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  • richardmitnick 4:39 pm on June 23, 2017 Permalink | Reply
    Tags: , , Gravitational waves, , NASA/DLR Grace   

    From Goddard: “ESA to Develop Gravitational Wave Space Mission with NASA Support” 

    NASA Goddard Banner
    NASA Goddard Space Flight Center

    June 22, 2017
    Francis Reddy
    NASA’s Goddard Space Flight Center, Greenbelt, Md.

    ESA (the European Space Agency) has selected the Laser Interferometer Space Antenna (LISA) for its third large-class mission in the agency’s Cosmic Vision science program. The three-spacecraft constellation is designed to study gravitational waves in space and is a concept long studied by both ESA and NASA.

    ESA’s Science Program Committee announced the selection at a meeting on June 20. The mission will now be designed, budgeted and proposed for adoption before construction begins. LISA is expected to launch in 2034. NASA will be a partner with ESA in the design, development, operations and data analysis of the mission.

    ESA/eLISA the future of gravitational wave research

    Gravitational radiation was predicted a century ago by Albert Einstein’s general theory of relativity. Massive accelerating objects such as merging black holes produce waves of energy that ripple through the fabric of space and time. Indirect proof of the existence of these waves came in 1978, when subtle changes observed in the motion of a pair of orbiting neutron stars showed energy was leaving the system in an amount matching predictions of energy carried away by gravitational waves.

    In September 2015, these waves were first directly detected by the National Science Foundation’s ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO).


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    ESA/eLISA the future of gravitational wave research

    The signal arose from the merger of two stellar-mass black holes located some 1.3 billion light-years away. Similar signals from other black hole mergers have since been detected.

    Seismic, thermal and other noise sources limit LIGO to higher-frequency gravitational waves around 100 cycles per second (hertz). But finding signals from more powerful events, such as mergers of supermassive black holes in colliding galaxies, requires the ability to detect frequencies much lower than 1 hertz, a sensitivity level only possible from space.

    LISA consists of three spacecraft separated by 1.6 million miles (2.5 million kilometers) in a triangular formation that follows Earth in its orbit around the sun. Each spacecraft carries test masses that are shielded in such a way that the only force they respond to is gravity. Lasers measure the distances to test masses in all three spacecraft. Tiny changes in the lengths of each two-spacecraft arm signals the passage of gravitational waves through the formation.

    For example, LISA will be sensitive to gravitational waves produced by mergers of supermassive black holes, each with millions or more times the mass of the sun. It will also be able to detect gravitational waves emanating from binary systems containing neutron stars or black holes, causing their orbits to shrink. And LISA may detect a background of gravitational waves produced during the universe’s earliest moments.

    For decades, NASA has worked to develop many technologies needed for LISA, including measurement, micropropulsion and control systems, as well as support for the development of data analysis techniques.

    For instance, the GRACE Follow-On mission, a U.S. and German collaboration to replace the aging GRACE satellites scheduled for launch late this year, will carry a laser measuring system that inherits some of the technologies originally developed for LISA.

    NASA/DLR Grace

    The mission’s Laser Ranging Interferometer will track distance changes between the two satellites with unprecedented precision, providing the first demonstration of the technology in space.

    In 2016, ESA’s LISA Pathfinder successfully demonstrated key technologies needed to build LISA.

    ESA/LISA Pathfinder

    Each of LISA’s three spacecraft must gently fly around its test masses without disturbing them, a process called drag-free flight. In its first two months of operations, LISA Pathfinder demonstrated this process with a precision some five times better than its mission requirements and later reached the sensitivity needed for the full multi-spacecraft observatory. U.S. researchers collaborated on aspects of LISA Pathfinder for years, and the mission carries a NASA-supplied experiment called the ST7 Disturbance Reduction System, which is managed by NASA’s Jet Propulsion Laboratory in Pasadena, California.

    For more information about the LISA project, visit:

    https://lisa.nasa.gov

    See the full article here.

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    NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

    Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.


    NASA/Goddard Campus

     
  • richardmitnick 7:56 pm on June 9, 2017 Permalink | Reply
    Tags: , Gravitational waves, , SDSC- San Diego Supercomputer Center,   

    From Science Node: “XSEDE cuts through the noise” 

    Science Node bloc
    Science Node

    06 June, 2017
    Alisa Alering

    3
    Courtesy LIGO; Caltech; MIT; Sonoma State; Aurore Simonnet.

    Over two billion years ago, when multicellular life had only just begun to evolve on Earth, two black holes collided and merged to form a new black hole.

    With a mass 49 times that of our sun, the massive collision set off ripples in space-time that radiated from the event like waves from a stone thrown into a pond. Predicted by Albert Einstein in 1916 and known as gravitational waves, those ripples are still traveling.


    Surf’s up! The Extreme Science and Engineering Discovery Environment (XSEDE) provides the HPC resources required to pluck gravitational waves from the noise found on LIGO detectors. Courtesy XSEDE.

    Able to pass through dust, matter, or anything else without being distorted, gravitational waves carry unique information about cosmic events that can’t be obtained in any other way. When the waves reach Earth, they give astrophysicists a completely new way to explore the universe.

    The first such waves were detected on September 14, 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration. In the months since, two more gravitational wave events have been confirmed, one in December 2015 and the most recent on January 4, 2017.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    ESA/eLISA the future of gravitational wave research

    Signal from noise

    When gravitational waves pass by, they change the distance between objects. The change is so infinitesimal that it can’t be felt, or seen with a microscope. But incredibly sensitive scientific instruments—interferometers—can detect a change that is a thousand times smaller than a proton.

    The LIGO Scientific Collaboration, a body of more than 1,000 international scientists who collectively perform LIGO research, operates two interferometers located over 2000 miles apart in Washington and Louisiana, USA.

    Despite the sensitivity of the instruments, it’s not easy to detect a gravitational wave. When a signal is received, scientists must determine what it means and how likely it is to be noise or a real gravitational wave. Making that determination requires high-performance computing.

    Since 2013, LIGO has collaborated with the Extreme Science and Engineering Discovery Environment (XSEDE), a National Science Foundation (NSF)-funded cyberinfrastructure network that includes not just high-performance computing systems but also experts who help researchers move projects forward.

    Better, faster, cheaper

    In order to validate the discovery of a gravitational wave, researchers measure the significance of the signal by calculating a false alarm rate for the event.


