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  • richardmitnick 2:08 pm on December 4, 2018 Permalink | Reply
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    From Lawrence Berkeley National Lab: “Topping Off a Telescope with New Tools to Explore Dark Energy” 

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    From Lawrence Berkeley National Lab

    December 4, 2018
    Glenn Roberts Jr.
    geroberts@lbl.gov
    (510) 486-5582


    In this time-lapse video, crews at the Mayall Telescope near Tucson, Arizona, lift and install the top-end components for the Dark Energy Spectroscopic Instrument, or DESI. The components, which include a stack of six lenses and other structures for positioning and support, weigh about 12 tons. DESI, scheduled to begin its sky survey next year, is designed to produce the largest 3-D map of the universe and produce new clues about the nature of dark energy. (Credit: Robert T. Sparks, NOAO/AURA)

    Key components for the sky-mapping Dark Energy Spectroscopic Instrument (DESI), weighing about 12 tons, were hoisted atop the Mayall Telescope at Kitt Peak National Observatory (KPNO) near Tucson, Arizona, and bolted into place Wednesday, marking a major project milestone.

    LBNL/DESI spectroscopic instrument on the Mayall 4-meter telescope at Kitt Peak National Observatory starting in 2018

    NOAO/Mayall 4 m telescope at Kitt Peak, Arizona, USA, Altitude 2,120 m (6,960 ft)

    Kitt Peak National Observatory of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O’odham Nation, 88 kilometers 55 mi west-southwest of Tucson, Arizona, Altitude 2,096 m (6,877 ft)

    DESI will create the largest 3-D map of the universe by gathering light from tens of millions of galaxies after its scheduled startup in late 2019. It is designed to provide more precise measurements of dark energy, which is accelerating the universe’s expansion and looms as one of the universe’s biggest mysteries.

    “Earlier this year we removed the old top-end of the Mayall Telescope, and Wednesday’s installation brings this telescope back to life with a new purpose,” said DESI Director Michael Levi of the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), which is leading the project’s international collaboration. “The more than 23,000 pounds of instrumentation that was installed represents the final top-end assembly.”

    The new top-end components include a 3.4-ton barrel-shaped, steel-framed structure, known as a corrector, that houses a precisely stacked array of large (the largest is 1.1 meters in diameter), delicate lenses.

    Also, the corrector is attached to a 1.1-ton six-axis “hexapod” that enables precise alignment adjustments; a surrounding ring, cage, and vanes support structure that weighs an additional 5.2 tons; and about 2 tons of other materials, including placeholder weights for DESI’s focal plane (see a related video), which is still under assembly and hasn’t yet arrived on site.

    A team at Fermi National Accelerator Laboratory built the corrector, hexapod, and other top-end support structures. The structures are designed to align the lenses with an accuracy of tens of microns (millionths of a meter) – similar to the width of the thinnest human hair.

    DESI involves more than 450 researchers from more than 70 institutions around the globe. KPNO is part of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation.

    The corrector will enable a larger field of view – covering an area more than 40 times larger than the telescope’s previous corrector, and more than 40 times the size of the full moon as seen from Earth’s surface – for a series of 5,000 robotically positioned fiber-optic cables that will gather light from sequences of targeted galaxies. This light will be analyzed to gather information about their distance and the rate at which the galaxies are moving away from us. Its large field of view will allow DESI to map one-third of the night sky during its planned 5-year survey.

    Each of the corrector’s six lenses began as a large, thick piece of glass made by either Corning Glass in New York, Ohara Corp. in Japan, or Schott AG in Germany (see a related video).

    One of the lenses housed in the corrector is among the largest to ever be fielded on a telescope, noted Berkeley Lab’s David Schlegel, a DESI project scientist. The lenses traveled the world for polishings and coatings at several companies, and were installed and precisely aligned inside the corrector barrel in a basement at the University College London last spring.

    The corrector was then disassembled for transport and flown on a chartered transport plane from England to Tucson, Arizona. Then, it was trucked to the Kitt Peak summit, at an elevation of 6,800 feet. Once reassembled, the corrector was connected to its mechanical support system.

