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  • richardmitnick 1:45 pm on March 29, 2017 Permalink | Reply
    Tags: , , , , , Gravitational waves,   

    From aeon: “Echoes of a black hole” 



    An infrared image from NASA’s Spitzer Space Telescope shows the centre of the Milky Way galaxy where the brightest white spot marks the site of a supermassive black hole

    Sabine Hossenfelder

    A billion years ago, two dancing black holes make a final spin, merge, and – in a matter of seconds – release a cataclysmic amount of energy. Much as a falling pebble spreads waves on the surface of a still lake, the merger initiates gravitational waves in the space-time continuum. Fast-forward to planet Earth and the year 2015. After an immense journey, the gravitational waves from the black-hole merger pass through our solar system. On the morning of 14 September, they oh-so-slightly wiggle the arms of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in Louisiana and Washington state.

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

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

    Niayesh Afshordi at the University of Waterloo in Canada first heard of LIGO’s seminal detection over lunch in a bistro. It was late 2015 and still weeks to go until the results were officially released. But rumours were buzzing, and a colleague who had seen the unpublished paper spilled the beans. Afshordi, an astrophysicist who also works at the Perimeter Institute in Waterloo, instantly appreciated the importance of the news – both for the physics community at large, and for his own unconventional theory about the construction of the Universe.

    ‘I had an existential crisis at some point. I thought all the problems in cosmology had been solved,’ Afshordi recalled. ‘But then I came up with this idea that dark energy is made by black holes.’ Studies of distant stellar explosions and other lines of evidence show that our universe grows at an accelerating pace, but nobody knows the cause. Matter alone cannot have this effect, so cosmologists blame the expansion on a peculiar type of energy, called dark energy. Its origin and nature were, and are, a mystery.

    In 2009, Afshordi, together with his colleagues Chanda Prescod-Weinstein and Michael Balogh, put forward a theory according to which black holes seed a long-range field that mimics dark energy. The field spills out from black holes and spans through the Universe. It’s an intriguing explanation for the origin of dark energy and, by Afshordi’s calculations, the number of black holes estimated to exist should create just about the right amount of field energy to fit the observations.

    But Afshordi’s idea overthrows what physicists believed they knew about black holes. In Albert Einstein’s theory of general relativity, the event horizon of a black hole – the surface beyond which there is no escape – is insubstantial. Nothing special happens upon crossing it, just that there is no turning around later. If Afshordi is right, however, the inside of the black hole past the event horizon no longer exists. Instead, a Planck-length away from where the horizon would have been, quantum gravitational effects become large, and space-time fluctuations go wild. (The Planck length is a minuscule distance: about 10-35 metres, or 10-20 times the diameter of a proton.) It’s a complete break with relativity.

    When he heard of the LIGO results, Afshordi realised that his so-far entirely theoretical idea could be observationally tested. If event horizons are different than expected, the gravitational-wave bursts from merging black holes should be different, too. Events picked up by LIGO should have echoes, a subtle but clear signal that would indicate a departure from standard physics. Such a discovery would be a breakthrough in the long search for a quantum theory of gravity. ‘If they confirm it, I should probably book a ticket to Stockholm,’ Afshordi said, laughing.

    Quantum gravity is the missing unification of general relativity with the quantum field theories of the standard model of particle physics. If just thrown together, the two theories lead to internal contradictions, and fail to make sense. Black holes are one of the most studied examples for such a contradiction. Use quantum field theory near the horizon and you find that the black hole emits particles, slowly evaporating. Those particles carry away mass but, as Stephen Hawking demonstrated in the 1970s, they cannot carry information about what formed the black hole. And so, if the black hole evaporates entirely, all the information about what fell in has been destroyed. In quantum field theory, however, information is always preserved. Something in the mathematics, therefore, doesn’t fit correctly.

    The culprit, most physicists think, is that the calculation doesn’t take into account the quantum behaviour of space and time because the theory for this – quantum gravity – is still unknown. For decades, physicists thought that the quantum gravitational effects necessary to solve the black-hole conundrum were hidden behind the event horizon. They thought that it is only near the singularity, at the centre of the black hole, that the effects of quantum gravity become relevant. But recently, they have had to rethink.

    In 2012, a group of researchers from the University of California, Santa Barbara, found an unexpected consequence of the currently favoured idea that information somehow escapes with the radiation from a black hole. To make the idea work, large deviations from general relativity are required, not only near the singularity but also at the event horizon. Those deviations would create what the researchers dubbed a ‘black hole firewall’, a barrier of high energy just outside the horizon.

    Such a firewall (if it exists) would become noticeable only for an infalling observer, and would not emit observable signatures that could show up in our telescopes. However, the firewalls lent support to Afshordi’s earlier idea that black holes create a field that acts as dark energy. If that was so, then the near-horizon region of black holes should be very different from what general relativity predicts; a firewall that solves the black hole information-loss problem could be one effect of that deviation. Afshordi’s proposal for how to modify general relativity might therefore hold the key to resolving the tension between quantum theory and general relativity. It was an idea that wouldn’t let him go.

    When he learned of LIGO’s first detection, Afshordi began to explore whether the gravitational waves emitted from a black-hole merger could reveal intimate details about what happens near the black-hole horizon. At first it seemed too much to hope for. ‘I didn’t really think we could see quantum gravity effects in the gravitational-wave signal because we had already looked in so many places,’ Afshordi said. ‘But I changed my mind about this.’

    What made Afshordi reconsider was work by Vítor Cardoso and colleagues at the Instituto Superior Técnico in Portugal on gravitational-wave echoes from black holes. Cardoso had laid out on general grounds that a merger of two objects that are compact but do not have an event horizon would produce gravitational waves very similar to those of black holes – similar, but not identical. The key feature indicating the horizon’s absence, Cardoso argued, would be a periodic recurrence in the signal from the merger. Instead of a single peak followed by a ringdown (think a big splash on a pond, and then rapidly dissipating ripples), the gravitational waves should come as a series of fading pulses – fainter echoes of the original event. Afshordi found that the near-horizon modification described by his theory would cause exactly such echoes. Moreover, he could calculate their recurrence time as a function of the final black hole’s mass, allowing a precise prediction.

    Nobody had ever sought such a signal before, and finding it would not be easy. So far there are only two public, well-defined gravitational-wave detections from LIGO. Together with a collaborator, Afshordi analysed the LIGO data for traces of echoes. By comparing the openly available recordings to random noise, they found an echo at the calculated recurrence time. The statistical significance is not high, however. In scientific terms, it has an estimated significance of 2.9 sigma. Such a signal can be caused by pure noise with a chance of about a one-in-200. In physics, an event of such low confidence is interesting but does not amount to a discovery.

    The LIGO experiment is really just getting started, however. The most remarkable thing about the first two gravitational wave events is that the facilities were able to record them at all. The technological challenges were tremendous. Each site, both in Louisiana and in Washington state, has an interferometer with two perpendicular arms about 4 kilometres long in which a laser beam bounces back and forth between mirrors; when recombined, the beams interfere with each other. Interference of the laser’s light-waves is sensitive to deformations in the arms’ relative length as little as a thousandth the diameter of a proton. That is the level of sensitivity required to pick up gravitational effects of colliding black holes.

    A gravitational wave passing through the interferometer deforms both arms at different times, thereby skewing the interference pattern. Requiring an event to be recorded at both sites provides protection against false alarms. By design, LIGO detects gravitational waves best at wavelengths of hundreds to thousands of kilometres, the range expected for black-hole mergers. Other gravitational-wave detectors are planned to cover different parts of the spectrum, each tuned to different types of phenomena.