    Making waves, taking names. The top part of the animation shows two black holes orbiting each other until they merge, and the lower part shows the two distinct gravitational waves emitted. Thanks to supercomputers at TACC and SDSC, researchers can pick out these waves from other detector noise. Courtesy Simulating eXtreme Spacetimes collaboration.

    TACC Maverick HP NVIDIA supercomputer

    TACC Lonestar Cray XC40 supercomputer

    Dell Poweredge U Texas Austin Stampede Supercomputer. Texas Advanced Computer Center 9.6 PF

    TACC HPE Apollo 8000 Hikari supercomputer

    TACC Maverick HP NVIDIA supercomputer

    SDSC Triton HP supercomputer

    SDSC Gordon-Simons supercomputer

    SDSC Dell Comet supercomputer

    Once confirmed, further supercomputer analysis is used to extract precise estimates of the physical properties of the event, including the masses of the colliding objects, position, orientation, and distance from the Earth, carefully checking millions of combinations of these characteristics and testing how well the predicted waveform matches the signal detected by LIGO.

    To draw larger conclusions about the nature of black holes requires careful modeling based on the received data. Each simulation can take from a week to one month to complete, depending upon the complexity.

    Such intensive data analysis requires large scale high-throughput computing with parallel workflows at the scale of tens of thousands of cores for long periods of time. LIGO has been allocated millions of hours on XSEDE’s high-performance computers, including Stampede at the Texas Advanced Computing Center (TACC) and Comet at the San Diego Supercomputer Center (SDSC).

    Over the first year of XSEDE’s collaboration with LIGO, XSEDE worked to increase the speed of the applications, making them 8-10x faster on average.

    “The strategic collaboration between the two NSF-funded projects allows for accelerated scientific discovery which also translates into cost-savings for LIGO on the order of tens of millions of dollars so far,” says Pedro Marronetti, Gravitational Physics program director at the NSF.

    Waves of the future

    LIGO plans to upgrade its observatories and improve the sensitivity of its detectors before the next observational period begins in late 2018. LIGO predicts that once its observatories reach their most sensitive state, they may able to detect as many as 40 gravitational waves per year.

    3

    More instruments like LIGO will soon be listening for waves around the world in Italy, Japan, and India. Scientists also hope to place interferometers in orbit in order to avoid interference from Earth noise.

    And that means much more computing power will be required to verify the signals and extract information about the nature and origins of our universe.

    See the full article here .

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    Science Node is an international weekly online publication that covers distributed computing and the research it enables.

    “We report on all aspects of distributed computing technology, such as grids and clouds. We also regularly feature articles on distributed computing-enabled research in a large variety of disciplines, including physics, biology, sociology, earth sciences, archaeology, medicine, disaster management, crime, and art. (Note that we do not cover stories that are purely about commercial technology.)

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  • richardmitnick 12:07 pm on June 1, 2017 Permalink | Reply
    Tags: , , , , , Gravitational waves, LIGO snags another set of gravitational waves   

    From ScienceNews: “LIGO snags another set of gravitational waves” 

    ScienceNews bloc

    ScienceNews

    June 1, 2017
    Emily Conover

    Spacetime vibrations arrive from black hole collision 3 billion light-years away.

    1
    THREE OF A KIND Scientists have made a third detection of gravitational waves. A pair of black holes, shown above, fused into one, in a powerful collision about 3 billion light-years from Earth. That smashup churned up ripples in spacetime that were detected by the LIGO experiment.

    For a third time, scientists have detected the infinitesimal reverberations of spacetime: gravitational waves.

    Two black holes stirred up the spacetime wiggles, orbiting one another and spiraling inward until they fused into one jumbo black hole with a mass about 49 times that of the sun. Ripples from that union, which took place about 3 billion light-years from Earth, zoomed across the cosmos at the speed of light, eventually reaching the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, which detected them on January 4.

    “These are the most powerful astronomical events witnessed by human beings,” Michael Landry, head of LIGO’s Hanford, Wash., observatory, said during a news conference May 31 announcing the discovery. As the black holes merged, they converted about two suns’ worth of mass into energy, radiated as gravitational waves.

    ________________________________________________________________________________
    Place in space

    Based on the time that signals arrived at each of LIGO’s two detectors, scientists were able to determine regions on the sky from which the gravitational waves came. LIGO’s three detections are shown, plus a fourth possible detection that was not strong enough to confirm. Lines indicate probabilities that the signal originated within each region. Outermost curves indicate 90 percent, while inner curves indicate 10 percent.

    2
    Leo Singer/LIGO, Caltech, MIT; Axel Mellinger (Milky Way image)

    ________________________________________________________________________________

    LIGO’s two detectors, located in Hanford and Livingston, La., each consist of a pair of 4-kilometer-long arms. They act as outrageously oversized rulers to measure the stretching of spacetime caused by gravitational waves.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    ESA/eLISA the future of gravitational wave research

    According to Einstein’s theory of gravity, the general theory of relativity, massive objects bend the fabric of space and create ripples when they accelerate — for example, when two objects orbit one another. Gravitational ripples are tiny: LIGO is tuned to detect waves that stretch and squeeze the arms by a thousandth of the diameter of a proton. Black hole collisions are one of the few events in the universe that are catastrophic enough to produce spacetime gyrations big enough to detect.

    The two black holes that spawned the latest waves were particularly hefty [Physical Review Letters], with masses about 31 and 19 times that of the sun, scientists report June 1 in Physical Review Letters. LIGO’s first detection, announced in February 2016, came from an even bigger duo: 36 and 29 times the mass of the sun (SN: 3/5/16, p. 6). Astrophysicists don’t fully understand how such big black holes could have formed. But now, “it seems that these are not so uncommon, so clearly there’s a way to produce these massive black holes,” says physicist Clifford Will of the University of Florida in Gainesville. LIGO’s second detection featured two smaller black holes, 14 and eight times the mass of the sun (SN: 7/9/16, p. 8).

