̽��ֱ�� of Cambridge - gravity

New findings that map the universe’s cosmic growth support Einstein’s theory of gravity

sc604 — Tue, 11 Apr 2023 14:00:00 +0000

̽��ֱ��findings, from the Atacama Cosmology Telescope collaboration involving researchers from the ̽��ֱ�� of Cambridge, provide further support to Einstein’s theory of general relativity, which has been the foundation of the standard model of cosmology for more than a century. ̽��ֱ��results offer new methods to demystify dark matter, the unseen mass thought to account for 85% of the matter in the universe.

For millennia, humans have been fascinated by the mysteries of the cosmos. From ancient civilisations such as the Babylonians, Greeks, and Egyptians to modern-day astronomers, the allure of the starry sky has inspired countless quests to unravel the secrets of the universe.

And although models that explain the cosmos have existed for centuries, the field of cosmology, where scientists use quantitative methods to understand the evolution and structure of the universe, is relatively new—having only formed in the early 20th century with the development of Albert Einstein’s theory of general relativity.

Now, a set of papers submitted to ̽��ֱ��Astrophysical Journal by researchers from the Atacama Cosmology Telescope (ACT) collaboration has produced a new image that reveals the most detailed map of matter distributed across a quarter of the entire sky, reaching deep into the cosmos. It confirms Einstein’s theory about how massive structures grow and bend light, with a test that spans the entire age of the universe.

“We have mapped the invisible dark matter across the sky to the largest distances, and clearly see features of this invisible world that are hundreds of millions of light-years across,” said co-author Professor Blake Sherwin from Cambridge’s Department of Applied Mathematics and Theoretical Physics, where he leads a group of ACT researchers. “It looks just as our theories predict.”

Although dark matter makes up a large chunk of the universe and shaped its evolution, it has remained hard to detect because it doesn’t interact with light or other forms of electromagnetic radiation. As far as we know, dark matter only interacts with gravity.

To track it down, the more than 160 collaborators who have built and gathered data from the National Science Foundation’s Atacama Cosmology Telescope in the high Chilean Andes observe light emanating following the dawn of the universe’s formation, the Big Bang—when the universe was only 380,000 years old. Cosmologists often refer to this diffuse light that fills our entire universe as the “baby picture of the universe,” but formally, it is known as the cosmic microwave background radiation (CMB).

̽��ֱ��team tracks how the gravitational pull of large, heavy structures including dark matter warps the CMB on its 14-billion year journey to us, like how a magnifying glass bends light as it passes through its lens.

“We’ve made a new mass map using distortions of light left over from the Big Bang,” said Mathew Madhavacheril from the ̽��ֱ�� of Pennsylvania, lead author of one of the papers. “Remarkably, it provides measurements that show that both the ‘lumpiness’ of the universe, and the rate at which it is growing after 14 billion years of evolution, are just what you’d expect from our standard model of cosmology based on Einstein's theory of gravity.”

“Our results also provide new insights into an ongoing debate some have called ‘ ̽��ֱ��Crisis in Cosmology’,” said Sherwin. This crisis stems from recent measurements that use a different background light, one emitted from stars in galaxies rather than the CMB. These have produced results that suggest the dark matter was not lumpy enough under the standard model of cosmology and led to concerns that the model may be broken. However, the team’s latest results from ACT were able to precisely assess that the vast lumps seen in this image are the exact right size.

“When I first saw them, our measurements were in such good agreement with the underlying theory that it took me a moment to process the results,” said Cambridge PhD candidate Frank Qu, lead author of one of the new papers. “But we still don’t know what the dark matter is, so it will be interesting to see how this possible discrepancy between different measurements will be resolved.”

“ ̽��ֱ��CMB lensing data rivals more conventional surveys of the visible light from galaxies in their ability to trace the sum of what is out there,” said Suzanne Staggs from Princeton ̽��ֱ��, Director of ACT. “Together, the CMB lensing and the best optical surveys are clarifying the evolution of all the mass in the universe.”

