Tag Archives: gravitational waves

At last, we’ve found gravitational waves from a collapsing pair of neutron stars

The Conversation

File 20171015 1505 1tylrql.jpg?ixlib=rb 1.1
Artist’s impression of the collision of two neutron stars, the source of the latest gravitational waves detected. National Science Foundation/LIGO/Sonoma State University/A. Simonnet, Author provided

David Blair, University of Western Australia

After weeks of rumour and speculation, scientists have today finally announced the death spiral of two neutron stars as a source of gravitational waves.

It’s among the biggest news for science in decades, because the findings help shed light on many aspects of astrophysics, including the origins of cosmic explosions known as gamma-ray bursts and of some heavy elements in the universe, such as gold.

The latest detection has scientists excited because most predictions had favoured the detection of gravitational waves from coalescing pairs of neutron stars. Yet the first and all subsequent detections prior to today’s announcement had only come from collisions of black holes.


Read more: We beat a cyber attack to see the ‘kilonova’ glow from a collapsing pair of neutron stars


The first detection

It was back in 2015 when the Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) detectors heard the whoop of the first gravitational wave signal ever detected.

The sound of two black holes colliding.
LIGO163 KB (download)

That came from the collision of a pair of black holes in the distant universe about 1.3 billion light years away. Suddenly we knew that our detectors worked; suddenly we knew that the black holes of Einstein’s theory are really out there. Suddenly the dream of gravitational wave astronomy became reality.

The first strong signal was so surprising that the international teams at the LIGO observatories spent weeks trying to work out if someone could have secretly put signals into the data!

Since then there have been more black hole signals, but there was no sign of the predicted neutron stars.

An artist’s conception of two merging black holes similar to those detected by LIGO. LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

The neutron star connection

Physicists have long considered neutron stars to be perfect sources of gravitational waves.

Neutron stars are balls of neutrons, about the size of a city but weighing in at about 1.4 times the mass of our Sun.

The first neutron star was discovered by Jocelyn Bell Burnell in 1967, and in 1974 Russell Hulse and Joseph Taylor found a pair of neutron stars spiralling slowly together in the Milky Way, a discovery that led to their Nobel Prize in Physics in 1993.

Caltech physicist Kip Thorne – one of three people awarded this year’s Nobel Prize for Physics – led a campaign to build huge laser interferometers, optimised for detecting the final death spiral of a pair of neutron stars.

Barry Barish (another of this year’s Nobel Prize winners) internationalised the LIGO observatories, bringing Britain, Germany and Australia into the collaboration.

More than just a wave

During the decades of development of gravitational wave detectors, astronomers had become fascinated by vast bursts of gamma rays coming in from the distant universe at the rate of about one every day.

Israeli physicist Tsvi Piran proposed in 1989 that some of these bursts could be created by coalescing neutron stars. If this was the case, then bursts of gravitational waves would be accompanied by bursts of gamma rays.

Many astrophysicists modelled the violent coalescence of merging neutron stars. Some of the superdense neutron rich matter would be flung into space, where it would be relieved of the massive pressure inside the neutron stars.

Uncompressed, it would go off like a vast nuclear fission bomb, creating a slew of heavy elements such as gold and platinum. Within minutes a hot fireball would shine brightly, powered by the decaying radioactivity of the new formed elements.

A new signal detected

Advanced LIGO‘s two 4km detectors in the United States have been operating since 2015. The 3km Advanced Virgo detector in Europe came online on August 1 this year.

Europe’s Virgo becomes the third detector in the hunt for gravitational waves. The Virgo collaboration

Many optical telescopes had signed up to receive any alerts from LIGO and Virgo.

Meanwhile, NASA’s orbiting gamma ray telescopes Fermi and Swift continued their continuous monitoring of the skies. Billions of dollars worth of astronomical hardware was poised and ready in August 2017.

Thursday August 17, 2017, was the day our detectors registered a slowly rising siren call that lasted for a minute and finished with a sharp crescendo.

It wasn’t the brief whoop of a pair of large black holes but the much slower death song of a pair of neutron stars with total mass about three times the mass of the Sun. Two seconds later the Fermi satellite detected a short gamma ray burst. Within minutes the source direction had been roughly localised.

The alert goes out

Within 30 minutes alerts went out to telescopes across the planet. Telescope schedules were interrupted, and before long a bright new object was found in galaxy NGC 4993, seen in the Hydra constellation, and visible in the southern hemisphere in August.

This simulation shows the final stages of the merging of two neutron stars.

The new object decayed away exponentially over a few days as might be expected for a radioactively powered nebula.

NGC 4993 is 130 million light years away. The arrival of gravity waves and gamma rays within 2 seconds of each other tells us that to a precision of a part in a million billion, both types of wave travel at the same speed.


Read more: After the alert: radio ‘eyes’ hunt the source of the gravitational waves


The fact that two completely different types of radiation, one that is a ripple of space itself, and the other that travels through space, should travel at exactly the same speed could seem astonishing, yet it is exactly what Einstein predicted.

