The ripples in spacetime, created by a stellar cataclysm in a distant galaxy, help explain the cosmic origins of gold and chart a course for a new era in astronomy, observing the electromagnetic spectrum and gravitational waves.
The beginning of a new era in astronomy and physics was announced on Monday by scientists that they had first detected ripples in spacetime, known as gravitational waves, which were formed by the collision of two neutron stars. On August 17, these waves from space reached Earth in the Indian Ocean region and were registered by two detector stations of the American Laser Interferometric Gravitational Wave Observatory (LIGO) and the European Virgo detector located in Italy.
This is the fifth time in the past two years that scientists have recorded such waves. Einstein was the first to predict this phenomenon, having done it more than 100 years ago. And this year, three leaders of LIGO received the Nobel Prize in physics for their discoveries in the field of gravitational waves.
However, all previously observed gravitational waves originated from the merging of black holes. These black holes are so dense that they don't release light. Therefore, such a merger of black holes is essentially impossible to detect with conventional telescopes, despite the incredibly powerful gravitational waves that they generate in the last moments of their frantic death spiral. Without a larger network of gravitational-wave observatories, astronomers are unable to pinpoint the exact location of the merging black holes, much less study and analyze them in depth.
Merging neutron stars, however, begins with objects that can be very light compared to black holes. A neutron star is the highly compressed core of an expiring massive star, and it forms after a supernova explosion. Its gravitational field is strong enough to squeeze and destroy matter as large as the entire Sun, turning it into a sphere of neutrons the size of a large city. Thus, it is not a star in the usual sense, but rather a core of an atom the size of Manhattan. However, the gravitational force of a neutron star is still too small to hold light, and therefore a flash from the collision of two such stars can penetrate into space, creating not only gravitational waves, but also one of the brightest fireworks in the Universe that anyone can see.
In this case, when the initial pulse of gravitational waves signaled the start of the merger, the fireworks consisted of a burst of gamma radiation two seconds long and afterglow of different wavelengths that lasted several weeks. Almost all astronomers and physicists on our planet who knew about this event were among "anyone who wants to". Project researcher Julie McEnery, working with the Fermi gamma-ray telescope, which recorded a burst of gamma rays, called August 17 "the most wonderful morning in all nine years of the telescope."
Astronomers working with physicists on the LIGO and Virgo telescope have taken an oath of secrecy. However, the huge number of observations around the world inevitably led to the spread of rumors, which have now been confirmed. This is a worldwide campaign to monitor the collision and its aftermath. The burst of new observations and the emergence of new theories after the collision is the most striking example of gravitational wave astronomy. It is a new branch of science that collects data and studies light, gravitational waves, and subatomic particles from astrophysical cataclysms.
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At the same time, a huge number of articles were published in several scientific journals, the authors of which connected the latest events with a wide variety of phenomena and proposed new ideas in various directions, from fundamental nuclear physics to the evolution of the Universe. Among other things, this merger gave observers the opportunity to trace the origin of a black hole, which could have formed in the collision of neutron stars. But one discovery is literally brilliant. This is compelling evidence that a neutron star merger is a cosmic melting pot in which heavy elements of our universe, including uranium, platinum and gold, appear.
So it says a lot about the fact that the radioactive material in a nuclear reactor, the catalytic converter in your car, and the precious metal in your wedding ring are the result of the collision of the smallest, densest, and most exotic stars in our universe, or at least the part of them that can escape from the black holes formed as a result of the merger. This discovery will help resolve the ongoing debate on the cosmic origin of heavy elements, which theorists have been engaged in for more than half a century. Most of the hydrogen and helium in our universe appeared in the first moments after the big bang. And most of the light elements, such as oxygen, carbon, nitrogen, and so on, were formed by nuclear fusion in stars. But the question of the origin of the heaviest elements has not yet been answered.
“We stumbled upon a gold mine! says Laura Cadonati, an astrophysicist at the Georgia Institute of Technology and LIGO's deputy press secretary. - In fact, we first discovered the gravitational-wave and electromagnetic phenomenon as a single astrophysical event. Gravity waves tell us the story of what happened before the cataclysm. Electromagnetic radiation tells about what happened after. " While these are not final conclusions, Kadonati says, the analysis of the gravitational waves of this phenomenon over time will help reveal the details of how matter is "splashed" inside neutron stars when merging, and scientists will receive new opportunities to study these strange objects, and also find out what size they can reach before they collapse and become a black hole. Kadonati also notes that there was some kind of mysterious delay of a couple of seconds between the end of the burst of the gravitational wave and the beginning of the gamma radiation. Perhaps this is the period of time when the structural integrity of merging neutron stars for a short time resisted the inevitable collapse.
