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  • richardmitnick 3:16 pm on February 16, 2017 Permalink | Reply
    Tags: , EHT,   

    From BBC: “Event Horizon Telescope ready to image black hole” 

    BBC
    BBC

    2.16.17
    Jonathan Amos

    1
    The EHT team has produced simulations of what Einstein’s theories predict the hole should look like. Hotaka Shiokawa/CFA/HARVARD

    Scientists believe they are on the verge of obtaining the first ever picture of a black hole.

    They have built an Earth-sized “virtual telescope” by linking a large array of radio receivers – from the South Pole, to Hawaii, to the Americas and Europe.

    There is optimism that observations to be conducted during 5-14 April could finally deliver the long-sought prize.

    In the sights of the so-called “Event Horizon Telescope” will be the monster black hole at the centre of our galaxy.

    Here is the Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    Sag A*  NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way
    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    Although never seen directly, this object, catalogued as Sagittarius A*, has been determined to exist from the way it influences the orbits of nearby stars.

    These race around a point in space at many thousands of km per second, suggesting the hole likely has a mass of about four million times that of the Sun.

    But as colossal as that sounds, the “edge” of the black hole – the horizon inside which an immense gravity field traps all light – may be no more than 20 million km or so across.

    And at a distance of 26,000 light-years from Earth, this makes Sagittarius A* a tiny pinprick on the sky.

    The Event Horizon Telescope (EHT) team is nonetheless bullish.

    “There’s great excitement,” said project leader Sheperd Doeleman from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

    “We’ve been fashioning our virtual telescope for almost two decades now, and in April we’re going to make the observations that we think have the first real chance of bringing a black hole’s event horizon into focus,” he told BBC News.

    EHT map

    2
    The eventual EHT array will have 12 widely spaced participating radio facilities

    The EHT’s trick is a technique called very long baseline array interferometry (VLBI).

    This combines a network of widely spaced radio antennas to mimic a telescope aperture that can produce the resolution necessary to perceive a pinprick on the sky.

    The EHT is aiming initially to get down to 50 microarcseconds. Team-members talk in analogies, describing the sharpness of vision as being the equivalent of seeing something the size of a grapefruit on the surface of the Moon.

    They emphasise the still complex years of work ahead, but also trail the prospect of an imminent breakthrough.

    The scientists certainly have an expectation of what they ought to see, if successful.

    Simulations rooted in Einstein’s equations predict a bright ring of light fringing a dark feature.

    The light would be the emission coming from gas and dust accelerated to high speed and torn apart just before disappearing into the hole.

    The dark feature would be the shadow the hole casts on this maelstrom.

    “Now, it could be that we will see something different,” Doeleman said.

    “As I’ve said before, it’s never a good idea to bet against Einstein, but if we did see something that was very different from what we expect we would have to reassess the theory of gravity.

    “I don’t expect that is going to happen, but anything could happen and that’s the beauty of it.”

    Over the years, more and more radio astronomy facilities have joined the project. A key recent addition is the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

    Its extraordinary state-of-the-art technology has at a stroke increased the EHT’s sensitivity by a factor of 10. Hence, the optimism ahead of April.

    Even so, scientists have had to install special equipment at all the radio facilities involved in the observations.

    This includes big hard drives to store colossal volumes of data, and atomic clocks to precisely timestamp it all.

    Nothing happens on the spot – the hard drives must first be flown to a large computing facility at MIT Haystack Observatory in Westford, just outside Boston, Massachusetts.

    “Our hard-drive modules hold the capacity of about 100 standard laptops,” said Haystack’s Vincent Fish.

    “We have multiple modules at each telescope and we have numerous telescopes in the array. So, ultimately, we’re talking about 10,000 laptops of data.”

    It is in Haystack’s correlator computer that the synthesis will begin.

    Some very smart imaging algorithms have had to be developed to make sense of the EHT’s observations, but it will not be a quick result.

    It could be the end of the year, perhaps the start of 2018, before the team releases an image in public.

    3
    All the data from the telescopes will be brought to MIT Haystack to be ingested in its correlator computer.

    Looking to the future, the scientists are already thinking about how to extend their techniques.

    For example, the matter closest to the event horizon and about to disappear into Sagittarius A* should take about 30 minutes to complete an orbit.

    Katie Bouman, from MIT’s Computer Science and Artificial Intelligence Laboratory, thinks it might be possible to capture this movement.

    “We want to push boundaries and to try to make movies from the data,” she told BBC News.

    “Maybe we can actually see some of the gas flowing around the black hole. That’s really the next stage of what we’re trying to accomplish with these imaging algorithms.”

    First and foremost, the team needs good weather at the participating observing stations in April.

    The strategy is to view the galactic centre at a wavelength of 1.3mm (230GHz). This has the best chance of piercing any obscuring gas and dust in the vicinity of the black hole. But if there is too much water vapour above the array’s receivers, the EHT will struggle even to see through Earth’s atmosphere.

    Just getting a resolved view of Sagittarius A* would be a remarkable triumph in itself. But the real objective here is to use the imaging capability to go test aspects of general relativity.

    If there are flaws to be found in Einstein’s ideas – and scientists suspects there are more complete explanations of gravity out there waiting to be discovered – then it is in the extreme environment of black holes that limitations should be exposed.

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

     
  • richardmitnick 1:05 pm on January 16, 2017 Permalink | Reply
    Tags: , , , , EHT, , ,   

    From Motherboard: “An Earth-Sized Telescope is About to ‘See’ a Black Hole For the First Time” 

    motherboard

    Motherboard

    January 13, 2017
    William Rauscher

    We were perched dizzyingly high in the Chilean Andes, ringed by a herd of sixty-six white giants. Through the broad windows of the low, nondescript building in which we stood, we could see massive white radio antennas outside against the Martian-red soil of the desolate Chajnantor Plateau, their dishes thrust towards a pure blue sky.