    In June, a 250-foot mobile crane was used to lift the old top-end off of the telescope and out of the dome. A 50-ton crane in the dome of the Mayall Telescope was used to lift all the DESI top-end components up to the dome floor from the ground level, where they were assembled. The same dome crane was then used to lift the assembled DESI top-end into position atop the telescope.

    David Sprayberry, the KPNO site director for DESI, said, “This was a complex lift that went without a hitch. We had a dozen of our technical personnel ensuring that the new top-end would be positioned exactly onto the center of the telescope. I’m very proud of my team for pulling this off flawlessly.”

    Early next year, researchers will mount a set of cameras and other instruments onto DESI’s focal plane to test how the lenses perform across the entire imaging field. This commissioning camera array was built at Ohio State University.

    “This will be a real test to determine if all the lenses are working together perfectly,” said Paul Martini, a professor at Ohio State University who oversaw the development of the commissioning camera.

    Then, other DESI systems will be tested over the next several months until the entire instrument is ready to begin its sky survey.

    DESI is supported by the U.S. Department of Energy’s Office of Science; the U.S. National Science Foundation, Division of Astronomical Sciences under contract to the National Optical Astronomy Observatory; the Science and Technologies Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the National Council of Science and Technology of Mexico; the Ministry of Economy of Spain; and DESI member institutions. The DESI scientists are honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

    Current DESI Member Institutions include: Aix-Marseille University; Argonne National Laboratory; Barcelona-Madrid Regional Participation Group; Brookhaven National Laboratory; Boston University; Carnegie Mellon University; CEA-IRFU, Saclay; China Participation Group; Cornell University; Durham University; École Polytechnique Fédérale de Lausanne; Eidgenössische Technische Hochschule, Zürich; Fermi National Accelerator Laboratory; Granada-Madrid-Tenerife Regional Participation Group; Harvard University; Korea Astronomy and Space Science Institute; Korea Institute for Advanced Study; Institute of Cosmological Sciences, University of Barcelona; Lawrence Berkeley National Laboratory; Laboratoire de Physique Nucléaire et de Hautes Energies; Mexico Regional Participation Group; National Optical Astronomy Observatory; Ohio University; Siena College; SLAC National Accelerator Laboratory; Southern Methodist University; Swinburne University; The Ohio State University; Universidad de los Andes; University of Arizona; University of California, Berkeley; University of California, Irvine; University of California, Santa Cruz; University College London; University of Michigan at Ann Arbor; University of Pennsylvania; University of Pittsburgh; University of Portsmouth; University of Queensland; University of Rochester; University of Toronto; University of Utah; University of Zurich; UK Regional Participation Group; Yale University. For more information, visit desi.lbl.gov.

    See the full article here .

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    In the world of science, Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the National Academy of Sciences (NAS), one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the National Academy of Engineering, and three of our scientists have been elected into the Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

    Berkeley Lab is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. It is managed by the University of California (UC) and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above the UC Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly $700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at $1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

    Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a UC Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

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  • richardmitnick 12:37 pm on February 12, 2018 Permalink | Reply
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    From LBNL: “Solving the Dark Energy Mystery: A New Assignment for a 45-Year-Old Telescope” 

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    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    February 12, 2018
    Glenn Roberts Jr.
    geroberts@lbl.gov
    (510) 486-5582

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    A view inside the dome at the Mayall Telescope near Tucson, Arizona. The 2-meter corrector barrel atop the telescope will be removed and replaced with a new corrector barrel for the Dark Energy Spectroscopic Instrument (DESI). DESI’s installation will begin soon. (Credit: P. Marenfeld and NOAO/AURA/NSF)

    NOAO/Mayall 4 m telescope at Kitt Peak, Arizona, USA, Altitude 2,120 m (6,960 ft)

    LBNL/DESI Dark Energy Spectroscopic Instrument for the Nicholas U. Mayall 4-meter telescope at Kitt Peak National Observatory near Tucson, Ariz, USA

    Forty-five years ago this month, a telescope tucked inside a 14-story, 500-ton dome atop a mile-high peak in Arizona took in the night sky for the first time and recorded its observations in glass photographic plates.

