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  • richardmitnick 9:11 am on October 11, 2017 Permalink | Reply
    Tags: , , , , ESA Planck All Sky Map 1, How to make the cosmic web give up the matter it’s hiding, ScienceNews   

    From ScienceNews: “How to make the cosmic web give up the matter it’s hiding” 

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    October 11, 2017
    Lisa Grossman

    Half the universe’s ordinary matter is missing. This new technique might have found it.

    A TANGLED SKEIN This computer simulation of the universe highlights its structure: long filaments of dark matter (blue) with galaxies strung along them like beads (pink). Most of the regular matter is probably stored in gas (orange). Illustris Collaboration

    Evidence is piling up that much of the universe’s missing matter is lurking along the strands of a vast cosmic web.

    A pair of papers report some of the best signs yet of hot gas in the spaces between galaxy clusters, possibly enough to represent the half of all ordinary matter previously unaccounted for. Previous studies have hinted at this missing matter, but a new search technique is helping to fill in the gaps in the cosmic census where other efforts fell short. The papers were published online at arXiv.org on September 15 and September 29.

    Two independent teams stacked images of hundreds of thousands of galaxies on top of one another to reveal diffuse filaments of gas connecting pairs of galaxies across millions of light-years. Measuring how the gas distorted the background light of the universe let the researchers determine the mass of ordinary matter, or baryons, that it held — the protons and neutrons that make up atoms.

    “It’s a very important problem,” says Dominique Eckert of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who has searched for the missing matter via X-rays emitted by individual strands. “If you want to understand how galaxies form and how everything forms within a galaxy, you have to understand the evolution of the baryon content.” That starts with knowing where it is.

    About 85 percent of the matter in the universe is mysterious, invisible stuff called dark matter, which physicists have yet to find (SN Online: 9/6/17).

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

    Weirdly, about half of the ordinary matter is also unaccounted for. When astronomers look around at the galaxies in the nearest few billion light-years, they find only about half the baryons that should have been produced in the Big Bang.

    The rest is probably hiding in long filaments of gas that connect galaxy clusters in a vast cosmic web (SN: 3/8/14, p. 8). Previous attempts to find the baryons focused on X-rays emitted by gas in the filaments (SN Online: 8/4/15) or on the light of distant quasars filtering through these cobwebby strands (SN: 5/13/00, p. 310). But those efforts were either inconclusive, or were sensitive to such a narrow range of gas temperatures that they missed much of the matter.

    Now there might be a way to find the rest. Two groups — cosmologist Hideki Tanimura, who did the work while at the University of British Columbia in Vancouver, and his colleagues, and Anna de Graaff of the University of Edinburgh and her colleagues — have sought the missing matter in a new way. Both teams found a way to look through the gas all the way back to the oldest light in the universe.

    “Filamentary gas is very difficult to detect, but now we have a technique to detect it,” says Tanimura, now with the Institute of Space Astrophysics in Orsay, France.

    That ancient light, called the cosmic microwave background, was emitted 380,000 years after the Big Bang. When this light passes though clouds of electrons in space — such as those found in filaments of hot gas — it gets deflected and distorted in a specific way. The Planck satellite released an all-sky map of these distortions in 2015 (SN: 3/21/15, p. 7).

    ESA Planck All Sky Map 1

    Tanimura and de Graaff separately figured that there would be more distortion along the filaments than in empty space. To locate the filaments, both teams chose pairs of galaxies from the Sloan Digital Sky Survey catalog that were at least 20 million light-years apart. De Graaff’s team chose roughly a million pairs, and Tanimura’s team chose 262,864 pairs. Both teams assumed that the galaxies were not part of the same cluster, but that they should be connected by a filament.

    The filaments were still too faint to see individually, so the teams used software to layer all the images and subtracted out distortion from electrons in the galaxies to see what was left. Both saw a residual distortion in the cosmic microwave background, which they attribute to the filaments.

    Next, de Graaff’s team calculated that those filaments account for 30 percent of the total baryon content of the universe. That’s surely an underestimate, since they didn’t examine every filament in the universe, the team writes — the rest of the missing matter is probably there too.

    “Both groups here took the obvious first step,” says Michael Shull of the University of Colorado Boulder, who was not involved in the new studies. “I think they’re on the right track.” But he worries that the gas they see might have been ejected from galaxies at high speeds, and so not actually the missing matter at all.

    Eckert also worries that the gas may belong more to the galaxies than to their intergalactic tethers. Future observations of the composition of the gas, as well as more sensitive X-ray observations, could help solve that part of the puzzle.

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  • richardmitnick 1:45 pm on October 10, 2017 Permalink | Reply
    Tags: , Antibiotics spiked with quantum dots fought off bacteria as effectively as 1000 times as much antibiotic alone, , , , , ScienceNews, Various superbugs are evolving too rapidly to be counteracted by traditional drugs   

    From ScienceNews: “Superbugs may meet their match in these nanoparticles” 

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    Science News

    October 9, 2017
    Maria Temming

    ‘Quantum dots’ mess with bacteria’s defenses, allowing antibiotics to work.

    ARMED AND DANGEROUS By producing a chemical that makes bacteria more vulnerable to antibiotic attack, quantum dots could help reboot medications that have lost their edge against hard-to-kill microbes. Kateryna Kon/Shutterstock

    Antibiotics may have a new teammate in the fight against drug-resistant infections.

    Researchers have engineered nanoparticles to produce chemicals that render bacteria more vulnerable to antibiotics. These quantum dots, described online October 4 in Science Advances, could help combat pathogens that have developed resistance to antibiotics (SN: 10/15/16, p. 11).

    “Various superbugs are evolving too rapidly to be counteracted by traditional drugs,” says Zhengtao Deng, a chemist at Nanjing University in China not involved in the research. “Drug resistant infections will kill an extra 10 million people a year worldwide by 2050 unless action is taken.”

    In the study, antibiotics spiked with quantum dots fought off bacteria as effectively as 1,000 times as much antibiotic alone. That’s “really impressive,” says Chao Zhong, a materials scientist at ShanghaiTech University who was not involved in the study. “This is a really comprehensive study.”

    Quantum dots, previously investigated as a tool to trace drug delivery throughout the body or to take snapshots of cells, are made of semiconductors — the same kind of material in such electronics as laptops and cellphones (SN: 7/11/15, p. 22). The new quantum dots are tiny chunks of cadmium telluride just 3 nanometers across, or about as wide as a strand of DNA.

    When illuminated by a specific frequency of green light, the nanoparticles’ electrons can pop off and glom onto nearby oxygen molecules — which are dissolved in water throughout the body — to create a chemical called superoxide. When a bacterial cell absorbs this superoxide, it throws the microbe’s internal chemistry so off-balance that the pathogen can’t defend itself against antibiotics, explains study coauthor Anushree Chatterjee, a chemical engineer at the University of Colorado Boulder.

    Chatterjee and colleagues mixed various amounts of quantum dots into different concentrations of each of five antibiotics, and then added these concoctions to samples of five drug-resistant bacterial strains, such as Salmonella and methicillin-resistant Staphylococcus aureus, or MRSA. In more than 75 percent of 480 tests of different antibiotic combinations on different bacteria, the researchers found that lower doses of antibiotics were required to kill or curb the growth of bacteria when the medicine was combined with quantum dots.

    One limitation of this treatment is that the green light that activates the nanoparticles can shine through only a few millimeters of flesh, says coauthor Prashant Nagpal, a chemical engineer also at the University of Colorado Boulder. So these quantum dots could probably be used only to treat skin or accessible wound infections.

    The researchers are now designing nanoparticles that absorb infrared light, which can pass through the body. “That could be really effective in deep tissue and bone infections,” Nagpal says.

    See the full article here .

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  • richardmitnick 1:31 pm on October 5, 2017 Permalink | Reply
    Tags: An early interest in tricks of light led Dionne to begin wielding it as a tool during graduate school at Caltech and then her postdoc at UC Berkeley, , Jennifer Dionne, , ScienceNews,   

    From ScienceNews: Women in STEM – “Jennifer Dionne harnesses light to illuminate nano landscapes” 

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    October 4, 2017
    Emily Conover

    Tiny particles could light the way to improved cancer tests or drugs with fewer side effects.

