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  • richardmitnick 11:02 am on April 11, 2018 Permalink | Reply
    Tags: , , , , , Dense stellar clusters may foster black hole megamergers, , LIGO/VIRGO   

    From Kavli MIT Institute For Astrophysics and Space Research: “Dense stellar clusters may foster black hole megamergers” 

    KavliFoundation

    http://www.kavlifoundation.org/institutes

    Kavli MIT Institute of Astrophysics and Space Research

    Kavli MIT Institute For Astrophysics and Space Research

    April 10, 2018
    Jennifer Chu

    When LIGO’s twin detectors first picked up faint wobbles in their respective, identical mirrors, the signal didn’t just provide first direct detection of gravitational waves — it also confirmed the existence of stellar binary black holes, which gave rise to the signal in the first place.

    1
    A snapshot of a simulation showing a binary black hole formed in the center of a dense star cluster. Credit: Northwestern Visualization/Carl Rodriguez. https://phys.org

    2
    A simulation showing an encounter between a binary black hole (in orange) and a single black hole (in blue) with relativistic effects. Eventually two black holes emit a burst of gravitational waves and merge, creating a new black hole (in red). Credit: Massachusetts Institute of Technology. https://phys.org


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    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

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    Stellar binary black holes are formed when two black holes, created out of the remnants of massive stars, begin to orbit each other. Eventually, the black holes merge in a spectacular collision that, according to Einstein’s theory of general relativity, should release a huge amount of energy in the form of gravitational waves.

    Now, an international team led by MIT astrophysicist Carl Rodriguez suggests that black holes may partner up and merge multiple times, producing black holes more massive than those that form from single stars. These “second-generation mergers” should come from globular clusters — small regions of space, usually at the edges of a galaxy, that are packed with hundreds of thousands to millions of stars.

    “We think these clusters formed with hundreds to thousands of black holes that rapidly sank down in the center,” says Carl Rodriguez, a Pappalardo fellow in MIT’s Department of Physics and the Kavli Institute for Astrophysics and Space Research. “These kinds of clusters are essentially factories for black hole binaries, where you’ve got so many black holes hanging out in a small region of space that two black holes could merge and produce a more massive black hole. Then that new black hole can find another companion and merge again.”

    If LIGO detects a binary with a black hole component whose mass is greater than around 50 solar masses, then according to the group’s results, there’s a good chance that object arose not from individual stars, but from a dense stellar cluster.

    “If we wait long enough, then eventually LIGO will see something that could only have come from these star clusters, because it would be bigger than anything you could get from a single star,” Rodriguez says.

    He and his colleagues report their results in a paper appearing in Physical Review Letters.

    Running stars

    For the past several years, Rodriguez has investigated the behavior of black holes within globular clusters and whether their interactions differ from black holes occupying less populated regions in space.

    Globular clusters can be found in most galaxies, and their number scales with a galaxy’s size. Huge, elliptical galaxies, for instance, host tens of thousands of these stellar conglomerations, while our own Milky Way holds about 200, with the closest cluster residing about 7,000 light years from Earth.

    In their new paper, Rodriguez and his colleagues report using a supercomputer called Quest, at Northwestern University, to simulate the complex, dynamical interactions within 24 stellar clusters, ranging in size from 200,000 to 2 million stars, and covering a range of different densities and metallic compositions. The simulations model the evolution of individual stars within these clusters over 12 billion years, following their interactions with other stars and, ultimately, the formation and evolution of the black holes. The simulations also model the trajectories of black holes once they form.

    “The neat thing is, because black holes are the most massive objects in these clusters, they sink to the center, where you get a high enough density of black holes to form binaries,” Rodriguez says. “Binary black holes are basically like giant targets hanging out in the cluster, and as you throw other black holes or stars at them, they undergo these crazy chaotic encounters.”

    It’s all relative

    When running their simulations, the researchers added a key ingredient that was missing in previous efforts to simulate globular clusters.

    “What people had done in the past was to treat this as a purely Newtonian problem,” Rodriguez says. “Newton’s theory of gravity works in 99.9 percent of all cases. The few cases in which it doesn’t work might be when you have two black holes whizzing by each other very closely, which normally doesn’t happen in most galaxies.”

    Newton’s theory of relativity assumes that, if the black holes were unbound to begin with, neither one would affect the other, and they would simply pass each other by, unchanged. This line of reasoning stems from the fact that Newton failed to recognize the existence of gravitational waves — which Einstein much later predicted would arise from massive orbiting objects, such as two black holes in close proximity.