    Fermilab scientists are managing key elements of the construction, including the heavy mechanical components (including the precise alignment of the structures that hold the lenses), the assembly of the charge-coupled devices, or CCDs, that capture light, and the online databases used for data acquisition. Fermilab is also providing software that maps between the locations of light sources in the sky and the positions of each of the 5,000 robotic fiber positioners that make up the DESI plane, drawing on more than 25 years of experience with the Sloan Digital Sky Survey and the Dark Energy Survey.

    For more information on Fermilab’s role in DESI, please contact Andre Salles at media@fnal.gov or 630-840-3351.

    Today, the dome closes on the previous science chapters of the 4-meter Nicholas U. Mayall Telescope so that it can prepare for its new role in creating the largest 3-D map of the universe. This map could help to solve the mystery of dark energy, which is driving the accelerating expansion of the universe.

    The temporary closure sets in motion the largest overhaul in the telescope’s history and sets the stage for the installation of the Dark Energy Spectroscopic Instrument (DESI), which will begin a five-year observing run next year at the National Science Foundation’s Kitt Peak National Observatory (KPNO) – part of the National Optical Astronomy Observatory (NOAO).

    NOAO Kitt Peak National Observatory on the Tohono O’odham reservation outside Tucson, AZ, USA

    “This day marks an enormous milestone for us,” said DESI Director Michael Levi of the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), which is leading the project’s international collaboration. “Now we remove the old equipment and start the yearlong process of putting the new stuff on.” More than 465 researchers from about 71 institutions are participating in the DESI collaboration.


    The Life of a Lens: This video chronicles the many steps it took to create a single lens for the Dark Energy Spectroscopic Instrument. (Credit: Google Earth, DESI Collaboration)

    The entire top end of the telescope, which weighs as much as a school bus and houses the telescope’s secondary mirror and a large digital camera, will be removed and replaced with DESI instruments. A large crane will lift the telescope’s top end through the observing slit in its dome.

    Besides providing new insights about the universe’s expansion and large-scale structure, DESI will also help to set limits on theories related to gravity and the formative stages of the universe, and could even provide new mass measurements for a variety of elusive yet abundant subatomic particles called neutrinos.

    “One of the primary ways that we learn about the unseen universe is by its subtle effects on the clustering of galaxies,” said DESI Collaboration Co-Spokesperson Daniel Eisenstein of Harvard University. “The new maps from DESI will provide an exquisite new level of sensitivity in our study of cosmology.”

    The Mayall Telescope has played an important role in many astronomical discoveries, including measurements supporting the discovery of dark energy and establishing the role of dark matter in the universe from measurements of galaxy rotation. Its observations have also been used in determining the scale and structure of the universe. Dark matter and dark energy together are believed to make up about 95 percent of all of the universe’s mass and energy.

    It was one of the world’s largest optical telescopes at the time it was built, and because of its sturdy construction it is perfectly suited to carry the new 9-ton instrument.

    “We started this project by surveying large telescopes to find one that had a suitable mirror and wouldn’t collapse under the weight of such a massive instrument,” said Berkeley Lab’s David Schlegel, a DESI project scientist.

    Arjun Dey, the NOAO project scientist for DESI, explained, “The Mayall was precociously engineered like a battleship and designed with a wide field of view.”

    The expansion of the telescope’s field-of-view will allow DESI to map out about one-third of the sky.

    Brenna Flaugher, a DESI project scientist who leads the Astrophysics Department at Fermi National Accelerator Laboratory, said DESI will transform the speed of science at the Mayall Telescope.

    “The telescope was designed to carry a person at the top who aimed and steered it, but with DESI it’s all automated,” she said. “Instead of one at a time we can measure the velocities of 5,000 galaxies at a time – we will measure more than 30 million of them in our five-year survey.”