    Gravitational waves are an unavoidable prediction of general relativity. Einstein recognised that space-time is dynamic – it stretches, it curves, and it wiggles in response to gravitational disturbances. When it wiggles, the waves can travel freely into the far distance, carrying away energy and manifesting themselves by a periodic expansion and contraction of space in orthogonal directions. We have long had indirect evidence for gravitational waves. Because they carry away energy, they cause a small but measurable decay in the mutual orbit of binary pulsars. This effect was first observed in the 1970s, and was awarded a Nobel Prize in 1993. But until LIGO’s detection, we had no direct evidence for the existence of gravitational waves.

    LIGO’s first event – the September 2015 detection that so excited Afshordi – was remarkable, and not only because it happened just a few days after a long-planned instrumental upgrade. It stood out also because the merging black holes were so heavy, with masses estimated at 29 and 36 times the Sun’s mass. ‘A lot of people expected the black hole events to have lower masses,’ said Ofek Birnholtz, a member of the LIGO collaboration’s group on compact binary coalescence and a physicist at the Max Planck Institute in Germany. The strikingly clean signal, together with the collaboration’s openness in sharing the data, has been an inspiration for physicists in other communities who, like Afshordi, are now exploring how to use the new observations for their own work.

    On 26 December 2015, LIGO recorded a second event. The age of gravitational-wave astronomy had officially begun, after many years of slow progress and false starts. ‘Some of my PhD colleagues had left the field of gravitational-wave astronomy,’ Birnholtz said and added, laughing, ‘but are returning because suddenly it’s hot again.’ This is uncharted territory, basic research at its finest. What kind of black hole and compact stellar systems are there? Where are they within the galaxies? What do the gravitational waves reveal about their origins? If a neutron star merges with a black hole, what can be learned about matter in such extreme conditions? Do black holes behave the way that our calculations predict?

    Afshordi’s theory of black holes and dark energy is just one example of the kinds of enquiries that are now possible. A wealth of information is waiting to be explored, openly, around the world.

    A few days after Afshordi’s result appears on the open-access server arXiv.org, members of the LIGO collaboration scrutinise the analysis. It takes only a few weeks until they publish a reply, criticise the methodology, and call for different statistical tools. Birnholtz is one of the authors of that criticism.

    ‘The claim is surprising,’ said Birnholtz. ‘I have no prior as to whether or not there should be echoes. That’s physics nobody can guess at. But I do have a strong intuition, working with LIGO data, that the amplitude is probably not large enough to claim such a significance at this stage.’ Birnholtz has suggestions for how to improve the analysis, but avoids making statements about the chances of confirming the result. Alex Nielsen, another member of the LIGO collaboration and one of Birnholtz’s co-authors, reiterates the need for caution: ‘As members of the LIGO collaboration, we have to be very careful about what statements we make in public, before we have full collaboration approval. But the data is public and people can do with it what they want.’

    The LIGO collaboration has an open science centre, where data recorded for one hour around the time of confirmed gravitational events is publicly available. ‘People are welcome to use it and contact us for any questions,’ Birnholtz said. ‘If they find anything interesting, they can share it with us, and we can work on it together. This is part of the scientific experience.’

    The collaboration has several thousand members worldwide, distributed at more than a hundred institutions. They meet twice a year; the most recent meeting was in March in Pasadena, California. Some members of the collaboration are now trying to reproduce Afshordi’s analysis. Birnholtz expects the effort to take several months. ‘The result might be disappointing,’ he warned. ‘Not in that it says there are no echoes, but that we can’t say whether there are echoes.’ Gravitational-wave astronomy is still a field in its infancy, though, and much more data are on the way. The collaboration estimates that by the completion of the third observing run in 2018, LIGO is likely to have made 40 high-quality detections of black-hole mergers. Each will offer another opportunity to test Afshordi’s theory.

    Because they interact so weakly and deposit so little energy as they pass by, gravitational waves are exceedingly difficult to measure. The deformation they cause is tiny, and enormous care is necessary to extract a clean signal. The discovery threshold used by the LIGO collaboration is 5 sigma, corresponding to a chance of less than one in 3 million that the signal was coincidence, which is far above the significance level of Afshordi’s signal. The weak interaction of gravitational waves, however, is also the reason why they are excellent messengers. Unlike particles or light, they are barely affected on their way to us, carrying with them pristine information about where and how they were generated. They allow entirely new precision tests of general relativity in a regime that has never before been explored.

    If black hole echoes should be confirmed, that would almost certainly indicate a stark deviation from general relativity. Finding echoes would not uniquely confirm Afshordi’s theory that black holes seed dark energy. But some truly novel idea would be needed to explain it. ‘I don’t know of such echoes in any simulation that we have done to date,’ Birnholtz said. ‘If we were to confirm that there was an echo, that would be very interesting. We would have to look into what could produce such an echo.’

    Afshordi has research plans in case the statistical significance of his signal increases. He wants to improve his model of black-hole mergers, and run a numerical simulation to support the analytical estimate of what the echoes should look like. The next step would then be to better understand the underlying theory of space-time that could give rise to such a behaviour of the black-hole horizon. Cosmologists would also want to look much more closely at the implications of this new explanation for dark energy.

    Afshordi is aware just how speculative it is to alter general relativity so drastically. But he’s a rebel with a mission: ‘I want to encourage people to keep an open mind and not to dismiss ideas because they don’t match their preconceived notions.’ With LIGO exposing the workings of the Universe in ways never before studied, a lot of preconceived notions may soon be set aside.

    See the full article here .

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  • richardmitnick 1:31 pm on March 28, 2017 Permalink | Reply
    Tags: , , , , Gravitational waves, , SUPERRADIANCE   

    From PI via GIZMODO: “Mind-Blowing New Theory Connects Black Holes, Dark Matter, and Gravitational Waves” 

    Perimeter Institute
    Perimeter Institute


    Ryan F. Mandelbaum

    The past few years have been incredible for physics discoveries. Scientists spotted the Higgs boson, a particle they’d been hunting for almost 50 years, in 2012, and gravitational waves, which were theorized 100 years ago, in 2016. This year, they’re slated to take a picture of a black hole. So, thought some theorists, why not combine all of the craziest physics ideas into one, a physics turducken? What if we, say, try to spot the dark matter radiating off of black holes through their gravitational waves?

    It’s really not that strange of an idea. Now that scientists have detected gravitational waves, ripples in spacetime spawned by the most violent physical events, they want to use their discovery to make real physics observations. They think they have a way to spot all-new particles that might make up dark matter, an unknown substance that accounts for over 80 percent of all of the gravity in the universe.

    The basic idea is that we’re trying to use black holes… the densest, most compact objects in the universe, to search for new kinds of particles,” Masha Baryakhtar, postdoctoral researcher at the Perimeter Institute for Theoretical Physics in Canada, told Gizmodo. Especially one particle: “The axion. People have been looking for it for 40 years.”

    Black holes are the universe’s sinkholes, so strong that light can’t escape their pull once it’s entered. They’ve got such powerful gravitational fields that they produce gravitational waves when they collide with each other. Dark matter might not be made from particles (specks of mass and energy), but if it was, we might observe it as axions, particles around one quintillion (a billion billion) times lighter than an electron, hanging around black holes. Now that you understand all the terms, here’s how the theory works.

    Baryakhtar and her teammates think that black holes are more than just bear traps for light, but nuclei at the center of a sort of gravitational atom. The axions would be the electrons, so to speak. If you already know about black holes, you know they have incredibly hot, high-energy discs of gas orbiting them, produced by the friction between particles accelerated by the black hole’s gravity. This theory ignores that stuff, since axions wouldn’t interact via friction.