Many researchers have long awaited this breakthrough discovery. “My dreams have come true,” says astrophysicist Szabolcs Marka at Columbia University and part of the LIGO research team. Back in the late nineties, this man became an adherent of gravitational wave astronomy, supplemented by observations of the electromagnetic spectrum. In those years, Mark recalls, he was considered a madman who was trying to prepare for future observations of gravitational waves, although there were still several decades before the direct discovery of this phenomenon. “Now my colleagues and I feel avenged,” he says. “We studied this system of colliding neutron stars in a very diverse set of signals. We saw it in gravitational waves, in gamma rays, in ultraviolet light, in visible and infrared light,as well as in X-rays and radio waves. This is the revolution and evolution in astronomy that I had pinned my hopes on 20 years ago."
The director of the National Science Foundation (the federal agency providing the bulk of LIGO's funding), France Córdova, said the latest achievement is a "historic moment in science" and that it was made possible by the sustained and longstanding government support of many astrophysical observatories … “The detection of gravity waves, from the first short vibroseismic signal heard around the world to the last, longer signal, not only justifies the risky but rewarding investment from the National Science Foundation, but also pushes us to do more in that direction, says Cordova. - I hope NSF will continue to support innovators and innovations,that will transform our knowledge and inspire generations to come."
What a great opportunity
When the initial gravitational waves from the merger were detected, followed by gamma rays (immediately detected by scientists using the Fermi telescope and INTEGRAL space telescopes), a race began to find out what was the source of the collision in space, as well as its afterglow. Very quickly, numerous teams of scientists aimed their existing telescopes at that part of the sky where, according to the calculations of the researchers with LIGO and Virgo, the source should have been. It was a patch of sky spanning 31 square degrees, containing hundreds of galaxies. (According to Kadonati, if only the LIGO observatory were used, these observations would be similar to searching for a golden ring lying on the bottom of the Pacific Ocean. But with the third data point from Virgo, she says, the researchers were able to calculate the location of the source.and as a result, the observations became more like “searching for the golden ring in the Mediterranean.”)
The main part of the observations was carried out by scientists in Chilean observatories. They began their work immediately after sunset, when the desired part of the sky came out of the horizon. Different teams of scientists used a wide variety of search strategies. Someone simply carried out continuous observation of a section of the sky, methodically moving from one side to the other; someone was targeting galaxies in which neutron stars were most likely to merge. Ultimately, the second strategy turned out to be a winning one.
The first to see the optical afterglow was a doctoral student and researcher at the University of California, Santa Cruz, Charles Kilpatrick. He sat at his desk in his office and looked through images of some galaxies, having received an assignment from one of his fellow astronomers Ryan Foley, who helped organize the project. The ninth image he began to study was a photograph, hastily taken and shared by colleagues on the other side of the world working on the huge Swope telescope at the Las Campanas observatory in Chile. It was on it that he saw what everyone was looking for: a bright blue dot at the center of a giant elliptical galaxy, which is a cluster of old red stars 10 billion years old, which were located at a distance of 120 million light years. They were all namelessexcept for the designations in the catalogs. It is believed that it is in such galaxies that mergers of neutron stars most often occur, since they are old, their stars have a high density, and there are quite a few young stars in such galaxies. Comparing this image with earlier images of the same galaxy, Kilpatrick did not see such a point on them. It was something new, just recently. “It really slowly dawned on me what a historic moment this was,” Kilpatrick recalls. "But at that time I was focused on my task, trying to work as quickly as possible."Comparing this image with earlier images of the same galaxy, Kilpatrick did not see such a point on them. It was something new, just recently. “It really slowly dawned on me what a historic moment this is,” Kilpatrick recalls. "But at the time, I was focused on my task, trying to work as quickly as possible."Comparing this image with earlier images of the same galaxy, Kilpatrick did not see such a point on them. It was something new, just recently. “It really slowly dawned on me what a historic moment this was,” Kilpatrick recalls. "But at the time, I was focused on my task, trying to work as quickly as possible."
