̽��ֱ�� of Cambridge - Christopher Moore

First detection of gravitational waves and light produced by colliding neutron stars

sc604 — Mon, 16 Oct 2017 13:17:01 +0000

It could be a scenario from science fiction, but it really happened 130 million years ago -- in the NGC 4993 galaxy in the Hydra constellation, at a time here on Earth when dinosaurs still ruled, and flowering plants were only just evolving.

Today, dozens of UK scientists – including researchers from the ̽��ֱ�� of Cambridge – and their international collaborators representing 70 observatories worldwide announced the detection of this event and the significant scientific firsts it has revealed about our Universe.

Those ripples in space finally reached Earth at 1.41pm UK time, on Thursday 17 August 2017, and were recorded by the twin detectors of the US-based Laser Interferometer Gravitational-wave Observatory (LIGO) and its European counterpart Virgo.

A few seconds later, the gamma-ray burst from the collision was recorded by two specialist space telescopes, and over following weeks, other space- and ground-based telescopes recorded the afterglow of the massive explosion. UK developed engineering and technology is at the heart of many of the instruments used for the detection and analysis.

Studying the data confirmed scientists’ initial conclusion that the event was the collision of a pair of neutron stars – the remnants of once gigantic stars, but collapsed down into approximately the size of a city. “These objects are made of matter in its most extreme, dense state, standing on the verge of total gravitational collapse,” said Michalis Agathos, from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “By studying subtle effects of matter on the gravitational wave signal, such as the effects of tides that deform the neutron stars, we can infer the properties of matter in these extreme conditions.”

There are a number of “firsts” associated with this event, including the first detection of both gravitational waves and electromagnetic radiation (EM) - while existing astronomical observatories “see” EM across different frequencies (eg, optical, infra-red, gamma ray etc), gravitational waves are not EM but instead ripples in the fabric of space requiring completely different detection techniques. An analogy is that LIGO and Virgo “hear” the Universe.

̽��ֱ��announcement also confirmed the first direct evidence that short gamma ray bursts are linked to colliding neutron stars. ̽��ֱ��shape of the gravitational waveform also provided a direct measure of the distance to the source, and it was the first confirmation and observation of the previously theoretical cataclysmic aftermaths of this kind of merger - a kilonova.

Additional research papers on the aftermath of the event have also produced a new understanding of how heavy elements such as gold and platinum are created by supernova and stellar collisions and then spread through the Universe. More such original science results are still under current analysis.

By combining gravitational-wave and electromagnetic signals together, researchers also used for the first time a new and novel technique to measure the expansion rate of the Universe.

While binary black holes produce “chirps” lasting a fraction of a second in the LIGO detector’s sensitive band, the August 17 chirp lasted approximately 100 seconds and was seen through the entire frequency range of LIGO — about the same range as common musical instruments. Scientists could identify the chirp source as objects that were much less massive than the black holes seen to date. In fact, “these long chirping signals from inspiralling neutron stars are really what many scientists expected LIGO and Virgo to see first,” said Christopher Moore, researcher at CENTRA, IST, Lisbon and member of the DAMTP/Cambridge LIGO group. “ ̽��ֱ��shorter signals produced by the heavier black holes were a spectacular surprise that led to the awarding of the 2017 Nobel prize in physics.”

UK astronomers using the VISTA telescope in Chile were among the first to locate the new source. “We were really excited when we first got notification that a neutron star merger had been detected by LIGO,” said Professor Nial Tanvir from the ̽��ֱ�� of Leicester, who leads a paper in Astrophysical Journal Letters today. “We immediately triggered observations on several telescopes in Chile to search for the explosion that we expected it to produce. In the end, we stayed up all night analysing the images as they came in, and it was remarkable how well the observations matched the theoretical predictions that had been made.”

“It is incredible to think that all the gold in the Earth was probably produced by merging neutron stars, similar to this event that exploded as kilonovae billions of years ago.”

“Not only is this the first time we have seen the light from the aftermath of an event that caused a gravitational wave, but we had never before caught two merging neutron stars in the act, so it will help us to figure out where some of the more exotic chemical elements on Earth come from,” said Dr Carlos Gonzalez-Fernandez of Cambridge’s Institute of Astronomy, who processed the follow-up images taken with the VISTA telescope.