    Keeping with the atom analogy, the axions can jump around the black hole, gaining and losing energy the same way that electrons do. But electrons interact via electromagnetism, so they let out electromagnetic waves, or light waves. Axions interact via gravity, so they let out gravitational waves. But like I said earlier, axions are tiny. Unlike a tiny atom, the black hole in these “gravity atoms” rotates, supercharging the space around it and coaxing it into producing more axions. Despite the axion’s tiny mass, this so-called superradiance process could generate 10^80 axions, the same number of atoms in the entire universe, around a single black hole. Are you still with me? Crazy spinning blob makes lots of crazy stuff.

    Craziest of all, we should be able to hear a gravitational wave hum from these axions moving around and releasing gravitational waves in our detectors, similar to the way you see spectral lines coming off of electrons in atoms in chemistry class. “You’d see this at a particular frequency which would be roughly twice the axion mass,” said Baryakhtar.

    There are giant gravitational wave detectors scattered around the world; presently there’s one called LIGO (Laser Interferometer Gravitational Wave Observatory) in Washington State, another LIGO in Louisiana, and one called Virgo in Italy that are sensitive enough to detect gravitational waves, and with upgrades, to detect axions and prove their theory right.

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

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

    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Scientists would essentially need to record data, play it back, and tune their analysis like a radio to pick up the signal at just the right frequency.

    There are other ways the team thinks it could spot this superradiance effect, by measuring the spins in sets of colliding black holes. If black holes really do produce axions, scientists would see very few quickly-spinning black holes in collisions, since the superradiance effects would slow down some of the colliding black holes and create a visible effect in the data, according to the research published this month in the journal Physical Review D. The black hole spins would have a specific pattern which we should be able to spot in the gravitational wave detector data.

    Other scientists were immediately excited about this paper. “I’m always super excited about new ways to detect my favorite pet particle, the axion! Also, SUPERRADIANCE!” Dr. Chanda Prescod-Weinstein, the University of Washington axion wrangler, told Gizmodo in an email. “It’s so cool, and I haven’t read a paper that talked about [superradiance] in years. So it was really fun to see superradiance and axions in one paper.”

    There are a few drawbacks, as there are with any theory. These theorized black hole atoms would have to produce axions of a certain mass, but that mass isn’t an ideal one for the axion to be a dark matter particle, said Prescod-Weinstein. Plus, the second detection idea, the one that looks at the spin rate of colliding black holes, might not work. “They say [in the paper] that they don’t take into account the potential influence of another black hole” in the colliding pair, Dr. Lionel London, a research associate at Cardiff University School of Physics and Astronomy specializing in gravitational wave modeling, told Gizmodo. “If this does turn out to be a significant effect and they’re not including it, this could cast doubt on their results.” But there’s hope. “There’s good reason to believe the effect of a companion [black hole] won’t be large.”

    When would we spot these kinds of events? As of now, the LIGO and Virgo gravitational wave detectors probably aren’t ready. “With the current sensitivity we’re on the edge” of detecting axions, said Baryakhtar. “But LIGO will continue improving their instruments and at design sensitivity we might be able to see as many as 1000s of these axion signals coming in,” she said. Thousands of hums from these black hole-atoms.

    So, if you’ve gotten all the way to this point of the story and still don’t understand what’s going on, a recap: We’ve got these gravitational wave detectors that cost hundreds of millions of dollars each, that are good at spotting really crazy things going on in the universe. Theorists have come up with an interesting way to use them to solve one of the most important interstellar mysteries: What the heck is dark matter? As with most new ideas in theoretical physics, this is something cool to think about and isn’t ready for the big time… yet.

    “I think that timescale is always a concern, but we’re just getting started with LIGO discoveries,” said Prescod-Weinstein. “So who knows what’s around the corner over the next 10 years.”

    See the full article here .

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    About Perimeter

    Perimeter Institute is the world’s largest research hub devoted to theoretical physics. The independent Institute was founded in 1999 to foster breakthroughs in the fundamental understanding of our universe, from the smallest particles to the entire cosmos. Research at Perimeter is motivated by the understanding that fundamental science advances human knowledge and catalyzes innovation, and that today’s theoretical physics is tomorrow’s technology. Located in the Region of Waterloo, the not-for-profit Institute is a unique public-private endeavour, including the Governments of Ontario and Canada, that enables cutting-edge research, trains the next generation of scientific pioneers, and shares the power of physics through award-winning educational outreach and public engagement.

  • richardmitnick 6:12 pm on March 7, 2017 Permalink | Reply
    Tags: , , , Gravitational waves, ,   

    From APS: “Gravitational Waves: Hints, Allegations, and Things Left Unsaid” 


    American Physical Society

    APS April Meeting 2017

    If the APS April Meeting 2016 was a champagne-soaked celebration for gravitational wave scientists, the 2017 meeting was more like spring training — there was lots of potential, but the real action is yet to come.

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

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

    The Laser Interferometer Gravitational-Wave Observatory, or LIGO, launched the era of gravitational wave astronomy in February 2016 with the announcement of a collision between two black holes observed in September 2015. “I’m contractually obligated to show the slide [of the original detection] at any LIGO talk for at least another year,” joked Jocelyn Read, a physicist at California State University, Fullerton, during her presentation at this year’s meeting.

    The scientific collaboration that operates the two LIGO detectors netted a second merger between slightly smaller black holes on December 26, 2015. (A third “trigger” showed up in LIGO data on October 12, 2015, but ultimately did not meet the stringent “five-sigma” statistical significance standard that physicists generally insist on.)

    The detectors then went offline in January 2016 for repairs and upgrades. The second observing run began on November 30, but due to weather-related shutdowns and other logistical hurdles, the two detectors had operated simultaneously on only 12 days as of this year’s meeting, which limited the experiment’s statistical power. Collaboration members said they had no new detections to announce.

    Instead, scientists focused on sharpening theoretical estimates of how often various events occur. In particular, they are eager to see collisions involving neutron stars, which lack sufficient mass to collapse all the way to a black hole. Neutron star collisions are thought to be plentiful, but would emit weaker gravitational waves than do mergers of more massive black holes, so the volume of space the LIGO detectors can scan for such events is smaller.

    Even with recent upgrades, failure to detect a neutron star merger during the current observing run would not rule out existing models, said Read. But she added that with future improvements and the long-anticipated addition of Virgo, a LIGO-like detector based in Cascina, Italy, neutron stars should soon come out of hiding.

    VIRGO Gravitational Wave interferometer, near Pisa, Italy [Not yet operational]

    “We’re expecting that with a little more volume and a little more time, we’re going to be starting to make some astrophysically interesting statements.”

    LIGO scientists are also looking for signals from individual pulsars — rapidly rotating neutron stars that are observed on earth as pulses of radio waves. A bump on a pulsar’s surface should produce gravitational waves, but so far, no waves with the right shape have been picked up. This absence puts a limit on the size of any irregularities and on the emission power of gravitational waves from nearby pulsars such as the Crab and Vela pulsars, said Michael Landry, head of the Hanford LIGO observatory, and could soon start putting limits on more distant ones.

    Presenters dropped a few hints of possible excitement to come. LIGO data taken through the end of January produced two short signals that were unusual enough to exceed the experiment’s “false alarm” threshold — signals with shapes and strengths expected to show up once a month or less by chance alone. Both LIGO collaboration members and astronomers at conventional telescopes are investigating the data to determine whether they represent real events.