The event is a treasure trove of astrophysics. From one faint gravitational sound, a momentary burst of gamma rays and the faint fading glow of exploding nuclear matter, we have the first direct measurement of the distance of galaxies.

This is because gravitational wave signals directly encode distance. And suddenly we know how gamma ray bursts are created. And suddenly we know that all our gold, our rings and treasures, was probably created in neutron star collisions.

The ConversationIt will take many years to fully explore the data, and meanwhile more and more data will flood in as we continue to open the gravitational wave spectrum with more observatories on earth and in space. The new era of multi-messenger astronomy has begun!

David Blair, Director, WA Node of the ARC Centre of Excellence for Gravitational Wave Discovery, and the Australian International Gravitational Research Centre, University of Western Australia

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

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2016: the year in space and astronomy

The Conversation

Alan Duffy, Swinburne University of Technology and Rebecca Allen, Swinburne University of Technology

The achievements of astrophysicists this year were as groundbreaking as they were varied. From reuniting a lander with a mothership on a comet, to seeing the most extreme cosmic events with gravitational waves, 2016 was truly out of this world for science.

Here are some of the highlights of the year that was.

1. Gravitational Waves

The spectacular announcement that ripples in the very fabric of spacetime itself had been found (and from surprisingly massive black holes colliding) sent similarly massive ripples through the scientific community. The discovery was made using the Laser Interferometer Gravitational-Wave Observatory (LIGO) and represents a fundamentally new sense with which to see the universe.

Animation showing how colliding black holes cause a ripple in spacetime that moves outwards into the universe as a gravitational wave.

The gravitational waves cause one arm of the LIGO detector to stretch relative to the other by less than a thousandth of the width of a proton in the centre of the atom. Relatively speaking, that’s like measuring a hair’s-width change in the distance to the nearest star.

This discovery was the end of a century-long quest to prove Einstein’s final prediction that these gravitational waves are real. It also allows us to directly “see” that famously and fundamentally invisible entity: the black hole (as well as definitively proving its existence). The fact that the two black holes collided 1.3 billion years ago and the waves swept through Earth just days after turning the detector on only add to the incredible story of this discovery.

The ‘sound’ of the black holes colliding where the measured signal from LIGO is converted to audio, the rising chirp sound towards the end is the two black holes spiralling together ever more quickly. A surprisingly wimpy sound for the most extreme collision ever detected.

2. SpaceX lands (and crashes) a rocket

The year started so well for SpaceX with the incredible achievement of sending a satellite into orbit, which is no mean feat itself at such low cost, before then landing that launch rocket on a barge in the ocean. A seemingly unstoppable sequence of launches and landings made it appear that a new era of vastly cheaper access to space through rockets that could be refuelled and reused was at hand.

A Falcon 9 first-stage automatically returns to the barge/droneship ‘Of Course I Still Love You’ in the middle of the Atlantic ocean.

Unfortunately, with the explosion of a Falcon 9 on the launchpad, the company was grounded, but apparently hopes for a resumed launch in early January.

SpaceX outlines a vision for travel to Mars with planned Interplanetary Transport System.

Add to that the visionary plans to settle Mars outlined by Elon Musk, albeit not without some audacious challenges, and it’s been a year of highs and lows for SpaceX.

3. Closest star may harbour Earth-like world

Proxima Centauri is our Sun’s nearest neighbour at just over four light years away, and it appears that its solar system may contain an Earth-like world. Until this year, astronomers weren’t even sure that any planets orbited the star, let alone ones that might harbour the best extrasolar candidate for life that spacecraft could visit within our lifetime.

What a trip to the Sun’s closet neighbour would look like.

The planet, creatively named “Proxima b”, was discovered by a team of astronomers at Queen Mary University in London. Using the light of Proxima Centauri, the astronomers were able to detect subtle shifts in the star’s orbit (seen as a “wobble”), which is the telltale sign that another massive object is nearby.

An artist’s impression of Proxima b’s landscape. ESO/M. Kornmesser

While Proxima Centauri is barely 10% the size of our Sun, Proxima b’s orbit is only 11 days long, meaning it is very close to the star and lies just within the so-called habitable zone. However, follow-up with either Hubble or the upcoming James Webb Space telescope is necessary to determine if the exoplanet is as well suited for life as Earth.

4. Breakthrough Listen listening and Starshot star-ted

With a potential Earth twin identified in Proxima b, now the challenge is to reach it within a human lifetime. With the breakthrough initiative starshot, which has been funded by Russian billionaire Yuri Milner and endorsed by none other than Stephen Hawking, lightweight nanosails can be propelled by light beams to reach speeds up to millions of kilometres an hour.

Such speeds would allow a spacecraft to arrive at Proxima b in about 20 years, thus enabling humans to send information to another known planet for the first time.

However, there are many challenges ahead, such as the fact that the technology doesn’t exist yet, and that high-speed collisions with gas and dust between stars may destroy it before it can reach its target.

But humans have proven to be resourceful, and key technology is advancing at an exponential rate. Incredibly the idea of sailing to another world is no longer science fiction, but rather an outrageously ambitious science project.