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    The first of 10 wedge-shaped petals for the DESI project is fully stocked with 500 slender robotic positioners. These positioners will each swivel independently to gather light from a preprogrammed sequence of space objects, including galaxies and quasars. The petals will fit snugly together to form DESI’s focal plane, which will be composed of about 600,000 individual parts. (Credit: DESI Collaboration)

    DESI will use an array of 5,000 swiveling robots, each carefully choreographed to point a fiber-optic cable at a preprogrammed sequence of deep-space objects, including millions of galaxies and quasars, which are galaxies that harbor massive, actively feeding black holes.

    The fiber-optic cables will carry the light from these objects to 10 spectrographs, which are tools that will measure the properties of this light and help to pinpoint the objects’ distance and the rate at which they are moving away from us. DESI’s observations will provide a deep look into the early universe, up to about 11 billion years ago.

    The cylindrical, fiber-toting robots, which will be embedded in a rounded metal unit called a focal plane, will reposition to capture a new exposure of the sky roughly every 20 minutes. The focal plane, which is now being assembled at Berkeley Lab, is expected to be completed and delivered to Kitt Peak this year.

    DESI will scan one-third of the sky and will capture about 10 times more data than a predecessor survey, the Baryon Oscillation Spectroscopic Survey (BOSS). That project relied on a manually rotated sequence of metal plates – with fibers plugged by hand into pre-drilled holes – to target objects.

    All of DESI’s six lenses, each about a meter in diameter, are complete. They will be carefully stacked and aligned in a steel support structure and will ultimately ride with the focal plane atop the telescope.

    Each of these lenses took shape from large blocks of glass. They have criss-crossed the globe to receive various treatments, including grinding, polishing, and coatings. It took about 3.5 years to produce each of the lenses, which now reside at University College London in the U.K. and will be shipped to the DESI site this spring.

    The Mayall Telescope has most recently been enlisted in a DESI-supporting sky survey known as the Mayall z-Band Legacy Survey (MzLS), which is one of four sky surveys that DESI will use to preselect its targeted sky objects. That survey wrapped up just days before the Mayall’s temporary closure, while the others are ongoing.

    Data from these surveys are analyzed at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility. Data from these surveys have been released to the public at legacysurvey.org.

    “We can see about a billion galaxies in the survey images, which is quite a bit of fun to explore,” Schlegel said. “The DESI instrument will precisely measure millions of those galaxies to see the effects of dark energy.”

    Levi noted that there is already a lot of computing work underway at NERSC to prepare for the stream of data that will pour out of DESI once it starts up.

    “This project is all about generating huge quantities of data,” Levi said. “The data will go directly from the telescope to NERSC for processing. We will create hundreds of universes in these computers and see which universe best fits our data.”

    Installation of DESI’s components is expected to begin soon and to wrap up in April 2019, with first science observations planned in September 2019.

    “Installing DESI on the Mayall will put the telescope at the heart of the next decade of discoveries in cosmology,” said Risa Wechsler, DESI Collaboration Co-Spokesperson and associate professor of physics and astrophysics at SLAC National Accelerator Laboratory and Stanford University. “The amazing 3-D map it will measure may solve some of the biggest outstanding questions in cosmology, or surprise us and bring up new ones.”

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  • richardmitnick 8:04 am on November 27, 2017 Permalink | Reply
    Tags: , , , , DESI spectroscopic instrument, , , , Simulating the universe using Einstein’s theory of gravity may solve cosmic puzzles   

    From ScienceNews: “Simulating the universe using Einstein’s theory of gravity may solve cosmic puzzles” 

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    ScienceNews

    November 25, 2017
    Emily Conover

    Until recently, simulations of the universe haven’t given its lumps their due.

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    UNEVEN TERRAIN Universe simulations that consider general relativity (one shown) may shift knowledge of the cosmos. James Mertens

    If the universe were a soup, it would be more of a chunky minestrone than a silky-smooth tomato bisque.

    Sprinkled with matter that clumps together due to the insatiable pull of gravity, the universe is a network of dense galaxy clusters and filaments — the hearty beans and vegetables of the cosmic stew. Meanwhile, relatively desolate pockets of the cosmos, known as voids, make up a thin, watery broth in between.