    LEADING LIGHT Jennifer Dionne, 35 Materials scientist, Stanford University.
    Materials scientist Jennifer Dionne melds purpose and play in her work with matter and light. Timothy Archibald

    To choose her research goals, Jennifer Dionne envisions conversations with hypothetical grandchildren, 50 years down the line. What would she want to tell them she had accomplished? Then, to chart a path to that future, “I work backward to figure out what are the milestones en route,” she says.

    That long-term vision has led the 35-year-old materials scientist on a quest to wrangle light and convince it to do her bidding in interactions with nanoparticles and various materials. Already, Dionne has created new nanomaterials that steer light in ways that are impossible with natural substances. Her new projects could eventually lead to light-based technologies used to improve drugs or to create new tests to find cancerous cells. There are even applications for renewable energy, for example, designing materials that help solar cells absorb more light.

    But the route to a scientific vision may not always be clear, so Dionne makes time for diversions. “A lot of the really amazing discoveries that we enjoy today came from just playing in the lab,” she says. Dionne encourages her team to let creativity be a guide, melding a serious sense of purpose with play.

    “She’s a very curious person, so she’s always learning new things,” says Paul Alivisatos, the vice chancellor for research at the University of California, Berkeley, who mentored Dionne when she was a postdoc there. Plus, “she’s an extremely deep and rigorous thinker.”

    Dionne, now at Stanford University, studies nanophotonics, the way that light interacts with matter on very small scales. Her interest in light and materials began in childhood, she recalls, when she was fascinated by the blue morpho butterfly.

    The insect’s wings sport an azure hue that comes not from pigments, like most colors found in living things, but from tiny nanostructures on the wings’ surface (SN: 6/7/08, p. 26). When light reflects off the structures, blue wavelengths are amplified, while wavelengths corresponding to other colors are canceled out.

    That early interest in tricks of light led Dionne to begin wielding it as a tool during graduate school at Caltech and then her postdoc at UC Berkeley. Then and now, says Alivisatos, “she has consistently done very beautiful work.”

    At Caltech, Dionne and colleagues created a bizarre optical material in which light bends backward. As light passes from one material to another — say, from air to water — the rays are deflected due to a property called the index of refraction. (That’s why a straw in a drinking glass appears to be broken at the water’s surface.) In natural materials, light always bends in the same direction. But that rule gets flipped around in oddball nanomaterials with a negative index of refraction.

    G. Dolling et al/Optics Express 2006
    Light rays bend as they pass from air into water, making a drinking straw look broken (illustrated in a computer-generated image, left). In materials with a negative index of refraction (right), light rays bend in the opposite direction they normally do, so that the straw appears flipped around.

    Dionne’s material, reported in Science in 2007, was the first that worked with visible light (SN: 3/24/07, p. 180). Because they can steer light around objects to hide the objects from view, such materials could be used to create rudimentary versions of invisibility cloaks — though so far all attempts are a far cry from Harry Potter’s version. Dionne is now working on a “squid skin” with an adjustable refractive index, which would mimic the shifting camouflage patterns of the stealthy cephalopod.

    Another focal point of Dionne’s research is harnessing light to separate mixtures of mirror-image molecules. Right- and left-handed versions of these molecules are perfect reflections of each other, like a person’s right and left hands. The two types are so similar that scientists struggle to separate them, which can cause problems for drugmakers. In drugs, these molecules can be two-faced; one might relieve pain, while the other causes unwanted side effects.

    To separate molecules and their mirror images, Dionne is developing techniques that use circularly polarized light, in which the light’s wiggling electromagnetic waves rotate over time. Such light can interact differently with right- and left-handed molecules, for example, breaking apart one version while leaving the other unscathed.

    Normally, the light’s effect is very weak. But in a theoretical study published in ACS Photonics last December, Dionne and colleagues showed that adding nanoparticles to the mix could enhance the process. These tiny particles behave like antennas that concentrate the light onto nearby molecules, helping break them apart. Dionne is now working to implement the technique.

    She and her colleagues have also created nanoparticles that, when illuminated with infrared light, emit visible light. The color of that light changes depending on how tightly the nanoparticle is squeezed, the team reported in Nano Letters in June. In keeping with her penchant for creative exploration in the lab, Dionne and colleagues fed these nanoparticles to roundworms, the nematode Caenorhabditis elegans, to study the forces exerted as a transparent worm squeezed a meal through its digestive tract.

    “You can see the nanoparticles change colors throughout,” Dionne says. She plans to use the technique to reveal more sinister squeezing. Cancer cells exert stronger mechanical forces on their environment than healthy cells, so such nanoparticles could one day be used to test for cancer, she says. Dionne is now cooking up other creative ways to use these nanoparticles. In collaboration with other researchers, she hopes to marshal her color-changing nanoparticles to understand how jellyfish move and how plants take a drink.

    Dionne’s work exploits light to reveal hidden forces — and as a force for good. “She’s done amazing work,” says materials scientist Prineha Narang of Harvard University. Narang was a graduate student at Caltech after Dionne left, and had heard chatter about Dionne before meeting her in person. “The legend of Jen Dionne was definitely all over,” Narang says. So Dionne has made a start at establishing her scientific legacy — even before that chat with her future grandchildren.

    At the full article many citations with links.

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  • richardmitnick 9:46 am on September 28, 2017 Permalink | Reply
    Tags: , , , , ScienceNews, , Technical University of Dresden, The spin Hall effect, The spin Nernst effect, The spin Peltier effect, The spin Seebeck effect, Turning up the heat on electrons reveals an elusive physics phenomenon, When things heat up spinning electrons go their separate ways   

    From ScienceNews: “Turning up the heat on electrons reveals an elusive physics phenomenon” 

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    September 26, 2017
    Emily Conover

    Spin Nernst effect could help scientists design new gadgets that store data using quantum property of spin.

    WHIRL AWAY Electrons in platinum move in different directions depending on their spin when the metal is heated at one end. Scientists have observed this phenomenon, called the spin Nernst effect, for the first time. Creativity103/Flickr (CC BY 2.0)

    When things heat up, spinning electrons go their separate ways.

    Warming one end of a strip of platinum shuttles electrons around according to their spin, a quantum property that makes them behave as if they are twirling around. Known as the spin Nernst effect, the newly detected phenomenon was the only one in a cadre of related spin effects that hadn’t previously been spotted, researchers report online September 11 in Nature Materials.

    “The last missing piece in the puzzle was spin Nernst and that’s why we set out to search for this,” says study coauthor Sebastian Goennenwein, a physicist at the Technical University of Dresden in Germany.

    The effect and its brethren — with names like the spin Hall effect, the spin Seebeck effect and the spin Peltier effect — allow scientists to create flows of electron spins, or spin currents. Such research could lead to smaller and more efficient electronic gadgets that use electrons’ spins to store and transmit information instead of electric charge, a technique known as “spintronics.”

    In the spin Nernst effect, named after Nobel laureate chemist Walther Nernst, heating one end of a metal causes electrons to flow toward the other end, bouncing around inside the material as they go. Within certain materials, that bouncing has a preferred direction: Electrons with spins pointing up (as if twirling counterclockwise) go to the right and electrons with spins pointing down (as if twirling clockwise) go to the left, creating an overall spin current. Although the effect had been predicted, no one had yet observed it.

    Finding evidence of the effect required disentangling it from other heat- and charge-related effects that occur in materials. To do so, the researchers coupled the platinum to a layer of a magnetic insulator, a material known as yttrium iron garnet. Then, they altered the direction of the insulator’s magnetization, which changed whether the spin current could flow through the insulator. That change slightly altered a voltage measured along the strip of platinum. The scientists measured how this voltage changed with the direction of the magnetization to isolate the fingerprints of the spin Nernst effect.

    “The measurement was a tour de force; the measurement was ridiculously hard,” says physicist Joseph Heremans of Ohio State University in Columbus, who was not involved with the research. The effect could help scientists to better understand materials that may be useful for building spintronic devices, he says. “It’s really a new set of eyes on the physics of what’s going on inside these devices.”