    _________________________________________________________________________________

    Sizing up gravitational waves

    LIGO’s three gravitational wave sightings all came from merging black holes. But those mergers varied in mass, distance and the amount of energy radiated in gravitational waves.
    First detection

    Date: September 14, 2015
    Mass of first black hole: 36.2 solar masses
    Mass of second black hole: 29.1 solar masses
    Merged mass: 62.3 solar masses
    Energy radiated as gravitational waves: 3 solar masses
    Distance from Earth: 1.4 billion light-years
    Second detection

    Date: December 26, 2015
    Mass of first black hole: 14.2 solar masses
    Mass of second black hole: 7.5 solar masses
    Merged mass: 20.8 solar masses
    Energy radiated as gravitational waves: 1 solar mass
    Distance from Earth: 1.4 billion light-years
    Third detection

    Date: January 4, 2017
    Mass of first black hole: 31.2 solar masses
    Mass of second black hole: 19.4 solar masses
    Merged mass: 48.7 solar masses
    Energy radiated as gravitational waves: 2 solar masses
    Distance from Earth: 2.9 billion light-years

    _________________________________________________________________________________

    Weighty black holes are difficult to explain, because the stars that collapsed to form them must have been even more massive. Typically, stellar winds steadily blow away mass as a star ages, leading to a smaller black hole. But under certain conditions, those winds might be weak — for example, if the stars contain few elements heavier than helium or have intense magnetic fields (SN Online: 12/12/16). The large masses of LIGO’s black holes suggest that they formed in such environments.

    Scientists also disagree about how black holes partner up. One theory is that two neighboring stars each explode and produce two black holes, which then spiral inward. Another is that black holes find one another within a dense cluster of stars, as massive black holes sink to the center of the clump (SN Online: 6/19/16).

    The new detection provides some support for the star cluster theory: The pattern of gravitational waves LIGO observed hints that one of the black holes might be spinning in the opposite direction from its orbit. Like a cosmic do-si-do, each black hole in a pair twirls on its own axis as it spirals inward. Black holes that pair up as stars are likely to have their spins aligned with their orbits. But if the black holes instead find one another in the chaos of a star cluster, they could spin any which way. The potentially misaligned black hole LIGO observed somewhat favors the star cluster scenario. The measurement is “suggestive, but it’s not definite,” says astrophysicist Avi Loeb of Harvard University.

    Scientists will need more data to sort out how the black hole duos form, says physicist Emanuele Berti of the University of Mississippi in Oxford. “Probably the truth is somewhere in between.” Various processes could contribute to the formation of black hole pairs, Berti says.

    As with previous detections of gravitational waves, the scientists used their measurements to test general relativity. For example, while general relativity predicts that gravitational waves travel at the speed of light, some alternative theories of gravity predict that gravitational waves of different energies travel at different speeds. LIGO scientists found no evidence of such an effect, vindicating Einstein once again.

    Now, with three black hole mergers under their belts, scientists are looking forward to a future in which gravitational wave detections become routine. The more gravitational waves scientists detect, the better they can test their theories. “There are already surprises that make people stop and revisit some old ideas,” Will says. “To me that’s very exciting.”

    See the full article here .

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  • richardmitnick 9:41 am on May 29, 2017 Permalink | Reply
    Tags: , , , Background of gravitational waves expected from binary black hole events like GW150914, , , , Gravitational waves   

    From LIGO: “Background of gravitational waves expected from binary black hole events like GW150914” 

    LSC LIGO Scientific Collaboration

    LIGO Scientific Collaboration

    5.29.17 Presentation
    No writer credit found


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    ESA/eLISA the future of gravitational wave research

    Introduction
    It is an amazing time in the field of gravitational-wave astronomy—the observation of gravitational waves by the Advanced LIGO detectors from a binary black hole merger is an event of tremendous scientific significance. A century ago, Einstein developed general relativity and predicted the existence of gravitational waves. This first direct detection of gravitational waves, the so-called “GW150914” event, is a confirmation of Einstein’s theory and is the first direct observation of a pair of black holes merging to form a single black hole. The observation of GW150914, and future observations of binary black hole mergers, will provide new insights about massive black holes in the relatively nearby part of our Universe.

    GW150914 will not be the only event of its type in the Universe. It can be expected that, on average, binary black hole mergers occur at some rate. When these mergers happen within a couple of billion light-years from the Earth, they will likely be directly observed by Advanced LIGO (and soon to come, Advanced Virgo, and ESA eLISA). Events that are further than this will just appear as random noise in the gravitational-wave detectors (like static on an old-fashioned TV), too small to be individually directly measured. However, it will be possible to observe the sum of binary black hole mergers that have happened throughout the history of our observable Universe.

    What is a gravitational-wave background?
    Gravitational waves are ripples in spacetime predicted by Einstein’s theory of general relativity. Gravitational waves are produced by accelerating objects of any size, including us humans. Most of these waves, however, are far too weak to experimentally detect. In general, we can only hope to observe gravitational waves produced by the most massive objects moving close to the speed of light, such as the binary black holes that produced GW150914.

    For every nearby, loud event like GW150914, there are many more that are too far away to be individually detected by Advanced LIGO. The gravitational waves from these distant binary black holes instead combine to create a relatively quiet “popcorn” background of gravitational waves. As a pair of black holes merges, it produces a short burst of gravitational waves lasting just a few tenths of a second. These mostly-quiet individual bursts are separated in time, and arrive at Earth at an average rate of about one every 15 minutes. Their exact arrival times, though, are randomly distributed, just like the random popping of individual kernels of popcorn.

    A popcorn background is an example of a broader category of gravitational-wave signal called a stochastic background. In general, stochastic backgrounds are formed by the combination of many unresolvable sources. Unresolvable sources are those which we cannot distinguish individually, either because they are too quiet (as in the case of a popcorn background described above) or because there are simply too many occurring at once. Detecting a stochastic background is something like listening to voices in a crowded room. Aside from the loudest people and those standing nearest to you, you can hear the remaining conversations blend together into a continuous hum.

    What can it tell us?
    Gravitational waves are expected to have been created throughout the history of the Universe. Depending on when they were produced, gravitational-wave backgrounds can be classified into two basic categories: cosmological and astrophysical. Cosmological backgrounds are predicted to have been produced by sources that existed in the very early Universe just a few seconds after the Big Bang, while astrophysical backgrounds are predicted to have been produced by systems of massive stars such as the neutron stars and black holes that we see today. Most likely, contributions to the gravitational-wave background from astrophysical sources will dominate cosmological ones. The gravitational-wave signal produced by a population of binary black holes like GW150914 is an example of an astrophysical background.

