From NASA Chandra: “A New Signal for a Neutron Star Collision Discovered”

NASA/Chandra Telescope

From NASA Chandra

April 16, 2019

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu

Credit: X-ray: NASA/CXC/Uni. of Science and Technology of China/Y. Xue et al; Optical: NASA/STScI

A bright burst of X-rays has been discovered by NASA’s Chandra X-ray Observatory in a galaxy 6.6 billion light years from Earth. This event likely signaled the merger of two neutron stars and could give astronomers fresh insight into how neutron stars — dense stellar objects packed mainly with neutrons — are built.

When two neutron stars merge they produce jets of high energy particles and radiation fired in opposite directions. If the jet is pointed along the line of sight to the Earth, a flash, or burst, of gamma rays can be detected. If the jet is not pointed in our direction, a different signal is needed to identify the merger.

The detection of gravitational waves — ripples in spacetime — is one such signal. Now, with the observation of a bright flare of X-rays, astronomers have found another signal, and discovered that two neutron stars likely merged to form a new, heavier and fast-spinning neutron star with an extraordinarily strong magnetic field.

“We’ve found a completely new way to spot a neutron star merger,” said Yongquan Xue of the University of Science and Technology of China and lead author of a paper appearing in Nature. “The behavior of this X-ray source matches what one of our team members predicted for these events.”

Chandra observed the source, dubbed XT2, as it suddenly appeared and then faded away after about seven hours.The source is located in the Chandra Deep Field-South, the deepest X-ray image ever taken that contains almost 12 weeks of Chandra observing time, taken at various intervals over several years. The source appeared on March 22nd, 2015 and was discovered later in analysis of archival data.

“The serendipitous discovery of XT2 makes another strong case that nature’s fecundity repeatedly transcends human imagination,”said co-author Niel Brandt of the Pennsylvania State University and principal investigator of the relevant Chandra Deep Field-South.

The researchers identified the likely origin of XT2 by studying how its X-ray light varied with time, and comparing this behavior with predictions made in 2013 by Bing Zhang from the University of Nevada in Las Vegas, one of the corresponding authors of the paper. The X-rays showed a characteristic signature that matched those predicted for a newly-formed magnetar — a neutron star spinning around hundreds of times per second and possessing a tremendously strong magnetic field about a quadrillion times that of Earth’s.

The team think that the magnetar lost energy in the form of an X-ray-emitting wind, slowing down its rate of spin as the source faded. The amount of X-ray emission stayed roughly constant in X-ray brightness for about 30 minutes, then decreased in brightness by more than a factor of 300 over 6.5 hours before becoming undetectable. This showed that the neutron star merger produced a new, larger neutron star and not a black hole.

This result is important because it gives astronomers a chance to learn about the interior of neutron stars, objects that are so dense that their properties could never be replicated on Earth.

“We can’t throw neutron stars together in a lab to see what happens, so we have to wait until the Universe does it for us,” said Zhang. “If two neutron stars can collide and a heavy neutron star survives, then this tells us that their structure is relatively stiff and resilient.”

Neutron star mergers have been prominent in the news since the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from one in 2017. That source, known as GW170817, produced a burst of gamma rays and an afterglow in light detected by many other telescopes, including Chandra. Xue’s team think that XT2 would also have been a source of gravitational waves, however it occurred before Advanced LIGO started its first observing run, and it was too distant to have been detected in any case.

Xue’s team also considered whether the collapse of a massive star could have caused XT2, rather than a neutron star merger. The source is in the outskirts of its host galaxy, which aligns with the idea that supernova explosions that left behind the neutron stars kicked them out of the center a few billion years earlier. The galaxy itself also has certain properties — including a low rate of star formation compared to other galaxies of a similar mass — that are much more consistent with the type of galaxy where the merger of two neutron stars is expected to occur.Massive stars are young and are associated with high rates of star formation.

“The host-galaxy properties of XT2 indeed boost our confidence in explaining its origin,”said co-author Ye Li from Peking University.

The team estimated the rate at which events like XT2 should occur, and found that it agrees with the rate deduced from the detection of GW170817. However, both estimates are highly uncertain because they depend on the detection of just one object each, so more examples are needed.

“We’ve started looking at other Chandra data to see if similar sources are present”, said co-author Xuechen Cheng, also of the University of Science and Technology of China. “Just as with this source, the data sitting in archives might contain some unexpected treasures.”

A paper describing these results appeared in the April 11th issue of Nature.

Other materials about the findings are available at:
http://chandra.si.edu

For more Chandra images, multimedia and related materials, visit:
http://www.nasa.gov/chandra

See the full article here.

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NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

From UC Santa Barbara: “The Standard Siren”

From UC Santa Barbara

Ten years before the detection of gravitational waves, two KITP postdocs at UC Santa Barbara had a novel idea.

April 2, 2019
Harrison Tasoff

Two neutron stars collide, sending out gravitational waves and electromagnetic radiation detected on Earth in 2017. Photo Credit: Fermilab

Scott Hughesz. Photo Credit: MIT

The history of science is filled with stories of enthusiastic researchers slowly winning over skeptical colleagues to their point of view. Astrophysicist Scott Hughes can relate to these tales.

“For the first 15 or 16 years of my career I was speaking to astronomers, and I always had the impression that they were politely interested in what I had to say, but regarded me as a little bit of a wild-eyed enthusiast who was telling them about a herd of unicorns that my friends and I were raising,” said Hughes.

“Now,” he continued, “there are people who are going, ‘Ooh, all those unicorns you found, can I use them to solve my problem? Do your unicorns have wings? Are they sparkly?’”

Daniel Holz. Photo Credit: University of Chicago

These unicorns are gravitational waves, an area of physics in which Hughes specializes. While working as postdoctoral researchers at UC Santa Barbara’s Kavli Institute for Theoretical Physics (KITP), Hughes and his colleague, Daniel Holz, were among the first to propose using the phenomena, in combination with telescope-based observations, to measure the Hubble constant, a fundamental quantity involved in describing the expansion of the universe.

As the universe expands, it carries celestial objects away from us. This stretches out the wavelength of light we detect from these objects, causing it to drop in frequency just like a siren on a passing ambulance. The faster the object is receding, the more its light will shift toward the red end of the spectrum. The Hubble constant relates an object’s distance from Earth to this redshift, and thus the object’s speed as it’s carried away.

One of an astronomer’s best tools for calculating this is a standard candle, any class of objects that always have the same, standard brightness.

Standard Candles to measure age and distance of the universe from supernovae NASA

If scientists know the brightness of an object, they can determine its distance by measuring how dim it appears to us on Earth.

For decades scientists have tried to get accurate measurements of the Hubble constant in order to investigate why the universe is expanding, and, in fact, accelerating. This ultimately resolves to measuring objects’ redshifts and matching them with independent measurements of the objects’ distances from us. However, these two most accurate measurements scientists currently have for the Hubble constant disagree — an endless source of frustration for cosmologists.

A Proposal

This was the cosmological landscape in the early 2000s when Holz and Hughes held positions as postdoctoral researchers at KITP. “Scott had been thinking about gravitational waves for a while,” said Holz. “He was the expert, and I was much more focused on cosmological questions.” But Hughes’ enthusiasm soon piqued Holz’s curiosity, and the two began to talk about gravitational wave cosmology in the office and on walks along the Santa Barbara bluffs.

