Neutron Stars Collide

Neutron Stars Collide and Are Seen[1]

The image below is linked to a video explaining the kilonova merging of two neutron stars.

For the first time, astronomers have seen and heard a pair of neutron stars collide in a crucible of cosmic alchemy.

Watch in Times Video »

Astronomers announced on Monday, October 16, 2017 that they had seen and heard a pair of dead stars collide, giving them their first glimpse of the violent process by which most of the gold and silver in the universe was created.

The collision, known as a kilonova, rattled the galaxy in which it happened 130 million light-years from here in the southern constellation of Hydra, and sent fireworks across the universe. On Aug. 17, 2017 the event set off sensors in space and on Earth, as well as producing a loud chirp in antennas designed to study ripples in the cosmic fabric. It sent astronomers stampeding to their telescopes, in hopes of answering one of the long-sought mysteries of the universe.

Such explosions, astronomers have long suspected, produced many of the heavier elements in the universe, including precious metals like gold, silver and uranium. All the atoms in your wedding band, in the pharaoh’s treasures and the bombs that destroyed Hiroshima and still threaten us all, so the story goes, have been formed in cosmic gong shows that reverberated across the heavens.

This gong show happened when a pair of neutron stars, the shrunken dense cores of stars that have exploded and died, collided at nearly the speed of light. These stars are masses as great as the sun packed into a region the size of Manhattan brimming with magnetic and gravitational fields.

An artist’s rendering of the merger of two neutron stars from Aug. 17. Credit Robin Dienel/The Carnegie Institution for Science

Studying the fireball from this explosion, astronomers have concluded that it had created a cloud of gold dust many times more massive than the Earth, confirming kilonovas as agents of ancient cosmic alchemy.

“For the first time ever, we have proof,” said Vicky Kalogera, an astronomer at Northwestern University.

She was one of thousands of astronomers that reported their results Monday (October 16, 2017) in a globe-girdling set of news conferences and academic conferences.

A blizzard of papers is being published, including one in Astrophysical Journal Letters that has some 4,000 authors . “That paper almost killed the paperwriting team,” said Dr. Kalogera, one of 10 people who did the actual writing.

An artist’s rendering of a neutron star compared with the skyline of Chicago. Neutron stars are about 12 miles in diameter and are extremely dense. Credit Daniel Schwen/Northwestern, via LIGO-Virgo

More papers are appearing in Nature, Physical Review Letters and in Science, on topics including nuclear physics and cosmology.

“It’s the greatest fireworks show in the universe,” said David Reitze of the California Institute of Technology and the executive director of the Laser Interferometer Gravitational-Wave Observatory, or LIGO.

Daniel Holz, an astrophysicist at the University of Chicago and a member of the LIGO Scientific Collaboration, a larger group that studies gravitational waves, said, “I can’t think of a similar situation in the field of science in my lifetime, where a single event provides so many staggering insights about our universe.”

The key to the discovery was the detection of gravitational waves, emanating like ripples in a pond vibrating the cosmic fabric, from the distant galaxy. It was a century ago that Albert Einstein predicted that space and time could shake like a bowl of jelly when massive things like black holes moved around. But such waves were finally confirmed only in 2016, when LIGO recorded the sound of two giant black holes colliding, causing a sensation that eventually led this month to a Nobel Prize.

For the researchers, this is in some ways an even bigger bonanza than the original discovery. This is the first time they have discovered anything that regular astronomers could see and study. All of LIGO’s previous discoveries have involved colliding black holes, which are composed of empty tortured space-time—there is nothing for the eye or the telescope to see.

But neutron stars are full of stuff, matter packed at the density of Mount Everest in a teaspoon. When neutron stars slam together, all kinds of things burst out: gamma rays, xrays, radio waves. Something for everyone who has a window on the sky.

“Joy for all,” said David Shoemaker, a physicist at the Massachusetts Institute of Technology who is the spokesman for the LIGO Scientific Collaboration.

This is the story of a gold rush in the sky.

It began on the morning of Aug. 17, Eastern time. Dr. Shoemaker was on a Skype call when alarms went off. One of the LIGO antennas, in Hanford, Wash., had recorded an auspicious signal and sent out an automatic alert. Twin antennas, in Washington and Livingston, Louisiana, monitor the distance between a pair of mirrors to detect the submicroscopic stretching and squeezing of space caused by a passing gravitational wave. Transformed into sound, the Hanford signal was a long 100-second chirp, that ended in a sudden whoop to 1000 cycles per second, two octaves above middle C. Such a high frequency indicated that whatever was zooming around was lighter than a black hole.