    “In Einstein’s theory of general relativity, where I can emit gravitational waves, then when one black hole passes near another, it can actually emit a tiny pulse of gravitational waves,” Rodriguez explains. “This can subtract enough energy from the system that the two black holes actually become bound, and then they will rapidly merge.”

    The team decided to add Einstein’s relativistic effects into their simulations of globular clusters. After running the simulations, they observed black holes merging with each other to create new black holes, inside the stellar clusters themselves. Without relativistic effects, Newtonian gravity predicts that most binary black holes would be kicked out of the cluster by other black holes before they could merge. But by taking relativistic effects into account, Rodriguez and his colleagues found that nearly half of the binary black holes merged inside their stellar clusters, creating a new generation of black holes more massive than those formed from the stars. What happens to those new black holes inside the cluster is a matter of spin.

    “If the two black holes are spinning when they merge, the black hole they create will emit gravitational waves in a single preferred direction, like a rocket, creating a new black hole that can shoot out as fast as 5,000 kilometers per second — so, insanely fast,” Rodriguez says. “It only takes a kick of maybe a few tens to a hundred kilometers per second to escape one of these clusters.”

    Because of this effect, scientists have largely figured that the product of any black hole merger would get kicked out of the cluster, since it was assumed that most black holes are rapidly spinning.

    This assumption, however, seems to contradict the measurements from LIGO, which has so far only detected binary black holes with low spins. To test the implications of this, Rodriguez dialed down the spins of the black holes in his simulations and found that in this scenario, nearly 20 percent of binary black holes from clusters had at least one black hole that was formed in a previous merger. Because they were formed from other black holes, some of these second-generation black holes can be in the range of 50 to 130 solar masses. Scientists believe black holes of this mass cannot form from a single star.

    Rodriguez says that if gravitational-wave telescopes such as LIGO detect an object with a mass within this range, there is a good chance that it came not from a single collapsing star, but from a dense stellar cluster.

    “My co-authors and I have a bet against a couple people studying binary star formation that within the first 100 LIGO detections, LIGO will detect something within this upper mass gap,” Rodriguez says. “I get a nice bottle of wine if that happens to be true.”

    This research was supported in part by the MIT Pappalardo Fellowship in Physics, NASA, the National Science Foundation, the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University, the Institute of Space Sciences (ICE, CSIC) and Institut d’Estudis Espacials de Catalunya (IEEC), and the Tata Institute of Fundamental Research in Mumbai, India.

    See the full article here .

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    The mission of the MIT Kavli Institute (MKI) for Astrophysics and Space Research is to facilitate and carry out the research programs of faculty and research staff whose interests lie in the broadly defined area of astrophysics and space research. Specifically, the MKI will

    Provide an intellectual home for faculty, research staff, and students engaged in space- and ground-based astrophysics
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    Communicate to students, educators, and the public an understanding of and an appreciation for the goals, techniques and results of MKI’s research.

    The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.

    The Foundation’s mission is implemented through an international program of research institutes, professorships, and symposia in the fields of astrophysics, nanoscience, neuroscience, and theoretical physics as well as prizes in the fields of astrophysics, nanoscience, and neuroscience.

     
  • richardmitnick 1:37 pm on June 22, 2016 Permalink | Reply
    Tags: , , , , , LIGO/VIRGO   

    From Ethan Siegel: “What Is The Future Of Gravitational Wave Astronomy?” 

    From Ethan Siegel

    Jun 22, 2016

    1
    Image credit: R. Hurt – Caltech/JPL.

    After turning on in September of 2015, the twin Laser Interferometer Gravitational-wave Observatories — the LIGO detectors in Hanford, WA and Livingston, LA — simultaneously detected not just one but two definitive black hole-black hole mergers during its first run, despite having reached only 30% of the sensitivity it was designed for.

    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

    These two events, one of a 36 and a 29 solar mass black hole merging on September 14, 2015, and one of a 14 and an 8 solar mass black hole merging on December 26, 2015, provided the first definitive, direct detections of the gravitational wave phenomena. It’s a remarkable fact, in and of itself, that it took a full century after their predictions for technology to catch up to the theory, and actually catch them.

    2
    The first gravitational wave event ever directly detected. Image credit: Observation of Gravitational Waves from a Binary Black Hole Merger B. P. Abbott et al., (LIGO Scientific Collaboration and Virgo Collaboration), Physical Review Letters 116, 061102 (2016).