    This is the Atacama Large Millimeter Array, also known as ALMA—one of the world’s largest radio telescope arrays, an international partnership that spans four continents. In spring of 2017, ALMA, along with eight other telescopes around the world, will aim towards the center of the Milky Way, around 25,000 light years from Earth, in an attempt to capture the first-ever image of a black hole. This is part of a daring astronomy project called the Event Horizon Telescope (EHT).

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres
    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    My partner Dave Robertson and I took turns huffing from a can of oxygen to stave off the altitude sickness that can come on at 16,500 feet. Our guide Danilo Vidal, an energetic Chilean who wore his dark hair in a ponytail, pointed to a grey metal door with a glass window. “If we open that door,” said Vidal, “everyone in science will hate us for the rest of our lives.” Confused by this cryptic statement, I took another hit from the oxygen and peered through the glass, into the heart of the experiment.

    Among a small forest of processors, I could see an eggshell-white box that resembled a dorm room refrigerator. Inside was the brand-new maser, an ultraprecise atomic clock that syncs up every antenna on-site, and then syncs ALMA itself to the Event Horizon Telescope’s global network, lending so much dish-space and processing power that it effectively doubles the entire network’s resolution.

    1
    Christophe Jacques of the NRAO inspects the wiring on ALMA’s new hydrogen maser atomic clock during installation. Image: Carlos Padilla/NRAO/AUI/NSF

    To keep equipment from overheating, the room is kept at an absurdly low temperature—very close to absolute zero. If we opened the door, Vidal explained, emergency systems would instantly shut down the maser to protect it, and ALMA’s beating heart would stop, ruining multiple international astronomy projects, including the EHT.

    Claudio Follert, an ALMA fiber-optic specialist in his mid-fifties, was there in 2014 when the maser first arrived—he told me it was a machine he had never seen before, carried in by strange men. The men were sent by the EHT, which is based out of MIT.

    The EHT is made possible by the maser’s astonishing precision—about one billion times more precise than the clock in your smartphone.

    Designed by an international team led by MIT scientist Shep Doeleman, the EHT is the first of its kind-a global telescope network that uses a technique called interferometry to synthesize astronomical data from multiple sources, each with its own maser—including ALMA in Chile, the Large Millimeter Telescope atop the Sierra Negra volcano in Mexico, and the National Radio Astronomy Observatory in Virginia.

    Together, these telescopes create a super-telescope that is quite literally the size of the Earth, with enough resolution to photograph an orange on the Moon.

    With ALMA recently added to this Avengers-like team of radio telescopes, the network is ten times more sensitive. As a result, Doeleman’s group believes it has the firepower to penetrate the interstellar gases that cloak their targets: supermassive black holes. Drawn into orbit by the black holes’ gravity, these gases form gargantuan clouds that yield nothing to optical telescopes.

    Faint radio signals from the black holes, on the other hand, slip through the gas clouds and are ultimately detected on Earth.

    Black holes are the folk legends of outer space. Since no light can escape them, they’re invisible to the eye, and we have no confirmation that they actually exist—only heaps of indirect evidence, particularly the gravitational wobbles in orbits of nearby stars, the behavior of interstellar gas clouds, and the gaseous jets that spew into space when an unseen source of extreme gravity appears to rip cosmic matter to shreds.

    Black holes challenge our most fundamental beliefs about reality. Visionary scientific minds, including the theoretical physicists Stephen Hawking and Kip Thorne, have devoted entire books to unpacking the hallucinatory scenarios thought to be induced by black holes’ gravitational forces—imagine the bottom of your body violently wrenched away from the top, physically stretching you like a Looney Tunes character, a scenario that Thorne’s Black Holes and Time Warps paints in stomach-churning detail.

    2
    An image from the heart of the Milky Way from NASA’s Chandra X-ray Observatory. The supermassive black hole is at the center. Image: NASA/CXC/MIT/F. Baganoff et al.

    NASA/Chandra Telescope
    NASA/Chandra Telescope

    Sag A*  NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way
    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    Black holes are thought to lurk at the centers of galaxies including our own. Prove the existence of Sagittarius A*, the supermassive black hole at the heart of the Milky Way, and you are one step closer to solving another mystery: the origin of humankind, and all life as we know it.

    “The black hole at the center of our galaxy has everything to do with our own origin,” said Violette Impellizzeri, an ALMA astronomer collaborating with Event Horizon Telescope. Supermassive black holes are thought to regulate the stars that surround them, influencing their formation and orbit. “Understanding how our galaxy was formed leads to our own origin directly,” she said.

    Scientists estimate the mass of Sagittarius A* to be four million times that of our Sun, yet its diameter is roughly equal to the distance from our sun to Mercury—not much, in cosmic terms. The resulting density produces gravity so strong that space and time distort around it, making it invisible.

    The current theory, espoused by Thorne, is that the distance from the center of a black hole, known as the singularity, to its edge, known as the event horizon, becomes so warped that it nears infinite length, and light simply runs out of energy as it tries to escape.

    It took Doeleman, the project leader at MIT, to decide that in order to see the unseeable, you would first have to create a new kind of vision. With ALMA as part of the giant EHT network, we can take a radio “photograph” of the matter that orbits Sagittarius A*—called the accretion disk—and finally see the black hole in shadow: its first-ever portrait.