Holz and Hughes credit their close collaboration to the construction of the new wing of Kohn Hall in 2001. Initially, all postdocs at KITP had their own offices, explained Hughes, but the construction forced them to double-up. “Suddenly we were spending a lot more time with each other.”

A 2002 KITP program on cosmological data fanned the flames of their interest in the topic. By the time Hughes left to join the faculty at MIT, they had finished the first draft of their paper detailing how to calculate the Hubble constant with gravitational waves. After two years gestating they finally published the study in The Astrophysical Journal.

“I had a great time writing that paper with Scott,” said Holz. “I learned an incredible amount. So much that I was convinced that gravitational waves were the future, and that I should get involved.”

The idea of using gravitational wave sources to measure the Hubble constant was not new. The concept was first proposed in a visionary paper back in 1986 by Bernard Schutz [Nature]. And a number of other notions regarding gravitational waves were also floating around the literature in the early 2000s. But what Holz and Hughes did was synthesize all these ideas and emphasize the feasibility of combining data from gravitational waves with follow-up observations using light.

The study also was the first to use the term “standard siren [Nature].” Hughes recalled discussing the paper with Caltech astrophysicist Sterl Phinney, who remarked, “Hmm. Kind of like a standard candle, but you hear it. You should call it a standard siren.” Holz independently had an almost identical conversation with physicist Sean Carroll, a former KITP postdoc himself. Holz and Hughes included the term in their paper, and it stuck. The phrase has since become ubiquitous in cosmology.

“The term ‘standard siren’ might be our most lasting contribution, Scott,” Holz remarked. “I’ll take it,” laughed Hughes.

Using gravitational waves to measure the Hubble constant has many advantages over other methods. Certain supernovae provide decent standard candles, “but, as a standard candle, supernovae are not very well understood,” said Holz. “The main thing that makes standard sirens interesting is that they’re understood from first principles, directly from the theory of general relativity.”

When using standard candles, scientists have to calibrate the distances of certain classes of objects using the information from other ones, effectively leapfrogging their way to a proper distance measurement. Astronomers call this method a “distance ladder,” and errors and uncertainty can creep in at many points in the calculations.

Getting accurate measurements of distance requires building up a distance ladder using a number of different techniques for various ranges. Photo Credit: MATT PERKO.

In contrast, gravitational waves can provide a direct measurement of an object’s distance. “You just write down the equations and solve them, and then you’re done,” said Holz. “We’ve tested general relativity for a hundred years; it really works, and it says ‘here’s how far that source is.’ There’s no distance ladder, there’s none of that fiddling around.”

All the early papers on measuring the Hubble constant using gravitational waves were somewhat speculative, according to Holz. They were proposals for the far future. “We hadn’t even detected gravitational waves yet, much less waves from two neutron stars, much less with an optical counterpart,” he said. But interest and enthusiasm for the technique were growing.

Hughes remembers colleagues coming up to him after his talks and asking about the likelihood of observing a standard siren in the next decade. He didn’t know, but he did say that with a better understanding of the optical counterpart, they could probably localize an event to within 10-20 square degrees. “And I think if you have that, every piece of large glass on Earth is going to stare at that spot on the sky,” Hughes had said. “And, in the end, that is exactly what happened.”

And Then It Happened

On August 17, 2017, less than two years after detecting the first gravitational waves, the LIGO and Virgo observatories recorded a signal from merging neutron stars. Thanks to an alert system, which Holz helped establish, a flurry of activity followed as nearly every major ground and space-based observatory trained their sights on the event. Scientists collected data on the merger in every region of the electromagnetic spectrum.

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

Gravity is talking. Lisa will listen. Dialogos of Eide

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

“It really is one of those things where, if it had happened before I retired, I would have been happy,” said Hughes. “But it actually happened before I turned 50.”

Suddenly, gravitational wave cosmology was a real field, and standard sirens were another part of the toolkit. “But for something to become part of the toolkit so quickly? That’s extraordinarily unusual,” said Holz.

It turns out that cosmologists need another tool, because they currently have two different values for the Hubble constant. Methods using the cosmic microwave background [CMB] — faint light left over from the big bang — yields a value of around 68. Meanwhile, calculations that use Type Ia supernovae — a variety of standard candle [above] — yield a bit more than 73.

CMB per ESA/Planck

Although they appear close, the two values actually differ by three standard deviations, and both have fairly tight error bars. The disagreement has cosmologists increasingly concerned as the error bars on these two values only get tighter. It could signal a fundamental problem in our understanding of the universe, and is the subject of a KITP conference this July.

There are a few intrinsic differences between the two techniques, though. The cosmic microwave background reflects the conditions of the early universe, while the supernovae paint a picture of the current universe. “There’s a chance that maybe something very strange and unexpected has happened between the early and late days of the universe, and that’s why these values don’t agree,” said Holz. But cosmologists simply don’t know for sure.

Getting another, independent value for the Hubble constant will help clear up this conundrum. “Because it’s so clean and so direct, that measurement will be a very compelling number,” Holz explained. “At the very least, it’ll inform this discussion, if not just completely resolve it.”

Holz and his colleagues, Hsin-Yu Chen and Maya Fishbach, have just published a paper in the journal Nature, finding that 20 to 30 observations would allow scientists to calculate the Hubble constant to within 2 percent accuracy, tight enough to begin comparing it to the two values from the cosmic microwave background and supernovae.

This summer, Holz is co-organizing a KITP program on the new era of gravitational wave physics and astrophysics, and the new field of standard siren cosmology will be a major topic of discussion. In fact, Holz also helped organize the KITP rapid response program that brought researchers together shortly after LIGO’s first detection of gravitational waves.

Holz and Hughes credit their success to their experiences at KITP. “While working together at the KITP the two of us got excited about measuring the Hubble constant using gravitational waves,” said Holz. “And that’s exactly what the KITP is about: bringing different people together with different backgrounds, stirring the pot and seeing what happens.”

For the past decade Holz’s career has focused on standard siren cosmology. “And the amazing thing is we’ve actually done it,” he said. “I helped write the paper that did the first standard siren measurement ever. This was exactly what Scott and I had hypothesized about years before.”

“If both of us hadn’t been at the KITP there’s no way I’d be spending a good fraction of my life on LIGO teleconferences right now,” said Holz. “But I wouldn’t have it any other way.”

See the full article here .

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The University of California, Santa Barbara (commonly referred to as UC Santa Barbara or UCSB) is a public research university and one of the 10 general campuses of the University of California system. Founded in 1891 as an independent teachers’ college, UCSB joined the University of California system in 1944 and is the third-oldest general-education campus in the system. The university is a comprehensive doctoral university and is organized into five colleges offering 87 undergraduate degrees and 55 graduate degrees. In 2012, UCSB was ranked 41st among “National Universities” and 10th among public universities by U.S. News & World Report. UCSB houses twelve national research centers, including the renowned Kavli Institute for Theoretical Physics.

From University of Chicago: “How to use gravitational waves to measure the expansion of the universe”

From University of Chicago

Mar 28, 2019
Louise Lerner

Prof. Daniel Holz discusses a new way to calculate the Hubble constant, a crucial number that measures the expansion rate of the universe and holds answers to questions about the universe’s size, age and history. Video by UChicago Creative

Ripples in spacetime lead to new way to determine size and age of universe.

On the morning of Aug. 17, 2017, after traveling for more than a hundred million years, the aftershocks from a massive collision in a galaxy far, far away finally reached Earth.