    The strength of the gravitational-wave background at different frequencies strongly depends on the type of sources that produce them. Thus, depending on the type of gravitational-wave background we detect, we may learn about the state of the Universe just a few moments after the Big Bang or how the Universe is evolving in more recent times. In addition, looking at whether the signal is stronger from certain directions on the sky or is uniformly spread out will give us information about the distribution of the sources that produced the background.

    What does GW150914 mean for a gravitational-wave background? Why is this interesting?
    GW150914 was a single event. Its gravitational waves were from the merger of two black holes, having masses of about 29 times and 36 times the mass of the Sun, forming a single black hole with a mass of about 62 times the Sun’s mass. The large component masses of the black holes making up GW150914 suggest that the unresolved popcorn signal from the population of binary black holes is probably stronger than most astrophysicists originally thought. This means we have a better chance of measuring this background with the Advanced LIGO and Virgo detectors. We will have to analyze the data for several years, but it may be possible to eventually measure the signal.

    How do we detect these gravitational waves?
    Although the individual signals from the binary black hole mergers have a characteristic shape, the signals from distant sources will be too weak to be individually detected and the arrival times of the popcorn-like bursts will be random. This means that the standard searches that look for single events like GW150914 won’t work for detecting the relatively quiet popcorn signal from the population of binary black hole mergers throughout the Universe. So we need to take a different approach (described below) to detect the waves from these unresolvable sources.

    It is hard to search for a weak random signal using data from a single detector because the detector noise itself is also random. So instead we compare (“correlate”) data from pairs of detectors, e.g., the two LIGO detectors in Hanford, WA and Livingston, LA. The random gravitational-wave signal will be the same (“correlated”) in both detectors, while the detector noise will not (since the detectors are widely separated and most noise sources due to the environment are local to the detectors). Thus we can use this similarity (“correlation”) to distinguish the gravitational-wave signal from unwanted detector noise.

    Moreover, by performing this correlation over the whole duration of the run (which could be months or years), we build up the signal relative to the noise by including the contribution of the events that occur (on average) once every 15 minutes. The more data we have to analyze, the better it is for this type of search.

    Estimating the gravitational-wave background based on what we’ve learned from GW150914
    A number of factors contribute to how many binary black hole systems will form in the Universe. An important long-term research question will be to try and describe how systems like GW150914 were created. For example, maybe the two original black holes in GW150914 evolved from a binary star system, where the two stars orbiting one another were very massive. Or perhaps the system was created in a globular cluster (a group of stars tightly bound together by gravity) where one could imagine many interactions taking place that would make initially small black holes evolve into larger ones. The most massive stars are short-lived and often produce black holes upon their death, so knowing the birth rate of massive stars is important. Star formation rates depend on the amount of matter present, and its constituents; was there just hydrogen and helium present when the star was formed, or were there other elements as well? Also, how long did the two black holes take to get close enough to collide? All of these ingredients contribute to the different binary black hole formation models that we considered.

    If we detect a stochastic gravitational-wave background in the future, we will not be able to distinguish between different models describing how these binary black hole systems were created. We will, however, be able to contribute to understanding how often these mergers happen in the far away Universe. Future measurements of individual mergering black holes will provide a better estimate of how often these types of events take place in the nearby Universe and more information on the masses of the black holes. Combining what we learn from a stochastic background with measurements from individual events may help distinguish between different formation pathways for binary black holes.

    Implications for the future
    The GW150914 event suggests that merger rates and masses of binary black holes are on the high end of the range of earlier predictions. This means that a gravitational-wave background due to merging black holes is expected to be larger as well. There are sizable uncertainties associated with the strength of this background, but detecting it may be within reach of the advanced detectors at their peak sensitivity. Looking to the future, the next generation of gravitational-wave detectors might be able combine measurements of a gravitational-wave background with measurements of individual black hole mergers to distinguish how black holes may come to orbit one another.

    Glossary

    Astrophysical background A stochastic background of gravitational waves produced by sources such as neutron stars and black holes.
    Black hole A region of space-time caused by an extremely compact mass where the gravity is so intense it prevents anything, including light, from leaving.
    Correlation The amount of similarity between two sets of data. For example, the comparison of data from pairs of gravitational-wave detectors in order to search for weak signals (like a stochastic background) that are shared (“correlated”) between the two instruments.
    Cosmological background A stochastic background of gravitational waves produced by sources such as those in the very early Universe in the instant after the Big Bang.
    Globular cluster A very dense group of stars bound together by gravity.
    Neutron star An extremely dense object which remains after the collapse of a massive star.
    Popcorn background The combination effects of bursts of gravitational waves from all binary black holes too distant to be directly observed. These bursts arrive at Earth at random times, like the popping of individual kernels of corn.
    Spacetime An interwoven continuum of space and time.
    Stochastic background A gravitational-wave signal formed from the combination of many individually unresolvable sources. Sources are unresolvable if they are too weak to be directly observed or if too many overlap at once, like conversations in a loud, crowded room.
    Stochastic Randomly determined; having a random pattern that may be analyzed statistically but may not be predicted precisely.

    Read more:

    Freely readable preprint of the publication describing the analysis
    Data used for this analysis
    Advanced LIGO
    Advanced Virgo

    See the full article here .

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    About the LSC

    The LIGO Scientific Collaboration (LSC) is a group of scientists seeking to make the first direct detection of gravitational waves, use them to explore the fundamental physics of gravity, and develop the emerging field of gravitational wave science as a tool of astronomical discovery. The LSC works toward this goal through research on, and development of techniques for, gravitational wave detection; and the development, commissioning and exploitation of gravitational wave detectors.

    The LSC carries out the science of the LIGO Observatories, located in Hanford, Washington and Livingston, Louisiana as well as that of the GEO600 detector in Hannover, Germany. Our collaboration is organized around three general areas of research: analysis of LIGO and GEO data searching for gravitational waves from astrophysical sources, detector operations and characterization, and development of future large scale gravitational wave detectors.

    Founded in 1997, the LSC is currently made up of more than 1000 scientists from dozens of institutions and 15 countries worldwide. A list of the participating universities.

    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 Gravitational Wave interferometer, near Pisa, Italy
    VIRGO Gravitational Wave interferometer, near Pisa, Italy

     
  • richardmitnick 4:40 pm on May 28, 2017 Permalink | Reply
    Tags: , , , , , , Gravitational waves,   

    From Monash: “Monash University researchers uncover new Gravitational Waves characteristics” 

    Monash Univrsity bloc

    Monash University

    23 May 2017

    1
    A visualization of a supercomputer simulation of merging black holes sending out gravitational waves. Credit: NASA/C. Henze/phys.org

    Monash University researchers have identified a new concept – ‘orphan memory’ – which changes the current thinking around gravitational waves.