Kilpatrick shared the sight with other members of his team, including Carnegie astronomer Josh Simon, who quickly captured a confirmation image with one of Chile's largest Magellan telescopes, six and a half meters in diameter. The blue dot was also present in these images. For an hour, Simon measured the spectrum of this point, that is, the various colors of the light it emits. He did this in paired shots with a shutter speed of five minutes. Simon believed that such spectral images would prove useful for further research. And if not, then in any case they will be able to prove that this is not just some ordinary supernova or some other cosmic impostor. Meanwhile, other teams of scientists also noticed this point and began to study it. But Foley's team was quicker than others to find confirmation and conduct a spectral analysis, securing the lead in this discovery. “We were the first to get the image, and we were the first to identify the source of that image,” says Simon. “And since we got both the first and the second very quickly, we were able to make the first spectral analysis of this merger, which no one in Chile could do that night. After that, we announced our discovery to the entire scientific community. "After that, we announced our discovery to the entire scientific community. "After that, we announced our discovery to the entire scientific community."
These first spectral observations proved to be extremely important for the subsequent analysis and solution of some mysteries. They showed that the remnants from the fusion quickly cool and lose their bright blue light, which turns into a deep ruby. These data were checked and confirmed in the course of observations in the following weeks, while the visible point faded and faded, and its afterglow shifted, and the bright light passed into the infrared region of the spectrum with a longer wavelength. The general patterns of color, cooling and expansion were very similar to what many theorists, working independently, had previously predicted. First of all, these are Brian Metzger of Columbia University and Dan Kasen of the University of California, Berkeley.
In short, Metzger explains, what the astronomers saw after this merger could be called "kilonova." It is an intense burst of light produced by the release and subsequent radioactive decay of white-hot, neutron-rich material from a neutron star. As this material expands and cools, most of its neutrons are captured by the nuclei of iron and other heavy elements left as ash from the supernova explosion and the formation of a neutron star. “This leads to the creation of even heavier elements within about one second, when the ejected particles capture these neutrons and expand in space. One of these mergers forms the bottom half of the periodic table, namely gold, platinum, uranium and so on,”says Metzger. At the final stage, the light from the kilonova sharply shifts to the infrared zone, when neutrons cascading out of the ejection form the heaviest elements that absorb visible light very effectively.
Measuring the spectral changes in the kilonova body, in turn, allows astronomers to determine the number of different elements formed during the fusion process. Edo Berger, who studies kilonovae at the Smithsonian Center for Astrophysics and led the many and most ambitious observations of this merger, says that the event produced heavy elements, weighing 16,000 Earth masses. “It's all there: gold, platinum, uranium, and other, weirdest elements that we know as letters on the periodic table, although we don't know their names,” he says. “As for the disintegration, the exact answer to this question is still unknown to us.”
Some theorists suggest that the amount of gold formed as a result of the merger is only a few tenths of the earth's mass. Metzger, for his part, believes that this number is equal to about 100 Earth masses. According to him, platinum was formed three times more than the earth's mass, and uranium - 10 times less. In any case, if we compare the new statistical estimates of the frequency of such mergers, based on the latest measurements, then we get a fairly large number of such events. “There are enough of them to form and accumulate the elements that form our own solar system and the variety of stars that we see,” says Metzger. “Based on what we have seen, these mergers can be explained in detail. There are probably other ways of forming heavy elements, but it seemsthat we don't need them. According to him, every 10 thousand years in the Milky Way there is only one merger of neutron stars.
Distant frontiers
Moreover, studying the process of fusion and formation of a kilonova can provide us with very important information about how the collision occurred. For example, the light from the initial ejection after the merger was bluer than scientists expected. Based on this, Metzger and other scientists concluded that they were looking at the kilonova from an angle, not directly. Based on this scenario, the initial blue ejection came from a spherical envelope or equatorial band of low neutron material that was blown out of neutron stars at an estimated speed of 10% of the speed of light. Later and redder emissions could have come from material with a high neutron content that was ejected from the poles of neutron stars when they collided at a speed two to three times faster - like toothpaste.squeezed out of the tube.
