Gravitational Wave Search Finds Tantalizing New Clue

 Pairs of black holes billions of times more massive than the Sun may be circling one another, generating ripples in space itself. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has spent more than a decade using ground-based radio telescopes to look for evidence of these space-time ripples created by behemoth black holes. 

This week, the project announced the detection of a signal that may be attributable to gravitational waves, though members aren't quite ready to claim success.

In a new paper published in the January 2021 issue of the Astrophysical Journal Supplements, the NANOGrav project reports the detection of unexplained fluctuations, consistent with the effects of gravitational waves, in the timing of 45 pulsars spread across the sky and measured over a span of 12 1/2 years.

Pulsars are dense nuggets of material left over after a star explodes as a supernova. As seen from Earth, pulsars appear to blink on and off. In reality, the light comes from two steady beams emanating from opposite sides of the pulsar as it spins, like a lighthouse. If gravitational waves pass between a pulsar and Earth, the subtle stretching and squeezing of space-time would appear to introduce a small deviation in the pulsar's otherwise regular timing.

 But this effect is subtle, and more than a dozen other factors are known to influence pulsar timing as well. A major part of the work done by NANOGrav is to subtract those factors from the timing data for each pulsar before looking for signs of gravitational waves.

LIGO and Virgo detect gravitational waves from individual pairs of black holes (or other dense objects called neutron stars). By contrast, NANOGrav is looking for a persistent gravitational wave "background," or the noiselike combination of waves created over billions of years by countless pairs of supermassive black holes orbiting one another across the universe.

 These objects produce gravitational waves with much longer wave lengths than those detected by LIGO and Virgo - so long that it might take years for a single wave to pass by a stationary detector. So while LIGO and Virgo can detect thousands of waves per second, NANOGrav's quest requires years of data.

As tantalizing as the latest finding is, the NANOGrav team isn't ready to claim they've found evidence of a gravitational wave background. Why the hesitation? In order to confirm direct detection of a signature from gravitational waves, NANOGrav's researchers will have to find a distinctive pattern in the signals between individual pulsars. According to Einstein's theory of general relativity, the effect of the gravitational wave background should influence the timing of the pulsars slightly differently based on their positions relative to one another.

At this point, the signal is too weak for such a pattern to be distinguishable. Boosting the signal will require NANOGrav to expand its dataset to include more pulsars studied for even longer lengths of time, which will increase the array's sensitivity. NANOGrav is also pooling its data with those from other pulsar timing array experiments in a joint effort by the International Pulsar Timing Array, a collaboration of researchers using the world's largest radio telescopes.

"Trying to detect gravitational waves with a pulsar timing array requires patience," said Scott Ransom with the National Radio Astronomy Observatory and the current chairperson of NANOGrav. "We're currently analyzing over a dozen years of data, but a definitive detection will likely take a couple more. It's great that these new results are exactly what we would expect to see as we creep closer to a detection.

"The NANOGrav team discussed their findings at a press conference on Jan. 11 at the 237th meeting of the American Astronomical Society, held virtually from Jan. 10 to 15. Michele Vallisneri and Joseph Lazio, both astrophysicists at NASA's Jet Propulsion Laboratory in Southern California, and Zaven Arzoumanian at NASA's Goddard Space Flight Center in Maryland are co-authors of the paper.

 Joseph Simon, a researcher at University of Colorado Boulder and the paper's lead author, conducted much of the analysis for the paper as a postdoctoral researcher at JPL. Multiple NASA postdoctoral fellows have participated in the NANOGrav research while at JPL. NANOGrav is a collaboration of U.S. and Canadian astrophysicists. The data in the new study was collected using the Green Bank antenna in West Virginia and the Arecibo dish in Puerto Rico before its recent collapse.

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GOTO telescope developed to hunt down sources of gravitational waves

 Current gravitational wave instruments such as the LIGO and VIRGO collaborations can only detect the passage of gravitational waves through the local spacetime, and are unable to trace the source of the events that send out the gravitational waves. To fill in this gap between gravitational wave astronomy and optical signals in the electromagnetic spectrum, an international collaboration of scientists have developed a new telescope known as Gravitational-wave Optical Transient Observer (GOTO). The instrument is made up of two identical arrays located on opposite sides of the planet, one located at the Siding Spring Observatory in Australia, and the other located Roque de Los Muchachos Observatory on La Palma island in the Canary Islands. 