    For now, potential events will continue to be scrutinized by collaboration members, and released to the public via announcements coming months after initial detection. But LIGO leaders expect to shorten the lag time as detections become more frequent, perhaps eventually putting out monthly updates. “We hope to make it quicker,” said LIGO collaboration spokesperson Gabriela González, a physicist at Louisiana State University in Baton Rouge.

    LIGO is not the only means by which scientists are searching for gravitational waves. Some scientists are using powerful radio telescopes to track signals emanating from dozens of extremely fast-rotating pulsars. A specific pattern of correlations between tiny hiccups in the arrival times of these pulses would be a signature of long-wavelength gravitational waves expected from mergers of distant supermassive black holes.

    Teams in the U.S., Europe, and Australia have monitored pulsars for more than a decade, so far without positive results. But in an invited talk, Laura Sampson of Northwestern University in Evanston, Illinois, coyly announced “hints of some interesting signals.” With 11 years of timing data from 18 pulsars tracked by the Green Bank Telescope in West Virginia and the Arecibo Telescope in Puerto Rico, Sampson and other scientists affiliated with a collaboration called NANOGrav have eked out a result with a statistical significance of around 1.5 to 2 sigma.

    GBO radio telescope, Green Bank, West Virginia, USA

    NAIC/Arecibo Observatory, Puerto Rico, USA

    Data from the Green Bank Telescope in West Virginia and Arecibo Telescope in Puerto Rico help researchers use pulsars to study gravitational waves.

    “It’s the first hint we’ve ever had that there might be a signal in the data,” Sampson said. “Everything we’ve done before was straight-up limits.”

    As NANOGrav continues to gather data, their signal could grow toward the 5-sigma gold standard, or it could vanish. Sampson and her colleagues hope to have an answer in the next year or two. “This is of course very exciting news,” said Gonzalez.

    See the full article here .

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    American Physical Society
    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries.

  • richardmitnick 12:15 pm on March 5, 2017 Permalink | Reply
    Tags: , , , , Gravitational waves,   

    From Nautilus: “Gravity’s Kiss – The third ripple. 



    Harry Collins

    Gravity’s Kiss by Harry Collins, is published by the MIT Press.

    LIGO bloc new
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    VIRGO Collaboration bloc
    VIRGO Gravitational Wave interferometer, near Pisa, Italy
    VIRGO Gravitational Wave interferometer, near Pisa, Italy [not yet operational]

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project
    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

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

    Gravitational waves are finding their way to the general public. There is massive television news coverage with never a doubt expressed; gravitational waves have simply “been detected”—exciting but no more dubious than, say, the moon landing. What is happening is that gravitational waves are being “domesticated” in the same way as black holes or the Higgs have been domesticated. Everyone knows what a black hole is—it is a feature of everyone’s day-to-day life embedded in a “semantic net” that includes “the cosmos,” “the big bang,” “Stephen Hawking,” “brilliant scientists,” “Einstein,” “space,” “alternative universes,” “time travel,” “worm-holes,” “astronomy,” “rockets,” and “being sucked into things”—and this is in spite of the fact that, before The Event, no black hole had been observed except by inference. As for the Higgs, everyone knows that it was found by the huge and brilliant team at CERN, but, familiar as it is, no one knows what it is. I know it is the last piece in the jigsaw puzzle of the particle “zoo” known as the standard model, but what I have is “beer-mat knowledge,” good for answering questions in Trivial Pursuit but that’s about it. On the other hand, the fact that we can imagine encountering questions about black holes and the Higgs while playing Trivial Pursuit is one of the things that makes them real: all this familiar knowledge makes stuff real. The moon landing, note, is pretty real for everyone but, just as in the case of what is building in respect of The Event, there are conspiracy theories about that too; and, just as in this case, you have to stray from the mainstream to find them.

    On Friday I gather a good selection of the United Kingdom print newspapers; they are big contributors to the domestication process. The Guardian is a broadsheet for the left-liberal middle classes and its news section is 38 pages long. It gives the story the lead and whole of page 11. It had also given it the whole of page 3 on Wednesday, building the story on the rumors. On Saturday, The Guardian’s regular political cartoon features the Syrian peace talks represented as some kind of funny-looking celestial object with the caption: “Not gravitational waving but gravitational drowning.” Thus do gravitational waves spread into the ordinary language.

    EXTRA! EXTRA!Newspaper front pages from around the world.
    Courtesy of LIGO

    The Independent has a similar readership to The Guardian but has a smaller tabloid format with 72 pages. It gives the story the entire page 1 and pages 6–8; it opines that this is “one of the greatest achievements in human history.”

    The Telegraph is another broadsheet, with 38 pages in its news section. It is a right-wing, patriotic paper for the educated. It makes gravitational waves the second story on page 1, leading with:

    A British scientist who was pivotal in the project to detect gravitational waves could not celebrate the momentous discovery with colleagues because he is suffering from dementia.

    This, of course, is Ron Drever. The paper also gives up pages 10 and 11 to the story.

    Martin Rees, the Astronomer Royal, writes columns in The Independent and the Telegraph. He opines that this is of similar importance to the discovery of the Higgs; most other commentators say it is much more important than the Higgs, but Rees has long been said by gravitational wave physicists to be less than enthusiastic about the enterprise.

    The Daily Mail is a “little Englander” tabloid with 92 pages serving those with strong right-wing opinions. It gives the story half of page 10, mistakenly claiming that Einstein predicted that colliding stars would generate gravitational waves that could be detected on Earth, whereas he actually thought they would remain completely undetectable.

    The Mirror is a left-leaning tabloid with 80 pages. It gives the story most of page 21 but says that LIGO was invented by Thorne and Weiss, missing out on Drever.

    The Sun is a tabloid with 60 pages that began its life by publishing photographs of topless models on the notorious “page 3” (now dropped). The only science I could find was on the bottom third of page 15, head-lined: “Top Prof Dies in Rubber Suit with Dog Lead Round Neck.” The “Top Prof” does not seem to have been one of the gravitational wave team.

    The Guardian website of February 12 includes a hilarious cartoon—one of a series called “First Dog on the Moon,” which anticipates one of my major sociological theses. The fourth panel of the cartoon opines:

    Obviously we can’t see these waves—the only way we know they are real is by using another extremely sensitive device which detects scientists having feelings of excitement.

    The excitement evoked in scientists by a gravitational wave is calibrated using the marginally smaller effect of a cheese salad sandwich as a standard candle.

    Later I will discover that my major thesis about social construction, which turns on pointing out that no gravitational waves were seen but merely a few numbers that were interpreted as gravitational waves, has been thoroughly anticipated (albeit on a strange, flat-earther YouTube channel that appears to treat conspiracy theories as an art form). It claims scientists have not seen gravitational waves, nor has their machine seen gravitational waves, but that the machine produces lots of glitchy noises out of which they have picked one and interpreted it as a gravitational wave.

    A member of the LIGO team has put together a collection of newspaper front pages from around the world. And, as though to put an indelible stamp on the soon to be taken for granted nature of this exotic phenomenon, in the United States the discovery is presented on Saturday Night Live and The Tonight Show, and, on Saturday, February 13, the humorous U.S. radio show A Prairie Home Companion devotes about five minutes to gravitational waves. Gravitational waves have arrived!

    The physicists continue to do my job for me by gathering more indications of the domestication of gravitational waves. On February 16, a French (presumably humorous) website normalizes the waves in contemporary fashion by calling for a ban on them and the distribution of protective helmets. This is a Google translation from the French with my minor edits:

    “For a Moratorium on Gravitational Waves

    Bringing together hundreds of independent researchers, the “Collective for a moratorium on gravitational waves” (COMOG) sent us this platform. We publish it verbatim in our columns:

    In recent days, highlighting the “gravitational waves” continues to make the press headlines. Everyone welcomes this alleged “scientific breakthrough,” which was published in the Physical Review Letters, a journal under orders of the nuclear lobby.

