News this Week

Science  01 Jun 2012:
Vol. 336, Issue 6085, pp. 1082

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  1. Around the World

    1 - Parma, Italy
    French GM Concerns Dismissed—Again
    2 - Washington, D.C.
    Senate Committee Wants to Sink Military's Biofuels Program
    3 - Harpenden, U.K.
    Test Crop Survives Protest
    4 - Brussels
    Horizon 2020: Battlefield for Open Access

    Parma, Italy

    French GM Concerns Dismissed—Again


    France's latest attempt to keep genetically modified (GM) crops from its fields was rebuked by a scientific panel at the European Food Safety Authority (EFSA) on 21 May. EFSA dismissed France's argument that a GM maize variety produced by Monsanto called MON810 might be harmful to the environment or human health.

    The European Commission approved the cultivation of MON810 in 1998, but the French government, faced with strong public opposition to GM crops, banned MON810 in 2008. EFSA rejected France's measure later that year.

    France again asked the European Commission in February for permission to ban MON810, suggesting that Cry1Ab, a protein produced by MON810 to ward off maize stalk borers (shown), could hurt non-target species such as bees and butterflies, and that it could linger in the soil.

    But the EFSA Panel on Genetically Modified Organisms said that it “could not identify any new science-based evidence indicating that maize MON 810 cultivation in the E.U. poses a significant and imminent risk to the human and animal health or the environment.”

    Washington, D.C.

    Senate Committee Wants to Sink Military's Biofuels Program

    The Senate Armed Services Committee narrowly voted on 24 May to ban the Department of Defense (DOD) from paying more for alternative fuels than for petroleum-based supplies and from building a biofuel refinery unless explicitly authorized to do so. The House of Representatives agreed to similar bans last month (Science, 25 May 2012, p. 971). Critics say the measures undermine efforts to diversify fuel sources and make the country less dependent on foreign oil. The measure is expected to be taken up by the full Senate in June.

    To reduce the impact of oil shortages and price spikes on its budget, DOD has pushed a program of investing in alternative energy sources since the Bush Administration. The Navy and Air Force set goals of using advanced biofuels, which can be blended with petroleum and burned in conventional engines, for 50% of their fuel use by the end of this decade. Military officials insist their investments in alternative energy are intended to improve energy security and drive down biofuel costs. But congressional Republicans have accused the Obama Administration of using the military to support its green agenda.

    Harpenden, U.K.

    Test Crop Survives Protest


    An experiment to test a strain of genetically modified wheat can continue after police prevented activists from destroying the crop. A group called Take the Flour Back had announced their intention to “decontaminate” the field trial at a day of protest on 27 May at the Rothamsted Research agricultural research station in Harpenden, U.K. Police lines blocked several hundred protesters from entering the test site, and after enjoying “a GM-free picnic” and listening to speeches, the protesters dispersed peacefully.

    Online, however, Rothamsted was subject to another, less peaceful, attack that took its Web site down for several hours shortly after the protest ended.


    Horizon 2020: Battlefield for Open Access

    Last week, the European Commission's research director, Robert-Jan Smits, said that open access “will be the norm” for studies funded through Europe's £80 billion Horizon 2020 research program. But what that will mean is unclear. Open access typically involves making research papers freely available, but the commission hasn't said whether it will require that papers be placed in a public repository immediately or within months or a year of publication. Horizon 2020, the successor of the current Framework Programme 7 (FP7), will start in 2014. Open access was partly mandatory in FP7, but the European Commission currently has no power to enforce compliance from scientists.

  2. Random Sample


    Tweeters, prepare your hashtags: The Innovative Medicines Initiative, a joint venture between the European Union and Europe's pharmaceutical industry, last week launched a new €220 million collaboration of companies and public partners to combat antimicrobial resistance—and they're calling it NewDrugs4BadBugs. The name might not bug the European Union for long, though: Like an adapting microorganism, it is starting to hide behind its own acronym: ND4BB.

    They Said It

    “We called it the banana standard. We thought this was a great idea, to show people that any radiation doses experienced by Americans from Fukushima would be small compared to eating a banana. But apparently some people in the banana industry took offense.”

    —U.S. presidential science adviser John Holdren describing a risk-communication strategy (which was never implemented) after the March 2011 Fukushima nuclear disaster, based on the natural radiation from potassium-40 in a banana.

    Hitchhikers From the Deep


    Are deep-sea submersibles bringing back creatures from the ocean depths? Scientists have long argued that immense pressure differences would kill any hangers-on. But, as reported online 24 May in Conservation Biology, some organisms from sea-floor vent systems may survive long enough to travel to another undersea spot.

    In September 2004, the submersible Alvin slurped up sediments from the sea floor at a site more than 2200 meters deep off the coast of Washington. But some of the creatures in the sediments—especially limpets (shown), a type of marine snail—turned out to be stowaways that had hitched a 635-kilometer ride with Alvin from a hydrothermal vent the researchers had visited 2 days earlier. The ratios of carbon and nitrogen in the creatures' tissues and shells, and the ratio of males to females in the sample—characteristics that vary among hydrothermal vent populations—gave the stowaways away.

