The LIGO observatory, whose creators received the 2017 Nobel Prize, has already changed the world of astronomy. When scientists from the international scientific community LIGO discovered the first gravitational waves in 2016, they discovered a new way of observing the universe. For the first time, scientists were able to "listen" to the fluctuations in space-time arising from the collision of large objects (for example, black holes).
But that was just the beginning. The goal was to combine the observation of gravitational waves with data from more conventional telescopes.
In October 2017, in Physical Review Letters, the LIGO team of scientists, which includes several thousand people around the world, published a series of papers about the incredible discovery. The researchers were able not only to detect gravitational waves from the collision of two neutron stars, but also to determine their coordinates in the sky, as well as observe the phenomenon through optical and electromagnetic telescopes.
“This is one of the most complete stories of an astrophysical phenomenon imaginable,” says physicist Peter Solson of Syracuse University and a member of the LIGO community.
Each source tells its own part of the story
Gravitational waves tell physicists the size and distance of objects, which allows them to recreate the moment before they collide. Observations of visible radiation and electromagnetic waves then fill in the gaps that gravitational waves cannot explain. They help astronomers find out what objects were made of and what chemical elements came from the collision. In our case, scientists were able to conclude that the explosion during the merger of neutron stars led to the appearance of heavy elements - gold, platinum and uranium (which was previously only assumed, but could not be confirmed by direct observation).
Now scientists have managed to see with their own eyes the alchemy of the universe in action. "I think the impact of this discovery on science will be more significant than the first detection of black holes through gravitational waves," said Duncan Brown, another scientist from the LIGO community and Syracuse University. "Many aspects of physics and astronomy are involved here." And all this is the result of a treasure hunt among the stars, in which the whole world is involved.
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Race against time. Place marked with a cross
On August 17 at 8:41 am, LIGO detected gravitational waves - the curvature of time and space - passing through the Earth. LIGO is two L-shaped observatories in the US states of Louisiana and Washington. They can register waves that compress and stretch the space-time continuum.
Over the past two years, LIGO has been able to detect gravitational waves generated by colliding black holes. But the signal on August 17 was quite different. It turned out to be much stronger than what was recorded when the black hole was discovered. The new signal lasted 100 seconds, while the signals from black holes only a few. This meant that the collision took place much closer to Earth.
When LIGO detects gravitational waves, it automatically sends notifications to hundreds of scientists around the world. Duncan Brown is one of them. “We received a telephone alert very quickly and realized that this was an unexpectedly strong signal of gravitational waves. It shocked us,”he recalls.
It immediately became clear that this was not a merger of black holes. Initial analysis showed that the waves originated from the collision of two neutron stars - objects with very high density. It is believed that heavy chemical elements are formed inside them.
When LIGO detects gravitational waves from colliding black holes, nothing can be seen in the sky: black holes, as their name suggests, are dark. What about a collision of two neutron stars? The spectacle should be like a colorful fireworks.
Sarah Wilkinson / Las Campanas Observatory
And so it happened: two seconds after the LIGO signal, NASA's Fermi space telescope detected a gamma-ray burst - one of the most powerful bursts of explosive energy in the universe known to us. For a long time, astronomers have built theories that the merger of neutron stars can cause gamma-ray bursts. And now it couldn't be a coincidence.
At the same time, the light from such an explosive fusion quickly dims. The count went on for minutes, and scientists from the international scientific community LIGO were forced to hurry. “The faster you get to the telescope, the more information you get,” says Brown. From studying light and how it changes, scientists can extract a wealth of information that will help them better understand neutron stars and how they merge change matter.
Brown and his colleagues got to work, organizing teleconferences with dozens of scientists around the world. The LIGO team worked with partners from VIRGO, an Italian gravitational wave observatory, to work with redoubled effort to map the sky and locate the source of the gravitational waves. They narrowed their search down to a fist-sized area at arm's length. (From an astronomical point of view, even this region is a huge space. An area of a map with a match head at arm's length can contain thousands of galaxies.) The VIRGO detector in Italy did not pick up the signal, which helped determine the position of the stars. VIRGO has zones of no reception, therefore the neutron stars should have been located near one of them.
This sky map is the result of combining information from Fermi, LIGO, VIRGO and Integral (another gamma ray observatory). Each detector provided an area in which a signal could occur. Where they overlapped, the place marked with a cross on the map of cosmic treasures was indicated.
Map in hand, the LIGO team sent out emails to astronomers around the world who could explore this region of the sky as night fell.
And luck did not pass them by! Several ground-based observatories were able to detect the position of the kilon (or macron) - an explosion from the collision of two neutron stars. The photo on the left shows what the astronomers captured on the opening night. On the right is how it looked a few days later. The explosion dimmed noticeably.