Checking the data from Livingston to find out why it had not also phoned in an alert, Dr. Shoemaker and his colleagues found a big glitch partly obscuring the same chirp.

Meanwhile, the Fermi Gamma-Ray Space Telescope, which orbits Earth looking at the highest-energy radiation in the universe, recorded a brief flash of gamma rays just two seconds after the LIGO chirp. Fermi sent out its own alert. The gamma-ray burst lasted about two seconds, which put it in a category of short gamma ray bursts, which astronomers suspect are neutron stars colliding.

“When we saw that,” Dr. Shoemaker said, “the adrenaline hit.”

Dr. Kalogera, who was in Utah hiking and getting ready for August’s total solar eclipse when she got the alarm, recalled thinking: “Oh my God, this is it. This 50-year-old mystery, the holy grail, is solved.”

Together the two signals told a tale of a pair of neutron stars spiraling around each other like the blades

But where?

Luckily the European Virgo antenna had joined the gravitational wave network only two weeks before, and it also showed a faint chirp at the same time. The fact that it was so weak allowed the group to localize the signal to a small region of the sky in the Hydra constellation that was in Virgo’s blind spot.

The hunt was on. By then Hydra was setting in the southern sky. It would be 11 hours before astronomers in Chile could take up the chase.

One of them was Ryan Foley, who was working with a team on the Swope telescope run by the Carnegie Institution on Cerro Las Campanas in Chile. His team made a list of the biggest galaxies in that region and set off to photograph them all systematically.

The fireball showed up in the ninth galaxy photographed, as a new bluish pinprick of light in the outer regions of NGC 4993, a swirl of stars about 130 million light-years from here. “These are the first optical photons from a kilonova humankind has ever collected,” Dr. Foley said.

Within 10 minutes, another group of astronomers, led by Marcelle Soares-Santos of Brandeis University and using the Dark Energy Camera, which could photograph large parts of the sky with a telescope at the nearby Cerro Tololo Interamerican Observatory, had also spotted the same speck of light.

Emails went flying about in the Chilean night.

When it was first identified, the fireball of 8,000-degree gas was about the size of Neptune’s orbit and radiating about 200 million times as much energy as the sun.

Nine days later, the orbiting Chandra X-ray Observatory detected x rays coming from the location of the burst, and a week after that, the Very Large Array in New Mexico recorded radio emissions. By then the fireball faded from blue to red.

From all this, scientists have begun patching together a tentative story of what happened in the NGC 4993 galaxy.

“It’s actually surprising how well we were able to anticipate what we’re seeing,” said Brian David Metzger, a theorist at Columbia University who coined the term kilonova back in 2010.

As they tell it, the merging objects were probably survivors of stars that had been orbiting each other and had each puffed up and then died in the supernova explosions in which massive stars end their luminous lives some 11 billion years ago, according to an analysis by Dr. Kalogera. Making reasonable assumptions about their spins, these neutron stars were about 1.1 and 1.6 times as massive as the Sun, smack in the known range of neutron stars.

As they approached each other swirling a thousand times a second, tidal forces bulged their surfaces outward. Quite a bit of what Dr. Metzger called “neutron star guts” were ejected and formed a fat doughnut around the merging stars.

At the moment they touched, a shock wave squeezed more material out of their polar regions, but the doughnut and extreme magnetic fields confined the material into an ultra-high-speed jet emitting a blitzkrieg of radiation, the gamma rays.

As the jet slowed down, encountering interstellar gas in the galaxy, it began to glow in x rays and then radio waves.

The subatomic nuggets known as neutrons meanwhile were working their cosmic alchemy. The atoms in normal matter are mostly empty space: a teeny tiny nucleus of positively charged protons and electrically neutral neutrons enveloped in a fluffy cloud of negatively charged electrons. Under the enormous pressures of a supernova explosion, however, the electrons get squeezed back into the protons turning them into neutrons packed into a ball denser than an atomic nucleus.

The big splat liberates these neutrons into space where they inundate the surrounding atoms, transmuting them into heavy elements. The radioactivity of these newly created elements keeps the fireball hot and glowing.

Dr. Metzger estimated that an amount of gold equal to 40 to 100 times the mass of the Earth could have been produced over a few days and blown into space. In the coming eons, it could be incorporated into new stars and planets and in some far, far day become the material for an alien generation’s jewels.

The discovery filled a long-known chink in the accepted explanation of how the chemistry of the universe evolved from pure hydrogen and helium into the diverse place it is today. Stars and supernovas could manufacture the elements up to iron or so, according to classic papers in the 1950s but heavier elements required a different thermonuclear chemistry called r-process and lots of free neutrons floating around. Where would they have come from?