“When we proposed this experiment in 2003, this measurement wasn’t even on our agenda; we had no idea the full extent of information that could be extracted from our telescope,” said Mark Devlin, from the ̽��ֱ�� of Pennsylvania, Deputy Director of ACT. “We owe this to the cleverness of the theorists, the many people who built new instruments to make our telescope more sensitive, and the new analysis techniques our team came up with.”

With ACT having been decommissioned in late 2022, further papers highlighting some of the other final results are slated for submission in the coming year. Observations will continue at the site with the Simons Observatory, including a new telescope due to begin in 2024 that can map the sky almost ten times faster.

̽��ֱ��pre-print articles highlighted in this release are available on act.princeton.edu and will appear on the open-access arXiv.org. They have been submitted to ̽��ֱ��Astrophysical Journal.

This work was supported by the U.S. National Science Foundation, Princeton ̽��ֱ��, the ̽��ֱ�� of Pennsylvania, and a Canada Foundation for Innovation award. Team members at the ̽��ֱ�� of Cambridge were supported by the European Research Council.

A new image reveals the most detailed map of dark matter distributed across a quarter of the entire sky, reaching deep into the cosmos.

We have mapped the invisible dark matter across the sky to the largest distances, and clearly see features of this invisible world that are hundreds of millions of light-years across

Blake Sherwin

ACT Collaboration

A new map of the dark matter made by the Atacama Cosmology Telescope. ̽��ֱ��orange regions show where there is more mass; purple where there is less.

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Sir Isaac Newton’s Cambridge papers added to UNESCO’s Memory of the World Register

sjr81 — Fri, 01 Dec 2017 09:36:33 +0000

Held at Cambridge ̽��ֱ�� Library, Newton’s scientific and mathematical papers represent one of the most important archives of scientific and intellectual work on universal phenomena. They document the development of his thought on gravity, calculus and optics, and reveal ideas worked out through painstaking experiments, calculations, correspondence and revisions.

In combination with alchemical papers at King’s College, Cambridge and his notebooks and correspondence at Trinity College, Cambridge and the Fitzwilliam Museum, this represents the largest and most important collection of Newton’s papers worldwide.

Katrina Dean, Curator of Scientific Collections at Cambridge ̽��ֱ�� Library said: “Newton’s papers are among the world’s most important collections in the western scientific tradition and are one of the Library’s most treasured collections. They were the first items to be digitised and added to the Cambridge Digital Library in 2011 and featured in our 600th anniversary exhibition Lines of Thought last year. In 2017, their addition to the UNESCO International Memory of the World Register recognises their unquestionable international importance.”

̽��ֱ��Memory of the World Project is an international initiative to safeguard the documentary heritage of humanity against collective amnesia, neglect, the ravages of time and climatic conditions, and wilful and deliberate destruction. It calls for the preservation of valuable archival, library and private collections all over the world, as well as the reconstitution of dispersed or displaced documentary heritage, and the increased accessibility to and dissemination of these items.

Newton’s Cambridge papers, and those at the Royal Society, now join the archive of Winston Churchill, held at Cambridge ̽��ֱ��’s Churchill Archives Centre, on the UNESCO Register. They also join Newton’s theological and alchemical papers at the National Library of Israel, which were added in 2015.

̽��ֱ��chief attractions in the Cambridge collection are Newton’s own copies of the first edition of the Principia (1687), covered with his corrections, revisions and additions for the second edition.

̽��ֱ��Cambridge papers also include significant correspondence with natural philosophers and mathematicians including Henry Oldenberg, Secretary of the Royal Society, Edmond Halley, the Astronomer Royal who persuaded Newton to publish Principia, Richard Bentley, the Master of Trinity College, and John Collins, mathematician and fellow of the Royal Society who became an important collector of Newton’s works.

Added Dean: “One striking illustration of Newton’s experimental approach is in his ‘Laboratory Notebook’, which includes details of his investigations into light and optics in order to understand the nature of colour. His essay ‘Of Colours’ includes a diagram that illustrates the experiment in which he inserted a bodkin into his eye socket to put pressure on the eyeball to try to replicate the sensation of colour in normal sight.”