    • Vidal and Follert, the guide and fiber-optic specialist, led us out onto the plateaus. There was work to do: one of the antennas was hobbled by a damaged radio receptor.

    It was blindingly bright and windy, not to mention dry—Chajnantor is located in Chile’s Atacama Desert, the driest place on Earth, if you don’t count the poles. Completely inhospitable for human beings, Chajnantor is an ideal setting for a radio telescope: the elevation puts it closer to the stars, and the strikingly low water vapor keeps the cosmic signals pristine.

    For some, like ALMA’s crew, as well as Doeleman, the extreme environment is part of the attraction. “I just love getting to the telescopes,” he said. At 50, Doeleman is fresh-faced, with glasses and thinning hair that make him look every part the bookish scientist. His outgoing personality and entrepreneurial vigor reflect an explorer’s spirit more at home in the field than behind a desk.

    Doeleman regularly travels to each EHT site around the world, many of them located in extreme environments like the Andes or the Sierra Negra. “The adventure part is what motivates me—driving along dirt roads, up the sides of mountains, to install new instruments, doing observations that have never been done before. It’s a little bit like Jacques Cousteau—we’re not sitting in armchairs in our offices.”

    Outside on Chajnantor, I felt light-headed. I tried to keep my breathing steady: low oxygen can quickly wreck your mental faculties. On the plateau, Dave and I were dwarfed by ALMA’s antennas, which blocked out the desert sun. They felt powerful and eerie, like Easter Island statues. Even when standing directly beneath these behemoths, it wasn’t clear how they were controlled—the white dishes seemed to twist and pivot without warning.

    3
    Using a technique called interferometry, ALMA’s antennas can be configured to act as one giant antenna, and ALMA itself can be synced up with telescopes worldwide. Image: Dave Robertson

    An ALMA antenna is useless when one of its radio receptors is out of tune. We followed Follert up several steel ladders, boots clanging on metal, until we were in a low-ceilinged maintenance room inside one of the antennas. We helped him remove the damaged receptor, a long metal cylinder resembling a futuristic bazooka.

    Vidal drove us back down the mountain to the Operations Support Facility (OSF), ALMA’s headquarters, so we could see the lab where receptors are maintained.

    Per strict international regulations, Vidal was required to breathe through an oxygen tube as he drove, lest the high altitude cause him to lose consciousness behind the wheel.

    As we descended, Vidal radioed at regular intervals to identify our location. All around us the mountain slopes were red, rocky and barren—no wonder that NASA regularly deploys expeditions to this desert to replicate conditions on Mars.

    Located at 9,000 ft, the OSF is where ALMA’s staff call home: a total of 600 scientists working in shifts are based here, including engineers and technicians, from over 20 countries. The working conditions can be extreme. Staff hole up in weeklong shifts separated from friends and family, and endure the short and long-term health risks of high elevation, including a stroke or pulmonary edema, where fluid fills your lungs and you suffocate.

    It is thus maybe not surprising to find out that the entire staff are monitored regularly by medical personnel, and that emergency oxygen and a hyperbaric chamber are on-hand.

    They unwind by exercising and watching movies, although certain sci-fi flicks are frowned upon. “We need a break from space sometimes,” said Follert. Alcohol consumption on site is strictly forbidden—have even a tipple and you risk amplifying the physical effects of high elevation.

    4
    Aerial picture of ALMA’s Operations Support Facility. Image: Carlos Padilla/NRAO/AUI/NSF

    The close teamwork at ALMA is absolutely essential for the life of the observatory. Detecting cosmic radio signals, including those sent from a black hole, requires constant cooperation across teams, who must obsessively calibrate, maintain and repair their instruments to fend off unwanted noise.

    ALMA and the other telescopes on the EHT will soon turn towards the center of the Milky Way to tune in to the black hole’s narrow radio frequency. The data that ALMA collects will be so large, it cannot be transferred online. Instead, physical hard drives will shipped by “sneakernet”: loaded into the belly of a 747 and flown directly to MIT.

    When ALMA’s data is correlated with the other telescopes later this year, Sagittarius A* should appear against the glowing gas of the accretion disk. Maybe.

    Actually, said Doeleman, “we don’t know what we’re going to see. Nature can be cruel. We may see something boring. But we’re not married to one outcome—we’re going to see nature the way nature is.”

    See the full article here .

    The full EHT:

    Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    The future is wonderful, the future is terrifying. We should know, we live there. Whether on the ground or on the web, Motherboard travels the world to uncover the tech and science stories that define what’s coming next for this quickly-evolving planet of ours.

    Motherboard is a multi-platform, multimedia publication, relying on longform reporting, in-depth blogging, and video and film production to ensure every story is presented in its most gripping and relatable format. Beyond that, we are dedicated to bringing our audience honest portraits of the futures we face, so you can be better informed in your decision-making today.

     
    • Jim Ruebush 1:51 pm on January 16, 2017 Permalink | Reply

      Very interesting. I look forward to seeing results. The radio telescopes at Atacama are the subject of a blog post of mine a few years ago. http://bit.ly/2jpp7hl

      Only 2 miles from my home in Iowa is a radio telescope part of the VLBA. I’ve been fortunate to go up inside and stand in the dish. What fun.

      Keep up the good work and posts.

      Like

  • richardmitnick 9:37 am on November 30, 2016 Permalink | Reply
    Tags: , , EHT, How an Earth-sized telescope will 'see' a supermassive black hole',   

    From COSMOS: “How an Earth-sized telescope will ‘see’ a supermassive black hole’ “ 

    Cosmos Magazine bloc

    COSMOS

    30 November 2016
    Jake Port

    1
    The centre of the Milky Way galaxy, with the supermassive black hole Sagittarius A* (Sgr A*) in the middle, is the first target for the Event Horizon Telescope.
    X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

    Around the world, observatories gaze at the sky. But what if you could combine these to make a single high-resolution image – and examine the event horizon of the supermassive black hole at the centre of the Milky Way galaxy while you’re at it?

