themselves. “Two black holes merge to form a new black hole Then they merge again, “Maka said.” this only happens when there are a lot of black holes, which are very dense. ” < p > < p > Maka hopes that the collision will take place near an AGN, where other objects can be anchored nearby due to the strong gravity of the AGN. Active galactic nucleus (AGN) is a dense region in the center of a galaxy. The radiation emitted by AGN is considered to be the accretion of matter from the supermassive black hole in the center of the galaxy. However, using the current data, astronomers have no way to determine the exact mechanism behind gw190521. < / P > < p > in the longer term, if we can discover more merging medium mass black holes, we may be able to solve a major mystery about supermassive black holes, that is, their origin. < / P > < p > “they’re like elephants in a room, with millions of masses of the sun,” Christopher berry, an astrophysicist at Northwestern University and a LIGO researcher, said in a separate statement. “Did they evolve from stellar mass black holes (produced by the collapse of a constant star), or by some undiscovered way? For a long time, we have been looking for a medium mass black hole to fill the gap between a stellar black hole and a supermassive black hole. Now, we have evidence that medium mass black holes do exist. ” < / P > < p > although astronomers are excited about this gravitational wave signal and the possibility of finding a medium mass black hole, they are not sure that the current hypothesis is correct. Of course, merging two black holes 85 and 66 times the mass of the sun is the best fit for the data, but astrophysicists are also considering other, more exotic explanations. < / P > < p > “what if some new event produced these gravitational waves?” “It’s an attractive prospect,” said Vicky kalogera, a physicist at Northwestern University and an expert on the LIGO team She added that current assumptions about the cause of the signal include the collapse of a galactic star and some ancient cosmic string. Currently, LIGO and Virgo detectors are offline. Due to the novel coronavirus pneumonia epidemic, they were forced to close by the end of March. However, astrophysicists are planning to upgrade the two detectors and their algorithms to continue to detect space-time ripples in the universe. The upgrade of the detector and its algorithm is very important for tracking more signals like gw190521. If the detector itself is more sensitive, scientists can capture more distant signals, and fine-tuning data processing algorithms will make it easier for them to identify shorter signals like this. According to Gonzalez, the detection of a collision event between two black holes and the fact that one of the black holes itself was formed by merging indicates that there are many signals to be observed in the universe. “I hope that means there are more black holes – maybe clusters of black holes, because they come together, so they merge more frequently,” she said. “I hope these clusters are large and distributed in many places, so that we can detect more black holes.” < / P > < p > of course, all this must depend on the future detection results. “Nature does what it should do, we can’t tell it what to do,” Gonzalez said. “We collect data, we make discoveries, and then theoretical astrophysicists speculate and come up with new theories to explain how these giant black holes are created.” The team of LIGO and Virgo published the results of the relevant research in the Physical Review Letters and the Astrophysical Journal Letters on September 2. The former described the discovery process of gravitational wave signal in detail, and the latter discussed the physical properties and astrophysical significance of the signal. Global Tech