The antipodal placement of the two arrays is to cover the entire sky at once. Most gravitational waves emerge from the violent interactions between massive objects such as black holes and neutron stars. While some of the gravitational waves detected by the VIRGO and LIGO collaborations have been linked to known optical sources, there have been none identified for some others events. The visual clues from these events are extremely fleeting, which is where GOTO comes into the picture. Events flagged by GOTO are expected to be observed by other astronomical assets on Earth and in space. GOTO is expected to act as an intermediary between gravitational wave instruments such as the LIGO and VIRGO collaborations, and other, more targetable multi-wavelength observatories that can follow up and investigate the optical source of an event. 

Both the Northern Node domes at La Palma. (Image credit: GOTO Collaboration)

Astronomer Martin Dyer says, “Gravitational waves are created when two black holes or neutron stars in close orbit – each tens of times heavier than the sun – violently collide. The detection of gravitational waves is like knowing that a truck has passed by feeling the rumble in a road’s surface and trying to work out where it came from based on that alone. This telescope will be crucial for scientists across the world in order to broaden our understanding of the universe. Having access to the telescope will allow our astronomers at the University of Sheffield to accelerate and enhance their pioneering research in this important area of physics.”

Principle investigator on GOTO, Danny Steeghs says “There are fleets of telescopes all over the world available to look towards the skies when gravitational waves are detected, in order to find out more about the source. But as the gravitational wave detectors are not able to pinpoint where the ripples come from, these telescopes do not know where to look. If the gravitational wave observatories are the ears, picking up the sounds of the events, and the telescopes are the eyes, ready to view the event in all the wavelengths, then GOTO is the bit in the middle, telling the eyes where to look.” 

The eight telescopes within a single dome. (Image credit: GOTO Collaboration)

Within the domes are telescope mount systems, each made up of eight 40 centimetre telescopes. Two of these arrays are now operational in La Palma, with a total of 16 telescopes capable of scanning the entire sky every few days. The team is preparing the site in Australia, which will also have two domes with 16 telescopes. The GOTO instruments are designed to perform their tasks autonomously, focusing on particular regions of the sky when there is an alert for a gravitational wave detection.

Steeghs says, “The hope is to catch the event quickly, then follow it as it fades, and also to trigger an alert to other, bigger telescopes so they can all collect more information and we can build a really detailed picture of these astronomical phenomena. It is a really dynamic and exciting time. In astronomy we are used to studying events which are millions of years old and aren’t going anywhere – this is a fast-paced, very different way of working where every minute counts.”

The GOTO instrument is expected to be an important enabler in the new era of multi-messenger astronomy, where the same event is observed by multiple astronomical instruments around the world, and in space. 

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Tina Kahniashvili Searches for Clues from Universe's First Moments

 An instant after the Big Bang burst the universe into existence, violent processes generated strong gravitational radiation that rippled away from the explosion.

Those primordial gravitational waves could provide deep insights into the early universe, and Carnegie Mellon University's Tina Kahniashvili(opens in new window) is at the forefront of an effort to understand more about its origin and evolution. A new grant from the NASA Astrophysics Theory Program (ATP) is helping to fund that effort.

"The work is high-risk but highly rewarding," said Kahniashvili, an associate research professor in the Department of Physics(opens in new window) and a member of the McWilliams Center for Cosmology(opens in new window). "We are looking for messengers from the very early epochs and reconstructing the newborn universe picture."

Axel Brandenburg(opens in new window), an adjunct faculty member at Carnegie Mellon and professor of astrophysics at Nordita and the University of Stockholm, is co-principal investigator with Kahniashvili for the grant "Gravitational Waves as a Probe of the Early Universe." The grant includes additional collaboration with Andrew Long at Rice University; and Mark Hindmarsh, of Sussex University in the United Kingdom and the University of Helsinki in Finland.

Their goal is to provide the astrophysics and cosmology community with robust predictions for gravitational wave spectra from early universe sources, guidance for distinguishing different potential sources given the observational data and a clear understanding of how new physics can be probed with gravitational wave detection.

Kahniashvili and colleagues are looking for relics of this primordial gravitational radiation. The radiation relics are present in the form of stochastic gravitational wave backgrounds. In the 13.7 billion years since the relic gravitational waves were formed, they have stretched as the universe expanded. They now form something akin to an isotropic background that does not interact with any other components of the universe.

"Gravitational radiation propagates almost freely throughout cosmic history," Kahniashvili said. "Primordial gravitational waves reflect a precise picture of the universe at their time of production in the first fractions of a second after the Big Bang. Detecting these gravitational waves today would open the possibility to test physical processes at energy scales far beyond what is reached by particle physics experiments and astrophysical observations."

Gravitational wave missions such as pulsar timing arrays (PTAs) including the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) are searching for this signal. The upcoming Laser Interferometer Space Antenna (LISA) will extend the search by several orders of magnitude in sensitivity.