    This is the idea behind the Event Horizon Telescope, a virtual telescope so big it spans continents and hemispheres thanks to an imaging technique called interferometry.

    It works as different telescopes scattered across the globe record data on the same subject, which is then combined and processed by a supercomputer. This fills in the gaps to produce a final image.

    For instance, astronomers can use this technique to take information from two telescopes 100 kilometres apart, which creates an image similar to that taken by a single telescope 100 kilometres wide.

    The accuracy gets better with more telescopes and the greater their vertical and horizontal separation.

    Interferometry is used at sites such as the Atacama Large Millimeter Array in Chile, which comprises 66 moveable antenna dishes.


    Access mp4 video here .

    Now astronomers are thinking bigger. Much, much bigger.

    By combining radio telescope observatories in Antarctica, Greenland, Chile, Hawaii and a number of other locations scattered across the globe, astronomers plan to image the event horizon of the supermassive black hole at the centre of our Milky Way galaxy.

    Black holes are the great consumers of the universe. Their event horizon is the point of no return – not even light can escape once over the boundary.

    Astronomers suspect that in the centre of each big galaxy lies a supermassive black hole. The one hosted by the Milky Way is thought to be around 4.5 million times the mass of the sun.

    Despite its size, it and other black holes (supermassive or not) are currently impossible to see directly. Astronomers must instead observe the effects they have on their surrounds, such as the motion of stars.

    So why radio telescopes over other types? Unlike optical light, radio waves can penetrate clouds of dust and other material between Earth and the target.

    Supermassive black holes, being in the centre of a galaxy, are surrounded by plenty of dust and gas.

    And specific wavelengths of light work better in different situations. Speaking to the BBC, University of Arizona astrophysicist Feryal Özel explained: “We’ve run upwards of a million simulations, for many different configurations of what that gas might look like. And in all cases, we think that the 1.3-millimetre wavelength is the right choice to see down to the event horizon.”

    The Event Horizon Telescope won’t just reveal what the event horizon of a black hole looks like, but may also test Einstein’s general theory of relativity, which describes gravitation.

    This theory has been tested by measuring the distortion of light by large astronomical bodies such as the sun. But black holes are gravity powerhouses. Will the theory stand up in this extreme setting?

    Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 5:38 pm on October 14, 2016 Permalink | Reply
    Tags: , , EHT, , ,   

    From PI: “10 Fascinating Facts About the World’s Most Powerful Telescope” 

    Perimeter Institute
    Perimeter Institute

    October 12, 2016

    It’s a telescope essentially as large as the Earth, and it is shedding light on some of the most mysterious phenomena in the universe.

    A supermassive black hole churns at the heart of the Milky Way galaxy. To see it clearly, we need a telescope the size of the Earth. But building that is impossible.

    So scientists have flipped the problem, and turned the Earth into a telescope.

    The Event Horizon Telescope (EHT) is a global array of interconnected radio telescopes that is expected to soon provide humanity’s first glimpse of a black hole. And that’s just one of the many amazing things about it.

    Here are some fascinating facts about the world’s most remarkable telescope.

    2

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    11


    Access mp4 video here .

    Further Exploration:

    The Event Horizon Telescope Initiative at Perimeter Institute
    Testing General Relativity with Black Holes (story and video)

    Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    About Perimeter

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

     
  • richardmitnick 11:07 am on September 5, 2016 Permalink | Reply
    Tags: , , , EHT,   

    From Ethan Siegel: “What Is The Biggest Black Hole As Seen From Earth?” 

    From Ethan Siegel

    9.5.16

    1
    The supermassive black hole at the core of galaxy NGC 1277 weighs in at 17 billion solar masses. But it’s too distant to be resolved from Earth. Image credit: NASA / ESA / Andrew C. Fabian / Remco C. E. van den Bosch (MPIA).

    If you collapse a large enough mass into a small enough volume, you’ll create a black hole.

    2
    The anatomy of a very massive star throughout its life, culminating in a Type II Supernova. Image credit: Nicole Rager Fuller for the NSF.

    Every object has a gravitational field, and without enough speed, you can’t leave it; you can’t reach escape velocity.

    For black holes, where escape velocity is bigger than the speed of light at the event horizon, nothing can escape, not even light.

    3
    Black holes may still emit light from outside the event horizon, as accelerated matter either falls in or is funneled into jets, but nothing inside the event horizon can ever escape. Image credit: ESO/L. Calçada, of an illustration of the quasar SDSS J1106-1939.

    Black holes are formed from the collapse of incredibly massive objects: ultramassive stars imploding in supernovae at the end of their lives.

    But common, stellar mass black holes, at 1-100 times the Sun’s mass, are surpassed by rarer, supermassive ones.

    4
    The core of galaxy NGC 4261, like the core of a great many galaxies, show signs of a supermassive black hole in both infrared and X-ray observations. Image credit: NASA / Hubble and ESA.

    Almost every galaxy has one, including our Milky Way.

    Sag A*  NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way
    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    4
    The largest flare ever observed from the supermassive black hole at our galaxy’s center. Image credit NASA/CXC/Stanford/I. Zhuravleva et al.

    At 4 million solar masses, our black hole is only 26,000 light years away.


    Andrea Ghez, UCLA

    Other, larger, more distant galaxies, like Messier 87, have even larger black holes, reaching into the billions of solar masses.