Another important item is Newton’s so-called ‘Waste Book’, a large notebook inherited from his stepfather. From 1664, he used the blank pages for optical and mathematical calculations and gradually mastered the analysis of curved lines, surfaces and solids. By 1665, he had invented the method of calculus. Newton later used the dated, documentary evidence provided by the Waste Book to argue his case in the priority dispute with Gottfried Wilhelm Leibniz over the invention of the calculus.

Cambridge ̽��ֱ�� Librarian Jess Gardner said: “Newton’s work and life continue to attract wonder and new perspectives on our place in the Universe. Cambridge ̽��ֱ�� Library will continue to work with scholars and curators worldwide to make Newton’s papers accessible now and for future generations.”

Isaac Newton entered Trinity College as an undergraduate in 1661 and became a Fellow in 1667. In 1669, he became Lucasian Professor of Mathematics at Cambridge ̽��ֱ��, a position he held until 1701.

Among the more personal items in the Cambridge collections are Newton’s daily concerns as recorded in an undergraduate notebook which records Newton’s expenditure on white wine, wafers, shoe-strings and ‘a paire of stockings’, along with a guide to Latin pronunciation.

A notebook of 1662-1669 records Newton’s sins before and after Whitsunday of 1662, written in a coded shorthand and first deciphered between 1872 and 1888. Among them are ‘Eating an apple at Thy house’, ‘Robbing my mothers box of plums and sugar’ along with the more serious ‘Wishing death and hoping it to some’ before a list of his expenses. These included chemicals, two furnaces and a recent edition of one of the most comprehensive compilations of alchemical writings in the western tradition Theatrum chemicum, edited by the publisher Lazarus Zetzner.

Cambridge ̽��ֱ�� Library is also hosting a series of talks open to the public by Sarah Dry and Patricia Fara on Newton’s manuscripts and Newton’s role in Enlightenment culture and polite society on December 7 and December 14 respectively. For details and bookings, see: http://www.lib.cam.ac.uk/using-library/whats

̽��ֱ��Cambridge papers of Sir Isaac Newton, including early drafts and Newton’s annotated copies of Principia Mathematica – a work that changed the history of science – have been added to UNESCO’s International Memory of the World Register.

Newton’s papers are among the world’s most important collections in the western scientific tradition and are one of the Library’s most treasured collections.

Katrina Dean

Cambridge ̽��ֱ�� Library

Image from Newton’s own annotated copy of Principia Mathematica

̽��ֱ��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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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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Planets similar to Jupiter are likely able to form on orbits shorter than the Earth’s

Anonymous — Fri, 15 Jul 2016 13:48:18 +0000

Warm Jupiters are large, gas-giant exoplanets—planets found around stars other than the Sun. They are comparable in size to Jupiter and Saturn in our Solar System. But unlike the Sun’s family of giant planets, Warm Jupiters orbit their parent stars at roughly the same distance that Mercury, Venus and the Earth circle the Sun. They take ten to two hundred days to complete a single orbit instead of decades. Their origin is heavily debated topic of research.

It is generally thought that Warm Jupiters cannot form on the orbits that we find them on today; they are too close to their parent stars to have accumulated enough mass, and sufficient gas. Instead, it appeared more likely that they had formed in the outer reaches of their planetary systems and had migrated inward to their current positions. However such a migratory history would also leave a trace. ̽��ֱ��large gravity of any migrating Warm Jupiter would have disturbed any neighbouring or companion planets, ejecting them from the system.

But, instead of finding “lonely”, companion-less Warm Jupiters, the team found that 11 of the 27 targets they studied have companions ranging in size from Earth-like to Neptune-like.

̽��ֱ��team’s analysis, published this week in the Astrophysical Journal, demonstrates the existence of two distinct types of warm Jupiters, each with their own formation and dynamical history.