One idea was neutron star collisions, or kilonovas, which now seem destined to take their place on the laundry list of cosmic catastrophes along with the supernova explosions and black hole collisions that have shaped the history of the universe.

Until now there was only indirect evidence of kilonovas. Astronomers found a fireball from a gamma-ray burst in 2013, but there was no proof that neutron stars were involved. Now astronomers know they are, completing the picture of the origin of bling.

One burning question is what happened to the remnant of this collision. According to the LIGO measurements, it was about as massive as 2.6 suns. Scientists say that for now they are unable to tell whether it collapsed straight into a black hole, formed a fat neutron star that hung around in this universe for a few seconds before vanishing, or remained as a neutron star. They may never know, Dr. Kalogera said.

Neutron stars are the densest form of stable matter known. Adding any more mass over a certain limit will cause one to collapse into a black hole, but nobody knows what that limit is.

Future observations of more kilonovas could help physicists understand where the line of no return actually is.

Dr. Holz, the University of Chicago astrophysicist, said, “I still can’t believe how lucky we all are,” reciting a list of fortuitous circumstances. They had three detectors running for only a few weeks, and it was the closest gamma-ray burst ever recorded and the loudest gravitational wave yet recorded. “It’s all just too good to be true. But as far as we can tell it’s really true. We’re living the dream.”

 

[1] See Dennis Overbye, “LIGO Detects Fierce Collision of Neutron Stars for the First Time,” New York Times (October 16, 2017). Accessed at https://www.nytimes.com/2017/10/16/science/ligo-neutron-stars-collision.html  The associated video may be linked at https://nyti.ms/2zc7Yfy as well as from the image at the top, or from the link in the body of the article.

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2017 Nobel Prize in Physics

2017 Nobel Prize in Physics[1]

Rainer Weiss, a professor at the Massachusetts Institute of Technology, and Kip Thorne and Barry Barish, both of the California Institute of Technology, were awarded the Nobel Prize in Physics on Tuesday for the discovery of ripples in space-time known as gravitational waves, which were predicted by Albert Einstein a century ago but had never been directly seen.

From left: Rainer Weiss, Barry Barish and Kip Thorne, the architects and leaders of LIGO, the Laser Interferometer Gravitational-wave Observatory. Credit Molly Riley/Agence France-Presse — Getty Images

In announcing the award, the Royal Swedish Academy called it “a discovery that shook the world.”

That shaking happened in February 2016, when an international collaboration of physicists and astronomers announced that they had recorded gravitational waves emanating from the collision of a pair of massive black holes a billion light years away, it mesmerized the world. The work validated Einstein’s longstanding prediction that space-time can shake like a bowlful of jelly when massive objects swing their weight around, and it has put astronomers on intimate terms with the deepest levels of physical reality, of a void booming and rocking with invisible cataclysms.

Why Did They Win?

Dr. Weiss, 85, Dr. Thorne, 77, and Dr. Barish, 81, were the architects and leaders of LIGO, the Laser Interferometer Gravitational-wave Observatory, the instrument that detected the gravitational waves, and of a sister organization, the LIGO Scientific Collaboration, of more than a thousand scientists who analyzed the data.

Dr. Weiss will receive half of the prize of 9 million Swedish Krona, or more than $1.1 million, and Dr. Thorne and Dr. Barish will split the other half.

Einstein’s General Theory of Relativity, pronounced in 1916, suggested that matter and energy would warp the geometry of space-time the way a heavy sleeper sags a mattress, producing the effect we call gravity. His equations described a universe in which space and time were dynamic. Space-time could stretch and expand, tear and collapse into black holes—objects so dense that not even light could escape them. The equations predicted, somewhat to his displeasure, that the universe was expanding from what we now call the Big Bang, and it also predicted that the motions of massive objects like black holes or other dense remnants of dead stars would ripple space-time with gravitational waves.

These waves would stretch and compress space in orthogonal directions as they went by, the same way that sound waves compress air. They had never been directly seen when Dr. Weiss and, independently, Ron Drever, then at the University of Glasgow, following work by others, suggested detecting the waves by using lasers to monitor the distance between a pair of mirrors. In 1975, Dr. Weiss and Dr. Thorne, then a well-known gravitational theorist, stayed up all night in a hotel room brainstorming gravitational wave experiments during a meeting in Washington.

Dr. Thorne went home and hired Dr. Drever to help develop and build a laser-based gravitational-wave detector at Caltech. Meanwhile, Dr. Weiss was doing the same thing at M.I.T.