    A relative of the spin Nernst effect called the spin Hall effect is much studied for its potential use in spintronic devices. In the spin Hall effect, an electric field pushes electrons through a material, and the particles veer off to the left and right depending on their spin. The spin Nernst effect relies on the same basic physics, but uses heat instead of an electric field to get the particles moving.

    “It’s a beautiful experiment. It shows very nicely the spin Nernst effect,” says physicist Greg Fuchs of Cornell University. “It beautifully unifies our understanding of the interrelation between charge, heat and spin transport.”

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  • richardmitnick 7:43 am on May 16, 2017 Permalink | Reply
    Tags: anti-de Sitter space, , , , , , Hypothetical universes, Naked singularity might evade cosmic censor, ScienceNews,   

    From ScienceNews: “Naked singularity might evade cosmic censor” 

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    May 15, 2017
    Emily Conover

    Spacetime singularities might exist unhidden in strangely curved universes

    LAID BARE Inside a black hole, the extreme curvature of space (shown) means that the standard rules of physics don’t apply. Such regions, called singularities, are thought to be shrouded by event horizons, but scientists showed that a singularity could be observable under certain conditions in a hypothetical curved spacetime. Henning Dalhoff/Science Source

    Certain stealthy spacetime curiosities might be less hidden than thought, potentially exposing themselves to observers in some curved universes.

    These oddities, known as singularities, are points in space where the standard laws of physics break down. Found at the centers of black holes, singularities are generally expected to be hidden from view, shielding the universe from their problematic properties. Now, scientists report in the May 5 Physical Review Letters that a singularity could be revealed in a hypothetical, saddle-shaped universe.

    Previously, scientists found that singularities might not be concealed in hypothetical universes with more than three spatial dimensions. The new result marks the first time the possibility of such a “naked” singularity has been demonstrated in a three-dimensional universe. “That’s extremely important,” says physicist Gary Horowitz of the University of California, Santa Barbara. Horowitz, who was not involved with the new study, has conducted previous research that implied that a naked singularity could probably appear in such saddle-shaped universes.

    In Einstein’s theory of gravity, the general theory of relativity, spacetime itself can be curved (SN: 10/17/15, p. 16). Massive objects such as stars bend the fabric of space, causing planets to orbit around them. A singularity occurs when the warping is so extreme that the equations of general relativity become nonsensical — as occurs in the center of a black hole. But black holes’ singularities are hidden by an event horizon, which encompasses a region around the singularity from which light can’t escape. The cosmic censorship conjecture, put forth in 1969 by mathematician and physicist Roger Penrose, proposes that all singularities will be similarly cloaked.

    According to general relativity, hypothetical universes can take on various shapes. The known universe is nearly flat on large scales, meaning that the rules of standard textbook geometry apply and light travels in a straight line. But in universes that are curved, those rules go out the window. To demonstrate the violation of cosmic censorship, the researchers started with a curved geometry known as anti-de Sitter space, which is warped such that a light beam sent out into space will eventually return to the spot it came from. The researchers deformed the boundaries of this curved spacetime and observed that a region formed in which the curvature increased over time to arbitrarily large values, producing a naked singularity.

    “I was very surprised,” says physicist Jorge Santos of the University of Cambridge, a coauthor of the study. “I always thought that gravity would somehow find a way” to maintain cosmic censorship.

    Scientists have previously shown that cosmic censorship could be violated if a universe’s conditions were precisely arranged to conspire to produce a naked singularity. But the researchers’ new result is more general. “There’s nothing finely tuned or unnatural about their starting point,” says physicist Ruth Gregory of Durham University in England. That, she says, is “really interesting.”

    But, Horowitz notes, there is a caveat. Because the violation occurs in a curved universe, not a flat one, the result “is not yet a completely convincing counterexample to the original idea.”

    Despite the reliance on a curved universe, the result does have broader implications. That’s because gravity in anti-de Sitter space is thought to have connections to other theories. The physics of gravity in anti-de Sitter space seems to parallel that of some types of particle physics theories, set in fewer dimensions. So cosmic censorship violation in this realm could have consequences for seemingly unrelated ideas.

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  • richardmitnick 1:14 pm on May 9, 2017 Permalink | Reply
    Tags: , , , , , ScienceNews   

    From ScienceNews: “Mars may not have been born alongside the other rocky planets” 

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    May 5, 2017
    Thomas Sumner

    New origin story could explain mystery of Red Planet’s makeup.

    FAR OUT Mars may have formed near what’s now the asteroid belt, much farther away from the sun than the other rocky planets. ESA & MPS for OSIRIS Team, UPD, LAM, IAA, RSSD, INTA, UPM, DASP, IDA (CC BY-SA 3.0 IGO)

    Mars may have had a far-out birthplace.

    Simulating the assembly of the solar system around 4.56 billion years ago, researchers propose that the Red Planet didn’t form in the inner solar system alongside the other terrestrial planets as previously thought. Mars instead may have formed around where the asteroid belt is now and migrated inward to its present-day orbit, the scientists report in the June 15 Earth and Planetary Science Letters. The proposal better explains why Mars has such a different chemical composition than Earth, says Stephen Mojzsis, a study coauthor and geologist at the University of Colorado Boulder.

    The new work is an intuitive next step in a years-long rethink of the early solar system, says Kevin Walsh, a planetary scientist at the Southwest Research Institute in Boulder, Colo., who was not involved with the new simulation. “We only became comfortable within the last 10 years with the idea that planets move around, possibly a lot,” he says. “Planets may not have formed where we see them today.”

    Mars, like Mercury, is a runt of the inner solar system, weighing in at only about a ninth of Earth’s mass. One of the reigning theories of planetary formation, the Grand Tack model, blames Jupiter for the Red Planet’s paltry size. In that scenario, the newly formed Jupiter migrated toward the sun until it reached Mars’ present-day orbit. A gravitational tug from Saturn then reversed Jupiter’s course, sending the gas giant back to the outer solar system (SN: 4/2/16, p. 7).

    Gravitational effects of Jupiter’s sunward jaunt acted like a snowplow, scientists believe, causing a pileup of material near where Earth’s orbit is today. The bulk of that material formed Venus and Earth, and the scraps created Mercury and Mars. This explanation predicts that all the terrestrial planets formed largely from the same batch of ingredients (SN: 4/15/17, p. 18). But studies of Martian meteorites suggest that the Red Planet contains a different mix of various elements and isotopes, such as oxygen-17 and oxygen-18, compared with Earth.

    Planetary scientist Ramon Brasser of the Tokyo Institute of Technology, Mojzsis and colleagues reran the Grand Tack simulations, keeping an eye on the materials that went into Mars’ creation to see if they could explain the different mix.

    As with previous studies, the researchers found that the most probable way of creating a solar system with the same planet sizes and positions as seen today is to have Mars form within Earth’s orbit and migrate outward. However, this explanation failed to explain Mars’ strikingly different composition.

    Another possible scenario, though seen in only about 2 percent of the team’s new simulations, is that Mars formed more than twice as far from the sun as its present-day orbit in the region currently inhabited by the asteroid belt. Then as Jupiter moved sunward, its gravitational pull yanked Mars into the inner solar system. Jupiter’s gravity also diverted planet-making material away from Mars, resulting in the planet’s relatively small mass. With Mars forming so far from the planetary feeding frenzy responsible for the other rocky planets, its composition would be distinct. While this scenario isn’t as likely as Mars forming in the inner solar system, it at least matches the reality of Mars’ makeup, Mojzsis says.

    Such a distant origin means that the fledgling Mars would have received far less sunlight than originally thought, a challenge to early Mars’ possible habitability. Without a sustained thick atmosphere of heat-trapping greenhouse gases, the planet would have been too cold to sustain liquid water on its surface for long periods of time, Mojzsis argues. Though large meteorite impacts could have temporarily warmed Mars above freezing, the planet wouldn’t have had a consistently warm and wet youth similar to that of the early Earth, he says.

    Confirming whether Mars really was born that far out in space will require taking a closer look at Venus’ mix of elements and isotopes, which the researchers predict would be similar to Earth’s. Venus’ composition is largely unknown because of a lack of Venusian meteorites found on Earth, and that mystery won’t be unlocked anytime soon: No missions to Venus are planned.