    The research, by the Monash Centre for Astrophysics, was published recently in Physical Review Letters.

    Einstein’s theory of general relativity predicts that cataclysmic cosmic explosions stretch the fabric of spacetime.

    The stretching of spacetime is called ‘gravitational waves.’ After such an event, spacetime does not return to its original state. It stays stretched out. This effect is called ‘memory.’

    The term ‘orphan’ alludes to the fact that the parent wave is not directly detectable.

    “These waves could open the way for studying physics currently inaccessible to our technology,” said Monash School of Physics and Astronomy Lecturer, Dr Eric Thrane, one of the authors of the study, together with Lucy McNeill and Dr Paul Lasky.

    “This effect, called ‘memory’ has yet to be observed,” said Dr Thrane.

    Gravitational-wave detectors such as LIGO only ‘hear’’ gravitational waves at certain frequencies, explains lead author Lucy McNeill.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    ESA/eLISA the future of gravitational wave research

    “If there are exotic sources of gravitational waves out there, for example, from micro black holes, LIGO would not hear them because they are too high-frequency,” she said.

    “But this study shows LIGO can be used to probe the universe for gravitational waves that were once thought to be invisible to it.”

    Study co-author Dr Lasky said LIGO won’t be able to see the oscillatory stretching and contracting, but it will be able to detect the memory signature if such objects exist.

    The researchers were able to show that high-frequency gravitational waves leave behind a memory that LIGO can detect.

    “This realisation means that LIGO [or e/Lisa] may be able to detect sources of gravitational waves that no one thought it could,” said Dr Lasky.

    See the full article here .

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    Monash U campus

    Monash University (/ˈmɒnæʃ/) is an Australian public research university based in Melbourne, Australia. Founded in 1958, it is the second oldest university in the State of Victoria. Monash is a member of Australia’s Group of Eight and the ASAIHL, and is the only Australian member of the influential M8 Alliance of Academic Health Centers, Universities and National Academies. Monash is one of two Australian universities to be ranked in the The École des Mines de Paris (Mines ParisTech) ranking on the basis of the number of alumni listed among CEOs in the 500 largest worldwide companies.[6] Monash is in the top 20% in teaching, top 10% in international outlook, top 20% in industry income and top 10% in research in the world in 2016.[7]

    Monash enrolls approximately 47,000 undergraduate and 20,000 graduate students,[8] It also has more applicants than any university in the state of Victoria.

    Monash is home to major research facilities, including the Australian Synchrotron, the Monash Science Technology Research and Innovation Precinct (STRIP), the Australian Stem Cell Centre, 100 research centres[9] and 17 co-operative research centres. In 2011, its total revenue was over $2.1 billion, with external research income around $282 million.[10]

    The university has a number of centres, five of which are in Victoria (Clayton, Caulfield, Berwick, Peninsula, and Parkville), one in Malaysia.[11] Monash also has a research and teaching centre in Prato, Italy,[12] a graduate research school in Mumbai, India[13] and a graduate school in Jiangsu Province, China.[14] Since December 2011, Monash has had a global alliance with the University of Warwick in the United Kingdom.[15] Monash University courses are also delivered at other locations, including South Africa.

    The Clayton campus contains the Robert Blackwood Hall, named after the university’s founding Chancellor Sir Robert Blackwood and designed by Sir Roy Grounds.[16]

    In 2014, the University ceded its Gippsland campus to Federation University.[17] On 7 March 2016, Monash announced that it would be closing the Berwick campus by 2018.

     
  • richardmitnick 9:26 am on May 21, 2017 Permalink | Reply
    Tags: , , , , , Gravitational waves,   

    From Manu Garcia: ” LIGO, Boxing Day” 


    Manu Garcia, a friend from IAC.

    The universe around us.
    Astronomy, everything you wanted to know about our local universe and never dared to ask.

    Author: Manu Astrologus – Update: 21/5/17
    Second detection of gravitational waves.

    1
    Artist illustration represents two binary black hole systems for molten GW150914 (left) and GW151226 (right). Pair of black holes are shown together in this illustration but actually detected at different times and in different parts of the sky. The images have been scaled to show the difference in the masses of black holes. In the event GW150914 , black holes were 29 and 36 times the mass of the sun, while GW151226 , the two black holes weighing between 14 and 8 solar masses. Image Credit: LIGO / A. Simonnet.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    ESA/eLISA the future of gravitational wave research

    The two gravitational wave detectors LIGO Hanford Washington and Livingston Louisiana have captured a second robust signal of two black holes in their final orbits then coalescence, fusion <>, in a single black hole. This event, called GW151226 , was seen on 26 December at 3:38:53 (coordinated universal time, also known as Greenwich Mean Time) near the end of the first LIGO observation period ( “O1”), and it was immediately nicknamed “the boxing day event”.

    As the first detection LIGO , this event was identified few minutes after passage of the gravitational wave. Subsequently, careful studies of tools and environments around the observatories showed that observed in the two detectors signal was truly distant black holes, about 1,400 million light years away, coinciding with the same distance as the first detected signal. However, the Boxing Day event differed from the first observation of gravitational waves LIGO in some important ways.

    The gravitational wave detectors came to the two almost simultaneously, indicating that the source is somewhere in heaven ring halfway between the two detectors. Knowing our pattern detector sensitivity, we can add that was a little more likely overhead or underfoot instead of west or east. With only two detectors, however, we can not reduce it much more than that. This differs from the first detected signal LIGO ( GW150914 , from 14 September 2015), which came from the southeast, hitting the detector Louisiana before Washington.
    The two black holes merged in the event of Boxing Day were less massive (14 and 8 times the mass of our sun) than those observed in the first detection GW150914 (36 and 29 times the mass of our sun). While this made the weakest signal that GW150914 , when these lighter black holes were combined, changed its signal at higher frequencies that bring in the sensitive band LIGO before the fusion event observed in September. This allowed us to observe more orbits that the first detection-orbits about 27 in about one second (this compares with only two tenths of observation in the first detection). Combined, these two factors (smaller and observed masses orbits) were keys to allow LIGO detect a weaker signal. They also allowed us to make more accurate comparisons with General Relativity. Note: the signal again coincides with Einstein’s theory.
    Last but not least, the event Boxing Day revealed that one of the first black holes was spinning like a top – and this is a first opportunity for LIGO can state this with confidence. A rotating black hole suggests that this object has a different story – p. Maybe “he sucked” the mass of a companion star before or after a star collapsing to form a black hole, achieving rotated in the process.