The technological odds were against both of them. The researchers calculated that a typical gravitational wave from out in space would change the distance between the mirrors by an almost imperceptible amount: one part in a billion trillion, less than the diameter of a proton. Dr. Weiss recalled that when he explained the experiment to his potential funders at the National Science Foundation, “everybody thought we were out of our minds.”

The foundation, which would wind up spending $1 billion over the next 40 years on the project, ordered the two groups to merge, with a troika of two experimentalists, Drs. Weiss and Drever, and one theorist Dr. Thorne, running things. The plan that emerged was to build a pair of L-shaped antennas, one in Hanford, Wash., and the other in Livingston, La., with laser light bouncing along 2.5-mile-long arms in the world’s biggest vacuum tunnels to monitor the shape of space.

In 1987, the original three-headed leadership of Drs. Weiss, Drever and Thorne was abandoned for a single director, Rochus Vogt of Caltech. Dr. Drever was subsequently forced out of the detector project. But LIGO still foundered until Dr. Barish, a Caltech professor with a superb pedigree in managing Big Science projects, joined in 1994 and then became director. He reorganized the project so that it would be built in successively more sensitive phases, and he created a worldwide LIGO Scientific Collaboration of astronomers and physicists to study and analyze the data. “The trickiest part is that we had no idea how to do what we do today,” he commented in an interview, giving special credit to the development of an active system to isolate the laser beams and mirrors from seismic and other outside disturbances.

“Without him there would have been no discovery,” said Sheldon Glashow, a Nobel Prize-winning theorist now at Boston University.

The most advanced version of LIGO had just started up in September 2015 when the vibrations from a pair of colliding black holes slammed the detectors in Louisiana and Washington with a rising tone, or “chirp,” for a fifth of a second.

It was also the opening bell for a whole new brand of astronomy. Since then LIGO (recently in conjunction with a new European detector, Virgo) has detected at least four more black hole collisions, opening a window on a new, unsuspected class of black holes, and rumors persist of even more exciting events in the sky.

“Many of us really expect to learn about things we didn’t know about,” Dr. Weiss said (October 3, 2017).

Who Are the Winners?

Dr. Weiss was born in Berlin in 1932 and came to New York by way of Czechoslovakia in 1939. As a high school student, he became an expert in building high-quality sound systems and entered M.I.T. intending to major in electrical engineering. He inadvertently dropped out when he went to Illinois to pursue a failing romance. After coming back, he went to work in a physics lab and wound up with a Ph.D. from M.I.T.

Dr. Thorne was born and raised in Logan, Utah, receiving a bachelor’s degree from Caltech and then a Ph.D. from Princeton under the tutelage of John Archibald Wheeler, an evangelist for Einstein’s theory who popularized the term black holes, and who initiated Dr. Thorne into their mysteries. “He blew my mind,” Dr. Thorne later said. Dr. Thorne’s enthusiasm for black holes is not confined to the scientific journals. Now an emeritus professor at Caltech, he was one of the creators and executive producers of the 2014 movie Interstellar, about astronauts who go through a wormhole and encounter a giant black hole in a search for a new home for humanity.

Dr. Barish was born in Omaha, Neb., was raised in Los Angeles and studied physics at the University of California, Berkeley, getting a doctorate there before joining Caltech. One of the mandarins of Big Science, he had led a team that designed a $1 billion detector for the giant Superconducting Supercollider, which would have been the world’s biggest particle machine had it not been canceled by Congress in 1993, before being asked to take over LIGO.

Subsequently, Dr. Barish led the international effort to design the International Linear Collider, which could be the next big particle accelerator in the world, if it ever gets built.

Reached by telephone by the Nobel committee, Dr. Weiss said that he considered the award as recognition for the work of about a thousand people over “I hate to say it—40 years.”

He added that when the first chirp came on Sept. 14, 2015, “many of us didn’t believe it,” thinking it might be a test signal that had been inserted into the data. It took them two months to convince themselves it was real.

In an interview from his home, Dr. Thorne said that as the resident theorist and evangelist on the project he felt a little embarrassed to get the prize. “It should go to all the people who built the detector or to the members of the LIGO-Virgo Collaboration who pulled off the end game,” he said.

“An enormous amount of rich science is coming out of this,” he added. “For me, an amazing thing is that this has worked out just as I expected when we were starting out back in the ‘80s. It blows me away that it all came out as I expected.”

Dr. Barish said he had awoken at 2:41 am in California and when the phone didn’t ring he figured he hadn’t won. Then it rang. “It’s a combination of being thrilled and humbled at the same time, mixed emotions,” he said. “This is a team sport, it gets kind of subjective when you have to pick out individuals.” LIGO, he said, is very deserving. “We happen to be the individuals chosen by whatever mechanism.”