One of the founders of the Breakthrough initiatives, Yuri Milner, discusses the technology needed for breakthrough starshot.

Perhaps, aliens are already sending out their own information in the form of radio transmissions. In another breakthrough initiative called Listen, also championed by Hawking, astronomers will be searching the habitable zones around the million closest stars to try to detect incoming radio transmissions. Involving Australia’s very own Parkes telescope (as well as the Green Bank Telescope and Lick Observatory at visible wavelengths of light), observations have been running through 2016 and the search for alien signals will continue for the next decade.

5. Philae reunited with Rosetta

In 2014 the Philae lander became the first space probe to land on a comet, and even though its crash landing dictated that its science transmission would be a one-off, its recent rediscovery by Rosetta has allowed it to continue to contribute to analysis of comet 67P.

Philae’s crash location, as well as the orientation of the doomed probe, has allowed astronomers to accurately interpret data taken by Rosetta regarding the composition of the comet.

Where’s Philae? ESA

While Philae has literally been living under (crashed on) a rock for the past two years, Rosetta has been the busy bee, taking numerous images, spectroscopy and other data of the comet.

In fact, data taken from Rosetta’s spectrometer has been analysed and revealed that the amino acid, glycine, is present in the comet’s outgassing, which breaks away from the surface of the comet as it becomes unstable from solar heating. Glycine is one of the fundamental building blocks of life; necessary for proteins and DNA, and its confirmed extraterrestrial confirms that the ingredients for life are unique to Earth, and that we may have comets to thank for providing our microbial ancestors with those crucial ingredients.

Dust and gas emitted from comet 67P reveal an amino acid. ESA

Outlook for Down Under

The future for astrophysics in Australia in 2017 looks particularly bright, with two ARC Centres of Excellence: CAASTRO-3D studying the build of atoms over cosmic time; and OzGRav exploring the universe with gravitational waves; as well as SABRE, the world’s first dark matter detector in the Southern Hemisphere, installed by end of the year.

If you thought 2016 was a great year in space, then you’re in for a treat in 2017.

The ConversationAlan Duffy, Research Fellow, Swinburne University of Technology and Rebecca Allen, PhD candidate researching galaxy formation and evolution, Swinburne University of Technology

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

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Gravitational waves discovered: top scientists respond

The Conversation

Keith Riles, University of Michigan; Alan Duffy, Swinburne University of Technology; Amanda Weltman, University of Cape Town; Daniel Kennefick, University of Arkansas; David Parkinson, The University of Queensland; Maria Womack, University of South Florida; Stephen Smartt, Queen’s University Belfast; Tamara Davis, The University of Queensland, and Tara Murphy, University of Sydney

One hundred years ago, Albert Einstein published his general theory of relativity, which described how gravity warps and distorts space-time.

While this theory triggered a revolution in our understanding of the universe, it made one prediction that even Einstein doubted could be confirmed: the existence of gravitational waves.

Today, a century later, we have that confirmation, with the detection of gravitational waves by the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) detectors.

Here we collect reactions and analysis from some of the leading astronomers and astrophysicists from around the world.


Keith Riles, University of Michigan

Keith Riles explains gravitational waves.

Einstein was skeptical that gravitational waves would ever be detected because the predicted waves were so weak. Einstein was right to wonder – the signal detected on September 14, 2015 by the aLIGO interferometers caused each arm of each L-shaped detector to change by only 2 billionths of a billionth of a meter, about 400 times smaller than the radius of a proton.

It may seem inconceivable to measure such tiny changes, especially with a giant apparatus like aLIGO. But the secret lies in the lasers (the real “L” in LIGO) that are projected down each arm.

Fittingly, Einstein himself indirectly helped make those lasers happen, first by explaining the photoelectric effect in terms of photons (for which he earned the Nobel Prize), and second, by creating (along with Bose) the theoretical foundation of lasers, which create coherent beams of photons, all with the same frequency and direction.

In the aLIGO arms there are nearly a trillion trillion photons per second impinging on the mirrors, all sensing the precise positions of the interferometer mirrors. It is this collective, coherent sensing that makes it possible to determine that one mirror has moved in one direction, while a mirror in the other arm has moved in a different direction. This distinctive, differential motion is what characterizes a gravitational wave, a momentary differential warp of space itself.

By normally operating aLIGO in a mode of nearly perfect cancellation of the light returning from the two arms (destructive interference), scientists can therefore detect the passage of a gravitational wave by looking for a momentary brightening of the output beam.

The particular pattern of brightening observed on September 14 agrees remarkably well with what Einstein’s General Theory of Relativity predicts for two massive black holes in the final moments of a death spiral. Fittingly, Einstein’s theory of photons has helped to verify Einstein’s theory of gravity, a century after its creation.


Amanda Weltman, University of Cape Town

The results are in and they are breathtaking. Almost exactly 100 years ago Einstein published “Die Feldgleichungen der Gravitation” in which he laid out a new theory of gravity, his General Theory of Relativity. Einstein not only improved on his predecessor, Newton, by explaining the unexpected orbit of the planet Mercury, but he went beyond and laid out a set of predictions that have shaken the very foundations of our understanding of the universe and our place in it. These predictions include the bending of light leading to lensed objects in the sky, the existence of black holes from which no light can escape as well as the entire framework for our modern understanding of cosmology.