“This is a spectacular discovery, and one of the first of many that we expect to come from combining together information from gravitational wave and electromagnetic observations,” said Nathan Johnson-McDaniel, researcher at DAMTP, who contributed to predictions of the amount of ejected matter using the gravitational wave measurements of the properties of the binary.

Though the LIGO detectors first picked up the gravitational wave in the United States, Virgo, in Italy, played a key role in the story. Due to its orientation with respect to the source at the time of detection, Virgo recovered a small signal; combined with the signal sizes and timing in the LIGO detectors, this allowed scientists to precisely triangulate the position in the sky. After performing a thorough vetting to make sure the signals were not an artefact of instrumentation, scientists concluded that a gravitational wave came from a relatively small patch of the southern sky.

“This event has the most precise sky localisation of all detected gravitational waves so far,” says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU ̽��ֱ�� Amsterdam, who is the spokesperson for the Virgo collaboration. “This record precision enabled astronomers to perform follow-up observations that led to a plethora of breath-taking results.”

Fermi was able to provide a localisation that was later confirmed and greatly refined with the coordinates provided by the combined LIGO-Virgo detection. With these coordinates, a handful of observatories around the world were able, hours later, to start searching the region of the sky where the signal was thought to originate. A new point of light, resembling a new star, was first found by optical telescopes. Ultimately, about 70 observatories on the ground and in space observed the event at their representative wavelengths. “What I am most excited about, personally, is a completely new way of measuring distances across the universe through combining the gravitational wave and electromagnetic signals. Obviously, this new cartography of the cosmos has just started with this first event, but I just wonder whether this is where we will see major surprises in the future,” said Ulrich Sperhake, Head of Cambridge’s gravitational wave group in LIGO.

In the weeks and months ahead, telescopes around the world will continue to observe the afterglow of the neutron star merger and gather further evidence about its various stages, its interaction with its surroundings, and the processes that produce the heaviest elements in the universe.

Reference:
Physical Review Letters
"GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral."

Science
"A Radio Counterpart to a Neutron Star Merger."
"Swift and NuSTAR observations of GW170817: detection of a blue kilonova."
"Illuminating Gravitational Waves: A Concordant Picture of Photons from a Neutron Star Merger."

Astrophysical Journal Letters
"Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A."
"Multi-Messenger Observations of a Binary Neutron Star Merger."

Nature
"A gravitational-wave standard siren measurement of the Hubble constant."

Adapted from STFC and LIGO press releases.

In a galaxy far away, two dead stars begin a final spiral into a massive collision. ̽��ֱ��resulting explosion unleashes a huge burst of energy, sending ripples across the very fabric of space. In the nuclear cauldron of the collision, atoms are ripped apart to form entirely new elements and scattered outward across the Universe.

What I am most excited about, personally, is a completely new way of measuring distances across the universe.

Ulrich Sperhake

ESO/L. Calçada/M. Kornmesser

Artist’s impression of merging neutron stars

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LIGO detects gravitational waves for third time

sc604 — Thu, 01 Jun 2017 16:09:44 +0000

̽��ֱ��Laser Interferometer Gravitational-wave Observatory (LIGO) has made a third detection of gravitational waves, ripples in space and time, demonstrating that a new window in astronomy has been firmly opened. As was the case with the first two detections, the waves were generated when two black holes collided to form a larger black hole.

̽��ֱ��newfound black hole formed by the merger has a mass about 49 times that of our sun. “With this third confirmed detection we are uncovering the population of black holes in the Universe for the first time,” said Christopher Moore from the ̽��ֱ�� of Cambridge’s Department of Applied Mathematics and Theoretical Physics (DAMTP), who is part of the LIGO Scientific Collaboration.

̽��ֱ��new detection occurred during LIGO’s current observing run, which began November 30, 2016, and will continue through the summer. LIGO is an international collaboration with members around the globe. Its observations are carried out by twin detectors—one in Hanford, Washington, and the other in Livingston, Louisiana—operated by Caltech and MIT with funding from the United States National Science Foundation (NSF).

̽��ֱ��LIGO group in Cambridge consists of seven researchers spread across DAMTP, the Cavendish Laboratory and the Institute of Astronomy.