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  • richardmitnick 1:14 pm on April 20, 2017 Permalink | Reply
    Tags: ScienceNews, There’s still a lot we don’t know about the proton   

    From ScienceNews: “There’s still a lot we don’t know about the proton” 

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    April 18, 2017
    Emily Conover

    Questions loom about the iconic particle’s size, spin and decay.

    PROTON PUZZLES Hidden secrets of the humble particle could have physicists rethinking some standard notions about matter and the universe. ARSCIMED/SCIENCE SOURCE

    Nuclear physicist Evangeline Downie hadn’t planned to study one of the thorniest puzzles of the proton.

    But when opportunity knocked, Downie couldn’t say no. “It’s the proton,” she exclaims. The mysteries that still swirl around this jewel of the subatomic realm were too tantalizing to resist. The plentiful particles make up much of the visible matter in the universe. “We’re made of them, and we don’t understand them fully,” she says.

    Many physicists delving deep into the heart of matter in recent decades have been lured to the more exotic and unfamiliar subatomic particles: mesons, neutrinos and the famous Higgs boson — not the humble proton.


    Protons have issues

    Three proton conundrums have scientists designing new experiments. Agreement eludes researchers on proton size, spin and stability.

    Current status: Two kinds of measurements of the proton’s radius disagree.

    Why do we care? Testing theories of how particles interact requires a precise radius measurement. If the discrepancy persists, it may mean that new, undiscovered particles exist.

    Current status: Scientists can’t account for the sources of the proton’s known spin.

    Why do we care? Understanding the spin would satisfy fundamental scientific curiosity about how the proton works.
    Life span

    Current status: Despite decades of searching, no one has ever seen a proton decay

    Why do we care? Proton decay would be a sign that three of nature’s forces — weak, strong and electromagnetic — were united early in the universe.


    But rather than chasing the rarest of the rare, scientists like Downie are painstakingly scrutinizing the proton itself with ever-higher precision. In the process, some of these proton enthusiasts have stumbled upon problems in areas of physics that scientists thought they had figured out.

    Surprisingly, some of the particle’s most basic characteristics are not fully pinned down. The latest measurements of its radius disagree with one another by a wide margin, for example, a fact that captivated Downie. Likewise, scientists can’t yet explain the source of the proton’s spin, a basic quantum property. And some physicists have a deep but unconfirmed suspicion that the seemingly eternal particles don’t live forever — protons may decay. Such a decay is predicted by theories that unite disparate forces of nature under one grand umbrella. But decay has not yet been witnessed.

    Like the base of a pyramid, the physics of the proton serves as a foundation for much of what scientists know about the behavior of matter. To understand the intricacies of the universe, says Downie, of George Washington University in Washington, D.C., “we have to start with, in a sense, the simplest system.”

    Sizing things up

    For most of the universe’s history, protons have been VIPs — very important particles. They formed just millionths of a second after the Big Bang, once the cosmos cooled enough for the positively charged particles to take shape. But protons didn’t step into the spotlight until about 100 years ago, when Ernest Rutherford bombarded nitrogen with radioactively produced particles, breaking up the nuclei and releasing protons.

    A single proton in concert with a single electron makes up hydrogen — the most plentiful element in the universe. One or more protons are present in the nucleus of every atom. Each element has a unique number of protons, signified by an element’s atomic number. In the core of the sun, fusing protons generate heat and light needed for life to flourish. Lone protons are also found as cosmic rays, whizzing through space at breakneck speeds, colliding with Earth’s atmosphere and producing showers of other particles, such as electrons, muons and neutrinos.

    In short, protons are everywhere. Even minor tweaks to scientists’ understanding of the minuscule particle, therefore, could have far-reaching implications. So any nagging questions, however small in scale, can get proton researchers riled up.

    A disagreement of a few percent in measurements of the proton’s radius has attracted intense interest, for example. Until several years ago, scientists agreed: The proton’s radius was about 0.88 femtometers, or 0.88 millionths of a billionth of a meter — about a trillionth the width of a poppy seed.


    Ladder of matter

    Protons make up a large part of the universe’s visible matter and play an essential role in atomic nuclei. But the building block is still revealing surprises.

    Core components

    Atoms are made of protons (red) and neutrons (blue), surrounded by a cloud of electrons. The proton number determines the element.

    Going inside

    Protons have two “up” quarks and one “down” quark. Neutrons have two downs and one up.


    Deeper dive

    But protons and neutrons contain much more. Quark-antiquark pairs constantly form and annihilate around the three persistent quarks. Gluons (yellow) hold the quarks together via the strong nuclear force. Quarks have a property called “color charge” — shown here as red, green and blue — which is related to the strong force.


    All images: Deutsches Elektronen-Synchrotron,
    Adapted by T. TibbitTs

    But that neat picture was upended in the span of a few hours, in May 2010, at the Precision Physics of Simple Atomic Systems conference in Les Houches, France. Two teams of scientists presented new, more precise measurements, unveiling what they thought would be the definitive size of the proton. Instead the figures disagreed by about 4 percent (SN: 7/31/10, p. 7). “We both expected that we would get the same number, so we were both surprised,” says physicist Jan Bernauer of MIT.

    By itself, a slight revision of the proton’s radius wouldn’t upend physics. But despite extensive efforts, the groups can’t explain why they get different numbers. As researchers have eliminated simple explanations for the impasse, they’ve begun wondering if the mismatch could be the first hint of a breakdown that could shatter accepted tenets of physics.

    The two groups each used different methods to size up the proton. In an experiment at the MAMI particle accelerator in Mainz, Germany, Bernauer and colleagues estimated the proton’s girth by measuring how much electrons’ trajectories were deflected when fired at protons. That test found the expected radius of about 0.88 femtometers (SN Online: 12/17/10).

    But a team led by physicist Randolf Pohl of the Max Planck Institute of Quantum Optics in Garching, Germany, used a new, more precise method. The researchers created muonic hydrogen, a proton that is accompanied not by an electron but by a heftier cousin — a muon.

    In an experiment at the Paul Scherrer Institute in Villigen, Switzerland, Pohl and collaborators used lasers to bump the muons to higher energy levels. The amount of energy required depends on the size of the proton. Because the more massive muon hugs closer to the proton than electrons do, the energy levels of muonic hydrogen are more sensitive to the proton’s size than ordinary hydrogen, allowing for measurements 10 times as precise as electron-scattering measurements.

    Pohl’s results suggested a smaller proton radius, about 0.841 femtometers, a stark difference from the other measurement. Follow-up measurements of muonic deuterium — which has a proton and a neutron in its nucleus — also revealed a smaller than expected size, he and collaborators reported last year in Science. Physicists have racked their brains to explain why the two measurements don’t agree. Experimental error could be to blame, but no one can pinpoint its source. And the theoretical physics used to calculate the radius from the experimental data seems solid.

    Now, more outlandish possibilities are being tossed around. An unexpected new particle that interacts with muons but not electrons could explain the difference (SN: 2/23/13, p. 8). That would be revolutionary: Physicists believe that electrons and muons should behave identically in particle interactions. “It’s a very sacred principle in theoretical physics,” says John Negele, a theoretical particle physicist at MIT. “If there’s unambiguous evidence that it’s been broken, that’s really a fundamental discovery.”

    But established physics theories die hard. Shaking the foundations of physics, Pohl says, is “what I dream of, but I think that’s not going to happen.” Instead, he suspects, the discrepancy is more likely to be explained through minor tweaks to the experiments or the theory.

    The alluring mystery of the proton radius reeled Downie in. During conversations in the lab with some fellow physicists, she learned of an upcoming experiment that could help settle the issue. The experiment’s founders were looking for collaborators, and Downie leaped on the bandwagon. The Muon Proton Scattering Experiment, or MUSE, to take place at the Paul Scherrer Institute beginning in 2018, will scatter both electrons and muons off of protons and compare the results. It offers a way to test whether the two particles behave differently, says Downie, who is now a spokesperson for MUSE.

    A host of other experiments are in progress or planning stages. Scientists with the Proton Radius Experiment, or PRad, located at Jefferson Lab in Newport News, Va., hope to improve on Bernauer and colleagues’ electron-scattering measurements. PRad researchers are analyzing their data and should have a new number for the proton radius soon.