̽��ֱ��two types include those that have smaller planetary companions and thus, likely formed near to where we find them today; and warm Jupiters without any companions indicating that they likely migrated to their current positions.

̽��ֱ��presence of many smaller sized planetary companions to warm Jupiters provides our strongest evidence that planets similar to Jupiter and Saturn can assemble on orbits shorter than the Earth’s. “And when we take into account that there is more analysis to come,” says lead-author Chelsea Huang, “the number of Warm Jupiters with smaller neighbours may be even higher. We may find that more than half have companions.”

This result came as a “surprise” according to Amaury Triaud, now a research fellow at the Institute of Astronomy, in Cambridge. “Before starting this study, we had not anticipated this remarkable result. This challenges our ideas about which conditions are sufficient for the formation of planets. It reinforces the view that planets form in richer and more diverse ways than what can be glimpsed from the sole study of the Solar system planets.”

Reference

Huang, C et al. Warm Jupiters are less lonely than Hot Jupiters: close neighbors. Astrophysical Journal; 6 July 2016; DOI:10.3847/0004-637X/825/2/98

Based on a press release prepared by the ̽��ֱ�� of Toronto.

After analyzing four years of Kepler space telescope observations, astronomers from the ̽��ֱ�� of Toronto, and of the ̽��ֱ�� of Cambridge have given us our clearest understanding yet of a class of exoplanets called “warm Jupiters”, showing that many have unexpected planetary companions.

We had not anticipated this remarkable result. This challenges our ideas about which conditions are sufficient for the formation of planets.

Amaury Triaud

Detlev Van Ravenswaay/Science Photo Library

An artist’s portrayal of a Warm Jupiter gas-giant planet (r.) in orbit around its parent star, along with smaller companion planets.

̽��ֱ��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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Gravitational vortex provides new way to study matter close to a black hole

Anonymous — Tue, 12 Jul 2016 15:01:00 +0000

Matter falling into a black hole heats up as it plunges to its doom. Before it passes into the black hole and is lost from view forever, it can reach millions of degrees. At that temperature it shines x-rays into space.

In the 1980s, astronomers discovered that the x-rays coming from black holes vary on a range of timescales and can even follow a repeating pattern with a dimming and re-brightening taking 10 seconds to complete. As the days, weeks and then months progress, the pattern’s period shortens until the oscillation takes place 10 times every second. Then it suddenly stops altogether.

This phenomenon was dubbed a Quasi Periodic Oscillation (QPO). During the 1990s, astronomers began to suspect that the QPO was associated with a gravitational effect predicted by Einstein’s general relativity which suggested that a spinning object will create a kind of gravitational vortex. ̽��ֱ��effect is similar to twisting a spoon in honey: anything embedded in the honey will be ‘dragged’ around by the twisting spoon. In reality, this means that anything orbiting around a spinning object will have its motion affected. If an object is orbiting at an angle, its orbit will ‘precess’ – in other words, the whole orbit will change orientation around the central object. ̽��ֱ��time for the orbit to return to its initial condition is known as a precession cycle.

In 2004, NASA launched Gravity Probe B to measure this so-called Lense-Thirring effect around Earth. By analysing the resulting data, scientists confirmed that the spacecraft would turn through a complete precession cycle once every 33 million years. Around a black hole, however, the effect would be much stronger because of the stronger gravitational field: the precession cycle would take just a matter of seconds to complete, close to the periods of the QPOs.

An international team of researchers, including Dr Matt Middleton from the Institute of Astronomy at the ̽��ֱ�� of Cambridge, has used the European Space Agency’s XMM-Newton and NASA’s NuSTAR, both x-ray observatories, to study the effect of black hole H1743-322 on a surrounding flat disc of matter known as an ‘accretion disk’.

Close to a black hole, the accretion disc puffs up into a hot plasma, a state of matter in which electrons are stripped from their host atoms – the precession of this puffed up disc has been suspected to drive the QPO. This can also explain why the period changes - the place where the disc puffs up gets closer to the black hole over weeks and months, and, as it gets closer to the black hole, the faster its Lense-Thirring precession becomes.