These ripples in the fabric of spacetime, called gravitational waves, tripped alarms at two ultra-sensitive detectors called LIGO, sending texts flying and scientists scrambling.

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

Gravity is talking. Lisa will listen. Dialogos of Eide

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

One of the scientists was Prof. Daniel Holz at the University of Chicago. The discovery had provided him the information he needed to make a groundbreaking new measurement of one of the most important numbers in astrophysics: the Hubble constant, which is the rate at which the universe is expanding.

The Hubble constant holds the answers to big questions about the universe, like its size, age and history, but the two main ways to determine its value have produced significantly different results. Now there was a third way, which could resolve one of the most pressing questions in astronomy—or it could solidify the creeping suspicion, held by many in the field, that there is something substantial missing from our model of the universe.

“In a flash, we had a brand-new, completely independent way to make a measurement of one of the most profound quantities in physics,” said Holz. “That day I’ll remember all my life.”

As LIGO and its European counterpart VIRGO turn back on on April 1, Holz and other scientists are preparing for more data that could shed light on some of the universe’s biggest questions.

Universal questions

We’ve known the universe is expanding for a long time (ever since eminent astronomer and UChicago alum Edwin Hubble made the first measurement of the expansion in 1929, in fact),

Edwin Hubble looking through a 100-inch Hooker telescope at Mount Wilson in Southern California, 1929 discovers the Universe is Expanding

but in 1998, scientists were stunned to discover that the rate of expansion is not slowing as the universe ages, but actually accelerating over time. In the following decades, as they tried to precisely determine the rate, it has become apparent that different methods for measuring the rate produce different answers.

One of the two methods measures the brightness of supernovae–exploding stars– in distant galaxies;

Standard Candles to measure age and distance of the universe from supernovae NASA

the other looks at tiny fluctuations in the cosmic microwave background [CMB], the faint light left over from the Big Bang.

CMB per ESA/Planck

ESA/Planck 2009 to 2013

Scientists have been working for two decades to boost the accuracy and precision for each measurement, and to rule out any effects which might be compromising the results; but the two values still stubbornly disagree by almost 10 percent.

A neutron star collision causes detectable ripples in the fabric of spacetime, which are called gravitational waves. Photo courtesy of Aurore Simonnet

Because the supernova method looks at relatively nearby objects, and the cosmic microwave background is much more ancient, it’s possible that both methods are right—and that something profound about the universe has changed since the beginning of time.

“We don’t know if one or both of the other methods have some kind of systematic error, or if they actually reflect a fundamental truth about the universe that is missing from our current models,” said Holz. “Either is possible.”

Holz saw the possibility for a third, completely independent way to measure the Hubble constant—but it would depend on a combination of luck and extreme feats of engineering.

The ‘standard siren’

In 2005, Holz wrote a paper [NJP] with Scott Hughes of Massachusetts Institute of Technology suggesting that it would be possible to calculate the Hubble constant through a combination of gravitational waves and light. They called these sources “standard sirens,” a nod to “standard candles”, which refers to the supernovae used to make the Hubble constant measurement.

But first it would take years to develop technology that could pick up something as ephemeral as ripples in the fabric of spacetime. That’s LIGO: a set of enormous, extremely sensitive detectors that are tuned to pick up the gravitational waves that are emitted when something big happens somewhere in the universe.

The Aug. 17, 2017 waves came from two neutron stars, which had spiraled around and around each other in a faraway galaxy before finally slamming together at close to the speed of light. The collision sent gravitational waves rippling across the universe and also released a burst of light, which was picked up by telescopes on and around Earth.

Neutron star collision-Robin Dienel-The Carnegie Institution for Science

Prof. Daniel Holz writes out the formula for the Hubble constant, which measures the rate at which the universe is expanding.

That burst of light was what sent the scientific world into a tizzy. LIGO had picked up gravitational wave readings before, but all the previous ones were from collisions of two black holes, which can’t be seen with conventional telescopes.

But they could see the light from the colliding neutron stars, and the combination of waves and light unlocked a treasure trove of scientific riches. Among them were the two pieces of information Holz needed to make his calculation of the Hubble constant.

How does the method work?

To make this measurement of the Hubble constant, you need to know how fast an object—like a newly collided pair of neutron stars—is receding away from Earth, and how far away it was to begin with. The equation is surprisingly simple. It looks like this: The Hubble constant is the velocity of the object divided by the distance to the object, or H=v/d.

Somewhat counterintuitively, the easiest part to calculate is how fast the object is moving. Thanks to the bright afterglow given off by the collision, astronomers could point telescopes at the sky and pinpoint the galaxy where the neutron stars collided. Then they can take advantage of a phenomenon called redshift: As a faraway object moves away from us, the color of the light it’s giving off shifts slightly towards the red end of the spectrum. By measuring the color of the galaxy’s light, they can use this reddening to estimate how fast the galaxy is moving away from us. This is a century-old trick for astronomers.

The more difficult part is getting an accurate measure of the distance to the object. This is where gravitational waves come in. The signal the LIGO detectors pick up gets interpreted as a curve, like this:

The signal picked up by the LIGO detector in Louisiana, as it caught the waves from two neutron stars colliding far away in space, forms a distinctive curve. Courtesy of LIGO

The shape of the signal tells scientists how big the two stars were and how much energy the collision gave off. By comparing that with how strong the waves were when they reached Earth, they could infer how far away the stars must have been.

The initial value from just this one standard siren came out to be 70 kilometers per second per megaparsec. That’s right in between the other two methods, which find about 73 (from the supernova method) and 67 (from the cosmic microwave background).

Of course, that initial standard siren measurement is only from one data point, and large uncertainties remain. But the LIGO detectors are turning back on after an upgrade to boost their sensitivity. Nobody knows how often neutron stars collide, but Holz (along with former student Hsin-Yu Chen and current student Maya Fishbach) wrote a paper estimating that the gravitational wave method may provide a revolutionary, extremely precise measurement of the Hubble constant within five years.

“As time goes on, we’ll observe more and more of these binary neutron star mergers, and use them as standard sirens to steadily improve our estimate of the Hubble constant. Depending on where our value falls, we might confirm one method or the other. Or we might find an entirely different value,” Holz said. “No matter what we find, it’s gonna be interesting—and will be an important step in learning more about our universe.”

See the full article here .

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The University of Chicago is an urban research university that has driven new ways of thinking since 1890. Our commitment to free and open inquiry draws inspired scholars to our global campuses, where ideas are born that challenge and change the world.

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From MIT News: Women in STEM “3 Questions: Lisa Barsotti on the new and improved LIGO”

From MIT News

April 1, 2019
Jennifer Chu

LIGO laboratory detection site near Hanford in eastern Washington. Image: Caltech/MIT/LIGO Laboratory

“If we are very lucky, we might observe something new … or maybe even something totally unexpected.”

The search for infinitely faint ripples in space-time is back in full swing. Today, LIGO, the Laser Interferometer Gravitational-wave Observatory, operated jointly by Caltech and MIT, resumes its hunt for gravitational waves and the immense cosmic phenomena from which they emanate.

Over the past several months, LIGO’s twin detectors, in Washington and Lousiana, have been offline, undergoing upgrades to their lasers, mirrors, and other components, which will enable the detectors to listen for gravitational waves over a far greater range, out to about 550 million light-years away — around 190 million light-years farther out than before.