    With these two detections confirmed, along with a third probable detection made in October 2015 (believed to also could be caused by a pair of coalescing black holes) we can now begin to estimate the rate of coalescence of black hole in the universe based not in theory, but in actual observations. Of course, with only a few signs, our estimate is large uncertainties, but maybe now is between 9 and 240 binary coalescence of black hole Gigaparsec cubic per year, or about one every 10 years in a volume a trillion times the volume galaxy of the Milky Way. Happily, in its first months of operation, they advanced LIGO detectors were sensitive enough to dig deep enough into space to see about an event every two months.

    Our next observation interval – Watching Round # 2, or “O2” – will begin in the fall of 2016. With improved sensitivity, we expect to see more coalescence of black holes and possibly detect gravitational waves from other sources, such as mergers of binary star neutrons. We also expect the Virgo detector will join us later in the race O2. Virgo will be enormously useful for locating sources in the sky, collapsing the ring until a patch, but also helping us to understand the sources of gravitational waves.

    LIGO releases its data to the public. This policy of open data allows others to analyze our data, ensuring that LIGO and Virgo collaborations do not lose anything in their analyzes, and hoping that others might be even more interesting events. Our data are shared in the Open LIGO Science Center. GW151226 has its own page there.

    We invite you to stroll the LIGO Laboratory website where you will find charts to help you understand the observation of Boxing Day, links to the press release and suggestions for scientific papers if you want to deepen further. There you will also find links to the website of LIGO Scientific Collaboration, and our collaboration sister, Virgo, which are essential for these scientific results.

    Credit:
    LIGO.

    See the full article here .

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  • richardmitnick 12:49 pm on May 19, 2017 Permalink | Reply
    Tags: , , , , ESA/Lisa, GEO600, Gravitational waves, ,   

    From Science Alert: “Einstein’s ‘Spooky’ Entanglement Is Guiding Next-Gen Gravitational Wave Detectors” 

    ScienceAlert

    Science Alert

    19 MAY 2017
    DAVID BLAIR

    1
    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    Breaking the standard quantum limit.

    The first direct detection of gravitational waves, a phenomenon predicted by Einstein’s 1915 general theory of relativity, was reported by scientists in 2016.

    Armed with this “discovery of the century”, physicists around the world have been planning new and better detectors of gravitational waves.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    ESA/eLISA the future of gravitational wave research

    Physicist Professor Chunnong Zhao and his recent PhD students Haixing Miao and Yiqiu Ma are members of an international team that has created a particularly exciting new design for gravitational wave detectors.

    The new design is a real breakthrough because it can measure signals below a limit that was previously believed to be an insurmountable barrier. Physicists call this limit the standard quantum limit. It is set by the quantum uncertainty principle.

    Proposal for gravitational-wave detection beyond the standard quantum limit through EPR entanglement
    Yiqiu Ma, Haixing Miao, Belinda Heyun Pang, Matthew Evans, Chunnong Zhao, Jan Harms, Roman Schnabel & Yanbei Chen

    The new design, published in Nature Physics this week, shows that this may not be a barrier any longer.

    Abstract

    In continuously monitored systems the standard quantum limit is given by the trade-off between shot noise and back-action noise. In gravitational-wave detectors, such as Advanced LIGO, both contributions can be simultaneously squeezed in a broad frequency band by injecting a spectrum of squeezed vacuum states with a frequency-dependent squeeze angle. This approach requires setting up an additional long baseline, low-loss filter cavity in a vacuum system at the detector’s site. Here, we show that the need for such a filter cavity can be eliminated, by exploiting Einstein–Podolsky–Rosen (EPR)-entangled signals and idler beams. By harnessing their mutual quantum correlations and the difference in the way each beam propagates in the interferometer, we can engineer the input signal beam to have the appropriate frequency-dependent conditional squeezing once the out-going idler beam is detected. Our proposal is appropriate for all future gravitational-wave detectors for achieving sensitivities beyond the standard quantum limit.
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    Figure 1
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    figure 2
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    Figure 3
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    Figure 4
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    Figure 5

    Using this and other new approaches may allow scientists to monitor black hole collisions and ‘spacequakes‘ across the whole of the visible Universe.

    4
    During a spacequake, Earth’s magnetic field shakes in a way that is analogous to the shaking of the ground during an earthquake. Image credit: Evgeny Panov, Space Research Institute of Austria.

    How gravitational wave detectors work

    Gravitational waves are not vibrations travelling through space, but rather vibrations of space itself.

    They have already told us about an unexpectedly large population of black holes. We hope that further study of gravitational waves will help us to better understand our Universe.

    But the technologies of gravitational wave detectors are likely to have enormous significance beyond this aspect of science, because in themselves they are teaching us how to measure unbelievably tiny amounts of energy.

    Gravitational wave detectors use laser light to pick up tiny vibrations of space created when black holes collide. The collisions create vast gravitational explosions.

    They are the biggest explosions known in the Universe, converting mass directly into vibrations of pure space.

    It takes huge amounts of energy to make space bend and ripple.

    Our detectors – exquisitely perfect devices that use big heavy mirrors with scarily powerful lasers – must measure space stretching by a mere billionth of a billionth of a metre over the four kilometre scale of our detectors. [LIGO, above.]

    These measurements already represent the smallest amount of energy ever measured.

    But for gravitational wave astronomers this is not good enough. They need even more sensitivity to be able to hear many more predicted gravitational ‘sounds’, including the sound of the moment the Universe was created in the big bang.

    This is where the new design comes in.

    A spooky idea from Einstein

    The novel concept is founded on original work from Albert Einstein.

    In 1935 Albert Einstein and co-workers Boris Podolsky and Nathan Rosen tried to depose the theory of quantum mechanics by showing that it predicted absurd correlations between widely spaced particles.