NASA’s Hubble Space Telescope captured gravitational lensing of light, as predicted by Einstein.
NASA, ESA, K. Sharon (Tel Aviv University) and E. Ofek (Caltech), CC BY

Einstein’s predictions have so far all proven true, and today, the final prediction has been directly detected, that of gravitational waves, the tiniest ripples through space; the energy radiated away by two massive heavenly bodies spiralling into each other. This is the discovery of the century, and it is perhaps poetic that one of the places it is being announced is Pisa, the very place where, according to legend, 500 years ago, Galileo dropped two massive objects to test how matter reacts to gravity.

As we bathe in the glory of this moment it is appropriate to ask, what is next for astronomy and physics and who will bring about the next revolution? Today’s discovery will become tomorrow’s history. Advanced LIGO brings a new way of testing gravity, of explaining the universe, but it also brings about the end of an era of sorts. It is time for the next frontier, with the Square Kilometre Array project finally afoot across Africa and Australia, the global South and indeed Africa itself is poised to provide the next pulse of gravity research.


Stephen Smartt, Queen’s University Belfast

Not only is this remarkable discovery of gravitational waves an extraordinary breakthrough in physics, it is a very surprising glimpse of a massive black hole binary system, meaning two black holes that are merging together.

Black holes are dark objects with a mass beyond what is possible for neutron stars, which are a type of very compact stars – about 10 km across and weighing up to two solar masses. To imagine this kind of density, think of the entire human population squeezed onto a tea spoon. Black holes are even more extreme than that. We’ve known about binary neutron stars for years and the first detection of gravitational waves were expected to be two neutron stars colliding.

What we know about black hole pairs so far comes from the study of the stars orbiting around them. These binary systems typically have black holes with masses five to 20 times that of the sun. But LIGO has seen two black holes with about 30 times the mass of the sun in a binary system that has finally merged. This is remarkable for several reasons. It is the first detection of two merging black holes, it is at a much greater distance than LIGO expected to find sources, and the total mass in the system is also much larger than expected.

This raises interesting questions about the stars that could have produced this system. We know massive stars die in supernovae, and most of these supernovae (probably at least 60%) produce neutron stars. The more massive stars have very large cores that collapse and are too massive to be stable neutron stars so they collapse all the way to black holes.

But a binary system with two black holes of around 30 solar masses is puzzling. We know of massive binary star systems in our own and nearby galaxies, and they have initial masses well in excess of 100 suns. But we see them losing mass through enormous radiation pressure and they are predicted, and often observed, to end their lives with masses much smaller – typically about ten times the sun.

If the LIGO object is a pair of 30 solar mass black holes, then the stars that formed it must have been at least as massive. Astronomers will be asking – how can massive stars end their lives so big and how can they create black holes so massive? As well as the gravitational wave discovery, this remarkable result will affect the rest of astronomy for some time.


Alan Duffy, Swinburne University

The detection of gravitational waves is the confirmation of Albert Einstein’s final prediction and ends a century-long search for something that even he believed would remain forever untested.

This discovery marks not the end, but rather the beginning, of an era in which we explore the universe around us with a fundamentally new sense. Touch, smell, sight and sound all use ripples in an electromagnetic field, which we call light, but now we can make use of ripples in the background field of space-time itself to “see” our surroundings. That is why this discovery is so exciting.

The Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) measured the tiny stretching of space-time by distant colliding black holes, giving them a unique view into the most extreme objects in general relativity.

The exact “ringing” of space-time as the ripples pass through the detector test this theory and our understanding of gravity in ways no other experiment can.

We can even probe the way galaxies grow and collide by trying to measure the gravitational waves from the even larger collisions of supermassive black holes as the galaxies they are contained in smash together.

Australia in particular is a leading nation in this search, using distant pulsars as the ruler at the Parkes telescope.

The LIGO detectors are sensitive to the minute ripples in space-time caused by the merging of two black holes.
University of Birmingham Gravitational Waves Group, Christopher Berry


Tara Murphy, University of Sydney

In addition to binary black holes, aLIGO will detect gravitational waves from other events such as the collision of neutron stars, which are the dense remnants left over when a massive stars collapse.

Astronomers think that two neutron stars colliding may trigger a gamma-ray burst, which we can detect with “regular” telescopes.

Simulation of neutron stars colliding. Credit: NASA

In Australia, we have been using the Murchison Widefield Array and the Australian Square Kilometre Array Pathfinder) to follow-up aLIGO candidates.

aLIGO is an incredibly sensitive instrument but it has very poor ability to determine where the gravitational waves are coming from. Our radio telescopes can scan large areas of sky extremely quickly, so can play a critical part in identifying the event.

This project has been like no other one I have worked on. When aLIGO identifies a candidate, it sends out a private alert to an international network of astronomers. We respond as quickly as possible with our telescopes, scanning the region the event is thought to have occurred in, to see if we can detect any electromagnetic radiation.