    But for now, the proton’s identity crisis, at least regarding its size, remains. That poses problems for ultrasensitive tests of one of physicists’ most essential theories. Quantum electrodynamics, or QED, the theory that unites quantum mechanics and Albert Einstein’s special theory of relativity, describes the physics of electromagnetism on small scales. Using this theory, scientists can calculate the properties of quantum systems, such as hydrogen atoms, in exquisite detail — and so far the predictions match reality. But such calculations require some input — including the proton’s radius. Therefore, to subject the theory to even more stringent tests, gauging the proton’s size is a must-do task.

    At the Max Planck Institute of Quantum Optics, researchers use lasers to study proton size. A. ANTOGNINI AND F. REISER/PSI

    Spin doctors

    Even if scientists eventually sort out the proton’s size snags, there’s much left to understand. Dig deep into the proton’s guts, and the seemingly simple particle becomes a kaleidoscope of complexity. Rattling around inside each proton is a trio of particles called quarks: one negatively charged “down” quark and two positively charged “up” quarks. Neutrons, on the flip side, comprise two down quarks and one up quark.

    Yet even the quark-trio picture is too simplistic. In addition to the three quarks that are always present, a chaotic swarm of transient particles churns within the proton. Evanescent throngs of additional quarks and their antimatter partners, antiquarks, continually swirl into existence, then annihilate each other. Gluons, the particle “glue” that holds the proton together, careen between particles. Gluons are the messengers of the strong nuclear force, an interaction that causes quarks to fervently attract one another.


    A new spin

    Scientists thought that a proton’s spin was due to the three main quarks (left, arrows indicate direction of a quark’s spin). Instead, gluons (yellow) and ephemeral pairs of quarks and antiquarks contribute through their spin and motion (gray arrows at right).


    As a result of this chaos, the properties of protons — and neutrons as well — are difficult to get a handle on. One property, spin, has taken decades of careful investigation, and it’s still not sorted out. Quantum particles almost seem to be whirling at blistering speed, like the Earth rotating about its axis. This spin produces angular momentum — a quality of a rotating object that, for example, keeps a top revolving until friction slows it. The spin also makes protons behave like tiny magnets, because a rotating electric charge produces a magnetic field. This property is the key to the medical imaging procedure called magnetic resonance imaging, or MRI.

    But, like nearly everything quantum, there’s some weirdness mixed in: There’s no actual spinning going on. Because fundamental particles like quarks don’t have a finite physical size — as far as scientists know — they can’t twirl. Despite the lack of spinning, the particles still behave like they have a spin, which can take on only certain values: integer multiples of ½.

    Quarks have a spin of ½, and gluons a spin of 1. These spins combine to help yield the proton’s total spin. In addition, just as the Earth is both spinning about its own axis and orbiting the sun, quarks and gluons may also circle about the proton’s center, producing additional angular momentum that can contribute to the proton’s total spin.

    Somehow, the spin and orbital motion of quarks and gluons within the proton combine to produce its spin of ½. Originally, physicists expected that the explanation would be simple. The only particles that mattered, they thought, were the proton’s three main quarks, each with a spin of ½. If two of those spins were oriented in opposite directions, they could cancel one another out to produce a total spin of ½. But experiments beginning in the 1980s showed that “this picture was very far from true,” says theoretical high-energy physicist Juan Rojo of Vrije University Amsterdam. Surprisingly, only a small fraction of the spin seemed to be coming from the quarks, befuddling scientists with what became known as the “spin crisis” (SN: 9/6/97, p. 158). Neutron spin was likewise enigmatic.

    Scientists’ next hunch was that gluons contribute to the proton’s spin. “Verifying this hypothesis was very difficult,” Rojo says. It required experimental studies at the Relativistic Heavy Ion Collider, RHIC, a particle accelerator at Brookhaven National Laboratory in Upton, N.Y.

    BNL RHIC Campus

    In these experiments, scientists collided protons that were polarized: The two protons’ spins were either aligned or pointed in opposite directions. Researchers counted the products of those collisions and compared the results for aligned and opposing spins. The results revealed how much of the spin comes from gluons. According to an analysis by Rojo and colleagues, published in Nuclear Physics B in 2014, gluons make up about 35 percent of the proton’s spin. Since the quarks make up about 25 percent, that leaves another 40 percent still unaccounted for.

    “We have absolutely no idea how the entire spin is made up,” says nuclear physicist Elke-Caroline Aschenauer of Brookhaven. “We maybe have understood a small fraction of it.” That’s because each quark or gluon carries a certain fraction of the proton’s energy, and the lowest energy quarks and gluons cannot be spotted at RHIC. A proposed collider, called the Electron-Ion Collider (location to be determined), could help scientists investigate the neglected territory.

    The Electron-Ion Collider could also allow scientists to map the still-unmeasured orbital motion of quarks and gluons, which may contribute to the proton’s spin as well.

    The PHENIX experiment at Brookhaven National Laboratory uses a giant detector to investigate spin. BROOKHAVEN NATIONAL LAB

    An unruly force

    Experimental physicists get little help from theoretical physics when attempting to unravel the proton’s spin and its other perplexities. “The proton is not something you can calculate from first principles,” Aschenauer says. Quantum chromo-dynamics, or QCD — the theory of the quark-corralling strong force transmitted by gluons — is an unruly beast. It is so complex that scientists can’t directly solve the theory’s equations.

    The difficulty lies with the behavior of the strong force. As long as quarks and their companions stick relatively close, they are happy and can mill about the proton at will. But absence makes the heart grow fonder: The farther apart the quarks get, the more insistently the strong force pulls them back together, containing them within the proton. This behavior explains why no one has found a single quark in isolation. It also makes the proton’s properties especially difficult to calculate. Without accurate theoretical calculations, scientists can’t predict what the proton’s radius should be, or how the spin should be divvied up.


    Cosmic time

    Protons have been around since the early moments of the universe. If certain theories are correct, the universe may eventually be devoid of protons.

    t = 0
    Big Bang
    t < 10-6 seconds
    Quarks and gluons roam freely
    t = 10-6 s
    Protons and neutrons form
    t = 10 s
    Protons and neutrons begin to form atomic nuclei
    t = 13.8 billion years
    In today’s universe, atoms have formed into stars, planets and intelligent life.
    t = 1034 years or later
    A substantial portion of protons may have decayed.


    To simplify the math of the proton, physicists use a technique called lattice QCD, in which they imagine that the world is made of a grid of points in space and time (SN: 8/7/04, p. 90). A quark can sit at one point or another in the grid, but not in the spaces in between. Time, likewise, proceeds in jumps. In such a situation, QCD becomes more manageable, though calculations still require powerful supercomputers.

    Lattice QCD calculations of the proton’s spin are making progress, but there’s still plenty of uncertainty. In 2015, theoretical particle and nuclear physicist Keh-Fei Liu and colleagues calculated the spin contributions from the gluons, the quarks and the quarks’ angular momentum, reporting the results in Physical Review D. By their calculation, about half of the spin comes from the quarks’ motion within the proton, about a quarter from the quarks’ spin, with the last quarter or so from the gluons. The numbers don’t exactly match the experimental measurements, but that’s understandable — the lattice QCD numbers are still fuzzy. The calculation relies on various approximations, so it “is not cast in stone,” says Liu, of the University of Kentucky in Lexington.

    Death of a proton

    Although protons seem to live forever, scientists have long questioned that immortality. Some popular theories predict that protons decay, disintegrating into other particles over long timescales. Yet despite extensive searches, no hint of this demise has materialized.

    A class of ideas known as grand unified theories predict that protons eventually succumb. These theories unite three of the forces of nature, creating a single framework that could explain electromagnetism, the strong nuclear force and the weak nuclear force, which is responsible for certain types of radioactive decay. (Nature’s fourth force, gravity, is not yet incorporated into these models.) Under such unified theories, the three forces reach equal strengths at extremely high energies. Such energetic conditions were present in the early universe — well before protons formed — just a trillionth of a trillionth of a trillionth of a second after the Big Bang. As the cosmos cooled, those forces would have separated into three different facets that scientists now observe.