    5
    Three views of the center of Messier 87 and its central, 6.6 billion solar mass black hole. Images credit: Top, optical, Hubble Space Telescope / NASA / Wikisky, via Wikimedia Commons user Friendlystar; lower left, radio, NRAO / Very Large Array (VLA); lower right, X-ray, NASA / Chandra X-ray telescope.

    Later this decade, an array of radio telescopes — the Event Horizon Telescope — comes online.

    Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    With a resolution of 10 micro-arc-seconds (μas), it should see the Milky Way’s supermassive black hole’s event horizon.

    6
    The expected view of the Milky Way’s supermassive black hole through the Event Horizon Telescope. It should be the only one directly visible. Image credit: S. Doeleman et al., via http://www.eventhorizontelescope.org/docs/Doeleman_event_horizon_CGT_CFP.pdf.

    With an angular size of 19 μas, no other black hole appears larger from Earth.

    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

     
  • richardmitnick 3:02 pm on August 28, 2016 Permalink | Reply
    Tags: , , , EHT, , ,   

    From SEEKER: “The Race to See Our Supermassive Black Hole” 

    Seeker bloc

    SEEKER

    May 26, 2016 [Article brought forward by ESO]
    No writer credit found

    Using the power of interferometry, two astronomical projects are, for the first time, close to directly observing the black hole in the center of the Milky Way.

    Sag A*  NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way
    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    There’s a monster living in the center of the galaxy.

    We know the supermassive black hole is there by tracking the motions of stars and gas clouds that orbit an invisible point. That point exerts an overwhelming tidal influence on all objects that get trapped in its gravitational domain and this force can be measured through stellar orbits to calculate its mass.

    It certainly isn’t the biggest black hole in the universe, but it isn’t the smallest either, it “weighs in” at an incredible 4 million times the mass of our sun.

    But this black hole behemoth, called Sagittarius A*, is over 20,000 light-years from Earth making direct observations, before now, nigh-on impossible. Despite its huge mass, the black hole is minuscule when seen from Earth; a telescope with an unprecedented angular resolution is needed.

    Though we already know a lot about Sagittarius A* from indirect observations, seeing is believing and there’s an international race, using the world’s most powerful observatories and sophisticated astronomical techniques, to zoom-in on the Milky Way’s black hole. This won’t only prove it’s really there, but it will reveal a region where space-time is so warped that we will be able to make direct tests of general relativity in the strongest gravity environment known to exist in the universe.

    The Event Horizon Telescope and GRAVITY

    A huge global effort is currently under way to link a network of global radio telescopes to create a virtual telescope that will span the width of our planet. Using the incredible power of interferometry, astronomers can combine the light from many distant radio antennae and collect it at one point, to mimic one large radio antenna spanning the globe.

    Event Horizon Telescope Array

    Event Horizon Telescope map
    Event Horizon Telescope map

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    This effort is known as the Event Horizon Telescope (EHT) and it is hoped the project will be able to attain the angular resolution and spatial definition required to soon produce its first radio observations of the bright ring just beyond Sagittarius A*’s event horizon — the point surrounding a black hole where nothing, not even light, can escape.

    However, another project has the same goal in mind, but it’s not going to observe in radio wavelengths, it’s going to stare deep into the galactic core to seek out optical and infrared light coming from Sagittarius A* and it just needs one observatory to make this goal a reality.

    1
    The ESO Very Large Telescope located atop Cerro Paranal in Chile. Ian O’Neill

    The GRAVITY instrument is currently undergoing commissioning at the ESO’s Very Large Telescope at Paranal Observatory high in the Atacama Desert in Chile (at an altitude of over 2,600 meters or 8,300 ft) and it will also use the power of interferometry to resolve our supermassive black hole. But rather than connecting global observatories like the EHT, GRAVITY will combine the light of the four 8 meter telescopes of the VLT Interferometer (collectively known as the VLTI) to create a “virtual” telescope measuring the distance between each individual telescope.

    ESO GRAVITY insrument
    ESO GRAVITY insrument

    “By doing this you can reach the same resolution and precision that you would get from a telescope that has a size, in this case, of roughly a hundred meters, simply because these eight meter-class telescopes are separated by roughly one hundred meters,” astronomer Oliver Pfuhl, of Max Planck Institute for Extraterrestrial Physics, Germany, told DNews. “If you combine the light from those you reach the same resolution as a virtual telescope of a hundred meters would have.”

    Strong Gravity Environment

    When GRAVITY is online it will be used to track features just outside Sagittarius A*’s event horizon.

    “For about ten years, we’ve known that this black hole is actually not black. Once in awhile it flares, so we see it brightening and darkening,” he said. This flaring is matter falling into the event horizon, generating a powerful flash of energy. The nature of these flares are poorly understood, but the instrument should be able to track this flaring material as it rapidly orbits the event horizon and fades away. These flares will also act as tracers, helping us see the structure of space-time immediately surrounding a black hole for the first time.

    2
    One of the four Very Large Telescope domes fires its new four-laser adaptive optics system. GRAVITY will make use of adaptive optics to improve observations of Sagittarius A* by compensating for the effects of atmospheric turbulence. ESO

    “Our goal is to measure these motions. We think that what we see as this flaring is actually gas which spirals into the black hole. This brightening and darkening is essentially the gas, when it comes too close to the black hole, the strong tidal forces make it heat up,” said Pfuhl.

    “If we can study these motions which happen so close to the black hole, we have a direct probe of the space time close to the black hole. In this way we have a direct test of general relativity in one of the most extreme environments which you can find in the universe.”