    Now, our collective, consisting of independent researchers who wish to remain anonymous for their own reasons, is concerned about the apparent toxicity of gravitational waves.

    To date, there is in fact no serious study establishing the actual safety of these waves. That is why we propose an action plan of four points.

    1. We recall, first, that the oscillations of the curvature of space-time can present health risks found, especially on the neurological system of employees too long exposed to the gravitational waves. It is appropriate in this case to call for the government to strictly enforce the Labour Code, to limit the time of exposure to gravitational waves, and equip the wage earners with protective helmets.

    2. To these health risks are added, as often environmental, of deleterious economic effects. The curvature of space-time is likely to cause untoward inconvenience, especially in the field of transport and travel. An example: If space-time is curved in the wrong direction when one performs a trip from Paris to Bordeaux, the journey can last more than 25 hours, according to our estimates. Gravitational waves expose the French economy to serious danger that cannot be underestimated.

    3. It appears that the production of gravitational waves calls for masses of matter and energy that are absolutely astounding: black holes, neutron stars, washing machines, etc. We ask that the environmental and climate impact of gravitational waves be measured in France by an independent body and a carbon footprint be determined as quickly as possible.

    4. As a result, we ask Ségolène Royal, Minister of Environment, Energy and the Sea, responsible for international relations on the climate, to apply the constitutional principle of precaution, and to take by decree related measures that are defined and recommended in principle 15 of the Rio Declaration. It seems to us urgent that France decide on a moratorium on gravitational waves.

    If the government does not abide by these basic precautions, peaceful COMOG teams will be forced to resort to direct action. Within six months, we will proceed to the systematic dismantling of gravitational wave antennas. Our teams of volunteer harvesters shall uproot the plants of space-time curvatures. Finally, we will not hesitate to leave Paris to set up a zone to defend Proxima Centauri, even against the advice of the prefect.

    We call upon our fellow citizens to join our fight. Gravitational waves, no thank you!”

    On February 18, the Huddersfield Daily Examiner (Huddersfield is a town in North England with a football club—“Huddersfield Town”—all about as provincially English as can be) carries a story about the “Huddersfield Town Supporters Association” (HTSA). It includes:

    Our HTSA column last week touched upon the subject of regional supporters groups and their recent cosmic rise in popularity. The Laser Interferometer Gravitational-Wave Observatory (LIGO) can probably demonstrate whether this is due to colliding Black Holes over Bexleyheath.

    And Barack Obama had tweeted on February 11:

    Einstein was right! Congrats to @NSF and @LIGO on detecting gravitational waves—a huge breakthrough in how we understand the universe.

    At the forthcoming American Physical Society (APS) meeting, Dave Reitze, the director of LIGO, will present a slide showing a woman wearing a dress patterned with the waveform, an Australian competition swimmer with the waveform on his swim-cap, and a New York advertisement for apartments.

    Fashionably wavy. More domestication of gravitational waves. No image credits.

    I attend two meetings in March: a general relativity 100th anniversary meeting at Caltech and the LIGO-Virgo collaboration meeting in Pasadena. Of course, the cat is now out of the bag so a big topic at the Caltech meeting is The Event. I follow Barry Barish onto the platform; he describes the technicalities and I talk about the way small science and big science had combined to create this possibility, with Barish bringing about the necessary transformation, and I talk about what it meant for me as a sociologist to be confronted by such a sudden and certain result. At neither meeting is there any criticism but the LVC is selling huge numbers of T-shirts and polo shirts with the waveform of The Event printed or embroidered on them: The waveform is becoming an icon! Many more such garments will be sold at the April APS meeting.

    At the March meeting of the APS—a much larger meeting than the April meeting I am going to attend—a group of physicists who have nothing to do with LIGO or gravitational waves performed a song based on the Neil Diamond/Monkees’ “I’m a Believer.” The lyrics are as follows:

    “I’m a LIGO Believer.” Lyrics: Marian McKenzie. Tune: “I’m a Believer,” by Neil Diamond (courtesy Marian McKenzie).

    I thought waves of gravity were fairy tales—fine for dilettantes, but not for me.
    What’s the use of searching?
    Noise is all you’ll find.
    I don’t want to clutter up my mind—

    [Chorus:] Then I saw the graph—Now I’m a believer!
    You can laugh, and hold me in scorn.
    I’m convinced, oooh, I’m a believer
    In Weiss, Reitze, Drever, Gonzalez, and Thorne!

    Einstein spoke of grav wave propagation.
    Weber tried to find them on the moon.
    BICEP2 announced them,
    Then said “Never mind.”
    —Do you wonder I was disinclined?

    [Sing chorus] [instrumental interlude] and repeat”

    The whole song can be seen and heard on YouTube.

    surfing the googleEnormous spike in interest in gravitational waves around February 11. No image credit.

    What about social media? Google Trends (see chart above) shows the huge spike in interest in gravitational waves around the February 11 press conferences by tracking hits on Google. Unfortunately, we have only the normalized trend, the scale having a maximum of 100, not absolute numbers.

    Breaking the internet. The enormous spike in interest in gravitational waves compared to hits for Kim Kardashian.

    Gravitational waves are not, however, about to take over the popular imagination. The chart above shows the same spike in comparison to Google hits for Kim Kardashian, the reality TV star. Gravitational waves’ enormous spike is a mere 2 percent as high as Kardashian’s peak performance and only about 5 percent as high as her average day-to-day score. Aside from the spike, gravitational waves score zero when compared to Kardashian’s average.

    See the full article here .

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  • richardmitnick 4:49 pm on February 12, 2017 Permalink | Reply
    Tags: , Gravitational waves, oscillon, U Basel   

    From U Basel: “Ancient Signals From the Early Universe” 


    U Basel

    February 12, 2017
    No writer credit

    The animation shows a computer simulation of an oscillon, a strong localized fluctuation of the inflaton field of the early universe. According to the calculations of Prof. Stefan Antusch and his team, oscillons produced a characteristic peak in the otherwise broad spectrum of gravitational waves. © Department of Physics, University of Basel

    For the first time, theoretical physicists from the University of Basel in Switzerland have calculated the signal of specific gravitational wave sources that emerged fractions of a second after the Big Bang. The source of the signal is a long-lost cosmological phenomenon called “oscillon”. The journal Physical Review Letters has published the results.

    Although Albert Einstein had already predicted the existence of gravitational waves, their existence was not actually proven until fall 2015, when highly sensitive detectors received the waves formed during the merging of two black holes. Gravitational waves are different from all other known waves. As they travel through the universe, they shrink and stretch the space-time continuum; in other words, they distort the geometry of space itself. Although all accelerating masses emit gravitational waves, these can only be measured when the mass is extremely large, as is the case with black holes or supernovas.

    Gravitational waves transport information from the Big Bang

    However, gravitational waves not only provide information on major astrophysical events of this kind but also offer an insight into the formation of the universe itself. In order to learn more about this stage of the universe, Prof. Stefan Antusch and his team from the Department of Physics at the University of Basel are conducting research into what is known as the stochastic background of gravitational waves. This background consists of gravitational waves from a large number of sources that overlap with one another, together yielding a broad spectrum of frequencies. The Basel-based physicists calculate predicted frequency ranges and intensities for the waves, which can then be tested in experiments.