As the LIGO detectors turn back on, they will be joined by Virgo, the European-based counterpart based in Italy, which also turns on today after undergoing upgrades that doubled its sensitivity. With both LIGO and Virgo back online, scientists anticipate that detections of gravitational waves from the farthest reaches of the universe may be a regular occurrence.

MIT News spoke with LIGO member Lisa Barsotti, principal research scientist at MIT’s Kavli Institute for Astrophysics and Space Research, about the potential discoveries that lie ahead.

Kavli MIT Institute For Astrophysics and Space Research

Q: Give us a sense of the new capabilities that the LIGO detectors now have. What sort of upgrades were made?

A: Both LIGO detectors are coming back online more sensitive than ever before, thanks to a wide range of improvements. In particular, we more than doubled the laser power in the interferometers to reduce one of the LIGO fundamental noise sources — quantum “shot noise,” caused by the uncertainty of the arrival time of photons onto the main photodetector. We also deployed a new technology, “squeezed” light, that uses quantum optics to further reduce shot noise.

Combined with other upgrades to mitigate technical noises (for example noises introduced by the control scheme or from stray light) we improved the sensitivity to binary neutron stars by 40 percent in each detector, with respect to the past observing run.

Q: What do these new capabilities mean for you, as a researcher who will be looking through the data from these upgraded detectors?

A: I am personally very excited to see the LIGO detectors operating with squeezed light! This new technology has been developed here at MIT after many years of research to make it compatible with the very stringent LIGO requirements, and our graduate students have been leading the commissioning of this new system at the observatories. It is particularly rewarding to see that we succeeded in making LIGO better.

Also, operation at high laser power has been enabled by another upgrade developed and built here at MIT — an “acoustic mode damper” glued to the main LIGO optics that mitigates instabilities originating with high laser power. We are looking forward to seeing many years of work in our labs pay off in this observing run!

Q: What new phenomena are you hoping to detect, and how soon could you detect them, with these new capabilities?

A: We hope to detect more binary neutron star systems (so far only one has been detected), and thanks to the improved LIGO sensitivity, we should be able to observe them with high signal-to-noise ratio. And more black holes, obviously! The more sources we detect, the more we can learn about the way these systems form and evolve.

If we are very lucky, we might observe something new, like a neutron star-black hole system, or maybe even something totally unexpected. Not only are the LIGO detectors better than before — the Virgo detector in Italy more than doubled its sensitivity with respect to the last observing run, and this will improve our ability to localize sources in the sky, facilitating the follow-up of telescopes at multiple wavelengths.

VIRGO Gravitational Wave interferometer, near Pisa, Italy

So, if the last observing run, “O2,” will be remembered as the one that started multimessenger astronomy, I hope the upcoming one, “O3,” will be the one in which multimessenger astronomy becomes the new normal!

See the full article here .

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From Discover Magazine: “Here’s What Scientists Hope to Learn as LIGO Resumes Hunting Gravitational Waves”

From Discover Magazine

March 29, 2019
Korey Haynes

After a year of downtime to perform hardware upgrades, the Laser Interferometer Gravitational-Wave Observatory (LIGO) is ready for action and will turn on its twin detectors, one in Washington state and the other in Louisiana, on April 1. This time, it will also be joined by the Virgo collaboration based out of Italy, and possibly also by the KAGRA detector in Japan later in the year.

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

Gravity is talking. Lisa will listen. Dialogos of Eide

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

Combined with the hardware upgrades, scientists expect these updates to allow LIGO to spot more observations and trace their origins more clearly. In 2016, LIGO made history with the first-ever direct detection of gravitational waves, produced in that case by colliding black holes.

A wrinkle in space-time confirms Einstein’s gravitation. Credit: Astronomy Magazine

New Hardware

“Most of the upgrades have been increasing the amount of laser power that’s used,” says Jolien Creighton, a University of Wisconsin Milwaukee professor and member of the LIGO collaboration. “That’s improved the sensitivity.” Each of LIGO’s detector is a giant L-shape, and instruments wait for passing gravitational waves to distort the length of each arm of the detector, measuring them by bouncing lasers across their lengths. Researchers are also pushing the physical limits of the detector, which Creighton says is limited by the quantum uncertainly principle. To increase sensitivity even more, the experiment will “quantum squeeze” the laser beam. “This puts it into an interesting quantum mechanical state that lets us detect the arm length of the detector,” to even greater precision than before.

The additional detectors from Virgo and KAGRA will let researchers triangulate sources on the sky more accurately than the two LIGO detectors can manage alone. Virgo will be online throughout the whole next year of observing, while KAGRA is still being commissioned, but could join as early as fall of 2018 [? Don’t ask me, I did not write this].

KAGRA gravitational wave detector, Kamioka mine in Kamioka-cho, Hida-city, Gifu-prefecture, Japan

New Detections

The upgraded LIGO will look for many of the same events it did before: collisions of two black holes, two neutron stars, or mixtures of both. Creighton says he’s personally excited about binary neutron stars, because those systems are the mostly likely to have counterparts that can be observed by traditional observatories at the same time, at wavelengths from radio waves to visible light to gamma rays. “Seeing more of those will give us more insight into the natures of gamma ray bursts and the formation of elements of the universe,” Creighton says. He points out the mergers can also teach astronomers how matter behaves when crunched down denser than an atom’s nucleus, a state that only exists in neutron stars. “The way we can probe that is by watching the interactions of neutron stars just before they merge. It’s a fundamental nuclear physics lab in space.”

Creighton says he’s confident they’ll see many more events from colliding black holes, a phenomenon LIGO has already observed more than once. “We’re hoping to see a binary of a neutron star and a black hole,” Creighton says, but since no one has ever seen one, it’s hard to calculate how common or rare they are, and what the odds are of LIGO spotting one in the next year. But LIGO will be peering farther into the universe, “so even rare things should start to be observed,” Creighton says.

Other possible objects LIGO might spy would be a supernova explosion, or an isolated neutron star spinning rapidly. “If it’s not perfectly symmetric, then that rotating distortion would produce gravitational waves,” Creighton says. The signal would be weak but constant, so the longer LIGO looks, the more likely finding a source like this becomes. Even more subtle would be a skywide, low-level reverberation from the Big Bang, similar to the microwave background that exists in radiation, and which researchers suspect might also exist in gravitational waves.

“There’s always the hope that we’ll see something entirely unexpected,” Creighton adds. “Those are the things that you really can’t predict in any way.”

LIGO’s upcoming run will last for roughly a year, at which point it will undergo more upgrades for a year, and then hopefully start the cycle over again, prepared to witness even more spectacular and invisible events.

See the full article here .

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From Ethan Siegel: “Ask Ethan: Why Haven’t We Found Gravitational Waves In Our Own Galaxy?”

From Ethan Siegel
Mar 30, 2019

Artist’s iconic conception of two merging black holes similar to those detected by LIGO Credit LIGO-Caltech/MIT/Sonoma State /Aurore Simonnet

LIGO and Virgo have now detected a total of 11 binary merger events.

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

Gravity is talking. Lisa will listen. Dialogos of Eide

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

But exactly 0 were in the Milky Way. Here’s why.

One of the most spectacular recent advances in all of science has been our ability to directly detect gravitational waves. With the unprecedented power and sensitivity of the LIGO and Virgo gravitational waves observatories at our disposal, these powerful ripples in the fabric of spacetime are no longer passing by undetected. Instead, for the first time, we’re able to not only observe them, but to pinpoint the location of the sources that generate them and learn about their properties. As of today, 11 separate sources have been detected.