    Until recently, simulations of the cosmos’s history haven’t given the lumps their due. The physics of those lumps is described by general relativity, Albert Einstein’s theory of gravity. But that theory’s equations are devilishly complicated to solve. To simulate how the universe’s clumps grow and change, scientists have fallen back on approximations, such as the simpler but less accurate theory of gravity devised by Isaac Newton.

    Relying on such approximations, some physicists suggest, could be mucking with measurements, resulting in a not-quite-right inventory of the cosmos’s contents. A rogue band of physicists suggests that a proper accounting of the universe’s clumps could explain one of the deepest mysteries in physics: Why is the universe expanding at an increasingly rapid rate?

    The accepted explanation for that accelerating expansion is an invisible pressure called dark energy. In the standard theory of the universe, dark energy makes up about 70 percent of the universe’s “stuff” — its matter and energy. Yet scientists still aren’t sure what dark energy is, and finding its source is one of the most vexing problems of cosmology.

    Perhaps, the dark energy doubters suggest, the speeding up of the expansion has nothing to do with dark energy. Instead, the universe’s clumpiness may be mimicking the presence of such an ethereal phenomenon.

    Most physicists, however, feel that proper accounting for the clumps won’t have such a drastic impact. Robert Wald of the University of Chicago, an expert in general relativity, says that lumpiness is “never going to contribute anything that looks like dark energy.” So far, observations of the universe have been remarkably consistent with predictions based on simulations that rely on approximations.

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    Growing a lumpy universe

    The universe has gradually grown lumpier throughout its history. During inflation, rapid expansion magnified tiny quantum fluctuations into minute density variations. Over time, additional matter glommed on to dense spots due to the stronger gravitational pull from the extra mass. After 380,000 years, those blips were imprinted as hot and cold spots in the cosmic microwave background, the oldest light in the universe. Lumps continued growing for billions of years, forming stars, planets, galaxies and galaxy clusters.

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    As observations become more detailed, though, even slight inaccuracies in simulations could become troublesome. Already, astronomers are charting wide swaths of the sky in great detail, and planning more extensive surveys. To translate telescope images of starry skies into estimates of properties such as the amount of matter in the universe, scientists need accurate simulations of the cosmos’s history. If the detailed physics of clumps is important, then simulations could go slightly astray, sending estimates off-kilter. Some scientists already suggest that the lumpiness is behind a puzzling mismatch of two estimates of how fast the universe is expanding.

    Researchers are attempting to clear up the debate by conquering the complexities of general relativity and simulating the cosmos in its full, lumpy glory. “That is really the new frontier,” says cosmologist Sabino Matarrese of the University of Padua in Italy, “something that until a few years ago was considered to be science fiction.” In the past, he says, scientists didn’t have the tools to complete such simulations. Now researchers are sorting out the implications of the first published results of the new simulations. So far, dark energy hasn’t been explained away, but some simulations suggest that certain especially sensitive measurements of how light is bent by matter in the universe might be off by as much as 10 percent.

    Soon, simulations may finally answer the question: How much do lumps matter? The idea that cosmologists might have been missing a simple answer to a central problem of cosmology incessantly nags some skeptics. For them, results of the improved simulations can’t come soon enough. “It haunts me. I can’t let it go,” says cosmologist Rocky Kolb of the University of Chicago.

    Smooth universe

    By observing light from different eras in the history of the cosmos, cosmologists can compute the properties of the universe, such as its age and expansion rate. But to do this, researchers need a model, or framework, that describes the universe’s contents and how those ingredients evolve over time. Using this framework, cosmologists can perform computer simulations of the universe to make predictions that can be compared with actual observations.

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    COSMIC WEB Clumps and filaments of matter thread through a simulated universe 2 billion light years across. This simulation incorporates some aspects of Einstein’s theory of general relativity, allowing for detailed results while avoiding the difficulties of the full-fledged theory.

