Gravitational waves

Third detection of gravitational waves: U of T scientists part of research team

ullahnor — Thu, 01 Jun 2017 16:10:56 +0000

Third detection of gravitational waves: U of T scientists part of research team

ullahnor Thu, 06/01/2017 - 12:10

Cutline

Illustration showing two merging black holes similar to those detected by LIGO. The black holes are spinning in a non-aligned fashion with different orientations (courtesy of LIGO/Caltech/MIT/Sonoma State/Aurore Simonnet)

Sean Bettam

Author legacy

Sean Bettam

Topic

Global Lens

Faculty of Arts & Science

Gravitational waves

Space

Astrophysicist

It's the third time gravitational waves – ripples in space and time – have been detected, paving the way to more information about black holes and proving once again that Einstein was correct.

The discovery announced today by the Laser Interferometer Gravitational-wave Observatory (LIGO) would not have been possible without the key contributions of a team of U of T astrophysicists.

The newfound black hole has a mass about 49 times that of our sun and, at 3 billion light-years away from Earth, seems to be the farthest away to date.

“With this latest detection of gravitational waves, we continue to learn that colliding black holes come in a variety of masses,” said Harald Pfeiffer, associate professor and Canada Research Chair in Gravitational Wave Astrophysics and Numerical Relativity at the Canadian Institute for Theoretical Astrophysics (CITA) in U of T's Faculty of Arts & Science. “These new black holes are the second-most massive stellar mass black holes ever detected, second only to our first LIGO discovery.”

“The fact that this latest detection is so far away enables us to test Einstein’s equations more precisely than ever before and to perform different tests of Einstein’s equations for the very first time.”

LIGO made the first-ever direct observation of gravitational waves in September 2015, followed by a second detection in December 2015. As with the previous discoveries, the latest gravitational waves were generated when two black holes collided and merged into a larger black hole.

The black hole involved in the first detection was 62 times the mass of the sun and 1.3 billion light years away, while the one that figured in the second detection was 21 times the sun’s mass and 1.4 billion light years away.

This third detection – called GW170104 and made on Jan. 4, 2017 – is described in a new paper accepted for publication in the journal Physical Review Letters. It was authored by the LIGO Scientific Collaboration (LSC), a body of more than 1,000 international scientists who perform LIGO research together with the Europe-based Virgo Collaboration.

In all three detections, each of LIGO’s twin detectors – one in Livingston, La., and one in Hanford, Wash., – picked up gravitational waves resulting from the tremendously energetic mergers of black hole pairs. These collisions produce more power than is radiated as light by all the stars and galaxies in the universe at any given time.

Pfeiffer, who is also a fellow of the Canadian Institute for Advanced Research (CIFAR), leads a team of seven researchers at U of T that constitutes Canada’s contribution to the LIGO project. The team had originally been largely responsible for performing simulations of black hole-collisions on high-performance supercomputers, and producing the gravitational waveforms – the shapes of the signals – that LIGO is searching. Having doubled in size over the past year, the team now includes researchers taking leading roles in estimating the masses and spins of the colliding black holes.

“Knowing more about the ways black holes spin will teach us how binary black holes form and whether or not the rotation of each are aligned with their orbits,” said Pfeiffer.

There are several models that explain how binary pairs of black holes can be formed, and the ongoing detections of gravitational waves will help scientists hone in on the best ones.

The new LIGO data points to the possibility that at least one of the black holes may have been non-aligned compared to the overall orbital motion. While more observations with LIGO are needed to be definitive these early data offer clues about how these pairs may form.

Heather Fong, a PhD candidate in the department of physics and a member of Pfeiffer’s team at CITA, has played a key role in the development of the gravitational waveforms. Though she was instrumental in the process of simulating collisions of black holes that led to the first two detections, this latest event is particularly exciting for her, having recently spent three months at LIGO’s Hanford site.

“I arrived just a week after we made this latest detection. Because I am mainly involved with the data analysis effort, it was amazing to be able to work on the experimental side and interact directly with the detector,” said Fong. “The time I spent there gave me a great deal of insight into how the LIGO detectors work, as well as a profound appreciation for how they're able to be as sensitive as they are.”

The study once again puts Einstein's theories to the test. For example, the researchers looked for an effect called dispersion, which occurs when light waves in a physical medium such as glass travel at different speeds depending on their wavelength. This is how a prism creates a rainbow. Einstein's general theory of relativity forbids dispersion from happening in gravitational waves as they propagate from their source to Earth. LIGO did not find evidence for this effect.

Pfeiffer believes that more results may also allow scientists to identify other unknown phenomena that are yet to be identified.

“The other really important source of gravitational waves that everybody is eagerly waiting for are binary pairs involving neutron stars, whether they be two neutron stars or a black hole colliding with a neutron star,” he said. “Either way, these detections are continuously proving Einstein’s century-old theory of relativity is correct. And they are doing so with objects we didn’t know existed before LIGO detected them.

