Violent origins of gravitational waves probed by new telescope array

 A new telescope array has begun to hunt for the most violent and cataclysmic events in the cosmos, clashes so powerful that they cause the very fabric of space and time to "ring."

The BlackGEM array, consisting of three new telescopes located at the European Southern Observatory's (ESO) La Silla Observatory in Chile, will search in visible light for events like neutron star collisions and black hole mergers, which launch ripples in space-time called gravitational waves. 

"With BlackGEM, we aim to scale up the study of cosmic events with both gravitational waves and visible light," BlackGEM Principal Investigator Paul Groot, of Radboud University in the Netherlands, said in a statement. "The combination of the two tells us much more about these events than just one or the other."

Thus far, only one explosive event has been detected in both gravitational waves and visible light, the collision between two neutron stars with masses between eight and 20 times that of the sun, which occurred 130 million years ago. 

By detecting both the gravitational waves and visible light generated by such events, scientists can not only hone in on their precise locations but can also learn more about their nature. For example, astronomers could confirm that collisions between neutron stars, also known as kilonovas, do indeed forge heavy elements like silver, platinum, and gold, as suspected. 

Following up ripples in space-time predicted by Einstein

Gravitational waves were first predicted by Albert Einstein in his theory of general relativity. This 1915 theory says that objects of mass "warp" the fabric of space-time, a four-dimensional unification of space and time, like objects placed on a stretched rubber sheet. This warping gives rise to gravity. General relativity also suggests that when massive objects accelerate, they generate gravitational waves.

Objects of tremendous mass circling each other, like binaries of black holes and neutron stars, produce gravitational waves that carry angular momentum away from the system, causing them to spiral together faster and faster. When the two objects finally collide and merge, they create a burst of gravitational waves that can be detected from Earth, even after traversing millions or billions of light-years at the speed of light to reach us.

The first detection of gravitational waves was made on Earth in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and came from the collision of two black holes 1.3 billion years ago. Since then, LIGO, based in the U.S., and its fellow gravitational wave detectors Virgo in Italy and KAGRA in Japan have detected gravitational waves from other black hole mergers, neutron star mergers and even collisions between black holes and neutron stars (which are the super-dense corpses of massive stars). 

As impressive as this is, however, gravitational wave detectors can't accurately pinpoint the location from which gravitational waves originate. Nor can they see the energetic blasts of light that are emitted with the gravitational wave bursts that occur during these collisions.

That's where BlackGEM will come in, quickly scanning large areas of the sky to hunt for gravitational-wave sources in visible light and more accurately honing in on their locations. 

How BlackGEM will fit in

Once BlackGEM identifies the source of gravitational waves, larger instruments like the Very Large Telescope (VLT) located on Cerro Paranal in the Atacama Desert of northern Chile will follow up on its findings by zooming in on the event. 

The three telescopes of the BlackGEM array are each 25.6 inches (65 centimeters) in diameter and can investigate different areas of the sky simultaneously. Eventually, these telescopes, built by Radboud University, the Netherlands Research School for Astronomy and KU Leuven in Belgium, may be joined in the array by a further 15 scopes. 

This should give an impressive boost to the sky-searching power of the BlackGEM array, the first system of its kind in the Southern Hemisphere.

"Despite the modest 65-centimeter primary mirror, we go as deep as some projects with much bigger mirrors because we take full advantage of the excellent observing conditions at La Silla," Groot said.

BlackGEM won't just be hunting for the sources of gravitational waves, however. The telescope array will also survey the southern sky in fully automated mode, allowing it to quickly find and identify events and objects with rapidly changing brightness, also known as "transients."

These could include supernovas, titanic explosions that accompany the deaths of massive stars and then quickly fade from view. 

"Thanks to BlackGEM, La Silla now has the potential to become a major contributor to transient research," La Silla Observatory site manager Ivo Saviane said. "We expect to see many outstanding results contributed by this project, which will expand the reach of the site for both the scientific community and the public at large."

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A neutron star collision may have emitted a fast radio burst

 

Gravitational waves from the smashup came from the same part of sky at almost the same time

A neutron star pileup may have emitted two different kinds of cosmic signals: ripples in spacetime known as gravitational waves and a brief blip of energy called a fast radio burst.

One of the three detectors that make up the gravitational wave observatory LIGO picked up a signal from a cosmic collision on April 25, 2019. About 2.5 hours later, a fast radio burst detector picked up a signal from the same region of sky, researchers report March 27 in Nature Astronomy.