If we compare this scenario with detailed observational data in the X-ray and radio range, then the very curious nature of the gamma-ray emission associated with such a merger becomes clearer. It was the closest gamma-ray burst on record, but also one of the weakest. The short-lived bursts of gamma rays are thought to be bipolar bursts of intense radiation that is accelerated and ejected at near the speed of light by magnetic fields inside colliding neutron stars as they merge and collapse into a black hole. If you look at this flash of gamma radiation directly (eye to eye, so to speak), it will be very bright. This happens in most cases of such emissions that astronomers observe in distant parts of the universe. But if you look at these bursts of gamma radiation from an angle, they seem rather dim, and they can only be detected if they are quite close, within a few hundred million light years.
Thus, using the abundant data accumulated by gravitational-wave astronomy, scientists will be able to determine over time the viewing angles of many kilonovs in the entire observable part of the universe, and this will allow them to more accurately measure large-scale cosmic structures and study their evolution. Scientists will have an opportunity to unravel those mysteries that are much deeper than the origin of heavy elements, say, the perplexing fact that the universe is not just expanding, but expanding with acceleration under the influence of a large-scale anti-gravity force known as dark energy.
Researchers in the field of cosmology hope that they will be able to better understand dark energy by accurately measuring its impact on the Universe, to trace objects in distant regions of the Universe, to understand how far away they are, and how fast they move in accelerating streams of dark energy. But for this, scientists need reliable "standard candles", that is, objects of known brightness, which could be used to calibrate this huge, all-encompassing field of space-time. Astrophysicist Daniel Holz of the University of Chicago and LIGO has demonstrated how merging neutron stars can contribute to this effort. In his work, he shows that the strength of the gravitational waves formed during the last merger,and also the kilonova emissions can be used to calculate the expansion rate of the nearest parts of the Universe. This method is limited to just one merge and therefore has significant uncertainty in its values, although it confirms the expansion rate data obtained with other methods. But in the coming years, gravitational-wave observatories, as well as new generation ground-based and space telescopes and large sizes, will work together, discovering hundreds or even thousands of collisions of neutron stars every year. In this case, the accuracy of estimates will increase markedly.although they confirm the data on expansion rates obtained using other methods. But in the coming years, gravitational-wave observatories, as well as new generation ground-based and space telescopes and large sizes, will work together, discovering hundreds or even thousands of collisions of neutron stars every year. In this case, the accuracy of estimates will increase markedly.although they confirm the data on expansion rates obtained using other methods. But in the coming years, gravitational-wave observatories, as well as new-generation ground-based and space telescopes and large sizes, will work together, discovering hundreds and even thousands of collisions of neutron stars every year. In this case, the accuracy of estimates will increase markedly.
“What does all this mean? And the fact that the measurements of gravitational waves from these mergers, carried out by LIGO and Virgo, will be supplemented by kilonova models, and then scientists will be able to understand what their inclinations and viewing angles are, examining their spectral evolution from blue to red. " This is stated by astrophysicist Richard O'Shaughnessy of the Rochester Institute of Technology and a member of the LIGO team. “This is a very powerful combination of efforts. If we know the inclination, we can calculate the distance, which will be very useful for cosmology. What has been done now is a prototype of what we will regularly do in the future."
“If you think about it, the universe is a kind of collider of cosmic particles, and the particles in this collider are neutron stars,” says O'Shaughnessy. - He pushes these particles, and now we have the opportunity to understand what comes out of this. We will see a large number of such events in the coming years. I don't know exactly how many there will be, but people are already calling it cosmic rain. This will give us real data that allow us to connect the very different and abrupt strands of astrophysics, which previously existed only in the minds of theorists or in the form of separate pieces of information in models of supercomputers. This will give us an opportunity to understand the reasons for the abundance of heavy elements in space. This will give us opportunities to study soft and easily compressible nuclear matter under conditions of enormous density. We will be able to measure the rate of expansion of the universe. Such collaborative efforts will provide vast opportunities for high energy astrophysics and pose many challenges for the coming decades. And the basis for such cooperation will be long-term investments. Today we are reaping the benefits of a huge mountain of gold, the mass of which is tens or even hundreds of times the mass of the Earth. This gift was presented to us by the Universe”.
Lee Billings is deputy editor-in-chief of Scientific American. He writes about space and physics.