But they’re all so far away! Why is that? That’s the question of Amitava Datta and Chayan Chatterjee, who ask:

Why are all the known gravitational wave sources (coalescing binaries) in the distant universe? Why none has been detected in our neighborhood? […] My guess (which is most probably wrong) is that the detectors need to be precisely aligned for any detection. Hence all the detection until now are serendipitous.

Let’s find out.

The way observatories like LIGO and Virgo work is that they have two long, perpendicular arms that have the world’s most perfect vacuum inside of them. Laser light of the same frequency is broken up to travel down these two independent paths, reflected back and forth a number of times, and recombined together at the end.

Light is just an electromagnetic wave, and when you combine multiple waves together, they generate an interference pattern. If the interference is constructive, you see one type of pattern; if it’s destructive, you see a different type. When LIGO and Virgo just hang out, normally, with no gravitational waves going through them, what you see is a relatively steady pattern, with only the random noise (mostly generated by the Earth itself) of the instruments to contend with.

When the two arms are of exactly equal length and there is no gravitational wave passing through, the signal is null and the interference pattern is constant. As the arm lengths change, the signal is real and oscillatory, and the interference pattern changes with time in a predictable fashion. (NASA’S SPACE PLACE)

But if you were to change the length of one of these arms relative to the other, the amount of time the light spent traveling down that arm would also change. Because light is a wave, a small change in the time light travels means you’re at a different point in the wave’s crest/trough pattern, and therefore the interference pattern that gets created by combining it with another light wave will change.

There could be many causes for a single arm to change: seismic noise, a jackhammer across the street, or even a passing truck miles away. But there’s an astrophysical source that could cause that change too: a passing gravitational wave.

When a gravitational wave passes through a location in space, it causes an expansion and a compression at alternate times in alternate directions, causing laser arm-lengths to change in mutually perpendicular orientations. Exploiting this physical change is how we developed successful gravitational wave detectors such as LIGO and Virgo. (ESA–C.CARREAU)

There are two keys that enable us to determine what’s a gravitational wave from what’s mere terrestrial noise.

Gravitational waves, when they pass through a detector, will cause both arms to change their distance together in opposite directions by a particular, in-phase amount. When you see a periodic pattern of arm lengths oscillating, you can place meaningful constraints on whether your signal was likely to be a gravitational wave or just an Earth-based source of noise.
We build multiple detectors at different points on Earth. While each one will experience its own noise due to its local environment, a passing gravitational wave will have very similar effects on each of the detectors, separated by at most milliseconds in time.

As you can see from the very first robust detection of these waves, dating back to observations taken on September 14, 2015, both effects are present.

The inspiral and merger of the first pair of black holes ever directly observed. The total signal, along with the noise (top) clearly matches the gravitational wave template from merging and inspiraling black holes of a particular mass (middle). Note how the frequency and amplitude change at the very end-stage of the merger. (B. P. ABBOTT ET AL. (LIGO SCIENTIFIC COLLABORATION AND VIRGO COLLABORATION))

If we come forward to the present day, we’ve actually detected a large number of mergers: 11 separate ones thus far. Events seem to come in at random, as it’s only the very final stages of inspiral and merger — the final seconds or even milliseconds before two black holes or neutron stars collide — that have the right properties to be picked up by even our most sensitive detectors.

If we look at the distances to these objects, though, we find something that might trouble us a little bit. Even though our gravitational wave detectors are more sensitive to objects the closer they are to us, the majority of objects we’ve found are many hundreds of millions or even billions of light-years away.

The 11 gravitational wave events detected by LIGO and Virgo, with their names, mass parameters, and other essential information encoded in Table form. Note how many events came in the last month of the second run: when LIGO and Virgo were operating simultaneously. The parameter dL is the luminosity distance; the closest object being the neutron star-neutron star merger of 2017, which corresponds to a distance of ~130 million light-years. (THE LIGO SCIENTIFIC COLLABORATION, THE VIRGO COLLABORATION; ARXIV:1811.12907)

Why is this? If gravitational wave detectors are more sensitive to closer objects, shouldn’t we be detecting them more frequently, in defiance of what we’ve actually observed?

There are a lot of potential explanations that could account for this mismatch between what you’d expect or not. As our questioners proposed, perhaps it’s due to orientation? After all, there are many phenomena in this Universe, such as pulsars or blazars, that only appear visible to us when the correct electromagnetic signal gets “beamed” directly to our line-of-sight.

Artist’s impression of an active galactic nucleus. The supermassive black hole at the center of the accretion disk sends a narrow high-energy jet of matter into space, perpendicular to the disc. A blazar about 4 billion light years away is the origin of many of the highest-energy cosmic rays and neutrinos. Only matter from outside the black hole can leave the black hole; matter from inside the event horizon can ever escape. (DESY, SCIENCE COMMUNICATION LAB)

It’s a clever idea, but it misses a fundamental difference between the gravitational and electromagnetic forces. In electromagnetism, electromagnetic radiation gets generated by the acceleration of charged particles; in General Relativity, gravitational radiation (or gravitational waves) are generated by the acceleration of massive particles. So far, so good.

But there are both electric and magnetic fields in electromagnetism, and electrically charged particles in motion generate magnetic fields. This allows you to create and accelerate particles and radiation in a collimated fashion; it doesn’t have to spread out in a spherical pattern. In gravitation, though, there are only gravitational sources (masses and energetic quanta) and the curvature of spacetime that results.

When you have two gravitational sources (i.e., masses) inspiraling and eventually merging, this motion causes the emission of gravitational waves. Although it might not be intuitive, a gravitational wave detector will be sensitive to these waves as a function of 1/r, not as 1/r², and will see those waves in all directions, regardless of whether they’re face-on or edge-on, or anywhere in between. (NASA, ESA, AND A. FEILD (STSCI))

As it turns out, it doesn’t really matter whether we see an inspiraling and merging gravitational wave source face-on, edge-on, or at an angle; they still emit gravitational waves of a measurable and observable frequency and amplitude. There may be subtle differences in the magnitude and other properties of the signal that arrives at our eyes that are orientation-dependent, but gravitational waves propagate spherically outward from a source that generates them, and can literally be seen from anywhere in the Universe so long as your detector is sensitive enough.

So why is it, then, that there aren’t gravitational waves from binary sources detected in our own galaxy?

It might surprise you to learn that there are binary sources of mass, like black holes and neutron stars, orbiting and inspiraling right now.

From the very first binary neutron star system ever discovered, we knew that gravitational radiation was carrying energy away. It was only a matter of time before we found a system in the final stages of inspiral and merger. (NASA (L), MAX PLANCK INSTITUTE FOR RADIO ASTRONOMY / MICHAEL KRAMER)

Long before gravitational waves were directly detected, we spotted what we thought was an ultra-rare configuration: two pulsars orbiting one another. We watched their pulse time vary in a way that showcased their orbital decay due to gravitational radiation. Many pulsars, including multiple binary pulsars, have since been observed. In every case where we’ve been able to measure them accurately enough, we see the orbital decay that shows yes, they are emitting gravitational waves.

Women in STEM – Dame Susan Jocelyn Bell Burnell

Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

Dame Susan Jocelyn Bell Burnell at work on first plusar chart 1967 pictured working at the Four Acre Array in 1967. Image courtesy of Mullard Radio Astronomy Observatory.

