̽��ֱ�� of Cambridge - Ulrich Sperhake

A force to be reckoned with

sc604 — Mon, 27 Nov 2017 16:04:01 +0000

Gravity is one of the universe's great mysteries. We decided to find out why.

Think you know what gravity is? Think again. New research is revealing how little we know about this most mysterious of forces.

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

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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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.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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New insights found in black hole collisions

sc604 — Fri, 27 Mar 2015 08:00:00 +0000

An international team of astronomers, including from the ̽��ֱ�� of Cambridge, have found solutions to decades-old equations describing what happens as two spinning black holes in a binary system orbit each other and spiral in toward a collision.

̽��ֱ��results, published in the journal Physical Review Letters, should significantly impact not only the study of black holes, but also the search for elusive gravitational waves – a type of radiation predicted by Einstein’s theory of general relativity – in the cosmos.

Unlike planets, whose average distance from the sun does not change over time, general relativity predicts that two black holes orbiting around each other will move closer together as the system emits gravitational waves.

“An accelerating charge, like an electron, produces electromagnetic radiation, including visible light waves,” said Dr Michael Kesden of the ̽��ֱ�� of Texas at Dallas, the paper’s lead author. “Similarly, any time you have an accelerating mass, you can produce gravitational waves.”

̽��ֱ��energy lost to gravitational waves causes the black holes to spiral closer and closer together until they merge, which is the most energetic event in the universe, after the big bang. That energy, rather than going out as visible light, which is easy to see, goes out as gravitational waves, which are much more difficult to detect.

While Einstein’s theories predict the existence of gravitational waves, they have not been directly detected. But the ability to ‘see’ gravitational waves would open up a new window to view and study the universe.

Optical telescopes can capture photos of visible objects, such as stars and planets, and radio and infrared telescopes can reveal additional information about invisible energetic events. Gravitational waves would provide a qualitatively new medium through which to examine astrophysical phenomena.

“Using gravitational waves as an observational tool, you could learn about the characteristics of the black holes that were emitting those waves billions of years ago, information such as their masses and mass ratios, and the way they formed” said co-author and PhD student Davide Gerosa, of Cambridge’s Department of Applied Mathematics and Theoretical Physics. “That’s important data for more fully understanding the evolution and nature of the universe.”

Later this year, upgrades to the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and VIRGO in Europe will be completed, and the first direct measurements of gravitational waves may be just around the corner. Around the same time, the LISA Pathfinder mission will be launched as a test mission for establishing a gravitational wave detector of unprecedented sensitivity in space.

“ ̽��ֱ��equations that we solved will help predict the characteristics of the gravitational waves that LIGO would expect to see from binary black hole mergers,” said co-author Dr Ulrich Sperhake, who, along with Gerosa, is also a member of Cambridge’s Centre for Theoretical Cosmology. “We’re looking forward to comparing our solutions to the data that LIGO collects.”

̽��ֱ��equations the researchers solved deal specifically with the spin angular momentum of binary black holes and a phenomenon called precession.

“Like a spinning top, black hole binaries change their direction of rotation over time, a phenomenon known as procession,” said Sperhake. “ ̽��ֱ��behaviour of these black hole spins is a key part of understanding their evolution.”

Just as Kepler studied the motion of the earth around the sun and found that orbits can be ellipses, parabola or hyperbolae, the researchers found that black hole binaries can be divided into three distinct phases according to their rotation properties.

̽��ֱ��researchers also derived equations that will allow statistical tracking of such spin phases, from black hole formation to merger, far more efficiently and quickly than was possible before.

“With these solutions, we can create computer simulations that follow black hole evolution over billions of years,” said Kesden. “A simulation that previously would have taken years can now be done in seconds. But it’s not just faster. There are things that we can learn from these simulations that we just couldn’t learn any other way.”

“With these tools, new insights into the dynamics of black holes will be unveiled,” said Gerosa. “Gravitational wave signals can now be better interpreted to unveil mysteries of the massive universe.”

Researchers from the Rochester Institute of Technology and the ̽��ֱ�� of Mississippi also contributed to the Physical Review Letters paper. ̽��ֱ��researchers were supported in part by the Science and Technology Facilities Council, the European Commission, the National Science Foundation, UT Dallas and the ̽��ֱ�� of Cambridge.

Inset image: Illustration of two rotating black holes in orbit. Both, the black hole spins (red arrows) and the orbital angular momentum (blue arrow) precess about the total angular momentum (grey arrow) in a manner that characterizes the black-hole binary system. Gravitational waves carry away energy and momentum from the system and the orbital plane (light blue) tilts and turns accordingly. Credit: Graphic by Midori Kitagawa

Adapted from ̽��ֱ�� of Texas at Dallas press release.

New research provides revelations about the most energetic event in the universe — the merging of two spinning, orbiting black holes into a much larger black hole.

̽��ֱ��behaviour of these black hole spins is a key part of understanding their evolution

Ulrich Sperhake

NASA's Marshall Space Flight Center

Black Holes Go 'Mano a Mano' (NASA, Chandra, 10/06/09)

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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