1M2H / UC Santa Cruz and Carnegie Observatory / Ryan Foley
This is how the galaxy looked a couple of weeks before the formation of the kilonova (top image). The bottom image shows an explosion.
The Dark Energy Camera GW-EM Collaboration and the DES / Berger Collaboration
Images may seem fuzzy, but there is a ton of information on them. With precise coordinates, scientists can tune the Hubble Space Telescope and the Chandra Space X-ray Observatory to explode a kilonova. With the help of these tools, astronomers will be able to look at the process of the universe with one eye.
How colliding neutron stars create gold
Neutron stars are unusual cosmic bodies. They are formed as a result of the gravitational collapse of stars (for example, during supernova explosions) and have a very high density. Imagine an object with a mass like the Sun, but only 25 kilometers in diameter. This is 333,000 masses of the entire Earth, compressed into a ball about the size of the Central District of Moscow. The pressure inside is so huge that only neutrons (protons fused with electrons) can exist there.
In a galaxy 130 million light years away, two such objects "danced" around each other, moving in orbit and getting closer and closer. They collided, and the released energy through the Universe sent a wave that distorts time and space, and a stream of particles (a gamma-ray burst detected along with gravitational waves). Both gravitational waves and gamma rays traveled at the speed of light. This is another proof of Albert Einstein's general theory of relativity. It is possible that after the merger, the neutron stars formed a new black hole, since they had sufficient mass. However, there is not enough information yet for an unambiguous statement.
V. Castown / T. Kawamura / B. Giacomazzo / R. Cholfi / A. Endrzzi
But one thing can already be said for sure: after the explosion, many of the remaining neutrons combined and formed chemical elements.
All of us and every element on Earth are made of stars. As a result of the Big Bang at the beginning of time, very light elements were formed - hydrogen and helium. These elements combined to form stars, inside which, during fusion reactions, elements with larger and larger masses appeared.
When stars went supernova (collapse and subsequent explosion), even heavier elements were created. However, according to Brown, the appearance of gold and platinum has long been a mystery. Even supernova explosions are not powerful enough to create them.
There have been theories that a kilon star (formed by the merger of two neutron stars) is capable of producing these metals. And since the astronomers were able to timely determine the place where the merger occurred, they confirmed this theory. The color and quality of the light left after the explosion confirmed the formation of gold and platinum. Scientists seemed to have watched alchemy in action.
“Gold on Earth was once created after a nuclear explosion from a merger [of neutron stars],” explains Brown. - Now I have a platinum wedding ring on my finger. Just think, it appeared due to the collision of neutron stars!"
A new era in astronomy is coming
The described discovery marks the beginning of a new era in astronomy. Scientists will be able to study celestial bodies not only with the help of light and radiation that they emit, but also combine these observations with information obtained during the analysis of gravitational waves. This information contains how the two neutron stars moved around each other when the collision occurred, as well as a huge body of information about its consequences.
On the right - visualization of the substance of neutron stars. On the left - distortion of space-time near explosions. Karan Janey / Georgia Institute of Technology
The combination of all sources of information is called multichannel astronomy, that is, astronomy based on the addition of observations of the electromagnetic spectrum with gravitational-wave observations. This has been the dream of LIGO scientists since the observatory was founded.
“Imagine living in a windowless room and all you can do is hear thunder but not see lightning,” explains Vicki Kalogera, an astrophysicist at Northwestern University and a member of the LIGO community. - Now imagine that you were moved to a room with a window. From now on, you not only hear thunder, but also see lightning. Lightning provides a completely new opportunity for studying thunderstorms and understanding what is really happening."
Gravitational waves are thunder. Observing explosions through a telescope - lightning.
Just a month ago, the three founders of LIGO received the Nobel Prize in Physics for their pioneering work. As Ed Young of The Atlantic observed, awarding the award to three out of hundreds who have made significant contributions to the LIGO project creates an awkward and controversial situation. However, recent results show that the prize for scientific work was well deserved.
The best thing about observing gravitational waves is that the process is passive. LIGO and VIRGO will "hear" any gravitational waves passing by the Earth on the same day. Each signal marks the beginning of a new search for "treasures", because scientists need to understand what created the fluctuations in space-time.
Astronomers hope to see more mergers of both black holes and neutron stars. But even more interesting phenomena can be discovered. If the LIGO and VIRGO observatories continue to improve, there is a chance that it will be possible to detect gravitational waves left over from the Big Bang. Or, more excitingly, these observatories will be able to detect sources of gravitational waves that were previously unknown and could not predict.
“I was sad that I was born after the first manned landing on the moon,” said Thomas Corbitt, physicist and member of the LIGO community at Louisina State University. - But when you become a witness of events like these, which serve as proof of the great success of joint activities, inspiration appears. They give us more knowledge about the Universe."
The original article in English is available here.