If strengthened by further observations, the finding could bolster the theory that mysterious fast radio bursts have multiple origins — and neutron star mergers are one of them.

“We’re 99.5 percent sure” the two signals came from the same event, says astrophysicist Alexandra Moroianu, who spotted the merger and its aftermath while at the University of Western Australia in Perth. “We want to be 99.999 percent sure.”

Unfortunately, LIGO’s two other detectors didn’t catch the signal, so it’s impossible to precisely triangulate its location. “Even though it’s not a concrete, bang-on observation for something that’s been theorized for a decade, it’s the first evidence we’ve got,” Moroianu says. “If this is true … it’s going to be a big boom in fast radio burst science.”

Mysterious radio bursts

Astronomers have spotted more than 600 fast radio bursts, or FRBs, since 2007. Despite their frequency, the causes remain a mystery. One leading candidate is a highly magnetized neutron star called a magnetar, which could be left behind after a massive star explodes (SN: 6/4/20). But some FRBs appear to repeat, while others are apparent one-off events, suggesting that there’s more than one way to produce them (SN: 2/7/20).

Theorists have wondered if a collision between two neutron stars could spark a singular FRB, before the wreckage from the collision produces a black hole. Such a smashup should emit gravitational waves, too (SN: 10/16/17).

Moroianu and colleagues searched archived data from LIGO and the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, a fast radio burst detector in British Columbia, to see if any of their signals lined up. The team found one candidate pairing: GW190425 and FRB20190425A.

Even though the gravitational wave was picked up only by the LIGO detector in Livingston, La., the team spotted other suggestive signs that the signals were related. The FRB and the gravitational waves came from the same distance, about 370 million light-years from Earth. The gravitational waves were from the only neutron star merger LIGO spotted in that observing run, and the FRB was particularly bright. There may even have been a burst of gamma rays at the same time, according to satellite data — another aftereffect of a neutron star merger.

“Everything points at this being a very interesting combination of signals,” Moroianu says. She says it’s like watching a crime drama on TV: “You have so much evidence that anyone watching the TV show would be like, ‘Oh, I think he did it.’ But it’s not enough to convince the court.”

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New methods will allow for better tests of Einstein's general theory of relativity using LIGO data

 Albert Einstein's general theory of relativity describes how the fabric of space and time, or spacetime, is curved in response to mass. Our sun, for example, warps space around us such that planet Earth rolls around the sun like a marble tossed into a funnel (Earth does not fall into the sun due to the Earth's sideways momentum).

One place to search for signatures of quantum gravity is in the mighty collisions between black holes, where gravity is at its most extreme. Black holes are the densest objects in the universe—their gravity is so strong that they squeeze objects falling into them into spaghetti-like noodles. When two black holes collide and merge into one larger body, they roil space-time around them, sending ripples called gravitational waves outward in all directions.

The National Science Foundation-funded LIGO, managed by Caltech and MIT, has been routinely detecting gravitational waves generated by black hole mergers since 2015 (its partner observatories, Virgo and KAGRA, joined the hunt in 2017 and 2020, respectively). So far, however, the general theory of relativity has passed test after test with no signs of breaking down.

Now, two new Caltech-led papers, in Physical Review X and Physical Review Letters, describe new methods for putting general relativity to even more stringent tests. By looking more closely at the structures of black holes, and the ripples in space-time they produce, the scientists are seeking signs of small deviations from general relativity that would hint at the presence of quantum gravity.

"When two black holes merge to produce a bigger black hole, the final black hole rings like a bell," explains Yanbei Chen (Ph.D. '03), a professor of physics at Caltech and a co-author of both studies. "The quality of the ringing, or its timbre, may be different from the predictions of general relativity if certain theories of quantum gravity are correct. Our methods are designed to look for differences in the quality of this ringdown phase, such as the harmonics and overtones, for example."

The work builds upon a ground-breaking equation developed 50 years ago by Saul Teukolsky (Ph.D. '73), the Robinson Professor of Theoretical Astrophysics at Caltech. Teukolsky had developed a complex equation to better understand how the ripples of space-time geometry propagate around black holes. In contrast to numerical relativity methods, in which supercomputers are required to simultaneously solve many differential equations pertaining to general relativity, the Teukolsky equation is much simpler to use and, as Li explains, provides direct physical insight into the problem.