    Einstein proved that if quantum theory was correct, then pairs of widely spaced objects could be entangled like two flies tangled up in a spider’s web. Weirdly, the entanglement did not diminish, however far apart you allowed the objects to move.

    Einstein called entanglement “spooky action at a distance”. He was sure that his discovery would do away with the theory of quantum mechanics once and for all, but this was not to be.

    Since the 1980s physicists have demonstrated time and again that quantum entanglement is real. However much he hated it, Einstein’s prediction was right and to his chagrin, quantum theory was correct. Things at a distance could be entangled.

    Today physicists have got used to the ‘spookiness’, and the theory of entanglement has been harnessed for the sending of secret codes that cannot be intercepted.

    Around the world, organisations such as Google and IBM and academic laboratories are trying to create quantum computers that depend on entanglement.

    And now Zhao and colleagues want to use the concept of entanglement to create the new gravitational wave detector’s design.

    A new way to measure gravitational waves

    The exciting aspect of the new detector design is that it is actually just a new way of operating existing detectors. It simply uses the detector twice.

    One time, photons in the detector are altered by the gravitational wave so as to pick up the waves. The second time, the detector is used to change the quantum entanglement in such a way that the noise due to quantum uncertainty is not detected.

    The only thing that is detected is the motion of the distant mirrors caused by the gravitational wave. The quantum noise from the uncertainty principle does not appear in the measurement.

    To make it work, you have to start with entangled photons that are created by a device called a quantum squeezer. This technology was pioneered for gravitational wave astronomy at Australian National University, and is now an established technique.

    Like many of the best ideas, the new idea is a very simple one, but one that took enormous insight to recognise. You inject a minuscule amount of squeezed light from a quantum squeezer, and use it twice!

    Around the world physicists are getting ready to test the new theory and find the best way of implementing it in their detectors.

    One of these is the GEO gravitational wave detector at Hannover in Germany, which has been a test bed for many of the new technologies that allowed last year’s momentous discovery of gravitational waves.

    6
    http://www.geo600.org GEO600 aims at the direct detection of Einstein’s gravitational waves by means of a laser interferometer.

    See the full article here .

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  • richardmitnick 6:57 am on May 16, 2017 Permalink | Reply
    Tags: , , EPR paradox, Gravitational waves, , , ,   

    From COSMOS: “Using Einstein’s ‘spooky action at a distance’ to hear ripples in spacetime” 

    Cosmos Magazine bloc

    COSMOS

    16 May 2017
    Cathal O’Connell

    1
    The new technique will aid in the detection of gravitational waves caused by colliding black holes. Henze / NASA

    In new work that connects two of Albert Einstein’s ideas in a way he could scarcely have imagined, physicists have proposed a way to improve gravitational wave detectors, using the weirdness of quantum physics.

    The new proposal, published in Nature Physics, could double the sensitivity of future detectors listening out for ripples in spacetime caused by catastrophic collisions across the universe.

    When the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves in late 2015 it was the first direct evidence of the gravitational waves Einstein had predicted a century before.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    Now it another of Einstein’s predictions – one he regarded as a failure – could potentially double the sensitivity of LIGOs successors.

    The story starts with his distaste for quantum theory – or at least for the fundamental fuzziness of all things it seemed to demand.

    Einstein thought the universe would ultimately prove predictable and exact, a clockwork universe rather than one where God “plays dice”. In 1935 he teamed up with Boris Podolsky and Nathan Rosen to publish a paper they thought would be a sort of reductio ad absurdum. They hoped to disprove quantum mechanics by following it to its logical, ridiculous conclusion. Their ‘EPR paradox’ (named for their initials) described the instantaneous influence of one particle on another, what Einstein called “spooky action at a distance” because it seemed at first to be impossible.

    Yet this sally on the root of quantum physics failed, as the EPR effect turned out not to be a paradox after all. Quantum entanglement, as it’s now known, has been repeatedly proven to exist, and features in several proposed quantum technologies, including quantum computation and quantum cryptography.

    2
    Artistic rendering of the generation of an entangled pair of photons by spontaneous parametric down-conversion as a laser beam passes through a nonlinear crystal. Inspired by an image in Dance of the Photons by Anton Zeilinger. However, this depiction is from a different angle, to better show the “figure 8” pattern typical of this process, clearly shows that the pump beam continues across the entire image, and better represents that the photons are entangled.
    Date 31 March 2011
    Source Entirely self-generated using computer graphics applications.
    Author J-Wiki at English Wikipedia

    Now we can add gravity wave detection to the list.

    LIGO works by measuring the minute wobbling of mirrors as a gravitational wave stretches and squashes spacetime around them. It is insanely sensitive – able to detect wobbling down to 10,000th the width of a single proton.

    At this level of sensitivity the quantum nature of light becomes a problem. This means the instrument is limited by the inherent fuzziness of the photons bouncing between its mirrors — this quantum noise washes out weak signals.

    To get around this, physicists plan to use so-called squeezed light to dial down the level of quantum noise near the detector (while increasing it elsewhere).

    The new scheme aids this by adding two new, entangled laser beams to the mix. Because of the ‘spooky’ connection between the two entangled beams, their quantum noise is correlated – detecting one allows the prediction of the other.

    This way, the two beams can be used to probe the main LIGO beam, helping nudge it into a squeezed light state. This reduces the noise to a level that standard quantum theory would deem impossible.

    The authors of the new proposal write that it is “appropriate for all future gravitational-wave detectors for achieving sensitivities beyond the standard quantum limit”.

    Indeed, the proposal could as much as double the sensitivity of future detectors.

    Over the next 30 years, astronomers aim to improve the sensitivity of the detectors, like LIGO, by 30-fold. At that level, we’d be able to hear all black hole mergers in the observable universe.

    ESA/eLISA, the future of gravitational wave research

    However, along with improved sensitivity, the proposed system would also increase the number of photons lost in the detector. Raffaele Flaminio, a physicist at the National Astronomical Observatory of Japan, points out in a perspective piece for Nature Physics [no link], Flaminio that the team need to do more work to understand how this will affect ultimate performance.

    “But the idea of using Einstein’s most famous (mistaken) paradox to improve the sensitivity of gravitational-wave detectors, enabling new tests of his general theory of relativity, is certainly intriguing,” Flaminio writes. “Einstein’s ideas – whether wrong or right – continue to have a strong influence on physics and astronomy.”

    See the full article here .