    A proton’s last moments

    If theories that unite fundamental forces are correct, protons should decay, with average lifetimes longer than the age of the universe. Scientists watch giant tanks of water for the telltale signatures of proton death. One possible type of decay is described below.

    A proton, made of three quarks, awaits its fate.

    In an extremely rare event, two quarks unite, producing a new particle.


    The new particle, an X boson, persists for a brief instant.


    The X boson releases a positron and an antiquark. No longer a proton, the two remaining particles are a pion.


    Finally, the pion decays into two photons, which can be detected, along with the positron.


    “We have a lot of circumstantial evidence that something like unification must be happening,” says theoretical high-energy physicist Kaladi Babu of Oklahoma State University in Stillwater. Beyond the appeal of uniting the forces, grand unified theories could explain some curious coincidences of physics, such as the fact that the proton’s electric charge precisely balances the electron’s charge. Another bonus is that the particles in grand unified theories fill out a family tree, with quarks becoming the kin of electrons, for example.

    Under these theories, a decaying proton would disintegrate into other particles, such as a positron (the antimatter version of an electron) and a particle called a pion, composed of a quark and an antiquark, which itself eventually decays. If such a grand unified theory is correct and protons do decay, the process must be extremely rare — protons must live a very long time, on average, before they break down. If most protons decayed rapidly, atoms wouldn’t stick around long either, and the matter that makes up stars, planets — even human bodies — would be falling apart left and right.

    Protons have existed for 13.8 billion years, since just after the Big Bang. So they must live exceedingly long lives, on average. But the particles could perish at even longer timescales. If they do, scientists should be able to monitor many particles at once to see a few protons bite the dust ahead of the curve (SN: 12/15/79, p. 405). But searches for decaying protons have so far come up empty.

    Still, the search continues. To hunt for decaying protons, scientists go deep underground, for example, to a mine in Hida, Japan. There, at the Super-Kamiokande experiment (SN: 2/18/17, p. 24), they monitor a giant tank of water — 50,000 metric tons’ worth — waiting for a single proton to wink out of existence. After watching that water tank for nearly two decades, the scientists reported in the Jan. 1 Physical Review D that protons must live longer than 1.6 × 1034 years on average, assuming they decay predominantly into a positron and a pion.

    Super-Kamiokande Detector, Japan

    After watching that water tank for nearly two decades, the scientists reported in the Jan. 1 Physical Review D that protons must live longer than 1.6 × 1034 years on average, assuming they decay predominantly into a positron and a pion.

    Experimental limits on the proton lifetime “are sort of painting the theorists into a corner,” says Ed Kearns of Boston University, who searches for proton decay with Super-K. If a new theory predicts a proton lifetime shorter than what Super-K has measured, it’s wrong. Physicists must go back to the drawing board until they come up with a theory that agrees with Super-K’s proton-decay drought.

    Many grand unified theories that remain standing in the wake of Super-K’s measurements incorporate supersymmetry, the idea that each known particle has another, more massive partner. In such theories, those new particles are additional pieces in the puzzle, fitting into an even larger family tree of interconnected particles. But theories that rely on supersymmetry may be in trouble. “We would have preferred to see supersymmetry at the Large Hadron Collider by now,” Babu says, referring to the particle accelerator located at the European particle physics lab, CERN, in Geneva, which has consistently come up empty in supersymmetry searches since it turned on in 2009 (SN: 10/1/16, p. 12).

    CERN/LHC Map

    CERN LHC Tube

    LHC at CERN

    Persnickety protons

    Scientists might solve some of their proton dilemmas with new data — for example, by spotting a proton decaying into a positron and two photons, as in the simulated data from the Super-Kamiokande detector below. But plenty more questions await exploration.


    Why are quarks confined within the proton? Scientists observe that quarks don’t live on their own, but no one has been able to mathematically demonstrate that they can’t.

    How are the quarks and gluons arranged inside the proton? Gluons might be more common on the proton’s outskirts than in its center, for example.

    Each quark and gluon carries a certain amount of the proton’s energy. How is that energy divvied up?

    Aside from their electric charges, protons and antiprotons appear the same. Do they differ on some level not yet measured?

    But supersymmetric particles could simply be too massive for the LHC to find. And some grand unified theories that don’t require supersymmetry still remain viable. Versions of these theories predict proton lifetimes within reach of an upcoming generation of experiments. Scientists plan to follow up Super-K with Hyper-K, with an even bigger tank of water. And DUNE, the Deep Underground Neutrino Experiment, planned for installation in a former gold mine in Lead, S.D., will use liquid argon to detect protons decaying into particles that the water detectors might miss.

    FNAL Dune & LBNF

    Surf-Dune/LBNF Caverns at Sanford

    FNAL DUNE Argon tank at SURF

    If protons do decay, the universe will become frail in its old age. According to Super-K, sometime well after its 1034 birthday, the cosmos will become a barren sea of light. Stars, planets and life will disappear. If seemingly dependable protons give in, it could spell the death of the universe as we know it.

    Although protons may eventually become extinct, proton research isn’t going out of style anytime soon. Even if scientists resolve the dilemmas of radius, spin and lifetime, more questions will pile up — it’s part of the labyrinthine task of studying quantum particles that multiply in complexity the closer scientists look. These deeper studies are worthwhile, says Downie. The inscrutable proton is “the most fundamental building block of everything, and until we understand that, we can’t say we understand anything else.”

    See the full article here .

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  • richardmitnick 12:39 pm on April 4, 2015 Permalink | Reply
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    From Science News: “Primordial stars left their imprint on dwarf galaxy” 

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    April 3, 2015
    Christopher Crockett

    Explosions of very massive stars in the early universe (illustrated) seeded a handful of stars in a nearby galaxy with unusual amounts of various elements. NASA, CXC, M.Weiss

    NASA Chandra Telescope

    A handful of ancient stars outside the Milky Way witnessed the explosive deaths of the first generation of stars, researchers report in the April 1 Astrophysical Journal. The eyewitnesses harbor unusual amounts of heavy elements, such as magnesium and silicon, which means they were probably bystanders to a few supernovas from primordial stars up to about 20 times as massive as the sun.

    The stars live about 290,000 light-years away in the puny Sculptor galaxy.

    The Sculptor Galaxy taken with the ESO VISTA telescope at the Paranal Observatory in Chile.

    ESO Vista Telescope

    Astronomers think that dwarf galaxies like Sculptor are relics from the early universe, which makes these galaxies useful laboratories for studying conditions from not too long after the Big Bang.

    See the full article here.

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  • richardmitnick 7:12 pm on March 28, 2015 Permalink | Reply
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    From SN: “Clean-up gene gone awry can cause Lou Gehrig’s disease” 

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    March 24, 2015
    Kate Baggaley

    Mutations on a gene necessary for keeping cells clean can cause Lou Gehrig’s disease, scientists report online March 24 in Nature Neuroscience. The gene is one of many that have been connected to the condition.

    In amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease, nerve cells that control voluntary movement die, leading to paralysis. Scientists have previously identified mutations in 29 genes that are linked with ALS, but these genes account for less than one-third of all cases.

    To track down more genes, a team of European researchers looked at the protein-coding DNA of 252 ALS patients with a family history of the disease, as well as of 827 healthy people. The team discovered eight mutations on a gene called TBK1 that were associated with ALS.

    TBK1 normally codes for a protein that controls inflammation and cleans out damaged proteins from cells. “We do not know which of these two principle functions of TBK1 is the more relevant one” to ALS, says coauthor Jochen Weishaupt, a neurologist at Ulm University in Germany. In cells with one of the eight TBK1 mutations, the protein either is missing or lacks components that it needs to interact with other proteins, the researchers found.

    TBK1 mutations may explain 2 percent of ALS cases that run in families, which make up about 10 percent of all incidences of the disease, Weishaupt says.

    See the full article here.

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  • richardmitnick 6:46 am on March 7, 2015 Permalink | Reply
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    From ScienceNews: “Sam Ting tries to expose dark matter’s mysteries” 

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    March 6, 2015
    Andrew Grant

    Physics Nobel laureate’s space-based detector is analyzing billions of cosmic rays

    EYES ON THE INVISIBLE PRIZE Designed to detect cosmic rays, the Alpha Magnetic Spectrometer cruises above Earth on the International Space Station.