    But detecting these waves is just the beginning, as a new era in astronomy is now dawning. 101 years ago, Einstein put forth a new theory of gravitation: General Relativity. Instead of distant masses instantaneously attracting one another across the Universe, the presence of matter and energy deformed the fabric of spacetime. This entirely new picture of gravity brought with it a slew of unexpected consequences, including gravitational lensing, an expanding Universe, gravitational time dilation and — perhaps most elusively — the existence of a new type of radiation: gravitational waves. As masses moved or accelerated relative to one another through space, the reaction of space itself causes the very fabric itself to ripple. These ripples travel through space at the speed of light, and when they pass through our detectors after a journey across the Universe, we can detect these disturbances as gravitational waves.

    3
    The spacetime in our local neighborhood, which can be ever so slightly perturbed by passing gravitational waves. Image Credit: T. Pyle/Caltech/MIT/LIGO Lab.

    The easiest things to detect are the things that emit the largest signals, which are:

    large masses,
    with small distances between them,
    orbiting quickly,
    where the orbital changes are severe and significant.

    This means collapsed objects, like black holes and neutron stars, are the prime candidates. We also need to consider the frequency at which we can detect these objects, which will be roughly equal to the path length of the detector (the arm length multiplied by the number of reflections) divided by the speed of light.

    For LIGO, with its 4 km arms with a thousand reflections of the light before creating the interference pattern, it can see objects with frequencies in the millisecond range. This includes coalescing black holes and neutron stars in the final stages of a merger, along with exotic events like black holes or neutron stars that absorb a large chunk of matter and undergo a “quake” to become more spherical. A highly asymmetric supernova could create a gravitational wave as well; a core-collapse event is unlikely to make detectable gravitational waves but perhaps nearby merging white dwarf stars could do it!

    4
    Image credit: Bohn et al 2015, SXS team, of two merging black holes and how they alter the appearance of the background spacetime in General Relativity.

    We’ve seen black hole-black hole mergers already, and as LIGO continues to improve, we can reasonably expect to make the first population estimates of stellar mass black holes (from a few to maybe 100 solar masses) over the next few years. LIGO is also highly anticipating finding neutron star-neutron star mergers; when it reaches the designed sensitivity, it may see up to three or four of these events each month if our estimates of their merger rates and LIGO’s sensitivity are correct. This could teach us the origin of short-period gamma ray bursts, which are suspected to be merging neutron stars, but this has never been confirmed.

    5
    Illustration of a starquake occurring on the surface of a neutron star, one cause of a pulsar “glitch.” Image credit: NASA.

    Asymmetric supernovae and exotic neutron star quakes are fun, if perhaps rare phenomena, but it’s exciting to have a shot at studying these in a new way. But the biggest new advances will come when more detectors are built. When the VIRGO detector in Italy comes online, it will finally be possible to do true position triangulation: to locate exactly where in space these gravitational wave events are originating, making follow-up optical measurements possible for the first time.

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

    With additional new gravitational wave interferometers scheduled to be built in Japan and India, our coverage of the gravitational wave sky is slated to improve rapidly in the next few years.

    ESA/LISA Pathfinder
    ESA/LISA Pathfinder

    ESA/eLISA
    ESA/eLISA

    But the biggest advances will come from taking our gravitational wave ambitions into space. In space, you’re not limited by seismic noise, rumbling trucks or plate tectonics; you have the quiet vacuum of space as your backdrop. You’re not limited by the curvature of the Earth for how long you can build your gravitational wave observatory’s arms; you can put it in orbit behind the Earth, or even in orbit around the Sun! Instead of milliseconds, we can measure objects with periods of seconds, days, weeks or even longer. We’ll be able to detect the gravitational waves from supermassive black holes, including from some of the largest known objects in the entire Universe.

    And finally, if we build a large enough, sensitive enough space observatory, we could see the leftover gravitational waves from before the Big Bang itself. We could directly detect the gravitational perturbations from cosmic inflation, and not only confirm our cosmic origin in a whole new way, but simultaneously prove that gravitation itself is a quantum force in nature. After all, these inflationary gravitational waves can’t be generated unless gravitation itself is a quantum field. The success of LISA Pathfinder more than proves this is possible; all it takes is the right investment.

    6
    llustration of the density (scalar) and gravitational wave (tensor) fluctuations arising from the end of inflation. Image credit: National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) – Funded BICEP2 Program.

    There’s currently a hotly contested race as to what will be chosen as the flagship NASA mission of the 2030s. Although many groups are proposing good missions, the biggest dream is a space-based, gravitational wave observatory in orbit around the Sun. A series of these could make our wildest gravitational wave dreams come true. We have the technology; we’ve proved the concept; we know the waves are there. The future of gravitational wave astronomy is limited only by what the Universe itself gives us, and how much we choose to invest in it. But this new era has already dawned. The only question is how bright this new field in astronomy is going to be. And that part of it is completely up to us.

    See the full article here .

    Please help promote STEM in your local schools.

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
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