“Answering key questions about the formation history of astrophysical black holes and their role in the evolution of the universe critically relies on applying a statistical analysis to a sufficiently large sample of observations,” said Dr Ulrich Sperhake, head of the group in DAMTP. “Each new detection not only strengthens our confidence in the theoretical modelling, but enables us to explore new phenomena of these mysterious and fascinating objects.”

One of the interests of the Cambridge group is testing Einstein’s theory of general relativity. “This particular source of gravitational waves is the furthest detected so far. This allows us to test our understanding of the propagation of gravitational waves across cosmological distances, by means of which we constrained any signs of wave dispersion to unprecedented precision,” said Dr Michalis Agathos, a postdoctoral researcher at DAMTP.

̽��ֱ��LIGO-Virgo team is continuing to search the latest LIGO data for signs of space-time ripples from the far reaches of the cosmos. They are also working on technical upgrades for LIGO’s next run, scheduled to begin in late 2018, during which the detectors’ sensitivity will be further improved.

“With the third confirmed detection of gravitational waves from the collision of two black holes, LIGO is establishing itself as a powerful observatory for revealing the dark side of the universe,” said David Reitze of Caltech, executive director of the LIGO Laboratory. “While LIGO is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars.”

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 UK (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. Additional partners are listed at: http://ligo.org/partners.php.

Results confirm new population of black holes.

Each new detection enables us to explore new phenomena of these mysterious and fascinating objects.

Ulrich Sperhake

LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

Artist's conception shows two merging black holes similar to those detected by LIGO.

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Yes

Using gravitational waves to catch runaway black holes

sc604 — Thu, 30 Jun 2016 08:47:41 +0000

Researchers have developed a new method for detecting and measuring one of the most powerful, and most mysterious, events in the Universe – a black hole being kicked out of its host galaxy and into intergalactic space at speeds as high as 5000 kilometres per second.

̽��ֱ��method, developed by researchers from the ̽��ֱ�� of Cambridge, could be used to detect and measure so-called black hole superkicks, which occur when two spinning supermassive black holes collide into each other, and the recoil of the collision is so strong that the remnant of the black hole merger is bounced out of its host galaxy entirely. Their results are reported in the journal Physical Review Letters.

Earlier this year, the LIGO Collaboration announced the first detection of gravitational waves – ripples in the fabric of spacetime – coming from the collision of two black holes, confirming a major prediction of Einstein’s general theory of relativity and marking the beginning of a new era in astronomy. As the sensitivity of the LIGO detectors is improved, even more gravitational waves are expected to be detected – the second successful detection was announced in June.

As two black holes circle each other, they emit gravitational waves in a highly asymmetric way, which leads to a net emission of momentum in some preferential direction. When the black holes finally do collide, conservation of momentum imparts a recoil, or kick, much like when a gun is fired. When the two black holes are not spinning, the speed of the recoil is around 170 kilometres per second. But when the black holes are rapidly spinning in certain orientations, the speed of the recoil can be as high as 5000 kilometres per second, easily exceeding the escape velocity of even the most massive galaxies, sending the black hole remnant resulting from the merger into intergalactic space.

̽��ֱ��Cambridge researchers have developed a new method for detecting these kicks based on the gravitational wave signal alone, by using the Doppler Effect. ̽��ֱ��Doppler Effect is the reason that the sound of a passing car seems to lower in pitch as it gets further away. It is also widely used in astronomy: electromagnetic radiation coming from objects which are moving away from the Earth is shifted towards the red end of the spectrum, while radiation coming from objects moving closer to the Earth is shifted towards the blue end of the spectrum. Similarly, when a black hole kick has sufficient momentum, the gravitational waves it emits will be red-shifted if it is directed away from the Earth, while they will be blue-shifted if it’s directed towards the Earth.

“If we can detect a Doppler shift in a gravitational wave from the merger of two black holes, what we’re detecting is a black hole kick,” said study co-author Davide Gerosa, a PhD student from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “And detecting a black hole kick would mean a direct observation that gravitational waves are carrying not just energy, but linear momentum as well.”