LIGO is funded by the National Science Foundation (NSF), and operated by MIT and Caltech, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project.

More than 1,000 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. LIGO partners with the Virgo Collaboration, a consortium including 280 additional scientists throughout Europe supported by the Centre National de la Recherche Scientifique (CNRS), the Istituto Nazionale di Fisica Nucleare (INFN), and Nikhef, as well as Virgo’s host institution, the European Gravitational Observatory.

Making waves: How the ��Ƶ made the discovery of gravitational waves possible

lavende4 — Tue, 04 Oct 2016 18:45:28 +0000

Making waves: How the ��Ƶ made the discovery of gravitational waves possible

lavende4 Tue, 10/04/2016 - 14:45

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Left to right: PhD candidate Heather Fong, CITA professor Harald Pfeiffer and CITA post doctoral fellow Prayush Kumar (Photo by Diana Tyszko)

Patchen Barss

Author legacy

Patchen Barss

Topic

Global Lens

LIGO

Gravitational waves

Faculty of Arts & Science

Canadian Institute for Theoretical Astrophysics

The discovery of gravitational waves in 2016 – predicted by Albert Einstein – was one of the biggest sciences stories of the decade. While the discovery didn’t take the Nobel Prize today, many in the scientific community think it is just a matter of time before the Nobel committee honours the Laser Interferometer Gravitational-Wave Observatory (LIGO) team for one of the most outstanding contributions in the field of physics. Science writer Patchen Barss explains the discovery and the key contributions of ��Ƶ’s Harald Pfeiffer and his team.

A billion years ago and a billion light years away, a star 36 times more massive than our Sun expended its remaining fuel in a final blast of nuclear fusion. With nothing left to burn, the star began to collapse under its own gravity. The atoms in its massive core collapsed like crushed soda cans. Protons and electrons ground together to form new neutrons.

The star’s density kept increasing. Its gravity became so concentrated and intense that not even light could escape any longer. Spacetime warped and ruptured. The star became a black hole.

But that wasn’t the end of the story.

A second black hole, the product of an only slightly smaller stellar cataclysm passed by. The two became trapped in each other’s mighty gravitational fields. They circled one another, slowly at first, but then more and more quickly. Their collision course became a high-speed death spiral that sent waves of gravitational energy rippling out across their galaxy and into the cosmos at the speed of light, stretching and squeezing space itself.

Back on present-day Earth, came a different kind of merger: a collision of ideas between observational cosmologists and numerical relativity experts.

In the 1970s, observational scientists had begun working on “laser interferometry” instruments that might detect gravitational waves. Decades of effort culminated in the construction of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which comprises two massive detectors, one in Washington and the other in Louisiana.

Concurrently, the ��Ƶ was leading an international effort to simulate black hole collisions and predict what the emerging gravitational wave patterns might look like.

The simulations belong to a field of study with the unglamorous name of “numerical relativity.” These supercomputer simulations nab few headlines, but without them, gravitational-wave research wouldn’t have gone far, even with LIGO’s whiz-bang technology. Scientists at U of T identified the need for powerful simulations early in LIGO’s planning stages, and drove the push to mature the theoretical science in time to make the most of LIGO’s observations.

“The development of these simulations was precisely designed to make us able to analyze the data collected by LIGO experiments,” says J. Richard Bond, a University Professor at the Canadian Institute for Theoretical Astrophysics (CITA) in the Faculty of Arts & Science. Bond drove the effort to recruit an expert devoted to numerical relativity.

“Detecting gravitational waves is a huge revolution. It will be front and centre in what’s going to happen over the next few decades,” he says. “You’re either on that bus or off it. Somebody here at the University had to be on the gravitational-wave bus.”

In fact, U of T attracted a whole busload of graduate students and postdoctoral fellows to work on numerical relativity. From the start, though, the person driving that bus has been Harald Pfeiffer.

Before Pfeiffer became an associate professor at CITA, he had already established his reputation in numerical relativity at Cornell University and Caltech.

“I have always been interested in black holes and Einstein and gravity and computers,” he says. “At Cornell, I worked with one of the world’s experts on solving Einstein’s equations on supercomputers. The relevance to LIGO was there all along.”

In the early 20^th century, Albert Einstein proposed his Theory of Relativity a model of gravity and the universe that scientists have been testing and exploring ever since. Many non-scientists can recite Einstein’s most famous equation: E=mc². But the so-called mass-energy equivalence equation is just one tiny part of the math behind relativity. Researchers are still finding new predictions based on Einstein’s equations, and using them to understand and simulate cosmic events that would otherwise defy imagination and intuition.