Everything is kept top secret – even the other people using our telescopes are not allowed to know where we are pointing them.

To make sure their complex processing pipeline was working correctly, someone in the aLIGO team inserted fake events into the process. Nobody on the team, or those of us doing follow-up, had any idea whether what we were responding to was real or one of these fake events.

We are truly in an era of big science. This incredible result has been the work of not only hundreds of aLIGO researchers and engineers, but hundreds more astronomers collaborating around the globe. We are eagerly awaiting the next aLIGO observing run, to see what else we can find.


Tamara Davis, University of Queensland

Rarely has a discovery been so eagerly anticipated.

When I was a university undergraduate, almost 20 years ago, I remember a physics lecturer telling us about the experiments trying to detect gravitational waves. It felt like the discovery was imminent, and it was one of the most exciting discoveries that could be made in physics.

Mass and energy warping the fabric of space is one of the pieces of general relativity that most captures the imagination. However, while it has enormous explanatory power, the reality of that curvature is hard to grasp or confirm.

For the last few months I’ve had to sit quietly and watch as colleagues followed up the potential gravitational wave signal. This is the one and only time in my scientific career that I wasn’t allowed to talk about a scientific discovery in progress.

But that’s because it is such a big discovery that we had to be absolutely sure about it before announcing it, lest we risk “crying wolf”.

Every last check had to be done, and of course, we didn’t know whether it was a real signal, or a signal injected by the experimenters to keep us on our toes, test the analysis and follow-up.

I work with a project called the Dark Energy Survey, and with our massive, wide-field, half-billion pixel camera on a four metre telescope in Chile, my colleagues took images trying to find the source of the gravitational waves.

The wide-field is important, because the gravitational wave detectors aren’t very good at pinpointing the exact location of the source.

Unfortunately if it was a black hole merger, we wouldn’t expect to see any visible light.

Now that we’re in the era of detecting gravitational waves, though, we’ll be able to try again with the next one.


Maria Womack, University of South Florida

This is a momentous change for astronomy. Gravitational-wave astronomy can now truly begin, opening a new window to the universe. Normal telescopes collect light at different wavelengths, such as Xray, ultraviolet, visible, infrared and radio, collectively referred to as electromagnetic radiation (EM). Gravitational waves are emitted from accelerating mass analogous to the way electromagnetic waves are emitted from accelerating charge; both are emitted from accelerating matter.

The most massive objects with the highest accelerations will be the first events detected. For example, Advanced LIGO, funded by the U.S. National Science Foundation, can detect binary black holes in tight, fast orbits. GWs carry away energy from the orbiting pair, which in turn causes the black holes to shrink their orbit and accelerate even more, until they merge in a violent event, which is now detectable on Earth as a whistling “chirp.”

An example signal from an inspired gravitational wave source.
A. Stuver/LIGO, CC BY-ND

The gravitational-wave sky is completely uncharted, and new maps will be drawn that will change how we think of the universe. GWs might be detected coming from cosmic strings, hypothetical defects in the curvature of space-time. They will also be used to study what makes some massive stars explode into supernovae, and how fast the universe is expanding. Moreover, GW and traditional telescopic observing techniques can be combined to explore important questions, such as whether the graviton, the presumed particle that transmits gravity, actually have mass? If massless, they will arrive at the same time as photons from a strong event. If gravitons have even a small mass, they will arrive second.


Daniel Kennefick, University of Arkansas

Almost 100 years ago, in February 1916, Einstein first mentioned gravitational waves in writing. Ironically it was to say that he thought they did not exist! Within a few months he changed his mind and by 1918 had published the basis of our modern theory of gravitational waves, adequate to describe them as they pass by the Earth. However his calculation does not apply to strongly gravitating systems like a binary black hole.

Albert Einstein was the original theorist who started the hunt for gravitational waves.

It was not until 1936 that Einstein returned to the problem, eventually publishing one of the earliest exact solutions describing gravitational waves. But his original sceptical attitude was carried forward by some of his former assistants into the postwar rebirth of General Relativity. In the 1950s, doubts were expressed as to whether gravitational waves could carry energy and whether binary star systems could even generate them.

One way to settle these disputes was to carry out painstaking calculations showing how the emission of gravitational waves affected the motion of the binary system. This proved a daunting challenge. Not only were the calculations long and tedious, but theorists found they needed a much more sophisticated understanding of the structure of space-time itself. Major breakthroughs included the detailed picture of the asymptotic structure of space-time, and the introduction of the concept of matched asymptotic expansions. Prior to breakthroughs such as these, many calculations got contradictory results. Some theorists even got answers that the binary system should gain, not lose, energy as a result of emitting gravitational waves!

While the work of the 1960s convinced theorists that binary star systems did emit gravitational waves, debate persisted as to whether Einstein’s 1918 formula, known as the quadrupole formula, correctly predicted the amount of energy they would radiate. This controversy lasted into the early 1980s and coincided with the discovery of the binary pulsar which was a real-life system whose orbit was decaying in line with the predictions of Einstein’s formula.