Detecting this elusive effect requires gravitational-wave experiments capable of observing black hole mergers with very high precision. A black hole kick cannot be directly detected using current land-based gravitational wave detectors, such as those at LIGO. However, according to the researchers, the new space-based gravitational wave detector known as eLISA, funded by the European Space Agency (ESA) and due for launch in 2034, will be powerful enough to detect several of these runaway black holes. In 2015, ESA launched the LISA Pathfinder, which is successfully testing several technologies which could be used to measure gravitational waves from space.

̽��ֱ��researchers found that the eLISA detector will be particularly well-suited to detecting black hole kicks: it will be capable of measuring kicks as small as 500 kilometres per second, as well as the much faster superkicks. Kick measurements will tell us more about the properties of black hole spins, and also provide a direct way of measuring the momentum carried by gravitational waves, which may lead to new opportunities for testing general relativity.

“When the detection of gravitational waves was announced, a new era in astronomy began, since we can now actually observe two merging black holes,” said study co-author Christopher Moore, a Cambridge PhD student who was also a member of the team which announced the detection of gravitational waves earlier this year. “We now have two ways of detecting black holes, instead of just one – it’s amazing that just a few months ago, we couldn’t say that. And with the future launch of new space-based gravitational wave detectors, we’ll be able to look at gravitational waves on a galactic, rather than a stellar, scale.”

Reference:
Davide Gerosa and Christopher J. Moore. ‘Black-hole kicks as new gravitational-wave observables.’ Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.117.011101

Black holes are the most powerful gravitational force in the Universe. So what could cause them to be kicked out of their host galaxies? Cambridge researchers have developed a method for detecting elusive ‘black hole kicks.’

We now have two ways of detecting black holes, instead of just one – it’s amazing that just a few months ago, we couldn’t say that.

Christopher Moore

SXS Lensing

Computer simulations motivated by GW150914

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Gravitational waves detected 100 years after Einstein’s prediction

sc604 — Thu, 11 Feb 2016 15:30:00 +0000

An international team of scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos.

̽��ֱ��gravitational waves were detected on 14 September 2015 at 09:51 UK time by both LIGO (Laser Interferometer Gravitational-wave Observatory) detectors in Louisiana and Washington State in the US. They originated from two black holes, each around 30 times the mass of the Sun and located more than 1.3 billion light years from Earth, coalescing to form a single, even more massive black hole.

̽��ֱ��LIGO Observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT. ̽��ֱ��discovery, published in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

“ ̽��ֱ��discovery of gravitational waves by the LIGO team is an incredible achievement,” said Professor Stephen Hawking, the Dennis Stanton Avery and Sally Tsui Wong-Avery Director of Research at the Department of Applied Mathematics and Theoretical Physics at the ̽��ֱ�� of Cambridge. “It is the first observation of gravitational waves as predicted by Einstein and will allow us new insights into our universe. ̽��ֱ��gravitational waves were released from the collision of two black holes, the properties of which are consistent with predictions I made in Cambridge in the 1970s, such as the black hole area and uniqueness theorems. We can expect this observation to be the first of many as LIGO sensitivity increases, keeping us all busy with many further surprises.”

Gravitational waves carry unique information about the origins of our Universe and studying them is expected to provide important insights into the evolution of stars, supernovae, gamma-ray bursts, neutron stars and black holes. However, they interact very weakly with particles and require incredibly sensitive equipment to detect. British and German teams, including researchers from the ̽��ֱ�� of Cambridge, working with US, Australian, Italian and French colleagues as part of the LIGO Scientific Collaboration and the Virgo Collaboration, are using a technique called laser interferometry.

Each LIGO site comprises two tubes, each four kilometres long, arranged in an L-shape. A laser is beamed down each tube to very precisely monitor the distance between mirrors at each end. According to Einstein’s theory, the distance between the mirrors will change by a tiny amount when a gravitational wave passes by the detector. A change in the lengths of the arms of close to 10^-19 metres (just one-ten-thousandth the diameter of a proton) can be detected.

According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole, converting a portion of the combined black holes’ mass to energy, according to Einstein’s formula E=mc². This energy is emitted as a final strong burst of gravitational waves. It is these gravitational waves that LIGO has observed.

Independent and widely separated observatories are necessary to verify the direction of the event causing the gravitational waves, and also to determine that the signals come from space and are not from some other local phenomenon.