̽��ֱ��plasma releases high energy radiation that strikes the matter in the surrounding accretion disc, making the iron atoms in the disc shine like a fluorescent light tube. Instead of visible light, the iron releases X-rays of a single wavelength – referred to as ‘a line’. Because the accretion disc is rotating, the iron line has its wavelength distorted by the Doppler effect: line emission from the approaching side of the disc is squashed – blue shifted – and line emission from the receding disc material is stretched – red shifted. If the plasma really is precessing, it will sometimes shine on the approaching disc material and sometimes on the receding material, making the line wobble back and forth over the course of a precession cycle.

It is this ‘wobble’ that has been observed by the researchers.

“Just as general relativity predicts, we’ve seen the iron line wobble as the accretion disk orbits the black hole,” says Dr Middleton. “This is what we’d expect from matter moving in a strong gravitational field such as that produced by a black hole.”

This is the first time that the Lense-Thirring effect has been measured in a strong gravitational field. ̽��ֱ��technique will allow astronomers to map matter in the inner regions of accretion discs around back holes. It also hints at a powerful new tool with which to test general relativity. Einstein’s theory is largely untested in such strong gravitational fields. If astronomers can understand the physics of the matter that is flowing into the black hole, they can use it to test the predictions of general relativity as never before - but only if the movement of the matter in the accretion disc can be completely understood.

“We need to test Einstein’s general theory of relativity to breaking point,” adds Dr Adam Ingram, the lead author at the ̽��ֱ�� of Amsterdam. “That’s the only way that we can tell whether it is correct or, as many physicists suspect, an approximation – albeit an extremely accurate one.”

Larger X-ray telescopes in the future could help in the search because they could collect the X-rays faster. This would allow astronomers to investigate the QPO phenomenon in more detail. But for now, astronomers can be content with having seen Einstein’s gravity at play around a black hole.

Adapted from a press release by the European Space Agency.

Image: ESA/ATG medialab.

An international team of astronomers has proved the existence of a ‘gravitational vortex’ around a black hole, solving a mystery that has eluded astronomers for more than 30 years. ̽��ֱ��discovery will allow astronomers to map the behaviour of matter very close to black holes. It could also open the door to future investigation of Albert Einstein’s general relativity.

We need to test Einstein’s general theory of relativity to breaking point

Adam Ingram, ̽��ֱ�� of Amsterdam

ESA/ATG medialab

Illustration of gravitational vortex

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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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Opinion: Inside Big Ben: why the world’s most famous clock will soon lose its bong

Anonymous — Wed, 04 May 2016 11:03:30 +0000

London is soon going to lose one of its most familiar sounds when the world-famous Big Ben falls silent for repairs. ̽��ֱ��“bonging” chimes that have marked the passing of time for Londoners since 1859 will fall silent for months beginning in 2017 as part of a three-year £29m conservation project.

Of course, “Big Ben” is the nickname of the Great Bell and the bell itself is not in bad shape – even though it does have a huge crack in it. ̽��ֱ��bell weighs nearly 14 tonnes and it cracked in 1859 when it was first bonged with a hammer that was way too heavy. ̽��ֱ��crack was never repaired. Instead the bell was rotated one eighth of a turn and a lighter (200kg) hammer was installed. ̽��ֱ��cracked bell has a characteristic sound which we have all grown to love, so maybe best leave it alone.

Big Ben strikes. UK Parliament918 KB (download)

Instead, it is the Elizabeth Tower (1859) and the clock mechanism (1854), designed by Denison and Airy, that need attention.

Big Ben in 1858. ̽��ֱ��Illustrated News of the World December 4 1858

Any building or machine needs regular maintenance – we paint our doors and windows when they need it and we repair or replace our cars quite routinely. It is convenient to choose a day when we’re out of the house to paint the doors, or when we don’t need the car to repair the brakes. But a clock just doesn’t stop – especially not a clock as iconic as the Great Clock at the Palace of Westminster.