“The first time people tried to simulate black-hole collisions on computers was in 1964,” says Pfeiffer. “But even when I started my PhD, nobody had yet figured out how to do it. We made steady progress but only on arcane technical sub-problems. The big problem eluded everybody until 2005 when finally all the pieces came together.”

LIGO faced hurdles of its own. Through the 1980s and 1990s, the project faced technological and budgetary delays. Between 2002 and 2010, the first major version of LIGO worked exactly as planned. But, during that time the cosmos failed to cooperate, sending no detectable waves our way. An international team of scientists continued to make refinements and improvements to increase LIGO’s sensitivity.

LIGO’s L-shaped detectors work by splitting a laser beam into two waves radiating at right angles to one another. Each beam travels precisely the same distance – four kilometres – through a vacuum, bounces off a fine-tuned mirror and returns along the same path to the split point. In the absence of gravitational waves, the returning beams cancel each other out. The detector stays quiet.

But passing gravitational waves would lengthen space in one direction and squeeze it in the other. Each beam would travel a slightly different distance, get out of sync with the other, and create a distinct, detectable interference pattern.

Researchers built two such detectors thousands of kilometres apart, which allowed them not just to detect waves, but also to triangulate them to determine the location of their source.

They still needed to know what to look for, though.

“In the first 10 years, my research and LIGO were not directly touching each other,” says Pfeiffer. “However, on both sides of the fence there was momentum building and building rapidly.”

Both sides were working toward a goal that nobody was sure would be achievable. Still, they were spiraling in on one another, circling toward an explosive discovery.

In September 2015 a new, vastly more sensitive iteration of LIGO came online. By then, Pfeiffer and his team had simulated thousands of collisions, creating a bank of “pattern templates” that gave observers clues about what to look for, and how to interpret what they found.

Not long after, gravitational waves from that distant, ancient black-hole collision finally reached the Earth.

Space compressed in one direction, stretched in another. The laser beams fell out of sync.

“Chirp!”

That chirp, revealed to the world at an international press conference in February 2016, was an audio interpretation of a laser interference pattern created by billion-year-old gravitational waves.

Using Pfeiffer’s simulations, researchers conclusively identified the pattern as the first-ever direct detection of gravitational waves.

“U of T’s key contribution was this waveform modeling,” says Pfeiffer. “If you know the shape of the signal you’re looking for, it’s like knowing the colour of a needle in a haystack. It’s easier to find.”

In June 2016, LIGO scientists announced that the detectors had chirped again: a second detection. In this case, though, the black holes involved had about one third the combined mass of the first collision. It was a “quieter” crash with a weaker signal, which meant the simulations played an even more important role in its interpretation.

“The second detection would have been an extremely marginal discovery without the simulations,” says Pfeiffer. “It would have been flagged as an interesting detection, possibly between two black holes, but nothing more precise.”

The pattern templates also save time – rather than deciphering data for days on end, observers can say right away, “You’ve got waves!”

“Real-time is important, because there’s a whole band of astronomers across the world who are not part of LIGO,” says Peter Martin, a CITA professor. “They want to turn optical or radio telescopes to the point of detection quickly to see whether any electromagnetic flash comes with gravitational radiation.”

Researchers continue to improve LIGO, with plans to double its sensitivity. That puts pressure on Pfeiffer to keep building simulations based on Einstein’s relativity equations.

“As boring as it sounds, there’s still a lot of work to be done in improving the waveforms that LIGO is looking for,” says Pfeiffer. “It’s really cool having this big breakthrough, but 99 percent of science is the tedious day-to-day work.”

LIGO plans to continue observations in 2016, and it will join forces with a French-Italian gravitational-wave detector in 2017. Plans include studying more colliding black holes, scoping out their properties in unprecedented detail, and checking whether Einstein’s theory continues to work flawlessly in light of ever more precise data.

Astronomers will also search for gravitational waves from sources other than black holes, including from less-massive-but-still-whoppingly-massive bodies like pulsars and other neutron stars that spin at high speed.

Bond, though, has his eye on another target.

“In Toronto, I and many others are heavily invested in discovering gravitational waves formed during the first moments of the universe,” Bond says.

“The sheer challenge of figuring out how to solve Einstein’s equations would have been enticing enough of a problem,” Pfeiffer says. But he found it doubly exciting when those equations allowed scientists to precisely reconstruct the story of that distant, cataclysmic collision from a billion years earlier.

“It is amazingly satisfying, to see the effort of thousands of people come together,” he says. “Building the LIGO instruments, developing the software to analyse the data, and also our own contribution toward detecting and deciphering the signals. It was only through this huge joint effort that we could discover black holes colliding.”

Gravitational waves

Third detection of gravitational waves: U of T scientists part of research team

Making waves: How the ������Ƶ made the discovery of gravitational waves possible

Making waves: How the ��Ƶ made the discovery of gravitational waves possible