    After Einstein introduced his theory in 1915, physicists set about figuring out how to use it to explain the universe. It wasn’t easy, thanks to general relativity’s unwieldy, difficult-to-solve suite of equations. Meanwhile, observations made in the 1920s indicated that the universe wasn’t static as previously expected; it was expanding. Eventually, researchers converged on a solution to Einstein’s equations known as the Friedmann-Lemaître-Robertson-Walker metric. Named after its discoverers, the FLRW metric describes a simplified universe that is homogeneous and isotropic, meaning that it appears identical at every point in the universe and in every direction. In this idealized cosmos, matter would be evenly distributed, no clumps. Such a smooth universe would expand or contract over time.

    A smooth-universe approximation is sensible, because when we look at the big picture, averaging over the structures of galaxy clusters and voids, the universe is remarkably uniform. It’s similar to the way that a single spoonful of minestrone soup might be mostly broth or mostly beans, but from bowl to bowl, the overall bean-to-broth ratios match.

    In 1998, cosmologists revealed that not only was the universe expanding, but its expansion was also accelerating (SN: 2/2/08, p. 74). Observations of distant exploding stars, or supernovas, indicated that the space between us and them was expanding at an increasing clip. But gravity should slow the expansion of a universe evenly filled with matter. To account for the observed acceleration, scientists needed another ingredient, one that would speed up the expansion. So they added dark energy to their smooth-universe framework.

    Now, many cosmologists follow a basic recipe to simulate the universe — treating the cosmos as if it has been run through an imaginary blender to smooth out its lumps, adding dark energy and calculating the expansion via general relativity. On top of the expanding slurry, scientists add clumps and track their growth using approximations, such as Newtonian gravity, which simplifies the calculations.

    In most situations, Newtonian gravity and general relativity are near-twins. Throw a ball while standing on the surface of the Earth, and it doesn’t matter whether you use general relativity or Newtonian mechanics to calculate where the ball will land — you’ll get the same answer. But there are subtle differences. In Newtonian gravity, matter directly attracts other matter. In general relativity, gravity is the result of matter and energy warping spacetime, creating curves that alter the motion of objects (SN: 10/17/15, p. 16). The two theories diverge in extreme gravitational environments. In general relativity, for example, hulking black holes produce inescapable pits that reel in light and matter (SN: 5/31/14, p. 16). The question, then, is whether the difference between the two theories has any impact in lumpy-universe simulations.

    Most cosmologists are comfortable with the status quo simulations because observations of the heavens seem to fit neatly together like interlocking jigsaw puzzle pieces. Predictions based on the standard framework agree remarkably well with observations of the cosmic microwave background — ancient light released when the universe was just 380,000 years old (SN: 3/21/15, p. 7). And measurements of cosmological parameters — the fraction of dark energy and matter, for example — are generally consistent, whether they are made using the light from galaxies or the cosmic microwave background [CMB].

    CMB per ESA/Planck


    ESA/Planck

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    An image from the Two-Micron All Sky Survey of 1.6 million galaxies in infrared light reveals how matter clumps into galaxy clusters and filaments. Future large-scale surveys may require improved simulations that use general relativity to track the evolution of lumps over time. T.H. Jarrett, J. Carpenter & R. Hurt, obtained as part of 2MASS, a joint project of Univ. of Massachusetts and the Infrared Processing and Analysis Center/Caltech, funded by NASA and NSF.


    Caltech 2MASS Telescopes, a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center (IPAC) at Caltech, at the Whipple Observatory on Mt. Hopkins south of Tucson, AZ, and at the Cerro Tololo Inter-American Observatory near La Serena, Chile.

    Dethroning dark energy

    Some cosmologists hope to explain the universe’s accelerating expansion by fully accounting for the universe’s lumpiness, with no need for the mysterious dark energy.

    These researchers argue that clumps of matter can alter how the universe expands, when the clumps’ influence is tallied up over wide swaths of the cosmos. That’s because, in general relativity, the expansion of each local region of space depends on how much matter is within. Voids expand faster than average; dense regions expand more slowly. Because the universe is mostly made up of voids, this effect could produce an overall expansion and potentially an acceleration. Known as backreaction, this idea has lingered in obscure corners of physics departments for decades, despite many claims that backreaction’s effect is small or nonexistent.