Repairs to the tower are long overdue. There is corrosion damage to the cast iron roof and to the belfry structure which keeps the bells in place. There is water damage to the masonry and condensation problems will be addressed, too. There are plumbing and electrical works to be done for a lift to be installed in one of the ventilation shafts, toilet facilities and the fitting of low-energy lighting.

Marvel of engineering

̽��ֱ��clock mechanism itself is remarkable. In its 162-year history it has only had one major breakdown. In 1976 the speed regulator for the chimes broke and the mechanism sped up to destruction. ̽��ֱ��resulting damage took months to repair.

Big Ben’s clock has had only one major breakdown, in 1976. UK Parliament, CC BY

̽��ֱ��weights that drive the clock are, like the bells and hammers, unimaginably huge. ̽��ֱ��“drive train” that keeps the pendulum swinging and that turns the hands is driven by a weight of about 100kg. Two other weights that ring the bells are each over a tonne. If any of these weights falls out of control (as in the 1976 incident), they could do a lot of damage.

̽��ֱ��pendulum suspension spring is especially critical because it holds up the huge pendulum bob which weighs 321kg. ̽��ֱ��swinging pendulum releases the “escapement” every two seconds which then turns the hands on the clock’s four faces. If you look very closely, you will see that the minute hand doesn’t move smoothly but it sits still most of the time, only moving on each tick by 1.5cm.

Pendulum suspension from a Smith of Derby clock. Hugh Hunt, Author provided

̽��ֱ��pendulum swings back and forth 21,600 times a day. That’s nearly 8m times a year, bending the pendulum spring. Like any metal, it has the potential to suffer from fatigue. ̽��ֱ��pendulum needs to be lifted out of the clock so that the spring can be closely inspected.

̽��ֱ��clock derives its remarkable accuracy in part from the temperature compensation which is built into the construction of the pendulum. This was yet another of John Harrison’s genius ideas (you probably know him from longitude fame). He came up with the solution of using metals of differing temperature expansion coefficient so that the pendulum doesn’t change in length as the temperature changes with the seasons.

In the Westminster clock, the pendulum shaft is made of concentric tubes of steel and zinc. A similar construction is described for the clock in Trinity College Cambridge and near perfect temperature compensation can be achieved. But zinc is a ductile metal and the tube deforms with time under the heavy load of the 321kg pendulum bob. This “creeping” will cause the temperature compensation to jam up and become less effective.

So stopping the clock will also be a good opportunity to dismantle the pendulum completely and to check that the zinc tube is sliding freely. This in itself is a few days' work.

What makes it tick

But the truly clever bit of this clock is the escapement. All clocks have one - it’s what makes the clock tick, quite literally. Denison developed his new gravity escapement especially for the Westminster clock. It decouples the driving force of the falling weight from the periodic force that maintains the motion of the pendulum. To this day, the best tower clocks in England use the gravity escapement leading to remarkable accuracy – better even than that of your quartz crystal wrist watch.

In Denison’s gravity escapement, the “tick” is the impact of the “legs” of the escapement colliding with hardened steel seats. Each collision causes microscopic damage which, accumulated over millions of collisions per year, causes wear and tear affecting the accuracy of the clock. It is impossible to inspect the escapement without stopping the clock. Part of the maintenance proposed during this stoppage is a thorough overhaul of the escapement and the other workings of the clock.

̽��ֱ��Westminster clock is a remarkable icon for London and for England. For more than 150 years it has reminded us of each hour, tirelessly. That’s what I love about clocks – they seem to carry on without a fuss. But every now and then even the most famous of clocks need a bit of tender loving care. After this period of pampering, “Big Ben” ought to be set for another 100 or so years of trouble-free running.

Hugh Hunt, Reader in Engineering Dynamics and Vibration, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

Hugh Hunt (Department of Engineering) discusses the mechanism that makes Big Ben chime, and why it needs repairing.

̽��ֱ��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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