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  • richardmitnick 8:43 am on May 2, 2017 Permalink | Reply
    Tags: , , , , , , Counterparts to Gravitational Wave Events: Very Important Needles in a Very Large Haystack, Gravitational waves   

    From astrobites: “Counterparts to Gravitational Wave Events: Very Important Needles in a Very Large Haystack” 

    Astrobites bloc

    Astrobites

    May 2, 2017
    Thankful Cromartie

    Title: Where and when: optimal scheduling of the electromagnetic follow-up of gravitational-wave events based on counterpart lightcurve models
    Authors: Om Sharan Salafia, Monica Colpi, Marica Branchesi, Eric Chassande-Mottin, Giancarlo Ghirlanda, Gabriele Ghisellini, & Susanna Vergani
    First Author’s Institutions: Universita degli Studi di Milano-Bicocca, Milano, Italy; INAF – Osservatorio Astronomico di Brera Merate, Merate, Italy; INFN – Sezione di Milano-Bicocca, Milano, Italy
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    Status: Submitted to ApJ [open access]

    The LIGO Scientific Collaboration’s historic direct detection of gravitational waves (GWs) brought with it the promise of answers to long-standing astrophysical puzzles that were unsolvable with traditional electromagnetic (EM) observations. In previous astrobites, we’ve mentioned that an observational approach that involves both the EM and GW windows into the Universe can help shed light on mysteries such as the neutron star (NS) equation of state, and can serve as a unique test of general relativity. Today’s paper highlights the biggest hinderance to EM follow-up of GW events: the detection process doesn’t localize the black hole (BH) and NS mergers well enough to inform a targeted observing campaign with radio, optical, and higher-frequency observatories. While EM counterparts to GW-producing mergers are a needle that’s likely worth searching an entire haystack for, the reality is that telescope time is precious, and everyone needs a chance to use these instruments for widely varying scientific endeavors.

    The first GW detection by LIGO, GW150914, was followed up by many observatories that agreed ahead of time to look for EM counterparts to LIGO triggers.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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


    The authors of this study propose to improve upon the near-aimless searches in swaths of hundreds of degrees that have been necessary following the first few GW candidate events (see Figure 1). Luckily, there are two key pieces of information we have a priori (in advance): information about the source of the GW signal that can be pulled out of the LIGO data, and an understanding of the EM signal that will be emitted during significant GW-producing events

    4
    Figure 1: Simplified skymaps for the two likely and one candidate (LVT151012) GW detections as 3-D projections onto the Milky Way. The largest contours are 90-percent confidence intervals, while the innermost are 10-percent contours. From the LIGO Scientific Collaboration.

    What are we even looking for?

    Mergers that produce strong GW signals include BH-BH, BH-NS, and NS-NS binary inspirals. GW150914 was a BH-BH merger, which is less likely to produce a strong EM counterpart due to a lack of circumbinary material. The authors of this work therefore focus on the two most likely signals following a BH-NS or NS-NS merger. The first is a short gamma-ray burst (sGRB), which would produce an immediate (“prompt”) gamma-ray signal and a longer-lived “afterglow” in a large range of frequencies. Due to relativistic beaming, it’s rare that prompt sGRB emission is detected, as jets must be pointing in our direction to be seen. GRB afterglows are more easily caught, however. The second is “macronova” emission from material ejected during the merger, which contains heavy nuclei that decay and produce a signal in the optical and infrared shortly after coalescence. One advantage to macronova events is that they’re thought to be isotropic (observable in all directions), so they’ll be more easily detected than the beamed, single-direction sGRBs.

    (Efficiently) searching through the haystack

    LIGO’s direct GW detection method yields a map showing the probability of the merger’s location on the sky (more technically, the posterior probability density for sky position, or “skymap”). The uncertainty in source position is partly so large because many parameters gleaned from the received GW signal, like distance, inclination, and merger mass, are degenerate. In other words, many different combinations of various parameters can produce the same received signal.

    An important dimension that’s missing from the LIGO skymap is time. No information can be provided about the most intelligent time to start looking for the EM counterpart after receiving the GW signal unless the search is informed by information about the progenitor system. In order to produce a so-called “detectability map” showing not only where the merger is possibly located but also when we’re most likely to observe the resulting EM signal at a given frequency, the authors follow an (albeit simplified) procedure to inform their searches.

    The first available pieces of information are the probability that the EM event, at some frequency, will be detectable by a certain telescope, and the time evolution of the signal strength. This information is available a priori given a model of the sGRB or macronova. Then, LIGO will detect a GW signal, from which information about the binary inspiral will arise. These parameters are combined with the aforementioned progenitor information to create a map that helps inform not only where the source will most likely be, but also when various observatories should look during the EM follow-up period. Such event-based, time-dependent detection maps will be created after each GW event, allowing for a much more responsive search for EM counterparts.

    6
    Figure 2: The suggested radio telescope campaign for injection 28840, the LIGO signal used to exemplify a more refined observing strategy. Instead of blindly searching this entire swath of sky, observations are prioritized by signal detectability as a function of time (see color gradient for the scheduled observation times). Figure 8 in the paper.

    Using these detectability maps to schedule follow-up observations with various telescopes (and therefore at different frequencies) is complicated to say the least. The authors present a potential strategy for follow-up using a real LIGO injection (a fake signal fed into data to test their detection pipelines) of a NS-NS merger with an associated afterglow. Detectability maps are constructed and observing strategies are presented for an optical, radio, and infrared follow-up search (see Figure 2 as an example). Optimizing the search for an EM counterpart greatly increased the efficiency of follow-up searches for the chosen injection event; for example, the example radio search would have found the progenitor in 4.7 hours, whereas an unprioritized search could have taken up to 47 hours.

    Conclusions

    The process of refining an efficient method for EM follow-up is distressingly complicated. Myriad unknowns, like EM signal strength, LIGO instrumental noise, observatory availability, and progenitor visibility on the sky all present a strategic puzzle that needs to be solved in the new era of multimessenger astronomy. This work proves that improvements in efficiency are readily available, and that follow-up searches for EM counterparts to GW events will likely be more fruitful as the process is refined.

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

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

    From aeon: “Echoes of a black hole” 

    1

    aeon

    1
    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
    http://www.spitzer.caltech.edu/images/1541-ssc2006-02a1-Spitzer-View-of-the-Center-of-the-Milky-Way

    3.29.17
    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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