For the National Science Foundation, the Nobel was a welcome victory lap for an investment of 40 years and about $1 billion. In a news release, France Córdova, the foundation’s director, said: “Gravitational waves contain information about their explosive origins and the nature of gravity that cannot be obtained from other astronomical signals. These observations have created the new field of gravitational wave astronomy.”

The prize was greeted with praise around the world. “Well done Sweden,” said Michael Turner, a cosmologist at the University of Chicago, adding about the result, “It took a village and 100 years to do this.”

The awarding of a Nobel to Drs. Weiss and Thorne completes a kind of scientific Grand Slam. In the last two years, along with Dr. Drever, they have shared a cavalcade of prestigious and lucrative prizes including the Kavli Prize for Astrophysics, the Gruber Cosmology Prize, the Shaw Prize in Astronomy and a Special Breakthrough Prize in Fundamental Physics. Dr. Drever died last March, and the Nobel is not awarded posthumously nor can more than three people share the prize.

[1] Dennis Overbye, “2017 Nobel Prize in Physics Awarded to LIGO Black Hole Researchers,” New York Times (October 3, 2017), accessed at https://www.nytimes.com/2017/10/03/science/nobel-prize-physics.html?_r=0 . A version of this article appears in print on October 4, 2017, on Page A8 of the New York edition with the headline: “Recording of Gravitational Waves Was ‘Discovery That Shook the World’”. Follow Dennis Overbye on Twitter: @overbye

 

New Gravitational Wave Detection From Colliding Black Hole

New Gravitational Wave Detection From Colliding Black Holes[1]

The LIGO and Virgo detectors in the United States and Europe identified gravitational waves emitted by the collision of two black holes 1.8 billion light years away. The location of the black holes in the night skies is visualized in this map. Credit LIGO/Virgo

In another step forward for the rapidly expanding universe of invisible astronomy, scientists said on Wednesday that on Aug. 14 they had recorded the space-time reverberations known as gravitational waves from the collision of a pair of black holes 1.8 billion light years away from here.

It was the fourth time, officially, in the last two years that astronomers have detected such ripples from the cataclysmic mergers of black holes—objects so dense that space and time are wrapped around them like a glove so that not even light can escape.

In the August event, one black hole with about 31 times the mass of the Sun and another with 25 solar masses, combined to make a hole of 53 solar masses. The remaining three solar masses were converted into gravitational waves that radiated more energy than all the stars in the known universe. The observation is in line with earlier gravitational wave detections, confirming an evolving view of the cosmic night full of monsters and violence.

The detection, announced at a G7 meeting of science ministers in Turin, Italy, and in a paper in the journal Physical Review Letters, marked the successful debut of a new gravitational wave detector known as Virgo, built by a European collaboration and located in Cascina, close to Pisa, Italy.

The first detections of gravitational waves had been made by a pair of L-shaped antennas, called LIGO, in Hanford, Wash. and Livingston, La., which monitor the squeezing and stretching of space between a pair of delicately positioned mirrors as a gravitational wave goes by. That announcement in February 2016 confirmed the existence of gravitational waves first predicted by Albert Einstein a century ago, and verified the nature of black holes, causing a sensation. LIGO’s leaders are now front-runners for the Nobel Prize in Physics, to be announced next week.

On Aug. 1, the Virgo antenna, built by the European Gravitational Observatory, came on line to join the existing LIGO antennas.

The addition paid off almost immediately, scientists for the observatories said on Wednesday (September 27, 2017), when a pair of black holes in collision rattled the antennas on Aug. 14, 2017. Although the Virgo antenna is still only about one-fourth as sensitive as the LIGO antennas, it greatly increases the network’s ability to triangulate the sources of gravitational waves so that optical telescopes can search for any accompanying fireworks in the visible sky.

To date none have been detected given that black holes are composed essentially of empty twisted space. But gravitational wave astronomers have hopes of finding other types of collisions, involving dense balls of matter called neutron stars, that will spark up the night aplenty.

The current observing run ended on Aug. 25, 2017. After a year of work improving the sensitivities of their instruments, a new run will begin in the fall of 2018. Hopes are, you might say, sky high.

In a news release from the University of Glasgow sent out before the G7 meeting, Sheila Rowan, director of the university’s Institute for Gravitational Research, said: “We now are demonstrating the capabilities of a network of gravitational wave detectors, which deepens the pool of data we’ll be able to draw from in future and will help to further expand our understanding of the universe.”