In her work, Kahniashvili is taking the search a step beyond that.

"A primordial gravitational wave background that's detectable by PTA or LISA — most likely — cannot arise from the Standard Model of particle physics and cosmology alone, but rather it would be strong evidence for new physics," she said.

Kahniashvili earned her bachelor's in physics (and certificate in physics education) and master's degree in theoretical physics at Tbilisi State University in Georgia. She earned a Ph.D. in physics and a doctor of sciences degree from the Russian Academy of Sciences. She came to the United States in 2000 through a National Science Foundation (NSF) Collaboration and Basic Science and Engineering grant that supported visiting scientists from the former Union of Soviet Socialist Republics. At the time, she was working on early-universe gravitational waves and primordial magnetic fields.

She joined Carnegie Mellon in 2009. Since 2010, her work has been supported through grants such as the NASA ATP and the NSF Astronomy and Astrophysics Grant programs. She also has received funding through Swedish and Georgian grants and holds positions as a professor of physics and astronomy at Ilia State University and is the main scientist at Abastumani Astrophysical Observatory, both in Georgia.

"My current NASA ATP grant is my first grant that I got in the USA on gravitational waves from the early universe," she said. "I am really pleased that my proposal was selected."

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News at a glance: Tracking gravitational waves, a Moon rover, and the ‘best fossil hunter’

 Researchers last week reached the midpoint in building a pair of observatories designed to pinpoint the location of cataclysmic events sensed by gravitational wave detectors so that other astronomers can quickly zoom in on the aftermath. The Gravitational-wave Optical Transient Observer (GOTO) uses two sets of 16 small telescopes, one in the Canary Islands—now operational—and one in Australia, whose construction has just started. They will swing into action automatically when gravitational wave detectors, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and Virgo in Italy, register a space-time ripple caused by events such as a merger of two black holes. Such detectors don’t give precise locations, so GOTO’s scopes will sweep the region of space for rapidly brightening objects that could be the source of the wave; operators will then send alerts, giving more sensitive optical telescopes a location to aim at. A neutron star merger, spotted gravitationally in 2017, produced visible signals, but telescopes took 11 hours to find them after the event became known, and missed the explosion’s early phases. The £4.4 million GOTO telescopes hope to do better and flag candidates in half an hour. Project leaders aim to be ready by March 2023, when upgrades to LIGO and Virgo are finished and they begin their next observing runs.

The World Health Organization (WHO) last week declared the global monkeypox outbreak, which has sickened more than 15,000 in at least 70 countries, a Public Health Emergency of International Concern (PHEIC), even though a sharply divided advisory committee had not recommended doing so. In June, WHO’s Emergency Committee for monkeypox first advised against giving the epidemic PHEIC status—which grants WHO extra powers and helps focus political attention on an outbreak—a decision that was widely criticized. At its second meeting on 21 July, the panel could not reach a consensus, with nine members opposing a PHEIC declaration and six supporting it. (The meeting was followed by tense exchanges between participants via email and text messages, Science has learned.) Opponents noted that monkeypox is not yet circulating in the general population; it has been observed mostly in men who have sex with men. But others argued for action now because there is a danger the virus could become established long-term in the wider population. On 23 July, WHO Director-General Tedros Adhanom Ghebreyesus said the outbreak met criteria in the International Health Regulations and, in an unprecedented move, declared a PHEIC anyway.

The first NASA robotic rover to visit the Moon will be delayed by a year, launching in November 2024, the agency announced last week. The Volatiles Investigating Polar Exploration Rover, known as VIPER, will land at the Moon’s south pole, hunting for water ice. The delay, which will nudge the $754 million mission closer to the planned launch by China of a similar device, will allow further testing of the lunar lander, developed by Astrobotic Technology, that will carry the rover in the lander’s first lunar touchdown.

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Gravitational wave ‘radar’ could help map the invisible universe

 

Such “GRADAR” signals could spot globs of dark matter or very distant neutron stars

It sounds like the setup for a joke: If radio waves give you radar and sound gives you sonar, what do gravitational waves get you?

The answer might be “GRADAR” — gravitational wave “radar” — a potential future technology that could use reflections of gravitational waves to map the unseen universe, say researchers in a paper accepted to Physical Review Letters. By looking for these signals, scientists may be able to find dark matter or dim, exotic stars and learn about their deep insides.

Astronomers routinely use gravitational waves — traveling ripples in the fabric of space and time itself, first detected in 2015 — to watch cataclysmic events that are hard to study with light alone, such as the merging of two black holes (SN: 2/11/16).