In the 1990s, with the beginnings of LIGO, theorists’ focus shifted to providing even more detailed corrections to formulas such as these. Researchers use descriptions of the expected signal as templates which facilitate the extraction of the signal from LIGO’s noisy data. Since no gravitational wave signals had ever been seen before, theorists found themselves unusually relevant to the detection project – only they could provide such data analysis templates.


David Parkinson, University of Queensland

Gravitational waves can be used to provide a direct probe of the very early universe. The further away we look, the further back in time we can see. But there is a limit to how far back we can see, as the universe was initially an opaque plasma, and remained so even as late as 300,000 years after the Big Bang.

This surface, from which the cosmic microwave background is emitted, represents the furthest back any measurement of electromagnetic radiation can directly investigate.

But this plasma is no impediment for gravitational waves, which will not be absorbed by any intervening matter, but come to us directly. Gravitational waves are predicted to be generated by a number of different mechanisms in the early universe.

For example, the theory of cosmic inflation, which suggests a period of accelerated expansion moments after the Big Bang, goes on to predict not just the creation of all structure that we see in the universe, but also a spectrum of primordial gravitational waves.

It is these primordial gravitational waves that the BICEP2 experiment believed it had detected in March 2014.

BICEP2 measured the polarisation pattern of the cosmic microwave background, and reported a strong detection of the imprint of primordial gravitational waves. These results turned out in fact to be contamination by galactic dust, and not primordial gravitational waves.

But there is every reason to believe that future experiments may be able detect these primordial gravitational waves, either directly or indirectly, and so provide a new and complementary way to understand the physics of the Big Bang.

The ConversationKeith Riles, Professor of Physics, University of Michigan; Alan Duffy, Research Fellow, Swinburne University of Technology; Amanda Weltman, SARChI in Physical Cosmology, Department of Mathematics and Applied Mathematics, University of Cape Town; Daniel Kennefick, Associate Professor of Physics, University of Arkansas; David Parkinson, Researcher in astrophysics, The University of Queensland; Maria Womack, Research Professor of Physics, University of South Florida; Stephen Smartt, Professor of Physics and Mathematics, Queen’s University Belfast; Tamara Davis, Professor, The University of Queensland, and Tara Murphy, Associate Professor and ARC Future Fellow, University of Sydney

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

 

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Timeline: the history of gravity

The Conversation

Geraint Lewis, University of Sydney

Our understanding of gravity has gone through a few permutations, from Newton’s equations through to Einstein’s general relativity. With today’s discovery of gravitational waves, we look back on how our grasp of gravity has evolved over the centuries.


1687: Newtonian gravity

Isaac Newton publishes Philosophiae Naturalis Principia Mathematica, giving a comprehensive account of gravity. This gave astronomers an accurate toolbox for predicting the motions of planets. But it was not without its problems, such as calculating the precise orbit of the planet Mercury.

All planets’ orbits precess – with the closest point of their orbit moving slightly with each revolution – due to the gravitational tugs from other planets.

Wes Mountain/The Conversation, CC BY-ND

The issue with Mercury’s orbit was that the amount of precession did not match what Newton’s theory predicted. It was only a small discrepancy, but big enough for astronomers to know it was there!

Wes Mountain/The Conversation, CC BY-ND

1859: Planet Vulcan

To explain Mercury’s odd behaviour, Urbain Le Verrier proposed the existence of an unseen planet called Vulcan, which orbited closer to the sun. He suggested that the gravity from Vulcan was influencing Mercury’s orbit. But repeated observations revealed no signs of Vulcan.

Wes Mountain/The Conversation, CC BY-ND

1905: Special relativity

Albert Einstein shakes up physics with his special theory of relativity. He then started incorporating gravity into his equations, which led to his next breakthrough.

1907: Einstein predicts gravitational redshift

What we now call gravitational redshift was first proposed by Einstein from his thoughts in the development of general relativity.

Wes Mountain/The Conversation, CC BY-ND

Einstein predicted that the wavelength of light coming from atoms in a strong gravitational field will lengthen as it escapes the gravitational force. The longer wavelength shifts the photon to the red end of the electromagnetic spectrum.

1915: General relativity

Albert Einstein publishes general theory of relativity. The first great success was its accurate prediction of Mercury’s orbit, including its previously inscrutable precession.

The theory also predicts the existence of black holes and gravitational waves, although Einstein himself often struggled to understand them.

Wes Mountain/The Conversation, CC BY-ND

1917: Einstein theorises stimulated emission

In 1917, Einstein publishes a paper on the quantum theory of radiation indicating stimulated emission was possible.

Einstein proposed that an excited atom could return to a lower energy state by releasing energy in the form of photons in a process called spontaneous emission.

In stimulated emission, an incoming photon interacts with the excited atom, causing it to move to a lower energy state, releasing photons that are in phase and have the same frequency and direction of travel as the incoming photon. This process allowed for the development of the laser (light amplification by stimulated emission of radiation).

1918: Prediction of frame dragging

Josef Lense and Hans Thirring theorise that the rotation of a massive object in space would “drag” spacetime around with it.