    A highly compressed universe

    Shortly after the Big Bang, the universe we see today was still very small, dense, and hot. “Picture something about the size of a football,” Antusch explains. The whole universe was compressed into this very small space, and it was extremely turbulent. Modern cosmology assumes that at that time the universe was dominated by a particle known as the inflaton and its associated field.

    Oscillons generate a powerful signal

    The inflaton underwent intensive fluctuations, which had special properties. They formed clumps, for example, causing them to oscillate in localized regions of space. These regions are referred to as oscillons and can be imagined as standing waves. “Although the oscillons have long since ceased to exist, the gravitational waves they emitted are omnipresent – and we can use them to look further into the past than ever before,” says Antusch.

    Using numerical simulations, the theoretical physicist and his team were able to calculate the shape of the oscillon’s signal, which was emitted just fractions of a second after the Big Bang. It appears as a pronounced peak in the otherwise rather broad spectrum of gravitational waves. “We would not have thought before our calculations that oscillons could produce such a strong signal at a specific frequency,” Antusch explains. Now, in a second step, experimental physicists must actually prove the signal’s existence using detectors.

    Original article

    Stefan Antusch, Francesco Cefalà, and Stefano Orani
    Gravitational Waves from Oscillons after Inflation
    Physical Review Letters (2017), doi: 10.1103/PhysRevLett.118.011303

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  • richardmitnick 11:17 am on February 12, 2017 Permalink | Reply
    Tags: , , Gravitational waves,   

    From Science Alert: “Surprise! LIGO Can Also Make Gravitational Waves” 


    Science Alert



    11 FEB 2017


    We can produce gravitational waves now.

    It’s been almost a year now since the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the greatest scientific discovery of 2016.

    LIGO bloc new
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Though the first gravitational waves were actually detected in September 2015, it was only after additional detections were made in June 2016 that LIGO scientists finally confirmed that the elusive waves exist, solidifying Albert Einstein’s major prediction in his theory of relativity.

    Now, the most sensitive detector of spacetime ripples in the world turns out to also be the best producer of gravitational waves.

    “When we optimise LIGO for detection, we also optimise it for emission [of gravitational waves],” said physicist Belinda Pang from the California Institute of Technology (Caltech) in Pasadena according to a report in Science.

    Pang was speaking at a meeting of the American Physical Society last week, representing her team of physicists.

    Gravitational waves are ripples that are produced when massive objects warp spacetime.

    They essentially stretch out space, and according to Einstein, they can be produced by certain swirling configurations of mass. Using uber-sensitive twin detectors in Hanford, Washington, and Livingston, Louisiana, LIGO is able to detect this stretching of space.

    Once they realised they could detect gravitational waves, the physicists posited that the sensitivity of their detectors would enable them to efficiently generate these ripples, too.

    “The fundamental thing about a detector is that it couples to gravitational waves,” said Fan Zhang, a physicist at Beijing Normal University.

    “When you have coupling, it’s going to go both ways.”

    The LIGO team tested their idea using a quantum mathematical model and found that they were right: their detectors did generate tiny, optimally efficient spacetime ripples.

    Quantum mechanics says that small objects, such as electrons, can be in two places at once, and some physicists think that it’s possible to coax macroscopic objects into a similar state of quantum motion.

    According to Pang, LIGO and these waves could be just the things to make it happen.

    Though that delicate state couldn’t be sustained for very long periods, any amount of time could give us added insight into quantum mechanics.

    We could measure how long it takes for decoherence to occur and see what role gravity might play in the existence of quantum states between macroscopic objects.

    “It’s an interesting idea, but experimentally it’s very challenging,” explained Caltech physicist Yiqui Ma, one of Pang’s colleagues.

    “It’s unbelievably difficult, but if you want to do it, what we’re saying is that LIGO is the best place to do it.”

    Any added insight into quantum activity could not only help us build better quantum computers, it could completely revolutionise our understanding of the physical universe.

    LIGO is already in the process of receiving upgrades that will help it detect even fainter gravitational waves, and eventually, the plan is to build the Evolved Laser Interferometer Space Antenna (eLISA), a gravitational wave observatory in space.

    ESA/LISA Pathfinder
    ESA/LISA Pathfinder


    Within the next decade, not only could LIGO be regularly detecting gravitational waves, it could also be finding ever more advanced ways to create them and furthering our understanding of the quantum world in unimaginable ways.

    See the full article here .

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  • richardmitnick 3:59 pm on December 13, 2016 Permalink | Reply
    Tags: , , , , Gravitational waves,   

    From ESA: “LISA Pathfinder’s pioneering mission continues” 

    ESA Space For Europe Banner

    European Space Agency

    13 December 2016
    Paul McNamara
    LISA Pathfinder Project Scientist
    European Space Agency
    Tel: +31 71 565 8239
    Email: paul.mcnamara@esa.int

    Oliver Jennrich
    LISA Pathfinder Deputy Mission Scientist
    L3 mission Study Scientist
    European Space Agency
    Tel: +31 71 565 6074
    Email: oliver.jennrich@esa.int

    ESA/LISA Pathfinder
    ESA/LISA Pathfinder

    On 7 December, LISA Pathfinder started the extended phase of its mission, an additional six months during which scientists and engineers will push the experiment to its limits in preparation for ESA’s future space observatory of gravitational waves.

    LISA Pathfinder, a demonstration mission to validate important technologies to observe gravitational waves – fluctuations in the fabric of spacetime – from space, was launched just over a year ago, on 3 December 2015.

    After a six-week-long journey, the spacecraft reached its operational orbit around the first Sun-Earth Lagrange point, L1 – 1.5 million km away from Earth towards the Sun – at the end of January. There, following commissioning of the on board instrumentation, LISA Pathfinder started its science mission on 1 March.

    Much to the team’s surprise, it did not take as long as expected to achieve the mission’s goal: demonstrating that two test masses – a pair of identical gold-platinum cubes – can be placed in the most precise freefall ever performed. In fact, the desired level of precision was already obtained within the first day of LISA Pathfinder’s scientific operations.

    Over the following months, scientists and engineers kept improving the performance of the experiment. They described these first results, including an analysis of the residual sources of disturbance on the cubes’ almost perfect freefall motion, in a paper published at the beginning of June in Physical Review Letters.

    LISA Pathfinder performance. Credit: spacecraft: ESA/ATG medialab; data: ESA/LISA Pathfinder Collaboration

    Then, on 25 June, the first operations phase, using the LISA Technology Package (LTP), was completed. The LTP is a European payload consisting of the test masses, inertial sensors, and laser interferometer, and uses a series of cold-gas micronewton thrusters to move the satellite and keep it centred on the cubes, in response to external and internal forces battering them around.

    Operations continued with NASA’s Disturbance Reduction System (DRS), an additional experiment which receives measurement input from the inertial sensors of the LTP but employs its own micronewton thrusters based on colloidal technology.

    Following completion of the DRS operations, the extended mission of LISA Pathfinder began on 7 December 2016, at 09:00 CET (08:00 UTC). It will last until 31 May 2017, making use of both the LTP and DRS payloads.

    “So far, we’ve been busy demonstrating the performance of LISA Pathfinder, which has been steadily improving as time went by,” says Paul McNamara, LISA Pathfinder Project Scientist at ESA, “but now we can spend the next six months learning everything we need to know to build and operate a gravitational-wave observatory in space.”

    Artist’s impression of a pair of merging black holes, releasing gravitational waves. Credit: ESA–C.Carreau

    Last October, ESA issued a call inviting European scientists to propose concepts for the third large mission (L3) in its Cosmic Vision plan, which will be a space observatory to study the gravitational Universe. The selection is expected to take place in the first half of 2017, with a preliminary internal study phase planned for later in the year.