    Backreaction continues to appeal to some researchers because they don’t have to invent new laws of physics to explain the acceleration of the universe. “If there is an alternative which is based only upon traditional physics, why throw that away completely?” Matarrese asks.

    Most cosmologists, however, think explaining away dark energy just based on the universe’s lumps is unlikely. Previous calculations have indicated any effect would be too small to account for dark energy, and would produce an acceleration that changes in time in a way that disagrees with observations.

    “My personal view is that it’s a much smaller effect,” says astrophysicist Hayley Macpherson of Monash University in Melbourne, Australia. “That’s just basically a gut feeling.” Theories that include dark energy explain the universe extremely well, she points out. How could that be if the whole approach is flawed?

    New simulations by Macpherson and others that model how lumps evolve in general relativity may be able to gauge the importance of backreaction once and for all. “Up until now, it’s just been too hard,” says cosmologist Tom Giblin of Kenyon College in Gambier, Ohio.

    To perform the simulations, researchers needed to get their hands on supercomputers capable of grinding through the equations of general relativity as the simulated universe evolves over time. Because general relativity is so complex, such simulations are much more challenging than those that use approximations, such as Newtonian gravity. But, a seemingly distinct topic helped lay some of the groundwork: gravitational waves, or ripples in the fabric of spacetime.

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    SPECKLED SPACETIME A lumpy universe, recently simulated using general relativity, shows clumps of matter (pink and yellow) that beget stars and galaxies. H. Macpherson, Paul Lasky, Daniel Price.

    The Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, searches for the tremors of cosmic dustups such as colliding black holes (SN: 10/28/17, p. 8).


    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

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    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)

    In preparation for this search, physicists honed their general relativity skills on simulations of the spacetime storm kicked up by black holes, predicting what LIGO might see and building up the computational machinery to solve the equations of general relativity. Now, cosmologists have adapted those techniques and unleashed them on entire, lumpy universes.

    The first lumpy universe simulations to use full general relativity were unveiled in the June 2016 Physical Review Letters. Giblin and colleagues reported their results simultaneously with Eloisa Bentivegna of the University of Catania in Italy and Marco Bruni of the University of Portsmouth in England.

    So far, the simulations have not been able to account for the universe’s acceleration. “Nearly everybody is convinced [the effect] is too small to explain away the need for dark energy,” says cosmologist Martin Kunz of the University of Geneva. Kunz and colleagues reached the same conclusion in their lumpy-universe simulations, which have one foot in general relativity and one in Newtonian gravity. They reported their first results in Nature Physics in March 2016.

    Backreaction aficionados still aren’t dissuaded. “Before saying the effect is too small to be relevant, I would, frankly, wait a little bit more,” Matarrese says. And the new simulations have potential caveats. For example, some simulated universes behave like an old arcade game — if you walk to one edge of the universe, you cross back over to the other side, like Pac-Man exiting the right side of the screen and reappearing on the left. That geometry would suppress the effects of backreaction in the simulation, says Thomas Buchert of the University of Lyon in France. “This is a good beginning,” he says, but there is more work to do on the simulations. “We are in infancy.”

    Different assumptions in a simulation can lead to disparate results, Bentivegna says. As a result, she doesn’t think that her lumpy, general-relativistic simulations have fully closed the door on efforts to dethrone dark energy. For example, tricks of light might be making it seem like the universe’s expansion is accelerating, when in fact it isn’t.

    When astronomers observe far-away sources like supernovas, the light has to travel past all of the lumps of matter between the source and Earth. That journey could make it look like there’s an acceleration when none exists. “It’s an optical illusion,” Bentivegna says. She and colleagues see such an effect in a simulation reported in March in the Journal of Cosmology and Astroparticle Physics. But, she notes, this work simulated an unusual universe, in which matter sits on a grid — not a particularly realistic scenario.