MIT’s David Shoemaker, spokesman for the LIGO Scientific Collaboration, said: “This is just the beginning of observations with the network enabled by Virgo and LIGO working together. With the next observing run planned for Fall 2018, we can expect such detections weekly or even more often.”

[1] Dennis Overbye, “New Gravitational Wave Detection from Colliding Black Holes,” New York Times (September 27, 2017) accessed at https://www.nytimes.com/2017/09/27/science/black-holes-collision-ligo-virgo.html?_r=0

Hints of Extra Dimensions in Gravitational Waves

Hints of Extra Dimensions in Gravitational Waves?[1]

Researchers from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Potsdam found that hidden dimensions – as predicted by string theory – could influence gravitational waves. In a recently published paper they study the consequences of extra dimensions on these ripples in space-time, and predict whether their effects could be detected.

Merging black holes generate gravitational waves. These ripples in space-time might be used to unveil hidden dimensions.

 

 

 

 

 

 

LIGO’s first detection of gravitational waves from a black-hole binary in September 2015 has opened a new window onto the universe. Now it looks like with this new observing tool physicists cannot only trace black holes and other exotic astrophysical objects but also understand gravity itself. “Compared to the other fundamental forces like, e.g. electromagnetism, gravity is extremely weak,” explains Dr. David Andriot, one of the authors of the study. The reason for this weakness could be that gravity interacts with more than the three dimensions in space and one dimension in time that are part of our everyday experience.

Extra dimensions

Extra dimensions that are hidden because they are very small are an indispensable part of string theory—one of the promising candidates for a theory of quantum gravity. A theory of quantum gravity, unifying quantum mechanics and general relativity, is sought after in order to understand what happens when very large masses at very small distances are involved, e.g. inside a black hole or at the Big Bang.

“Physicists have been looking for extra dimensions at the Large Hadron Collider at CERN but up to now this search has yielded no results,” says Dr. Gustavo Lucena Gómez, the second author of the paper. “But gravitational wave detectors might be able to provide experimental evidence.”

The researchers discovered that extra dimensions should have two different effects on gravitational waves: they would modify the “standard” gravitational waves and would cause additional waves at high frequencies above 1000 Hz. However, the observation of the latter is unlikely since the existing ground-based gravitational wave detectors are not sensitive enough at high frequencies.

On the other hand, the effect that extra dimensions can make a difference in how “standard” gravitational waves stretch and shrink space-time might be easier to detect by making use of more than one detector. Since the Virgo detector will join the two LIGO detectors for the next observing run this might happen after late 2018/beginning of 2019.

[1] Max Planck Institute for Gravitational Physics, Albert Einstein Institute,.“Hints of Extra Dimensions in Gravitational Waves?,”(June 28, 2017)

Black Hole Merger

Third Gravitational Wave Detection, From Black-Hole Merger[1]

An artist’s conception shows two merging black holes similar to those detected by LIGO. Astronomers said Thursday (June 7, 2017) that they had felt space-time vibrations known as gravitational waves from the merger of a pair of mammoth black holes resulting in a pit of infinitely deep darkness. (Credit Aurore Simonnet/Sonoma State/Caltech/MIT/LIGO)

The void is rocking and rolling with invisible cataclysms.

Astronomers said Thursday (June 7, 2017) that they had felt space-time vibrations known as gravitational waves from the merger of a pair of mammoth black holes resulting in a pit of infinitely deep darkness weighing as much as 49 suns, some 3 billion light-years from here.

This is the third black-hole smashup that astronomers have detected since they started keeping watch on the cosmos back in September 2015, with LIGO, the Laser Interferometer Gravitational-Wave Observatory. All of them are more massive than the black holes that astronomers had previously identified as the remnants of dead stars.

In less than two short years, the observatory has wrought twin revolutions. It validated Einstein’s longstanding prediction that space-time can shake like a bowlful of jelly when massive objects swing their weight around, and it has put astronomers on intimate terms with the most extreme objects in his cosmic zoo and the ones so far doing the shaking: massive black holes.

“We are moving in a substantial way away from novelty towards where we can seriously say we are developing black-hole astronomy,” said David Shoemaker, a physicist at the Massachusetts Institute of Technology and spokesman for the LIGO Scientific Collaboration, an international network of about 1,000 astronomers and physicists who use the LIGO data. They and a similar European group named Virgo are collectively the 1,300 authors of a report on the most recent event that will be published in the journal Physical Review Letters on Thursday.

“We’re starting to fill in the mass spectrum of black holes in the universe,” said David Reitze, director of the LIGO Laboratory, a smaller group of scientists headquartered at Caltech and M.I.T. who built and run the observatory.