    In the near vacuum of outer space, each rare morsel of matter tells a story. A speedy proton may have been propelled by the shock wave of an exploding star. A stray electron may have teetered on the precipice of a black hole, only to be flung away in a powerful jet of searing gas.

    Since 2011, the International Space Station has housed an experiment that aims to decipher those origin stories. The Alpha Magnetic Spectrometer has already cataloged more than 60 billion protons, electrons and other spaceborne subatomic particles, known as cosmic rays, as they zip by.

    Other experiments sample the shower of particles produced when cosmic rays strike atoms and molecules in Earth’s atmosphere. But the spectrometer scrutinizes pristine cosmic rays — some of which have traveled undisturbed over millions of light-years — from its perch some 400 kilometers above Earth. The Alpha Magnetic Spectrometer is by far the most sensitive cosmic ray detector ever to fly in space, and with a price tag of about $2 billion, it’s also the most expensive.

    The detector’s unprecedented particle census could unmask the identity of dark matter, the mysterious, invisible substance that is five times as abundant in the universe as ordinary matter. Some of the cosmic rays snatched by the instrument may have been produced by particles of dark matter colliding and annihilating each other in the center of the galaxy.

    The spectrometer could also help scientists determine why planets, stars and other structures in the universe are made of matter rather than antimatter. Particles of antimatter have the opposite charge as their matter counterparts but are identical in nearly every other way. It’s uncertain why most of the antimatter particles disappeared just after the Big Bang 13.8 billion years ago. Physicists would love to discover primordial antimatter to test their theories on what hastened its demise.

    Sam Ting

    Nearly four years into the mission, the Alpha Magnetic Spectrometer is delivering precise data and arguably providing a few hints about the nature of dark matter. But it’s unclear whether the mission will ever deliver on its ambitious goals. Cosmic rays are charged particles that get whipped around by magnetic fields, so they don’t travel in straight lines and cannot be traced back to their source. To pin the origin of particular cosmic rays to dark matter, scientists will have to rule out every other possible explanation. Critics say the chances of identifying dark matter are very slim. And finding primordial antimatter, they say, is nearly impossible.

    Such criticism barely registers with the mission’s leader, particle physicist Samuel Ting. The 79-year-old Nobel laureate has made a career of designing elegant experiments and, despite frequent opposition, successfully lobbying to get them built. Then he has patiently collected and analyzed data, meticulous to the extreme, before revealing the often-impressive findings. Though results may come later than most scientists would prefer, Ting is confident that conducting a powerful particle physics experiment in space will expand scientists’ understanding of the cosmos.

    Full focus

    Ting’s home base these days is at CERN, the European physics laboratory outside Geneva that partially funds the Alpha Magnetic Spectrometer and is home to the mission’s command center. But on one afternoon in December, Ting is at MIT, where he still runs a lab. His office is housed in a building marked with a capital J that honors his Nobel Prize–winning discovery, the J particle. The alleged reason for Ting’s U.S. visit was to meet with a contractor to discuss renovating his Cambridge, Mass., home. But the contractor confab was brief. For Ting, matters outside of physics take a backseat.

    “You really can’t get into this field without thinking this is the most important thing in your life,” Ting says.

    Two high-definition monitors on his office wall reinforce his obsession. One shows a live feed from the space station, a grainy black-and-white image capturing the spectrometer and our imperceptibly spinning planet below. The other screen plays a computer reconstruction of the instrument in action. In nearly real time, cosmic rays pass through its magnet, triggering a slate of sensors that determine the particles’ identity, energy and trajectory.

    Ting doesn’t have a background in astrophysics, but he has plenty of experience sorting through a glut of particles to find really cool stuff.

    He pulls up a 1965 New York Times article on his computer. The article describes Ting’s first major discovery, when he, Leon Lederman (who won the 1988 Nobel Prize in physics) and colleagues produced and detected antimatter nuclei for the first time. (A team at CERN made a similar discovery soon after.) It’s difficult enough to observe single particles of antimatter because they disappear in a burst of energy when they encounter ordinary matter. Ting and Lederman managed to observe bound pairs of antimatter particles, called antideuterons, in a particle accelerator at Brookhaven National Laboratory in Upton, N.Y.

    Ting’s childlike curiosity quickly comes across as he describes the possibility that antideuterons and other large chunks of antimatter, relics of the first moments after the Big Bang, could be drifting in the cosmos, waiting to be found. But beneath the inquisitiveness is also an extreme confidence, even an arrogance, that he alone knows the way to probe the big questions.

    Those qualities were on display in the early 1970s when Ting became interested in quarks, tiny parcels that compose such particles as protons and neutrons. Physicists had proposed and discovered evidence for three kinds of quarks. But Ting, eager to unravel every detail about matter’s makeup, joined a group of physicists who wondered whether there were other quark varieties. He proposed colliding particles at high energies, which would create short-lived matter that ultimately decayed into electrons and their antimatter counterparts, positrons. By analyzing the electrons and positrons, he could determine the composition of the intermediate particles.

    Ting says many physicists scoffed at his proposal; they believed that the three quarks could explain all the more complex particles in physics. Multiple labs turned him down before Brookhaven let him give it a try.

    In the summer of 1974, Ting and his team saw convincing signs of a new subatomic particle with an unusual composition. But Ting refused to release the data until he was sure everything was correct. He split his team into two groups that independently analyzed the data again and again. Only in November of that year, when a colleague at a meeting told Ting that particle physicist Burton Richter had seen the same signal at the Stanford Linear Accelerator Center, did Ting share his finding. The confirmation of a fourth quark, the charm, embedded in a particle that Ting called J and Richter called Psi earned Ting a share (with Richter) of the 1976 Nobel Prize in physics. Ting’s experimental design skill, combined with large doses of meticulousness, smarts and stubbornness, had netted him the ultimate physics honor. He was 40 years old.

    From there, Ting kept pursuing big projects. In the late 1980s, he organized a team to design a detector for the multibillion-dollar Superconducting  Super Collider, an 87-kilometer-around particle accelerator slated for construction near Waxahachie, Texas. Ting wanted to build a $750 million instrument; the U.S. Department of Energy said the detector should not cost more than $500 million. So Ting quit. “He was very determined to do it his way,” says Gary Sanders, a high-energy physicist and former Ting graduate student who was part of that team.

    In 1993, Congress dealt American physicists a devastating blow by canceling the Super Collider. Ting, however, had moved on. In 1994, he pitched perhaps the most ambitious project of his career.

    Like his first major experiment, it would hunt for antideuterons and other antimatter nuclei. And similar to his Nobel-winning research, it would use electrons and positrons as probes to identify undiscovered parent particles. Except instead of sorting through shrapnel created in carefully orchestrated particle collisions, he wanted to go after particles produced naturally in the universe. The Alpha Magnetic Spectrometer experiment would collect and analyze particles in space.

    Both NASA and the Department of Energy, the same agency that rejected Ting’s plan for the detector in Texas, pledged their support.

    From lab to liftoff

    Scientists have studied cosmic rays for a century in hope of learning about the objects that produce them. But Ting’s proposal offered the rare chance to create a robust census of cosmic rays from well above Earth’s meddlesome atmosphere. Most previous experiments took place on balloons, which fly only briefly and don’t leave the atmosphere, or on the ground, forcing scientists to analyze cascading showers of particles triggered by cosmic rays striking atoms in the atmosphere.

    Those past experiments still delivered some tantalizing results. In 1997, the High-Energy Antimatter Telescope, or HEAT, a cosmic ray detector tethered to a high-altitude balloon, revealed an unexpectedly high concentration of positrons in space. At the time, physicists didn’t know of many processes in the universe that could produce positrons, so theorists quickly came up with some ideas. The most intriguing possibility was that the positrons were created by particles of dark matter in the galaxy. Though the dark matter particles would be invisible, they would occasionally collide and annihilate each other to produce gamma radiation and detectable particles, including electrons and positrons. If these dark matter theories were correct, then a precise measurement of cosmic ray positrons would enable physicists to pin down the nature and mass of dark matter particles.