"If one wants to solve all the Einstein equations of a black hole merger to accurately simulate it, they must turn to supercomputers," Li says. "Numerical relativity methods are incredibly important for accurately simulating black hole mergers, and they provide a crucial foundation for interpreting LIGO data. But it is extremely hard for physicists to draw intuitions directly from the numerical results. The Teukolsky equation gives us an intuitive look at what is going on in the ringdown phase."

Li was able to take Teukolsky's equation and adapt it for black holes in the beyond-general-relativity regime for the first time. "Our new equation allows us to model and understand gravitational waves propagating around black holes that are more exotic than Einstein predicted," he says.

The second paper, published in Physical Review Letters, led by Caltech graduate student Sizheng Ma, describes a new way to apply Li's equation to actual data acquired by LIGO and its partners in their next observational run. This data analysis approach uses a series of filters to remove features of a black hole's ringing predicted by general relativity, so that potentially subtle, beyond-general-relativity signatures can be revealed.

"We can look for features described by Dongjun's equation in the data that LIGO, Virgo, and KAGRA will collect," Ma says. "Dongjun has found a way to translate a large set of complex equations into just one equation, and this is tremendously helpful. This equation is more efficient and easier to use than methods we used before."

The two studies complement each other well, Li says. "I was initially worried that the signatures my equation predicts would be buried under the multiple overtones and harmonics; fortunately, Sizheng's filters can remove all these known features, which allows us to just focus on the differences," he says.

Chen added: "Working together, Li and Ma's findings can significantly boost our community's ability to probe gravity."

The first study, titled "Perturbations of spinning black holes beyond General Relativity: Modified Teukolsky equation," was funded by the Simons Foundation, the Brinson Foundation, and the National Science Foundation (NSF). Other authors include Nicolás Yunes of the University of Illinois at Urbana-Champaign. The second study, titled "Black Hole Spectroscopy by Mode Cleaning," was funded by the Brinson Foundation, the Simons Foundation, NSF, and the Australian Research Council Center of Excellence for Gravitational Wave Discovery (OzGrav). Ling Sun of the Australian National University is also a co-author.

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Strange Quark Matter: Gravitational Waves Hold Clues to the Universe’s Densest Matter

 Gravitational waves could reveal whether the quark soup that existed in the early Universe is created in neutron-star mergers.

RIKEN researchers suggest that gravitational-wave signals from merging neutron stars could reveal the existence of ultra-dense quark-gluon matter. By simulating these mergers and analyzing the resultant gravitational waves, they propose that next-gen detectors, due within the next decade, could confirm this theory.

Telltale signatures in gravitational-wave signals from merging neutron stars should reveal what happens to matter at the extreme pressures generated during such mergers, calculations by RIKEN researchers predict.

If you took some water and compressed it with a piston, it would shrink as the molecules get pushed closer together.

If you continued ramping up the pressure, you’d reach a point where the atoms collapse and form an ultra-dense soup of neutrons and protons. The only place in the Universe where this happens is neutron stars, the collapsed remnants of burned-out stars, and it produces mind-boggling densities—one teaspoon of such material weighs several hundred billion kilograms.

But what would happen if you continued to increase the pressure still further? Not even astrophysicists know the answer to that.

If it does exist, there are two possibilities for how protons and neutrons would disintegrate into their constituent quarks during mergers. They could go through a sharp transition, much like liquid water turns into vapor at its boiling point at normal pressures. Or there could be a fuzzy transition, analogous to how water becomes vapor at pressures above its critical point.

Now, Nagataki and co-workers have stimulated mergers between two neutron stars and calculated the gravitational waves that would be produced by them to explore the second possibility.

The frequency of the gravitational waves from neutron-star mergers typically depends on how fast the neutron star rotates. Larger neutron stars typically rotate slower, and vice-versa.

The team found that it should be possible to probe whether the quark phase exists in a neutron star by analyzing the frequency of its gravitational waves. If it does exist, the gravitational waves can also reveal how the quark phase appears.

While current gravitational-wave detectors can’t detect this, the next generation of detectors, which will be coming online in the next decade or so, should be able to.

“It’s amazing to think we should be able to detect the type of transition by detecting gravitational waves,” says Nagataki.

Reference: “Merger and Postmerger of Binary Neutron Stars with a Quark-Hadron Crossover Equation of State” by Yong-Jia Huang, Luca Baiotti, Toru Kojo, Kentaro Takami, Hajime Sotani, Hajime Togashi, Tetsuo Hatsuda, Shigehiro Nagataki and Yi-Zhong Fan, 26 October 2022, Physical Review Letters.

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