    For most other simulations, the effect of optical illusions remains small. That leaves many cosmologists, including Giblin, even more skeptical of the possibility of explaining away dark energy: “I feel a little like a downer,” he admits.

    6
    Lumps (gray) within this simulated universe change the path light takes (yellow lines), potentially affecting observations. Matter bends space, slightly altering the light’s trajectory from that in a smooth universe. James Mertens.

    Surveying the skies

    Subtle effects of lumps could still be important. In Hans Christian Andersen’s The Princess and the Pea, the princess felt a tiny pea beneath an impossibly tall stack of mattresses. Likewise, cosmologists’ surveys are now so sensitive that even if the universe’s lumps have a small impact, estimates could be thrown out of whack.

    The Dark Energy Survey, for example, has charted 26 million galaxies using the Victor M. Blanco Telescope in Chile, measuring how the light from those galaxies is distorted by the intervening matter on the journey to Earth.

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

    In a set of papers posted online August 4 at arXiv.org, scientists with the Dark Energy Survey reported new measurements of the universe’s properties, including the amount of matter (both dark and normal) and how clumpy that matter is (SN: 9/2/17, p. 32). The results are consistent with those from the cosmic microwave background [CMB] — light emitted billions of years earlier.

    To make the comparison, cosmologists took the measurements from the cosmic microwave background, early in the universe, and used simulations to extrapolate to what galaxies should look like later in the universe’s history. It’s like taking a baby’s photograph, precisely computing the number and size of wrinkles that should emerge as the child ages and finding that your picture agrees with a snapshot taken decades later. The matching results so far confirm cosmologists’ standard picture of the universe — dark energy and all.

    “So far, it has not yet been important for the measurements that we’ve made to actually include general relativity in those simulations,” says Risa Wechsler, a cosmologist at Stanford University and a founding member of the Dark Energy Survey. But, she says, for future measurements, “these effects could become more important.” Cosmologists are edging closer to Princess and the Pea territory.

    Those future surveys include the Dark Energy Spectroscopic Instrument, DESI, set to kick off in 2019 at Kitt Peak National Observatory near Tucson; the European Space Agency’s Euclid satellite, launching in 2021; and the Large Synoptic Survey Telescope in Chile, which is set to begin collecting data in 2023.

    LBNL/DESI spectroscopic instrument on the Mayall 4-meter telescope at Kitt Peak National Observatory, Altitude 2,120 m (6,960 ft)

    LBNL/DESI spectroscopic instrument on the Mayall 4-meter telescope at Kitt Peak National Observatory starting in 2018

    NOAO/Mayall 4 m telescope at Kitt Peak, Arizona, USA, Altitude 2,120 m (6,960 ft)

    ESA/Euclid spacecraft

    LSST


    LSST Camera, built at SLAC



    LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    If cosmologists keep relying on simulations that don’t use general relativity to account for lumps, certain kinds of measurements of weak lensing — the bending of light due to matter acting like a lens — could be off by up to 10 percent, Giblin and colleagues reported at arXiv.org in July. “There is something that we’ve been ignoring by making approximations,” he says.

    That 10 percent could screw up all kinds of estimates, from how dark energy changes over the universe’s history to how fast the universe is currently expanding, to the calculations of the masses of ethereal particles known as neutrinos. “You have to be extremely certain that you don’t get some subtle effect that gets you the wrong answers,” Geneva’s Kunz says, “otherwise the particle physicists are going to be very angry with the cosmologists.”

    Some estimates may already be showing problem signs, such as the conflicting estimates of the cosmic expansion rate (SN: 8/6/16, p. 10). Using the cosmic microwave background, cosmologists find a slower expansion rate than they do from measurements of supernovas. If this discrepancy is real, it could indicate that dark energy changes over time. But before jumping to that conclusion, there are other possible causes to rule out, including the universe’s lumps.

    Until the issue of lumps is smoothed out, scientists won’t know how much lumpiness matters to the cosmos at large. “I think it’s rather likely that it will turn out to be an important effect,” Kolb says. Whether it explains away dark energy is less certain. “I want to know the answer so I can get on with my life.”

    See the full article here .

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