    While GRAVITY will be able to track these flaring events very close to the black hole, the Event Horizon Telescope will see the shadow, or silhouette, of the dark event horizon surrounded by radio wave emissions. Both projects will be able to measure different components of the region directly surrounding the event horizon, so combined observations in optical and radio wavelengths will complement one other.

    It just so happens that the Atacama Large Millimeter/submillimeter Array (ALMA), the largest radio observatory on the planet — also located in the Atacama Desert — will also be added to the EHT.

    “The Event Horizon Telescope will combine ALMA with telescopes around the world like Hawaii and other locations, and with that power you can look at really fine details especially in the black hole in the center of our galaxy and perhaps in some really nearby other galaxies that also have black holes in their centers,” ESO astronomer Linda Watson told DNews.

    3
    The ALMA antenna in a clustered formation on Chajnantor plateau during the #MeetESO event on May 11, 2016. The extreme location of the observatory can produce unpredictable weather and, as depicted here, a blizzard descended on the plateau cutting the visit short.
    Ian O’Neill

    ALMA itself is an interferometer combining the collecting power of 66 radio antennae located atop Chajnantor plateau some 5,000 meters (16,400 ft) in altitude. Watson uses ALMA data to study the cold dust in interstellar space, but when added to the EHT, its radio-collecting power will help us understand the dynamics of the environment surrounding Sagittarius A*.

    “ALMA’s an interferometer with 66 antennas, (the EHT) will treat ALMA as just one telescope and will combine it with other telescopes around the world to be another interferometer,” she added.

    Black Hole Mysteries

    Many black holes are thought to possess an accretion disk of swirling gas and dust. ALMA, when combined with the EHT, will be able to measure this disk’s structure, speed and direction of motion. Lacking direct observations, many of these characteristics have only been modeled by computer simulations or inferred from indirect observations. We’re about to enter an era when we can truly get to answer some of the biggest mysteries surrounding black hole dynamics.

    “The first thing we want to see is we want to understand how accretion works close to the black hole,” said Pfuhl. “This is also true for the Event Horizon Telescope. Another thing we want to learn is does our black hole have spin? That means, does it rotate?”

    Though the EHT and GRAVITY are working at different wavelengths, observing phenomena around Sagittarius A* will reveal different things about the closest supermassive black hole to Earth. By extension it is hoped that we may observe smaller black holes in our galaxy and other supermassive black holes in neighboring galaxies.

    3
    Computer simulation of what theoretical physiicists expect to see with the EHT — a round, dark disk surrounded by radio emissions.
    Avery E. Broderick/Univ. of Waterloo/Perimeter Institute (screenshot from the Convergence meeting)

    But as we patiently wait for the first direct observations of the black hole monster lurking in the center of our galaxy, an event that some scientists say will be as historic as the “Pale Blue Dot” photo of Earth as captured by Voyager 1 in 1990, it’s hard not to wonder which project will get there first.

    “I think it’s a very tight race,” said Pfuhl. “Let’s see.”

    See the full article here .

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  • richardmitnick 11:42 am on July 25, 2016 Permalink | Reply
    Tags: EHT, , VLBI   

    From blueshift: “Thirty Years of Space VLBI” 

    NASA Blueshift

    NASA Blueshift

    July 25, 2016
    Koji Mukai

    As I write this in July 2016, it has been 30 years since the first successful space very long baseline interferometry (VLBI) observations were made. VLBI is the radio astronomy technique to use widely separated radio dishes to produce exquisite images of celestial radio sources – and space VLBI allows separation between dishes larger than the diameter of the earth, potentially producing higher resolution images.

    But astute readers might be questioning my sanity. Many sources, including a page on our Imagine the Universe site will tell you that the first space VLBI satellite was Japan’s HALCA, which was launched in 1997. And 1997 was less than 20 years ago. Both these statements cannot be true – can they? Actually, yes, they can be, and they are. The actual sentence on the linked page reads: “The first mission dedicated to space interferometry was the Japanese HALCA mission which ran from 1997 to 2005.” The key phrase is “dedicated to” – you see, we sometimes use somewhat awkward phrasing in communicating with the general public when we don’t want to bother you with all the details, at least not initially. The hidden detail behind the sentence above is that well before HALCA, there was an earlier satellite which was used to demonstrate that space VLBI is possible, even though it was not specially designed for that purpose.

    1
    Top: This radio image of the galaxy M87, taken with the Very Large Array (VLA) radio telescope in February 1989, shows giant bubble-like structures where radio emission is thought to be powered by the jets of subatomic particles coming from the the galaxy’s central black hole. The false color corresponds to the intensity of the radio energy being emitted by the jet. M87 is located 50 million light-years away in the constellation Virgo. Bottom: A Very Long Baseline Array (VLBA) radio image of the region close to the black hole, where an extragalactic jet is formed into a narrow beam by magnetic fields. The false color corresponds to the intensity of the radio energy being emitted by the jet. The red region is about 1/10 light-year across. The image was taken in March 1999. Credit: NASA, National Radio Astronomy Observatory/National Science Foundation, John Biretta (STScI/JHU), and Associated Universities, Inc.

    But let’s back up and start with a refresher on the basics. Professional astronomers and the general public alike like to have the sharpest, the most detailed images of astronomical objects. For UV and optical telescopes, we need bigger telescope mirrors for this, and to preferably launch them into space so the images are not blurred by the Earth’s atmosphere. With these telescopes, we can approach the diffraction limit – the fundamental limit on the sharpness of images set by the physics of light. You see, light is a wave, and there is an intrinsic fuzziness in how it goes through a slit, is reflected by a mirror, etc. The minimum angular size of an image – the diffraction limit – is proportional to the wavelength and inversely proportional to the diameter of the telescope mirror.