    But dark matter wasn’t the only explanation. Other theorists proposed positron-forming mechanisms that have far less relevance for deciphering the universe. Atop the list were pulsars — dense, rapidly spinning cores left over after massive stars explode. A pulsar’s rapid rotational speed generates an intense electromagnetic field strong enough to rip electrons from its surface. Those electrons interact with photons and create pairs of electrons and positrons. Calculations suggested that just one or two pulsars, which are difficult to detect, within hundreds of light-years of the solar system would be enough to litter Earth with positrons.

    Despite the intriguing quandary exposed by HEAT, some scientists doubted that the Alpha Magnetic Spectrometer could add much to the positron origin debate or resolve any big physics mysteries. But Ting was determined to see his project fly. He assembled a 16-country collaboration to divide the work and the ballooning costs. When the 2003 explosion of the space shuttle Columbia led NASA to rescind its offer of a ride to the space station, Ting lobbied members of Congress, teasing at the wonders that could be hidden in cosmic rays and stressing the International Space Station’s not-so-stellar reputation for housing serious science.

    “If you told Sam that to get what he wanted he had to win the Indy 500, he’d become the world’s best race car driver,” says Richard Milner, the director of MIT’s Laboratory for Nuclear Science, who oversees Ting’s group. Ting wouldn’t let up on government officials in Washington, even as many of his collaborators focused on other projects.

    He was very persuasive, says Kay Bailey Hutchison, at the time a U.S. Senator from Texas. She says Ting convinced her and others that the mission was worth the cost and safety concerns of extending the beleaguered shuttle program. “He’s such a visionary,” she says. She was inspired enough to switch appropriations subcommittees to find funding for the project. In October 2008, President George W. Bush signed a bill adding shuttle flights so that the Alpha Magnetic Spectrometer would hitch a ride on one of them. “Without [Ting’s] absolute unwillingness to give up, we would not have gotten it,” Hutchison says.

    By the time Ting’s brainchild reached the space station in May 2011, a couple of space-based cosmic ray experiments had beaten his spectrometer to the punch. In 2008, PAMELA, a cosmic ray detector attached to a Russian reconnaissance satellite, revealed the same positron excess hinted at by HEAT. NASA’s Fermi Gamma-ray Space Telescope, which also carries a cosmic ray detector, came up with similar results in 2011. Neither probe discerned the source of the positrons, however.

    PAMELA Cosmic Ray Detector

    NASA Fermi Telescope
    NASA’s Fermi Gamma-ray Space Telescope

    POSITRON PUZZLE The positron measurements (as a fraction of the total number of positrons and electrons) made by the Alpha Magnetic Spectrometer (AMS) are shown with solid red circles in this graph. Measurements made by previous instruments (see legend) had much larger margins of error, as indicated by the lines above and below each data point. [Source: L. Accardo et al/Phys. Rev. Lett. 2014]

    Ting’s instrument began its cosmic ray survey almost immediately after installation, collecting as much data in one day as PAMELA did in 50. It sifted through positively charged particles, most of which are protons, and picked out the more valuable positrons. Ting, true to form, took his time before releasing the first results. “I doubt in the next 20 years anyone will be able to repeat the experiment,” he says. “There’s nobody to check us. It’s of the utmost importance to get it correct.”

    Ting broke his silence with a news conference at CERN in April 2013. After again employing two separate teams to comb through the data, he confirmed the positron excess detected by HEAT, PAMELA and Fermi (SN: 5/4/13, p. 14). Analyzing the properties of 6.8 million positrons and electrons, Ting’s team found that the number of positrons keeps rising as the particle energies increase. The clear excess of positrons, Ting said, reinforces that something relatively nearby must be producing them. He pushed the dark matter explanation but admitted it was not the only possibility.

    Ting returned for another news conference in September. This time, after poring over 10.9 million positrons and electrons, Ting’s team pinpointed the energy, about 275 billion electron volts, at which the concentration of positrons stops increasing (see graph above). That’s an interesting number, says Peter McIntyre, a high-energy physicist at Texas A&M University in College Station, because it indicates that the mass of hypothetical dark matter particles limits the energy of the positrons they can produce. Theorists could use the peak positron energy to estimate dark matter’s mass. But again, the experiment did not come close to proving that dark matter actually produced the positrons.


    X-ray: NASA/CXC/Univ. of Toronto/M. Durant et al; Optical: DSS/Davide De Martin

    Pulsars, like the Vela pulsar located about 1,000 light-years away, are rapidly spinning dense cores of former stars. Nearby pulsars may produce the unexplained excess of positrons detected by the Alpha Magnetic Spectrometer and other experiments.
    What is it?

    Dark matter A form of matter that accounts for most of the mass in a galaxy but does not consist of the ordinary kind of matter found on Earth.

    Pulsar A dense, rapidly spinning remnant of a star that was initially much more massive than the sun.
    How would it produce positrons?

    Dark matter In theory, two dark matter particles can collide and annihilate each other to produce electrons and positrons.

    Pulsar The collision of photons with speedy electrons ripped from a pulsar’s surface by intense electromagnetic fields produces electrons and positrons.

    What are the implications?

    Dark matter Finding positrons from dark matter would help scientists to determine the type and mass of dark matter particles, resolving a decades-long mystery.

    Pulsar Positrons from pulsars would reveal something about particles that pulsars create. But it would not lead to big-picture understanding of the universe.

    In fact, some physicists argue that the Alpha Magnetic Spectrometer, despite its unmatched particle-detecting prowess, can never definitively distinguish between dark matter annihilation, pulsars or a yet-to-be-discovered process that might be producing those surplus shards of antimatter.

    “A pulsar could explain any observation that AMS could ever make,” says Gregory Tarlé, a particle astrophysicist at the University of Michigan in Ann Arbor. No matter what the positron data, physicists will not be able to definitively isolate the alleged signal of dark matter, he argues.

    Katherine Freese, a theoretical astrophysicist at the Nordic Institute for Theoretical Physics in Stockholm, agrees that conclusively proving dark matter from positrons will be very difficult. “My bet is on pulsars,” she says.

    Other experiments also suggest that AMS has a slim chance of making a compelling case for dark matter. In a study posted online in January at arXiv.org, physicists pored over Fermi telescope measurements to look for gamma radiation, which should also be produced when dark matter particles annihilate each other. The data ruled out most dark matter collision mechanisms proposed by theorists. And in December, scientists with the Planck satellite announced that their survey of the universe’s most ancient light revealed no signs of detritus from colliding dark matter, which if self-annihilating now also should have been when the cosmos was young (SN: 12/27/14, p. 11).

    ESA Planck

    Ting says he pays about as much attention to other experiments as he does to his critics. He monitors the scientific literature, but doesn’t put much stock in blanket conclusions based on one set of data. “I learned a long time ago: Only look at your own experiment,” he says.

    He expects to learn more by studying positrons at higher energies. If the mass of a dark matter particle is, say, one trillion electron volts, then it probably wouldn’t produce positrons with more than a quarter of that energy. So if the positron concentration falls off a cliff after the newly identified peak, Ting says, that would suggest a dark matter origin. Pulsars, on the other hand, should produce positrons with a spectrum of energies that wouldn’t drop so precipitously.

    Within the next year or two, the AMS team will release its first analysis of antiprotons, antimatter particles that Ting says are too heavy to be manufactured by pulsars but should be produced in dark matter collisions. Ting calls the preliminary results “intriguing.” But of course, he won’t offer more until all the cross-checks are complete.

    He’s confident that future measurements will allow him to definitively pin down the origin of positrons, whether from dark matter or something else.

    Even if the dark matter picture remains muddled, there is a possibility that AMS will detect primordial antimatter. One of the biggest mysteries in physics is why matter won out in a universe that presumably began with equal parts of matter and antimatter. Ting hopes to find complex antimatter — perhaps antihelium (two antiprotons and two antineutrons) or antideuterons — that was forged just after the Big Bang. Tarlé and other scientists say the chances of detecting these antinuclei are extremely low because the antimatter would have to navigate through the matter-rich galaxy and solar system without being destroyed.

    Ting is undeterred. Gathering insights about the cosmos takes time. Anticipating that funding will run as long as the space station operates, Ting simply wants to see what nature throws at him. “If you don’t look,” he says, “you do not know.”

    See the full article here.

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