Dame Susan Jocelyn Bell Burnell 2009

Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

Similarly, we’ve observed X-ray emissions from systems that indicate there must be a black hole at the center. While binary black holes have only been discovered in two instances from electromagnetic observations, the stellar-mass black holes we know of have been discovered as they accrete or siphon matter from a companion star: the X-ray binary scenario.

LIGO and Virgo have discovered a new population of black holes with masses that are larger than what had been seen before with X-ray studies alone (purple). This plot shows the masses of all ten confident binary black hole mergers detected by LIGO/Virgo (blue), along with the one neutron star-neutron star merger seen (orange). LIGO/Virgo, with the upgrade in sensitivity, should detect multiple mergers every week beginning this April. (LIGO/VIRGO/NORTHWESTERN UNIV./FRANK ELAVSKY)

These systems are:

abundant within the Milky Way,
inspiraling and radiating gravitational waves away to conserve energy,
which means there are gravitational waves of specific frequencies and amplitudes passing through our detectors,
with the sources generating those signals destined to someday merge and complete their coalescence.

But again, we have not observed them in our ground-based gravitational wave detectors. And there’s a simple, straightforward reason for that: our detectors are in the wrong frequency range!

The sensitivities of a variety of gravitational wave detectors, old, new, and proposed. Note, in particular, Advanced LIGO (in orange), LISA (in dark blue), and BBO (in light blue). LIGO can only detect low-mass and short-period events; longer-baseline, lower-noise observatories are needed for either more massive black holes or for systems that are in an earlier stage of gravitational inspiral. (MINGLEI TONG, CLASS.QUANT.GRAV. 29 (2012) 155006)

It’s only in the very, very last seconds of coalescence that gravitational waves from merging binaries fall into the LIGO/Virgo sensitivity range. For all the millions or even billions of years that neutron stars or black holes orbit one another and see their orbits decay, they do so at larger radial separations, which means they take longer to orbit each other, which means lower frequency gravitational waves.

The reason we don’t see the binaries orbiting in our galaxy today is because LIGO’s and Virgo’s arms are too short! If they were millions of kilometers long instead of 3–4 km with many reflections, we’d have already seen them. As it stands right now, this will be a significant advance of LISA [above]: it can show us these binaries that are destined to merge in the future, even enabling us to predict where and when it will happen!

It’s true: during the time that LIGO and Virgo have been operating, we haven’t seen any mergers of black holes or neutron stars in our own galaxy. This is no surprise; the results from our gravitational wave observations have taught us that there are somewhere around 800,000 merging black hole binaries throughout the Universe in any year. But there are two trillion galaxies in the Universe, meaning that we need to observe millions of galaxies in order to just get one event!

This is why our gravitational wave observatories need to be sensitive to distances that go out billions of light-years in all directions; there simply won’t be enough statistics otherwise.

The range of Advanced LIGO and its capability of detecting merging black holes. Note that even though the amplitude of the waves will fall off as 1/r, the number of galaxies increases with volume: as r³. (LIGO COLLABORATION / AMBER STUVER / RICHARD POWELL / ATLAS OF THE UNIVERSE)

There are plenty of neutron stars and black holes orbiting one another all throughout the Universe, including right here in our own Milky Way galaxy. When we look for these systems, with either radio pulses (for the neutron stars) or X-rays (for the black holes), we find them in great abundances. We can even see the evidence for the gravitational waves they emit, although the evidence we see is indirect.

If we had more sensitive, lower-frequency gravitational wave observatories, we could potentially detect the waves generated by sources within our own galaxy directly. But if we want to get a true merger event, those are rare. They might be aeons in the making, but the actual events themselves take just a fraction of a second. It’s only by casting a very wide net that we can see them at all. Incredibly, the technology to do so is already here.

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

From Louisiana State University: “Hello, Quantum Vacuum, Nice to See You”

From Louisiana State University

March 25, 2019

Elsa Hahne
LSU Office of Research & Economic Development
504-610-1950
ehahne@lsu.edu

Mimi LaValle
LSU Department of Physics & Astronomy
225-439-5633
mlavall@lsu.edu

Thomas Corbitt, associate professor at the LSU Department of Physics & Astronomy, and his team of researchers measure quantum behavior at room temperature, visible to the naked eye, as reported today in the journal Nature.

Thomas Corbitt in his lab, setting up a complex sequence of lasers.Elsa Hahne/LSU

Since the historic finding of gravitational waves from two black holes colliding over a billion light years away was made in 2015, physicists are advancing knowledge about the limits on the precision of the measurements that will help improve the next generation of tools and technology used by gravitational wave scientists.

Artist’s iconic conception of two merging black holes similar to those detected by LIGO Credit LIGO-Caltech/MIT/Sonoma State /Aurore Simonnet

LSU Department of Physics & Astronomy Associate Professor Thomas Corbitt and his team of researchers now present the first broadband, off-resonance measurement of quantum radiation pressure noise in the audio band, at frequencies relevant to gravitational wave detectors, as reported today in the scientific journal Nature. The research was supported by the National Science Foundation, or NSF, and the results hint at methods to improve the sensitivity of gravitational-wave detectors by developing techniques to mitigate the imprecision in measurements called “back action,” thus increasing the chances of detecting gravitational waves.

Corbitt and researchers have developed physical devices that make it possible to observe—and hear—quantum effects at room temperature. It is often easier to measure quantum effects at very cold temperatures, while this approach brings them closer to human experience. Housed in miniature models of detectors like LIGO (the Laser Interferometer Gravitational-Wave Observatory, one located in Livingston, La., and the other in Hanford, Wash.), these devices consist of low-loss, single-crystal micro-resonators—each a tiny mirror pad the size of a pin prick, suspended from a cantilever. A laser beam is directed at one of these mirrors, and as the beam is reflected, the fluctuating radiation pressure is enough to bend the cantilever structure, causing the mirror pad to vibrate, which creates noise.

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

Gravity is talking. Lisa will listen. Dialogos of Eide

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

Gravitational wave interferometers use as much laser power as possible in order to minimize the uncertainty caused by the measurement of discrete photons and to maximize the signal-to-noise ratio. These higher power beams increase position accuracy but also increase back action, which is the uncertainty in the number of photons reflecting from a mirror that corresponds to a fluctuating force due to radiation pressure on the mirror, causing mechanical motion. Other types of noise, such as thermal noise, usually dominate over quantum radiation pressure noise, but Corbitt and his team, including collaborators at MIT, have sorted through them. Advanced LIGO and other second and third generation interferometers will be limited by quantum radiation pressure noise at low frequencies when running at their full laser power. Corbitt’s paper in Nature offers clues as to how researchers can work around this when measuring gravitational waves.

Thomas Corbitt looks through the custom-built device used to measure quantum radiation pressure noise. Elsa Hahne/LSU

“Given the imperative for more sensitive gravitational wave detectors, it is important to study the effects of quantum radiation pressure noise in a system similar to Advanced LIGO, which will be limited by quantum radiation pressure noise across a wide range of frequencies far from the mechanical resonance frequency of the test mass suspension,” Corbitt said.