    Radio waves have wavelengths often measured in centimeters, much larger than the wavelength of visible light, by a factor of almost a million. While it is easier to build a bigger radio dish than a bigger optical telescope, there is a practical limit. The giant Arecibo radio telescope, famously featured in the film Contact, based on a book by Carl Sagan, used to be the biggest radio telescope in the world.

    NAIC/Arecibo Observatory, Puerto Rico, USA
    NAIC/Arecibo Observatory, Puerto Rico, USA

    Now China just completed what is considered to be the world’s biggest radio telescope.

    FAST Chinese Radio telescope under construction, Guizhou Province, China
    FAST Chinese Radio telescope

    Though the diameters of these big radio dishes are on the order of 100 times the diameters of the biggest visible light mirrors, the wavelengths of radio waves are still so large that the diffraction limited images from any of these single dish telescopes are not very sharp.

    4
    Primary mirror size comparisons. Note Arecibo is so big that it is only represented by a dark gray arc at the bottom of the image. [FAST is not represented at all.] Credit: Cmglee, creative commons.

    Interferometry to the rescue. If you have an array of radio dishes, they can be combined to increase the effective size of the telescope and obtain sharp images. In technical terms, a baseline is the separation between a pair of radio dishes; you want long (and short) baselines in a variety of directions to make a sharp image. For example, the Karl G. Jansky Very Large Array (VLA) has 27 movable dishes in a Y shaped configuration, each arm of which is 21 km (13 miles) long. Image-wise, its performance is similar to a single, 40 km diameter, telescope.

    NRAO/VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA
    NRAO/VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

    VLBI is when you combine signal from multiple radio telescopes on Earth. Space VLBI allows you to have baselines that are longer than the diameter of the Earth. With VLBI (Earth-bound or including a satellite), you tend to have fewer participating telescopes, and you may have to rely on the rotation of the Earth or the orbital motion of the satellite to give you a variety of baselines. HALCA allowed baselines up to about 30,000 km (3 times the diameter of the Earth), and the Russian RadioAstron satellite has an orbit that takes it up to distances equivalent to halfway to the distance to the Moon.

    6
    Active galaxy (PKS 1519-273) as imaged with HALCA satellite, along with the National Science Foundation’s VLBA and VLA ground-based radio telescopes. This is the first VLBI image ever made using an orbiting radio-astronomy satellite. Credit: NRAO

    But space VLBI started 30 years ago, before these purposefully built satellites. What they used prior to them was the tracking and data relay satellite system (TDRSS), which NASA started in the 1980s for communication between the Space Shuttles and other satellites and ground stations. The communication is via radio waves in some of the same frequency bands used for astronomical radio observations. Back in July and August of 1986, astronomers and engineers used the TDRSS satellite (there was only one in orbit back then) together with the 64-m antenna of the NASA Deep Space Network at Tidbinbilla, Australia and the 64-m antenna of the Institute for Space and Astronautical Science in Usuda, Japan. They demonstrated space VLBI was possible, and that the three quasars they observed were very compact and beaming radio sources. This success opened the way for HALCA and RadioAstron.

    So, here’s to the 30th anniversary of the first successful space VLBI observations!

    See the full article here .

    [This article suffers from no mention of the Event Horizon Telescope (EHT), a new adventure in VLBI. Aimed specifically at exploration of supermassive black hole Sagittarius A*, at the center of the Milky Way, this new adventure will surely take on other projects in a life of its own. That is how science works.

    So, here is the EHT

    Event Horizon Telescope Array

    Event Horizon Telescope map
    Event Horizon Telescope map

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL ]

    Please help promote STEM in your local schools.
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    Blueshift is produced by a team of contributors in the Astrophysics Science Division at Goddard. Started in 2007, Blueshift came from our desire to make the fascinating stuff going on here every day accessible to the outside world.

    NASA Goddard Banner

    NASA image

     
  • richardmitnick 6:26 pm on January 19, 2016 Permalink | Reply
    Tags: , , EHT, eLISA, , ,   

    From PI: “Preparing for a cosmological challenge” 

    Perimeter Institute
    Perimeter Institute

    January 19, 2016
    Rose Simone

    Einstein’s theory of general relativity may soon be put to the ultimate test through measurements of a black hole’s shadow, say a pair of Perimeter researchers.
    __________________________________________________________________________________________________________________________________________
    Even though it is over 100 years old, Albert Einstein’s theory of general relativity is still a formidable prizefighter.

    The theory, which successfully describes gravity as a consequence of the curvature of spacetime itself, has withstood all the experimental tests that physicists have been able to throw at it over the decades.

    So now, to have any hope of challenging general relativity, they need to bring in a heavyweight. Enter the closest challenger: the smallish but still formidable 4.5-million- at the centre of our own Milky Way galaxy.

    The challenge will be assisted by the Event Horizon Telescope (EHT), a radio telescope array as large as the Earth, being configured to take precise images of the silhouette (or the shadow) of that black hole, known as Sagittarius A*.

    1
    Sag A*. This image was taken with NASA’s Chandra X-Ray Observatory. Ellipses indicate light echoes.

    NASA Chandra Telescope
    NASA/Chandra

    Event Horizon Telescope map
    EHT map

    Meanwhile, Tim Johannsen, a postdoctoral fellow at Perimeter Institute and the University of Waterloo, who works with Avery Broderick, an Associate Faculty member at Perimeter Institute jointly appointed at Waterloo, has led a group of researchers in calculating the measurements that will be used to determine whether general relativity really does stand up in the strong gravity regime of that black hole.