But physicists have also known about a seemingly useless property of gravitational waves: They can change course. Einstein’s theory of gravity says that spacetime gets warped by matter, and any wave passing through these distortions will change course. The upshot is that when something emits gravitational waves, part of the signal comes straight at Earth, but some might arrive later — like an echo — after taking longer paths that bend around a star or anything else heavy.

Scientists have always thought these later signals, called “gravitational glints,” should be too weak to detect. But physicists Craig Copi and Glenn Starkman of Case Western Reserve University in Cleveland, Ohio, took a leap: Working off Einstein’s theory, they calculated how strong the signal would be when waves scatter through the gravitational field inside a star itself.

“The shocking thing is that you seem to get a much larger result than you would have expected,” Copi says. “It’s something we’re still trying to understand, where that comes from — whether it’s believable, even, because it just seems too good to be true.”

If gravitational glints can be so strong, astronomers could possibly use them to trace the insides of stars, the team says. Researchers could even look for massive bodies in space that would otherwise be impossible to detect, like globs of dark matter or lone neutron stars on the other side of the observable universe.

“That would be a very exciting probe,” says Maya Fishbach, an astrophysicist at Northwestern University in Evanston, Ill., who was not involved in the study.

There are still reasons to be cautious, though. If this phenomenon stands up to more detailed scrutiny, Fishbach says, scientists would have to understand it better before they could use it — and that will probably be difficult.

“It’s a very hard calculation,” Copi says.But similar challenges have been overcome before. “The whole story of gravitational wave detection has been like that,” Fishbach says. It was a struggle to do all the math needed to understand their measurements, she says, but now the field is taking off (SN: 1/21/21). “This is the time to really be creative with gravitational waves.”

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Gravitational waves confirm a black hole law predicted by Stephen Hawking

 Despite their mysterious nature, black holes are thought to follow certain simple rules. Now, one of the most famous black hole laws, predicted by physicist Stephen Hawking, has been confirmed with gravitational waves.

According to the black hole area theorem, developed by Hawking in the early 1970s, black holes can’t decrease in surface area over time. The area theorem fascinates physicists because it mirrors a well-known physics rule that disorder, or entropy, can’t decrease over time. Instead, entropy consistently increases (SN: 7/10/15).

That’s “an exciting hint that black hole areas are something fundamental and important,” says astrophysicist Will Farr of Stony Brook University in New York and the Flatiron Institute in New York City.

The surface area of a lone black hole won’t change — after all, nothing can escape from within. However, if you throw something into a black hole, it will gain more mass, increasing its surface area. But the incoming object could also make the black hole spin, which decreases the surface area. The area law says that the increase in surface area due to additional mass will always outweigh the decrease in surface area due to added spin.

To test this area rule, MIT astrophysicist Maximiliano Isi, Farr and others used ripples in spacetime stirred up by two black holes that spiraled inward and merged into one bigger black hole. A black hole’s surface area is defined by its event horizon — the boundary from within which it’s impossible to escape. According to the area theorem, the area of the newly formed black hole’s event horizon should be at least as big as the areas of the event horizons of the two original black holes combined.

The team analyzed data from the first gravitational waves ever spotted, which were detected by the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, in 2015 (SN: 2/11/16). The researchers split the gravitational wave data into two time segments, before and after the merger, and calculated the surface areas of the black holes in each period. The surface area of the newly formed black hole was greater than that of the two initial black holes combined, upholding the area law with a 95 percent confidence level, the team reports in a paper to appear in Physical Review Letters.

“It’s the first time that we can put a number on this,” Isi says.

The area theorem is a result of the general theory of relativity, which describes the physics of black holes and gravitational waves. Previous analyses of gravitational waves have agreed with predictions of general relativity, and thus already hinted that the area law can’t be wildly off. But the new study “is a more explicit confirmation,” of the area law, says physicist Cecilia Chirenti of the University of Maryland in College Park, who was not involved with the research.

So far, general relativity describes black holes well. But scientists don’t fully understand what happens where general relativity — which typically applies to large objects like black holes — meets quantum mechanics, which describes small stuff like atoms and subatomic particles. In that quantum realm, strange things can happen.

For example, black holes can release a faint mist of particles called Hawking radiation, another idea developed by Hawking in the 1970s. That effect could allow black holes to shrink, violating the area law, but only over extremely long periods of time, so it wouldn’t have affected the relatively quick merger of black holes that LIGO saw.

Physicists are looking for an improved theory that will combine the two disciplines into one new, improved theory of quantum gravity. Any failure of black holes to abide by the rules of general relativity could point physicists in the right direction to find that new theory.

So physicists tend to be grumpy about the enduring success of general relativity, Farr says. “We’re like, ‘aw, it was right again.’”

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