    The future observatory will detect gravitational waves with frequencies from 1 Hz down to 0.1 mHz. These are about a hundred to a million times lower than the frequencies of waves that can be measured with ground-based experiments like the Laser Interferometer Gravitational-Wave Observatory (LIGO), which obtained the first direct detection of gravitational waves in September 2015.

    LIGO bloc new
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib
    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    During the extended mission of LISA Pathfinder, the team will run a series of long duration experiments to better characterise the mission performance especially at the lowest frequencies that will be probed by the future observatory.

    “We are thrilled to be pushing the limits of LISA Pathfinder, a unique physics laboratory in space giving us confidence that we can definitely build a space-borne observatory of gravitational waves”, says Oliver Jennrich, LISA Pathfinder deputy mission scientist and L3 study scientist at ESA.

    One of the operations that will be attempted in the coming weeks concerns the station-keeping manoeuvres that mission operators have been regularly conducting to keep the satellite on its operational orbit.

    LISA Pathfinder orbits around L1, but if left unattended, it would slowly drift away from the Lagrangian point under the gravitational pull of Earth. To avoid that, it is sufficient to fire the micro-newton thrusters once every one to two weeks.

    Between 25 December and 14 January, however, the team decided to apply no correction manoeuvres. This will allow the scientists to run uninterrupted experiments for almost three weeks, exploring what happens in the range of very low frequencies that are of interest to detect gravitational wave from space.

    The LISA Technology Package core assembly at the heart of LISA Pathfinder. Credit: ESA/ATG medialab

    Another experiment concerns slightly higher frequencies, around 1–60 mHz. At these frequencies, the main source of disturbance seems to be gas molecules that are present in the test mass enclosures and bouncing off the two cubes – an effect that has been reducing as more molecules are being vented into space.

    The team is now curious to see whether additional sources of noise are lurking underneath, something that will be important for the future L3 mission. One possible way of testing this entails simply waiting until most molecules are vented into space, but there is an alternative: to switch off many of the heaters on board, reducing the temperature by ten degrees, and thereby reducing the pressure inside the enclosure. The team will run this experiment in late January.

    These are some examples of the range of experiments that will be conducted during LISA Pathfinder’s extended mission. Eventually, at the end of the mission, the spacecraft will be gently pushed towards a heliocentric orbit.

    See the full article here .

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  • richardmitnick 12:52 pm on December 13, 2016 Permalink | Reply
    Tags: , Einstein's general theory of relativity, Gravitational waves,   

    From Science Alert: “Echoes in gravitational waves hint at a breakdown of Einstein’s general relativity” 


    Science Alert

    Dana Berry/NASA

    12 DEC 2016

    The physics discovery of 2016 just got more tantalising.

    In February, astronomers made a monumental discovery. Almost 100 years after Albert Einstein first predicted them, researchers detected gravitational waves – ripples in space time that had radiated out from the merging of two black holes.

    It was a striking confirmation of Einstein’s general theory of relativity – but in an unexpected twist, the discovery has also provided the first tantalising evidence that his theory breaks down at the event horizon of black holes.

    Since February, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has observed three gravitational wave events in total. And now researchers have pored over this data, and claim they’ve found evidence of ‘echoes’ in the waves that defy Einstein’s predictions of black holes.

    For now, the new claims have been published on ArXiv.org, where they can be examined by the rest of the physics community before being submitted for peer-review – so there’s a very real possibility that these echoes could go away with more scrutiny of the data.

    And the evidence so far falls short of the crucial 5-sigma error margin, which is the gold-standard in the physics world that means there’s a one in 3.5 million chance that the result could be a fluke.

    But if further research shows that these ‘echoes’ are really there, it would be a huge day for physics. It’s already thought that general relativity breaks down at the centre of black holes, but this would show that it fails at their edges, too. And that could herald the birth of new physics.

    “The LIGO detections, and the prospect of many more, offer an exciting opportunity to investigate a new physical regime,” Steve Giddings, a black-hole researcher from the University of California, Santa Barbara, who wasn’t involved in the study, told Zeeya Merali at Nature.

    If the echoes disappear, general relativity would have just withstood yet another test. For decades, physicists have been attempting to poke holes in the theory, trying to find ways it could be more compatible with quantum mechanics, but so far, Einstein’s original version has held up incredibly well.

    But before we get too carried away, what are these ‘echoes’, and what do they have to do with general relativity?

    It all comes down to something called the black hole information paradox.

    According to Einstein’s general theory of relativity, anything that crosses a black hole’s event horizon should disappear, with no trace left behind. The traditional thinking is that not even light can escape a black hole, hence the name ‘black’.

    More recently, though, researchers have questioned that idea. Because according to quantum mechanics, matter swallowed by a black hole would actually leave a trace of itself on the outside.

    So how can an event horizon satisfy both general relativity (and destroy everything that passes its boundary) and quantum mechanics (leave some trace of it)?

    This is one of the biggest problems in physics, and scientists still don’t have an answer.

    One explanation is the firewall hypothesis, which was proposed in 2012, and suggests that there’s a ring of high-energy particles surrounding a black hole’s event horizon that burns up any matter that passes through.

    Physicist Stephen Hawking has a different idea – that black holes are surrounded by soft ‘hair’. The ‘hair’ is low-energy quantum excitations that store a signature pattern of everything that’s been sucked up by a black hole.

    Regardless of which hypothesis you think is the most convincing, the overarching message is the same: instead of the clean event horizon predicted by general relativity, black holes might be ‘fluffier’ than we imagined.

    The only problem is we’ve had no way to actually test any of these ideas, until LIGO detected gravitational waves earlier this year.

    Now, with their latest data in hand, a team of international researchers has proposed a way to measure what’s going on around black holes.

    The prediction is that, if the edges of black holes really do defy general relativity and are fuzzy, then a series of echoes should be released after an initial gravitational wave burst.

    That’s because the team predicts the fuzziness surrounding a black hole will act kind of like a hall of mirrors, trapping some of the gravitational waves escaping the black hole merger and bouncing them around for a bit, so only a few escape at a time, meaning they would have hit LIGO slightly later.

    According to the team’s calculations, these echoes would have been detected by LIGO at 0.1, 0.2 and 0.3 seconds after the initial gravitational wave burst, and that’s exactly what the data show.

    Not only after the first gravitational wave burst detected in February, but all three events seen since.

    Of course, three gravitational wave bursts is still a pretty tiny sample size. And there’s a chance that these echoes are just some kind of background noise – around a 1 in 270 chance, to be precise, or 2.9 sigma – but further observations should help narrow down those odds.

    “The good thing is that new LIGO data with improved sensitivity will be coming in, so we should be able to confirm this or rule it out within the next two years,” lead research Niayesh Afshordi told Nature.

    Even if the echoes can be confirmed, they still don’t provide any insight into what kind of fuzzy boundary black holes might actually have, so we’ve got a long way to go before that information paradox is solved.

    But one thing’s for sure: the physics discovery of the year just got even more tantalising.

    See the full article here .

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  • richardmitnick 5:18 pm on December 1, 2016 Permalink | Reply
    Tags: , Gravitational waves,   

    From MIT: “LIGO back online, ready for more discoveries” 

    MIT News

    MIT Widget
    MIT News

    November 30, 2016
    Jennifer Chu

    Upgrades make detectors more sensitive to gravitational waves.