    3
    Perimeter postdoctoral researcher Tim Johannsen.

    4
    Perimeter Associate Faculty member Avery Broderick.

    Their paper was recently published in Physical Review Letters, along with an accessible synopsis of the work.

    When the images from the black hole come in and the measurements outlined in the recent paper are actually taken, it will be the first truly broad test of general relativity in the strong gravity regime.

    “That is very exciting and we expect to be able to do that within the next few years,” Johannsen says.

    Black holes are regions of spacetime, where gravity is so strong that not even light can escape once it has passed the threshold of no return − the event horizon. So as the name implies, they are dark.

    But owing to its immense gravity, the black hole pulls in vast quantities of dust and gas from surrounding stars. These accrete into a hot swirling plasma disk that illuminates the silhouette of the black hole. The EHT will be able to capture this, in images that will be historic firsts.

    A lot of physics will be done with the data gleaned from those images, but putting general relativity to the test is perhaps the most exciting challenge.

    General relativity has been fantastically successful. In every experiment that has been done to test how the sun and stars in our cosmos affect spacetime and exert gravitational pull on other objects, its predictions have held up.

    But the question is whether the theory will continue to hold up in a strong gravity environment, such as the surroundings of a black hole.

    Black holes are so massive and compact that the spacetime-warping effects, predicted by general relativity, would be more evident than around the sun or other stars. They are “orders of magnitude” different as gravitational environments go, Broderick says.

    “That means that this is terra incognita and we don’t know what we are going to find,” Broderick says. The EHT provides “an opportunity to begin probing in a critical way the non-linear nature of general relativity in the strong gravity regime.”

    This is important to physicists because even though general relativity has been enormously successful in explaining the cosmos that we can see, there are a number of difficulties with it. “It is not clear, for example, exactly how it should be combined with the quantum theory that we have, and in fact, it is very difficult to reconcile the two in a grand unification scheme,” Johannsen says.

    Moreover, there is the problem of the mysterious “dark energy” driving the accelerated expansion of spacetime, as well as the conundrum about the nature of “dark matter,” unseen mass theorized as an explanation for observed galaxy rotation rates that prevent galaxy clusters from flying apart. Physicists are hoping for some insights about general relativity in the strong gravity regime to make sense of these mysteries.

    Johannsen’s team has developed a way of checking how much the gravitational environment of this black hole might deviate from the theory of general relativity and other gravity theories.

    The paper sets constraints on the parameters of the size of the shadow to fit with general relativity. Other gravity models also propose modifications to the theory of general relativity, such as the Modified Gravity Theory (MOG) and the Randall-Sundrum-type braneworld model (RS2). The paper sets the constraints for the black hole to fit with these gravity models as well.

    “We have made the first realistic estimate of the high precision with which the EHT can detect the size of the shadow,”Johannsen says. “We show that such a measurement can be a precise test of general relativity.”

    A nice bonus from this work is that researchers will also get much more precise measurements of the mass of the black hole and its distance. “Sharpening the precision is great because that will enable us to get even more precise constraints on deviations from general relativity,” Johannsen adds.

    There are already good measurements of how far away Sagittarius A* is and how massive it is, based on other experiments that have looked at the motion of stars as they orbit the black hole, as well as of masers throughout the Milky Way, Johannsen explains. “People have been doing this for about 20 years.”

    This can be used to figure out what it should look like. But once the images from the EHT are available, it will be possible to check: “Do we get what we expect? Or do we get something else?” Johannsen says.

    Getting the measurements is really a matter of drawing a series of lines from the centre of the black hole image to the edge of its shadow. On the image, it looks like a pie shape with slices. Measuring the lines of each slice and calculating an average “gives us the angular radius of the shadow and then we know how big it is,” Johannsen says.

    6
    A reconstructed image of Sgr A* for an EHT observation at 230 GHz with a seven-station array.

    From the measurements of the size of the shadow, it is possible to see how closely the gravity in the black hole environment matches the predictions of general relativity and of other theories of gravity.

    “If general relativity is not correct, there can be significant change in the size. The shadow can also become asymmetric so that it is no longer circular, but egg-shaped, for example,” Johannsen says.

    Getting to the point of making these measurements will take a couple more years because at least seven or eight of the telescopes in the EHT array must be coordinated to get the data at the same time in a massive worldwide collaboration.

    The amount of raw data that has to be gathered to get the images is so enormous, it can’t even be transmitted over the internet.

    “These are humongous data sets. So they literally have to save all this data on hard drives and put them in a box and ship them,” Johannsen says.

    The hard drives get shipped to the MIT Haystack Observatory, which is the headquarters for the EHT. From there, the raw data is analyzed and the images are produced.

    After the images are produced, Johannsen gets to use his measurement technique to find out if general relativity is correct for the strong gravity environment around this black hole.

    This isn’t the only test of general relativity in the strong gravity regime in the works. There are other sophisticated experiments to detect, for example, the gravitational waves that are predicted by general relativity. But the prime experimental candidate to confirm the existence of gravitational waves would be the Evolved Laser Interferometer Space Antenna (eLISA), a space-based telescope with an estimated launch date of 2034.

    LISA graphic
    NASA LISA
    LISA

    The EHT will produce images in the next few years.

    If it turns out that the measurements yield what was expected and general relativity holds up, that would be interesting, “because Einstein had this theory 100 years ago, and then we will know that it is true,” Johannsen says.

    But if the challenger should prevail, and strong gravity does strike a blow to the theory of general relativity, “that would be big,” he adds.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

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    About Perimeter

    Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

     
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