Corbitt’s former academic advisee and lead author of the Nature paper, Jonathan Cripe, graduated from LSU with a Ph.D. in Physics last year and is now a postdoctoral research fellow at the National Institute of Standards and Technology:

“Day-to-day at LSU, as I was doing the background work of designing this experiment and the micro-mirrors and placing all of the optics on the table, I didn’t really think about the impact of the future results,” Cripe said. “I just focused on each individual step and took things one day at a time. [But] now that we have completed the experiment, it really is amazing to step back and think about the fact that quantum mechanics—something that seems otherworldly and removed from the daily human experience—is the main driver of the motion of a mirror that is visible to the human eye. The quantum vacuum, or ‘nothingness,’ can have an effect on something you can see.”

Pedro Marronetti, a physicist and NSF program director, notes that it can be tricky to test new ideas for improving gravitational wave detectors, especially when reducing noise that can only be measured in a full-scale interferometer:

“This breakthrough opens new opportunities for testing noise reduction,” he said. The relative simplicity of the approach makes it accessible by a wide range of research groups, potentially increasing participation from the broader scientific community in gravitational wave astrophysics.”

For more information from LSU Physics & Astronomy, visit http://www.phys.lsu.edu.

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

Louisiana State University (officially Louisiana State University and Agricultural and Mechanical College, commonly referred to as LSU) is a public coeducational university located in Baton Rouge, Louisiana. The university was founded in 1853 in what is now known as Pineville, Louisiana, under the name Louisiana State Seminary of Learning & Military Academy. The current LSU main campus was dedicated in 1926, consists of more than 250 buildings constructed in the style of Italian Renaissance architect Andrea Palladio, and occupies a 650-acre (2.6 km²) plateau on the banks of the Mississippi River.

LSU is the flagship institution of the Louisiana State University System. In 2017, the university enrolled over 25,000 undergraduate and over 5,000 graduate students in 14 schools and colleges. Several of LSU’s graduate schools, such as the E.J. Ourso College of Business and the Paul M. Hebert Law Center, have received national recognition in their respective fields of study. Designated as a land-grant, sea-grant and space-grant institution, LSU is also noted for its extensive research facilities, operating some 800 sponsored research projects funded by agencies such as the National Institutes of Health, the National Science Foundation, the National Endowment for the Humanities, and the National Aeronautics and Space Administration.

LSU’s athletics department fields teams in 21 varsity sports (9 men’s, 12 women’s), and is a member of the NCAA (National Collegiate Athletic Association) and the SEC (Southeastern Conference). The university is represented by its mascot, Mike the Tiger.

From insideHPC: “Nor-Tech Powers LIGO and IceCube Nobel-Physics Prize-Winning Projects”

From insideHPC

March 28, 2019

Today HPC integrator Nor-Tech announced participation in two recent Nobel Physics Prize-Winning projects. The company’s HPC gear will help power the Laser Interferometer Gravitational-Wave Observatory (LIGO) project as well as the IceCube neutrino detection experiment.

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

Gravity is talking. Lisa will listen. Dialogos of Eide

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

U Wisconsin IceCube neutrino observatory

U Wisconsin ICECUBE neutrino detector at the South Pole

U Wisconsin IceCube experiment at the South Pole

U Wisconsin ICECUBE neutrino detector at the South Pole

IceCube Gen-2 DeepCore PINGU

IceCube reveals interesting high-energy neutrino events

“We are excited about the amazing discoveries these enhanced detectors will reveal,” said Nor-Tech Executive Vice President Jeff Olson. “This is an energizing time for all of us at Nor-Tech—knowing that the HPC solutions we are developing for two Nobel projects truly are changing our view of the world.”

LIGO just announced that their detectors are about to come online after a one-year shutdown for hardware upgrades. In preparation for this, LIGO Consortium member University of Wisconsin-Milwaukee upgraded their clusters with Nor-Tech hardware to assist with the computing demands. At UWM they design, build and maintain computational tools, such as Nor-Tech’s supercomputer, that handle LIGO’s massive amounts of data. Nor-Tech completed the most recent update-including Intel Skylake processors-in 2018. The new Skylake-equipped technology is proving to be almost 10 times faster.

LIGO was awarded a Nobel Prize in 2017. Prior to this, at a Feb. 11, 2016 national media conference, National Science Foundation (NSF) researchers announced the first direct observation of a gravitational wave. This was a paradigm-shifting achievement in the science community. Subsequent gravitational wave detections have confirmed those results.

In 2018, the LIGO team announced the first visible detection of a neutrino event. This was made possible, in part, by the powerful HPC technology Nor-Tech has been providing to multiple LIGO Consortium institutions since 2005.

The first Nor-Tech client to win a Nobel Prize in Physics was the IceCube research team, headquartered at the University of Wisconsin-Madison. IceCube is designed specifically to identify neutrinos from space. It’s a cubic kilometer of ice, laced with photo-detectors, located at a dedicated Antarctic research facility.

Nor-Tech has been working with several of the world’s leading research institutions involved with the IceCube project for more than 10 years; designing, building, and upgrading HPC technology that made exciting neutrino discoveries possible.

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

Founded on December 28, 2006, insideHPC is a blog that distills news and events in the world of HPC and presents them in bite-sized nuggets of helpfulness as a resource for supercomputing professionals. As one reader said, we’re sifting through all the news so you don’t have to!

If you would like to contact me with suggestions, comments, corrections, errors or new company announcements, please send me an email at rich@insidehpc.com. Or you can send me mail at:

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From MIT Caltech Advanced aLIGO: “LIGO and Virgo Resume Search for Ripples in Space and Time”

From MIT Caltech Advanced aLIGO

Detector engineers Hugh Radkins (foreground) and Betsy Weaver (background) are pictured here inside the vacuum system of the detector at LIGO Hanford Observatory, beginning the hardware upgrades necessary for Advanced LIGO’s third observing run. Image credit: LIGO/Caltech/MIT/Jeff Kissel

March 26, 2019

The National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory) is set to resume its hunt for gravitational waves—ripples in space and time—on April 1, after receiving a series of upgrades to its lasers, mirrors, and other components. LIGO—which consists of twin detectors located in Washington and Louisiana—now has a combined increase in sensitivity of about 40 percent over its last run, which means that it can survey an even larger volume of space than before for powerful, wave-making events, such as the collisions of black holes.

Joining the search will be Virgo, the European-based gravitational-wave detector, located at the European Gravitational Observatory (EGO) in Italy, which has almost doubled its sensitivity since its last run and is also starting up April 1.

“For this third observational run, we achieved significantly greater improvements to the detectors’ sensitivity than we did for the last run,” says Peter Fritschel, LIGO’s chief detector scientist at MIT. “And with LIGO and Virgo observing together for the next year, we will surely detect many more gravitational waves from the types of sources we’ve seen so far. We’re eager to see new events too, such as a merger of a black hole and a neutron star.”

In 2015, after LIGO began observing for the first time in an upgraded program called Advanced LIGO, it soon made history by making the first direct detection of gravitational waves. The ripples traveled to Earth from a pair of colliding black holes located 1.3 billion light-years away. For this discovery, three of LIGO’s key players—Caltech’s Barry C. Barish, the Ronald and Maxine Linde Professor of Physics, Emeritus, and Kip S. Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus, along with MIT’s Rainer Weiss, professor of physics, emeritus—were awarded the 2017 Nobel Prize in Physics.

Since then, the LIGO-Virgo detector network has uncovered nine additional black hole mergers and one explosive smashup of two neutron stars. That event, dubbed GW170817, generated not just gravitational waves but light, which was observed by dozens of telescopes in space and on the ground.