1919: First observation of gravitational lensing

Gravitational lensing is the bending of light around massive objects, such as a black hole, allowing us to view objects that lie behind it. During a total solar eclipse in May 1919, stars near the sun were observed slightly out of position. This indicated that light was bending due to the sun’s mass.

Wes Mountain/The Conversation, CC BY-ND

1925: First measurement of gravitational redshift

Walter Sydney Adams examined light emitted from the surface of massive stars and detected a redshift, as Einstein predicted.

1937: Prediction of a galactic gravitational lensing

Swiss astronomer Fritz Zwicky proposed that an entire galaxy could act as a gravitational lens.

1959: Gravitational redshift verified

The theory was conclusively tested by Robert Pound and Glen Rebka by measuring the relative redshift of two sources at the top and bottom of Harvard University’s Jefferson Laboratory tower. The experiment accurately measured the tiny change in energies as photons travelled between the top and the bottom.

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1960: Laser invented using stimulated emission

Theodore H. Maiman, a physicist at Hughes Research Laboratories in California, builds the first laser.

1960s: First evidence for black holes

The 1960s was the beginning of the renaissance of general relativity, and saw the discovery of galaxies that were powered by the immense pull of black holes in their centres.

There is now evidence of massive black holes in the hearts of all large galaxies, as well as there being smaller black holes roaming between the stars.

1966: First observation of gravitational time delays

American astrophysicist Irwin Shapiro proposed that if general relativity is valid, then radio waves will be slowed down by the sun’s gravity as they bounce around the solar system.

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The effect was observed between 1966-7 by bouncing radar beams off the surface of Venus and measuring the time taken for the signals to return to Earth. The delay measured agreed with Einstein’s theory.

We now use time-delays on cosmological scales, looking at the time differences in flashes and flares between gravitationally lensed images to measure the expansion of the universe.

1969: False detection of gravitational waves

American physicist Joseph Weber (a bit of a rebel) claimed the first experimental detection of gravitational waves. His experimental results were never reproduced.

Wes Mountain/The Conversation, CC BY-ND

1974: Indirect evidence for gravitational waves

Joseph Taylor and Russell Hulse discover a new type of pulsar: a binary pulsar. Measurements of the orbital decay of the pulsars showed they lost energy matching the amounts predicted by general relativity. They receive the 1993 Nobel Prize for Physics for this discovery.

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1979: First observation of a galactic gravitational lens

The first extragalactic gravitational lens was discovered, when observers Dennis Walsh, Bob Carswell and Ray Weymann saw two identical quasi-stellar objects, or “quasars”. It turned out to be one quasar that appears as two separate images.

Since the 1980s, gravitational lensing has become a powerful probe of the distribution of mass in the universe.

1979: LIGO receives funding

US National Science Foundation funds construction of the Laser Interferometer Gravitational-Wave Observatory (LIGO).

1987: Another false alarm for gravitational waves

A false alarm on direct detection from Joseph Weber (again) with claimed signal from the supernova SN 1987A using his torsion bar experiments, which consisted of large aluminium bars designed to vibrate when a large gravitational wave passed through it.

1994: LIGO construction begins

It took a long time, but the construction of LIGO finally began in Hanford, Washington, and Livingston, Louisiana.

 

2002: LIGO starts first search

In August 2002, LIGO starts searching for evidence of gravitational waves.

2004: Frame dragging probe

NASA launches Gravity Probe B to measure the spacetime curvature near the Earth. The probe contained gyroscopes that rotated slightly over time due to the underlying spacetime. The effect is stronger around a rotating object which “drags” spacetime around with it.

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The gyroscopes in Gravity Probe B rotated by an amount consistent with Einstein’s theory of general relativity.

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2005: LIGO hunt ends

After five searches, the first phase of LIGO ends with no detection of gravitational waves. The sensors then undergo an interim refit to improve sensitivity, called Enhanced LIGO.

2009: Enhanced LIGO

An upgraded version called Enhanced LIGO starts new hunt for gravitational waves.

2010: Enhanced LIGO hunt ends

Enhanced LIGO fails to detect and gravitational waves. A major upgrade, called Advanced LIGO begins.

2014: Advanced LIGO upgrade completed

The new Advanced LIGO has finished installation and testing and is nearly ready to begin a new search.

2015: False alarm #3 for gravitational waves

The indirect signature of gravitational waves in the early universe was claimed by the BICEP2 experiment, looking at the cosmic microwave background. But it looks like this was dust in our own galaxy spoofing the signal.

2015: LIGO upgraded again

Advanced LIGO starts a new hunt for gravitational waves with four times the sensitivity of the original LIGO. In September, it detects a signal that looks likely to be from the collision between two black holes.

2016: Gravitational wave detection confirmed

After rigorous checks, the Advanced LIGO team announce the detection of gravitational waves.

Wes Mountain/The Conversation, CC BY-ND

The ConversationGeraint Lewis, Professor of Astrophysics, University of Sydney

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

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ASU’s Krauss hails discovery, which he predicted, as important as the invention of the telescope

Everything shifted this morning.