To ensure absolute accuracy, the consortium of nearly 1,000 scientists from 16 countries spent several months carefully checking and re-checking the data before submitting their findings for publication.

Christopher Moore, a PhD student from Cambridge’s Institute of Astronomy, was part of the discovery team who worked on the data analysis.

“Since September, we’ve known that something was detected, but it took months of checking to confirm that it was actually gravitational waves,” he said. “This team has been looking for evidence of gravitational waves for decades – a huge amount of work has gone into it, and I feel incredibly lucky to be part of the team. This discovery will change the way we do astronomy.”

Over coming years, the Advanced LIGO detectors will be ramped up to full power, increasing their sensitivity to gravitational waves, and in particular allowing more distant events to be measured. With the addition of further detectors, initially in Italy and later in other locations around the world, this first detection is surely just the beginning. UK scientists continue to contribute to the design and development of future generations of gravitational wave detectors.

̽��ֱ��UK Minister for Universities and Science, Jo Johnson MP, said: “Einstein’s theories from over a century ago are still helping us to understand our universe. Now that we have the technological capability to test his theories with the LIGO detectors his scientific brilliance becomes all the more apparent. ̽��ֱ��Government is increasing support for international research collaborations, and these scientists from across the UK have played a vital part in this discovery.”

LIGO was originally proposed as a means of detecting these gravitational waves in the 1980s by Kip Thorne, Caltech’s Richard P. Feynman Professor of Theoretical Physics, Emeritus; Ronald Drever, professor of physics, emeritus also from Caltech; and Rainer Weiss, professor of physics, emeritus, from MIT.

“ ̽��ֱ��description of this observation is beautifully described in the Einstein theory of General Relativity formulated 100 years ago and comprises the first test of the theory in strong gravitation. It would have been wonderful to watch Einstein’s face had we been able to tell him,” said Weiss.

“With this discovery, we humans are embarking on a marvelous new quest: the quest to explore the warped side of the universe—objects and phenomena that are made from warped spacetime. Colliding black holes and gravitational waves are our first beautiful examples,” said Thorne.

̽��ֱ��discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed—and the discovery of gravitational waves during its first observation run.

̽��ֱ��US National Science Foundation leads in financial support for Advanced LIGO. Funding organisations in Germany (Max Planck Society), the UK (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project.

Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse ̽��ֱ��, and the ̽��ֱ�� of Wisconsin-Milwaukee.

Several universities designed, built, and tested key components for Advanced LIGO: ̽��ֱ��Australian National ̽��ֱ��, the ̽��ֱ�� of Adelaide, the ̽��ֱ�� of Florida, Stanford ̽��ֱ��, Columbia ̽��ֱ�� of New York, and Louisiana State ̽��ֱ��.

Cambridge has a long-standing involvement in the field of gravitational wave science, and specifically with the LIGO experiment. Until recently these efforts were spearheaded by Dr Jonathan Gair, who left last year for a post at the ̽��ֱ�� of Edinburgh and who has made significant contributions to a wide range of gravitational wave and LIGO science; he is one of the authors on the new paper. Several scientists in Cambridge are current members of the collaboration, including PhD students Christopher Moore and Alvin Chua from the Institute of Astronomy; Professor Anthony Lasenby and PhD student Sonke Hee from the Cavendish Laboratory and the Kavli Institute of Cosmology; and Professor Mike Hobson from the Cavendish Laboratory.

Further members of the collaboration until recently based at Cambridge, include Dr Philip Graff (author on the detection paper) and Dr Farhan Feroz, who, jointly with Mike Hobson and Anthony Lasenby, developed a machine learning method of analysis used currently within LIGO, as well as Dr Christopher Berry (author) and Dr Priscilla Canizares.

These findings will be discussed at next month's Cambridge Science Festival during the open afternoon at the Institute of Astronomy.

Reference:
B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration) 'Observation of Gravitational Waves from a Binary Black Hole Merger.' Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.116.061102.

New window on the universe is opened with the observation of gravitational waves – ripples in spacetime – caused by the collision of two black holes.

I feel incredibly lucky to be part of the team - this discovery will change the way we do astronomy.

Christopher Moore

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Yes