The National Science Foundation, which poured $1 billion into LIGO over 40 years, responded with pride. “This is exactly what we hoped for from N.S.F.’s investment in LIGO: taking us deeper into time and space in ways we couldn’t do before the detection of gravitational waves,” France Cordova, the foundation’s director, said in a statement. “In this case, we’re exploring approximately 3 billion light-years away!”

In the latest LIGO event, a black hole 19 times the mass of the Sun and another black hole 31 times the Sun’s mass, married to make a single hole of 49 solar masses. During the last frantic moments of the merger, they were shedding more energy in the form of gravitational waves than all the stars in the observable universe.

After a journey lasting 3 billion years, that is to say, a quarter of the age of the universe, those waves started jiggling LIGO’s mirrors back and forth by a fraction of an atomic diameter 20 times a second. The pitch rose to 180 cycles per second in about a tenth of a second before cutting off.

Zsuzsanna Marka, an astronomer at Columbia University, was sitting in an office on the morning of Jan. 4, 2017 when she got an email alert. She started to smile but then remembered she was not alone and the other person was not a member of LIGO, so she couldn’t say why she was smiling.

“I just kept smiling,” she said.

Upon further analysis it proved to be a perfect chirp, as predicted by Einstein’s equations. Because of the merger’s great distance, the LIGO scientists were able to verify that different frequencies of gravity waves all travel at the same speed, presumably the speed of light. As Dr. Reitze said, “Once again Einstein triumphs.”

“That’s not surprising,” Dr. Reitze went on, adding, “at some point he’s going to be wrong, and we’ll be looking.”

Poor Einstein.

Black holes were an entirely unwelcome consequence of his theory of general relativity that ascribes gravity to the warping of space-time geometry by matter and energy. Too much mass in one place, the equations said, could cause space to wrap itself around in a ball too tight and dense for even light to escape. In effect, Einstein’s theory suggested, matter, say a dead star, could disappear from the universe, leaving behind nothing but its gravitational ghost.

Einstein thought that nature would have more sense than that. But astronomers now agree that the sky is dotted with the dense dark remnants of stars that have burned up all their fuel and collapsed, often in gigantic supernova explosions. Until now, they were detectable only indirectly by the glow of x rays or other radiation from doomed matter heated to stupendous degrees as it swirls around a cosmic drain.

But what telescopes cannot see, gadgets like LIGO now can feel, or “hear.”

Gravitational waves alternately stretch and squeeze space as they travel along at the speed of light. LIGO was designed to look for these changes by using lasers to monitor the distances between mirrors in a pair of L-shaped antennas in Hanford, Wash., and in Livingston, La. There is another antenna in Italy known as Virgo now undergoing its final testing. When it is online, possibly later this summer (2017), having three detectors will greatly improve astronomers’ ability to tell where the gravitational waves are coming from.

The detectors were designed and built and rebuilt over 40 years to be able to detect collisions of neutron stars—the superdense remnants of some kinds of supernova explosions. Astronomers know such pairs exist in abundance, doomed someday for a fiery ending.

Colliding black holes, being more massive, would be even easier to detect, but LIGO’s founders and funders at the National Science Foundation mostly did not know if there were any around to detect.

Now they know.

The current version of the observatory, known as Advanced LIGO, was still preparing for its first official observing run, in September 2015, when it recorded the collision of a pair of black holes 36 and 29 times as massive as the sun. A second collision, on Dec. 26, 2015, was also confirmed to be massive black holes. A third event in October of that year was probably a black hole merger, the collaboration said.

The burning question now is: Where did such massive black holes come from?

“How were such large black-hole binaries created? How did they form?” Szabolcs Marka, a physics professor at Columbia and LIGO member, said recently. “This is indeed one of the big questions of our field today.”

One possibility is that they were born that way, from a pair of massive stars orbiting each other that evolved, died, blew up and then collapsed again into black holes — all without either star getting kicked out of the system during one of those episodes of stellar violence.

Another idea is that two pre-existing black holes came together by chance and captured each other gravitationally in some crowded part of the galaxy, such as near the center, where black holes might naturally collect.

Astronomers won’t say which explanation is preferred, pending more data, but what Dr. Reitze calls a “tantalizing hint” has emerged from analysis of the Jan. 4 (2017) chirp, namely how the black holes were spinning.

If the stars that gave rise to these black holes had been lifting and evolving together in a binary system, their spins should be aligned, spinning on parallel axes like a pair of gold medal skating dancers at the Olympics, Dr. Reitze explained.

Examination of the January chirp, Dr. Reitze said, gives hints that the spins of the black holes were not aligned, complicating the last motions of their mating dance.