    After making several upgrades, scientists have restarted the twin detectors of LIGO, the Laser Interferometer Gravitational-wave Observatory. The Livingston detector site, located near Livingston, Louisiana, is pictured here. Photo: Caltech/MIT/LIGO Lab

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

    LSC LIGO Scientific Collaboration

    Today, scientists restarted the twin detectors of LIGO, the Laser Interferometer Gravitational-wave Observatory, after making several improvements to the system. Over the last year, they have made enhancements to LIGO’s lasers, electronics, and optics that have increased the observatory’s sensitivity by 10 to 25 percent. The detectors, scientists hope, will now be able to tune in to gravitational waves — and the extreme events from which they arise — that emanate from farther out in the universe.

    On Sept. 14, 2015, LIGO’s detectors made the very first direct detection of gravitational waves, just two days after scientists restarted the observatory as Advanced LIGO — an upgraded version of LIGO’s two large interferometers, one located at Hanford, Washington, and the other 3,000 kilometers away in Livingston, Lousiana. After analyzing the signal, scientists determined that it was indeed a gravitational wave, which arose from the merger of two massive black holes 1.3 billion light years away.

    Three months later, on Dec. 26, 2015, the detectors picked up another signal, which scientists decoded as a second gravitational wave, rippling out from yet another black hole merger, slightly farther out in the universe, 1.4 billion light years away.

    Now with LIGO’s latest upgrades, members of the LIGO Scientific Collaboration are hoping to detect more frequent signals of gravitational waves, arising from colliding black holes and other extreme cosmic phenomena. MIT News spoke with Peter Fritschel, the associate director for LIGO at MIT, and LIGO’s chief detector scientist, about LIGO’s new view.

    Q: What sort of changes have been made to the detectors since they went offline?

    A: There were different sorts of activities at the two observatories. With the detector in Livingston, Louisiana, we did a lot of work inside the vacuum system, replacing or adding new components. As an example, each detector contains four test masses that respond to a passing gravitational wave. These test masses are mounted in complex suspension systems that isolate them from the local environment. Previous testing had shown that two of the vibrational modes of these suspensions could oscillate to a degree that would prevent the detector from operating with its best sensitivity. So, we designed and installed some tuned, passive dampers to reduce the oscillation amplitude of these modes. This will help the Livingston detector operate at its peak sensitivity for a greater fraction of the data run duration.

    On the Hanford, Washington, detector, most of the effort was geared toward increasing the laser power stored in the interferometer. During the first observing run, we had about 100 kilowatts of laser power in each long arm of the interferometer. Since then we worked on increasing this by a factor of two, to achieve 200 kilowatts of power in each arm. This can be quite difficult because there are thermal effects and optical-mechanical interactions that occur as the power is increased, and some of these can produce instabilities that must be tamed. We actually succeeded in solving these types of problems and were able to operate the detector with 200 kilowatts in the arms. However, there were other problems that cost sensitivity, and we didn’t have time to solve these, so we are now operating with 20 to 30 percent higher power than we had in the first observing run. This modest power increase gives a small but noticeable increase in sensitivity to gravitational wave frequencies higher than about 100 hertz.

    We also gathered a lot of important information that will be used to plan out the next detector commissioning period, which will commence at the end of this six-month observation period. We still have a lot of challenging work ahead of us to get to our final design sensitivity.

    Q: How sensitive is LIGO with these new improvements?

    A: The metric we most commonly use is the sensitivity to gravitational waves produced by the merger of two neutron stars, because we can easily calculate what we should see from such a system — but note we have not yet detected gravitational waves from a neutron star-neutron star merger. The Livingston detector is now sensitive enough to detect a merger from as far away as 200 million parsecs (660 million light years). This is about 25 percent farther than it could “see” in the first observing run. For the Hanford detector the corresponding sensitivity range is pretty much on par with what it was during the first run and is about 15 percent lower than these figures.

    Of course in the first observing run we detected the merger of two black holes, not neutron stars. The sensitivity comparison for black hole mergers is nonetheless about the same: Compared to last year’s observing run, the Livingston detector is around 25 percent more sensitive and the Hanford detector is about the same. But even small improvements in sensitivity can help, since the volume of space being probed, and thus the rate of gravitational-wave detections, grows as the cube of these distances.

    Q: What do you hope to “hear” and detect, now that LIGO is back online?

    A: We definitely expect to detect more black hole mergers, which is still a very exciting prospect. Recall that in the first run we detected two such black hole binary mergers and saw strong evidence for a third merger. With the modest improvement in sensitivity and the plan to collect more data than we did before, we should add to our knowledge of the black hole population in the universe.

    We would also love to detect gravitational waves from the merger of two neutron stars. We know these systems exist, but we don’t know how prevalent they are, so we can’t be sure how sensitivity we need to start seeing them. Binary neutron star mergers are interesting because (among other things) they are thought to be the producers and distributers of the heavy elements, such as the precious metals, that exist in our galaxy.

    See the full article here .

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  • richardmitnick 11:06 am on October 28, 2016 Permalink | Reply
    Tags: Gravitational waves, ,   

    From Phys.org: “Shocks in the early universe could be detectable today” 


    October 27, 2016

    Simulation showing cosmological initial conditions (left) evolving into shocks (right). Credit: Pen and Turok. ©2016 American Physical Society

    Physicists have discovered a surprising consequence of a widely supported model of the early universe: according to the model, tiny cosmological perturbations produced shocks in the radiation fluid just a fraction of a second after the big bang. These shocks would have collided with each other to generate gravitational waves that are large enough to be detected by today’s gravitational wave detectors.

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

    The physicists, Ue-Li Pen at the Canadian Institute for Theoretical Astrophysics in Toronto, and Neil Turok at the Perimeter Institute for Theoretical Physics in Waterloo, have published a paper on the shocks in the early universe and their aftermath in a recent issue of Physical Review Letters.

    As the scientists explain, the most widely supported model of the early universe is one with a radiation-dominated background that is almost perfectly homogeneous, except for some tiny waves, or perturbations, in the radiation.

    In the new study, Pen and Turok have theoretically shown that some of these early, tiny perturbations, which are small-amplitude waves, would have spiked to form large-amplitude waves, or shocks. These shocks would have formed only at very high temperatures, like those that occur immediately after the big bang.

    The physicists also showed that, when two or more shocks collide with each other, they generate gravitational waves.

    The results suggest that both colliding shocks and merging black holes—like those detected earlier this year by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment—contribute to the gravitational wave background.

    LIGO bloc new
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Some researchers have previously speculated that the mergining black holes may have formed from the same perturbations that created the shocks and, further, that black holes of this size may make up the dark matter in our galaxy.

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project
    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    However, it would be possible to distinguish between merging black holes and colliding shocks because the gravitational waves emitted by shocks would be detected at far lower frequencies today since the wavelength would have been stretched by the expansion of the universe. Today the gravitational waves from shocks would have frequencies of 3 nHz, as opposed to the 100 Hz regime in which the LIGO experiment currently operates.

    Based on their analysis, the scientists think that both current and future planned gravitational wave detectors will be able to detect the frequencies of gravitational waves emitted by shocks.


    These frequencies correspond to emission times of around 10-4 to 10-30 seconds after the big bang.

    Another interesting consequence of shocks in the early universe is that their interactions would have caused the surrounding radiation fluid to rotate, generating vorticity. This means that shocks in the early universe would have generated entropy in an otherwise perfect radiation fluid, in which normally the entropy cannot increase.

    The possibility that shocks in the early universe could have generated gravitational waves, vorticity, and entropy could help scientists solve some of the more perplexing puzzles of the early universe, such as why the universe has more matter than antimatter (the baryogenesis problem), as well as the origins of the magnetic fields that are observed in many astrophysical objects.

    See the full article here .

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