In the 100th-anniversary year of Einstein’s theory of relativity, scientists announced they have proved it.

Using a stunning display of technological prowess, a group of physicists measured gravitational waves, a ripple in the fabric of space caused by the collision of two immense objects far out in the universe.

The discovery is on par with the invention of the telescope, said Lawrence Krauss, a theoretical physicist and cosmologist at Arizona State University.

“It heralds what I think is the beginning of the new astronomy for the 21st century,” Krauss said. “Gravitational-wave astronomy will be the astronomy of the 21st century. It’s opened a new window on the universe, just like the telescope in some sense or when we first used radio waves to explore the universe.”

Researchers at the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), a joint project between the Massachusetts Institute of Technology and the California Institute of Technology, used two detectors at opposite ends of the country to measure a change in length down to a tolerance of one ten-thousandth of a proton.

“Using gravitational waves to explore the universe will allow us to see things we could have never seen before,” Krauss said. “We’ll be exploring science in a domain we’ve never seen before. It will also allow us to explore objects in the universe we’ve never seen before.”

The LIGO experiment observed the collision of two black holes. Black holes are at the center of virtually every large galaxy, and their dynamics may be related to the dynamics of galaxy formation. It’s a chicken-and-egg question: Which formed first?

Two incredibly immense black holes collided, converting a mass three times the size of the sun into energy in a single second, sending out a ripple in space and time.

“It allows us to see things that are just truly mind-boggling,” Krauss said. “A black hole with a mass 39 times the mass of our sun collides with another black hole 26 times the mass of the sun, comes together to make one big black hole that’s 62 times the mass of the sun. If you do your addition, 62 is not 39 plus 26, it’s three solar masses smaller. Three solar masses of energy went in a second into gravitational waves. … Our sun over 10 billion years is only going to convert a small fraction of its mass into energy by burning 100 million hydrogen bombs every second. But in one second or so, in a very short time — BOOM — three times the mass of the sun was converted by E=mc² into energy. It’s unfathomable. It just disappeared. Imagine our sun just disappearing in a second.”

“Gravitational-wave astronomy will be the astronomy of the 21st century. It’s opened a new window on the universe, just like the telescope in some sense or when we first used radio waves to explore the universe.” — tweet by Lawrence Krauss, ASU theoretical physicist and cosmologist

They fired lasers down the arms and bounced them off mirrors at either end. The laser path lengths are equal under normal circumstances, but not when a gravitational wave passes through.

“It’s a testament to human perseverance and ingenuity,” Krauss said. “What was required to build a detector to detect gravitational waves is unbelievable.”

It’s like being in California and detecting a leaf falling in Virginia. The system is so sensitive a truck hitting a pothole miles away threw it off in the early years when it started operating in 2002. The arms are so long that the curvature of the Earth is a measurable 1 meter (vertical) difference over the 4-kilometer length of each arm. “The most precise concrete pouring and leveling imaginable was required to counteract this curvature and ensure that LIGO’s vacuum chambers were truly ‘flat’ and level,” the lab’s website said. The detectors are so precise continental drift had to be taken into consideration, Krauss said.

“They had to be able to measure the change in length of a 4-kilometer-long tunnel by an amount equal to one ten-thousandth the size of a proton,” he said. “When you say that, it’s just amazing. It’s just amazing that human beings could do that. They had to push quantum technology to its limits. Even the quantum fluctuations of atoms in the mirror are such that even those have to be controlled. It’s just amazing what they can do. It’s proof that truth is stranger than fiction. Science-fiction writers wouldn’t dare to even propose it, but it’s been done. It’s taken 20 years of hard work by thousands of physicists; there are more than 1,000 people working on that collaboration.”

LIGO will allow the laws of physics to be tested in domains never seen before, like the event horizon of black holes, “which is that region inside of which you never get out, and which, if you’re near, strange things happen — if you’ve seen the movie ‘Interstellar,’ time dilates and everything else,” Krauss said. “It’ll be a whole new type of astronomy.”

He appreciated the poetics of the discovery happening in the anniversary year of relativity.

“It’s beautifully fitting that on the 100th anniversary of the development of general relativity, when Einstein first proposed the existence of gravitational waves, that they’ve finally been directly discovered,” he said. “It’s superlatives all over. It’s an amazing piece of work by an amazing group of scientists who were dedicated to doing something that appeared impossible, to discover something that opens a new window on the universe. And every time we open a new window on the universe, we’ve been surprised. I’m sure there will be surprises.”

Krauss has taken abuse from some quarters for teasing the announcement on his Twitter feed, once this past September and again in January. (One astrophysicist claimed to be “appalled” by the tweets.) Krauss thought drumming up interest in a major discovery was the right thing to do.

“If scientists are excited, I didn’t see why the public shouldn’t be,” he said. “No one on the project told me anything in confidence. I just heard the rumor, and it turned out to be true. … The net result was hundreds of articles are being prepared now for this result that wouldn’t be there if I hadn’t in some sense laid the groundwork. … I think I’d say I was doing God’s work, if I believed in God.”

Reblogged from Arizona State University weh site.

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