“It was not a simple waltz, it was more like a couple of break dancers,” he said.

As for the original stellar identities of these dark dancers, the consensus, said Daniel Holz of the University of Chicago, is that they were probably very massive and primitive stars at least 40 times heavier than the Sun.

According to theoretical calculations, stars composed of primordial hydrogen and helium and lacking heavier elements like oxygen and carbon, which astronomers with their knack for nomenclature call “metals,” can grow monstrously large. They could collapse directly into black holes when their brief violent lives were over without the benefit of a supernova explosion or other cosmic fireworks.

Dr. Holz said in an email: “It is indeed odd to think that some of the most dramatic stellar collapse do not result in massive stellar explosions outshining galaxies, but instead just involve a star winking out of existence. But that’s what the theory says should happen.”

As if on cue, just last week (June, 2017) astronomers from Ohio State reported that a massive star called N6946-BH1 had suddenly disappeared. The star was in a spiral galaxy 22 million light-years away that is nicknamed the “Fireworks Galaxy” because so many supernova explosions happen in it.

The star, estimated to weigh as much as 25 suns had been brightening since 2009 and was presumably on its way to being a supernova. Instead it winked out in 2015. After a search for remains with the Hubble and Spitzer space telescopes, the astronomers concluded that the supernova had probably fizzled and the star had instead collapsed into a black hole.

In a news release from Ohio State, Kris Stanek, a co-leader of this discovery, said it could help explain the LIGO results and why astronomers didn’t see supernovas from really massive stars. “I suspect it’s much easier to make a very massive black hole if there is no supernova,” he said.

In an email Dr. Stanek wrote, “I am obviously biased, but I think this is a very important discovery, and one that the community is not yet fully ‘groking’ in how it will impact a number of things, including LIGO results.”

Dr. Holz agreed. He said, “We think this might be a channel for ‘heavy’ black hole formation, and it’s amazing to see it actually happening in real time.” Noting that the LIGO observations were in some sense the deaths of the black holes that collided, he added, “so now in some sense we get to watch both the birth and the death of the black holes.”

[1] Dennis Overbye, “Third Gravitational Wave Detection, From Black-Hole Merger 3 Billion Light Years Away ,” New York Times (June 1, 2017). A version of this article appears in print on June 2, 2017, on Page A19 of the New York edition with the headline: “From Cheap Seats on Earth, Sensing a Tiny Echo of a Tumultuous Spectacle.”

Gravitational Waves

Gravitational Waves[1]

The Laser Interferometer Gravitational-Wave Observatory, or LIGO, launched the era of gravitational wave astronomy in February 2016 with the announcement of a collision between two black holes observed in September 2015.

The scientific collaboration that operates the two LIGO detectors netted a second merger between slightly smaller black holes on December 26, 2015. (A third “trigger” showed up in LIGO data on October 12, 2015, but ultimately did not meet the stringent statistical significance standard that physicists generally insist on.)

Instead, scientists focused on sharpening theoretical estimates of how often various events occur. In particular, they are eager to see collisions involving neutron stars, which lack sufficient mass to collapse all the way to a black hole. Neutron star collisions are thought to be plentiful, but would emit weaker gravitational waves than do mergers of more massive black holes, so the volume of space the LIGO detectors can scan for such events is smaller.

LIGO scientists are also looking for signals from individual pulsars—rapidly rotating neutron stars that are observed on earth as pulses of radio waves. A bump on a pulsar’s surface should produce gravitational waves, but so far, no waves with the right shape have been picked up. This absence puts a limit on the size of any irregularities and on the emission power of gravitational waves from nearby pulsars such as the Crab and Vela pulsars, said Michael Landry, head of the Hanford LIGO observatory, and could soon start putting limits on more distant ones.

A few hints of possible excitement to come: LIGO data taken through the end of January, 2017 produced two short signals that were unusual enough to exceed the experiment’s “false alarm” threshold—signals with shapes and strengths expected to show up once a month or less by chance alone. Both LIGO collaboration members and astronomers at conventional telescopes are investigating the data to determine whether they represent real events.

LIGO is not the only means by which scientists are searching for gravitational waves. Some scientists are using powerful radio telescopes to track signals emanating from dozens of extremely fast-rotating pulsars. A specific pattern of correlations between tiny hiccups in the arrival times of these pulses would be a signature of long-wavelength gravitational waves expected from mergers of distant supermassive black holes.

[1] See Gabriel Popkin, “Gravitational Waves: Hints, Allegations, and Things Left Unsaid,” in APSNEWS (36, 3, March 2017, p. 1)