“With our three detectors now operational at a significantly improved sensitivity, the global LIGO-Virgo detector network will allow more precise triangulation of the sources of gravitational waves,” says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration. “This will be an important step toward our quest for multi-messenger astronomy.”

Now, with the start of the next joint LIGO-Virgo run, the observatories are poised to detect an even greater number of black hole mergers and other extreme events, such as additional neutron-neutron star mergers or a yet-to-be-seen black hole-neutron star merger. One of the metrics the team uses for measuring increases in sensitivity is to calculate how far out they can detect neutron-neutron star mergers. In the next run, LIGO will be able to see those events out to an average of 550 million light-years away, or more than 190 million light-years farther out than before.

A key to achieving this sensitivity involves lasers. Each LIGO installation consists of two long arms that form an L shaped interferometer. Laser beams are shot from the corner of the “L” and bounced off mirrors before traveling back down the arms and recombining. When gravitational waves pass by, they stretch and squeeze space itself, making imperceptibly tiny changes to the distance the laser beams travel and thereby affecting how they recombine. For this next run, the laser power has been doubled to more precisely measure these distance changes, thereby increasing the detectors’ sensitivity to gravitational waves.

Other upgrades were made to LIGO’s mirrors at both locations, with a total of five of eight mirrors being swapped out for better-performing versions.

“We had to break the fibers holding the mirrors and very carefully take out the optics and replace them,” says Calum Torrie, LIGO’s mechanical-optical engineering head at Caltech. “It was an enormous engineering undertaking.”

LIGO team members install in-vacuum equipment that is part of the squeezed-light upgrade. Image credit: LIGO/Caltech/MIT/Matt Heintze

This next run also includes upgrades designed to reduce levels of quantum noise. Quantum noise occurs due to random fluctuations of photons, which can lead to uncertainty in the measurements and can mask faint gravitational-wave signals. By employing a technique called “squeezing,” initially developed for gravitational-wave detectors at the Australian National University, and matured and routinely used since 2010 at the GEO600 detector, researchers can shift the uncertainty in the photons around, making their amplitudes less certain and their phases, or timing, more certain. The timing of photons is what is crucial for LIGO’s ability to detect gravitational waves.

Torrie says that the LIGO team has spent months commissioning all of these new systems, making sure everything is aligned and working correctly. “One of the things that is satisfying to us engineers is knowing that all of our upgrades mean that LIGO can now see farther into space to find the most extreme events in our universe.”

The Collaborations

LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. More than 1,200 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.

The Virgo collaboration consists of more than 300 physicists and engineers belonging to 28 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; 11 from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with IFAE and the Universities of Valencia and Barcelona; two in Belgium with the Universities of Liege and Louvain; Jena University in Germany; and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef. A list of the Virgo Collaboration can be found at http://public.virgo-gw.eu/the-virgo-collaboration/. More information is available on the Virgo website at http://www.virgo-gw.eu.

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VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

From AAS NOVA: “Merging Eccentric Pairs of Black Holes”

From AAS NOVA

27 March 2019
Susanna Kohler

This scene from a computer simulation shows the dense, chaotic center of a stellar cluster. What happens when black-hole binaries encounter each other in this extreme environment? [Carl Rodriguez/Northwestern Visualization (Justin Muir, Matt McCrory, Michael Lannum)]

The dense, chaotic centers of star clusters may be a birthplace for binary pairs of black holes like those observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

A Question of Origin

Since the discovery of the first gravitational-wave signal in September 2015, LIGO and its European counterpart Virgo have detected nine more merging black-hole binaries. After a brief pause for upgrades, the detectors are slated to come back online in April with significantly improved sensitivities — promising many more detections to come.

A new study now explores how eccentric binaries might arise and merge in these extreme environments.

The ten black-hole mergers detected thus far by LIGO/Virgo.[Teresita Ramirez/Geoffrey Lovelace/SXS Collaboration/LIGO-Virgo Collaboration]

VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation

Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

Gravity is talking. Lisa will listen. Dialogos of Eide

ESA/eLISA the future of gravitational wave research

Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

Though the gravitational-wave signals provide a wealth of information about the pre-merger binaries, we haven’t yet been able to determine how these black-hole binaries formed in the first place. Did these pairs evolve in isolation? Or were they born from interactions in the dense centers of star clusters?

One overlooked piece of data might shed light on these questions in the future: eccentricity. Since black-hole binaries in isolation take a long time to merge, any initial eccentricity in the orbit will be damped by gravitational-wave emission by the time the merger happens. But what if the binary doesn’t evolve in isolation? Could we see an imprint of eccentricity on the gravitational-wave signal then?

A new study led by scientist Michael Zevin (Northwestern University and CIERA) explores one possible channel for eccentric mergers: chaotic interactions between multiple black-hole binaries in the centers of star clusters.

Two examples of the complex evolution of binary–binary encounters, both eventually leading to a gravitational-wave capture. An animation of the second example is shown in the video at the end of the post [only available at the full article]. [Adapted from Zevin et al. 2019]

Complex Interactions

Zevin and collaborators use models to explore what happens during strong interactions between pairs of black-hole binaries and between black-hole binaries and single black holes.

These interactions are incredibly complex (don’t believe me? Check out the video below!). Systems with more than two bodies evolve chaotically, with small changes in initial conditions leading to vastly different outcomes. To make matters worse, simple Newtonian physics won’t accurately describe these systems; to capture the effects of gravitational-wave dissipation, we must model these interactions taking general relativity into account.

Zevin and collaborators find that these complexities lead to surprising results. Though binary–binary interactions occur 10–100 times less frequently than binary–single interactions in the centers of globular clusters, the long life and complexity of binary–binary interactions means that they are significantly more likely to result in a gravitational-wave capture — the rapid inspiral and merger of a binary pair, which occurs quickly enough that the pair may still have measurable eccentricity at merger time.

Predicted eccentricity distributions and delay times for three populations of binary–binary produced gravitational-wave mergers. The horizontal black lines show minimum measurable eccentricities predicted for LIGO/Virgo and LISA. Solid colored lines show the eccentricities for the three populations at 10 Hz (LIGO/Virgo’s lower limit) and 0.1 Hz (the most sensitive frequency predicted for LISA). [Zevin et al. 2019]

An Eccentric Result

Predicted eccentricity distributions and delay times for three populations of binary–binary produced gravitational-wave mergers. The horizontal black lines show minimum measurable eccentricities predicted for LIGO/Virgo and LISA. Solid colored lines show the eccentricities for the three populations at 10 Hz (LIGO/Virgo’s lower limit) and 0.1 Hz (the most sensitive frequency predicted for LISA). [Zevin et al. 2019]
The authors demonstrate that binary–binary interactions contribute a significant fraction (~25–45%) of the eccentric mergers that result when black holes strongly interact in cluster centers. But what are our prospects for being able to detect these eccentric collisions?

The outlook is promising! Gravitational-wave captures generally have eccentricities at merger that should be measurable by LIGO/Virgo, and binary–binary-produced mergers that occur later, either in-cluster or after being ejected from the cluster, could have eccentricities detectable by the future Laser Interferometer Space Antenna (LISA). With enough observations, eccentric binaries may soon help us better understand the origin of black-hole pairs.

Citation

“Eccentric Black Hole Mergers in Dense Star Clusters: The Role of Binary–Binary Encounters,” Michael Zevin et al 2019 ApJ 871 91.
https://iopscience.iop.org/article/10.3847/1538-4357/aaf6ec/meta

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The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
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