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)