    Bivalve Digging Inspires ‘RoboClams’


    The speedy digging strategy of a humble clam may help underwater robots stay parked over the sea floor.

    As any clam-digger knows, Atlantic razor clams (Ensis directus) are quite spry, despite being legless. They can burrow 70 centimeters deep into sediment at up to a centimeter per second, although their muscles suggest they should be much slower.

    So what keeps them from slowing down as they tunnel deeper? The trick, discovered mechanical engineer Amos G. Winter V of the Massachusetts Institute of Technology in Cambridge, is that the clams surround themselves with a pocket of quicksand. Winter and his team placed razor clams inside a tank filled with small glass beads. The clams first wriggled part of their shells into the “sediment” with a fleshy organ called a foot. Then, they contracted their shells so that the surrounding beads started to cave in. By pulling their shells in even closer, the clams drew surrounding water into the spaces that opened up between the beads. The resulting water-bead mixture reduced the resistance clams encountered while digging, the researchers reported online 23 May in The Journal of Experimental Biology.

    Based on the animals' technique, Winter successfully tested a prototype burrowing robot, RoboClam, and is now busy building small robots that mimic the clam's motions for inclusion on autonomous underwater vehicles. The goal is to use them as anchors that dig themselves in and out of the sea floor, eliminating the need for battery-draining motors while still keeping the vehicle stationary in the water.

    By the Numbers

    450 — Minimum number of California condors required for delisting under the U.S. Endangered Species Act. The population hit 405 on 30 April.

    35% — Percentage of the U.S. Southern High Plains that, at current groundwater depletion rates, won't sustain irrigation within 30 years, putting crop production at risk, according to a Proceedings of the National Academy of Sciences study.

    31.6 billion — Metric tons of carbon dioxide emitted globally in 2011—a record, the International Energy Agency said on 24 May. A 9.3% increase in emissions from China offset decreases of 1.7% in the United States and 1.9% in Europe.


    Join us Thursday, 7 June, at 3 p.m. EDT for a live chat with leading experts on a hot topic in science.

  3. Newsmakers

    Radioactive Waste Expert Nominated to Lead NRC



    Allison Macfarlane, an academic geologist and nuclear waste expert, is in line to be the next head of the U.S. Nuclear Regulatory Commission (NRC). President Barack Obama's choice last week of Macfarlane, a professor at George Mason University (GMU) in Fairfax, Virginia, is drawing positive reviews from key members of Congress and both supporters and critics of nuclear power.

    Macfarlane is a veteran of nuclear power policy debates. She earned a 1992 doctorate in geology from the Massachusetts Institute of Technology in Cambridge, and joined GMU in 2006 after a succession of academic posts at top-tier institutions. In 2010 she was named to the presidential Blue Ribbon Commission on America's Nuclear Future.

    If confirmed by the Senate, Macfarlane would replace Gregory Jaczko, a physicist and former aide to Senator Harry Reid (D-NV), the Senate's Majority Leader. Jaczko announced on 21 May that he would step down following controversy over his mdanagement style and policy positions. Reid said he hopes the Senate will vote on Macfarlane's nomination by the end of June.

  4. Mysteries of Astronomy

    1. Robert Coontz

    Endless mysteries lurk in the depths of space. To pare the list down to eight—now, there's a challenge. In deciding what to include in this section, Science's news staff teamed up with Science Associate Editor Maria Cruz and consulted researchers on the Board of Reviewing Editors and elsewhere. From the outset, the team decided that true mysteries must have staying power (as opposed to mere “questions” that researchers might resolve in the near future). Some of the finalists are obvious shoo-ins; others have received less of the popular limelight. The final selection spans the entire history of the universe on scales ranging from our sun and its planetary system to the entire cosmos. Each mystery is sure to be solved largely through astronomical observations—if it is solved: In at least one case, experts aren't sure that a seemingly simple question will ever be answered.

  5. Astronomy

    What Is Dark Energy?

    1. Adrian Cho

    The nature of the "dark energy" that is causing the expansion of the universe to accelerate is now perhaps the most profound mystery in cosmology and astrophysics. And it may remain forever so.


    Fourteen years ago, the discovery of dark energy rocked astronomy. Two teams of astronomers and astrophysicists studied distant stellar explosions called type Ia supernovae to measure how the universe has expanded over its 13.7-billion-year lifetime. They expected that the expansion would be slowing as galaxies pull toward one another with their gravity. To their shock, they found that the expansion is accelerating as if some bizarre “dark energy” is stretching space. The nature of dark energy is now perhaps the most profound mystery in cosmology and astrophysics. And it may remain forever so. “Part of the mystery is that we have no clue whether we will be able to find an answer,” says Simon White, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany.

    Dark energy could be one of three things. It could simply be a property of empty space itself. Einstein's theory of gravity, known as general relativity, allows for just such a “cosmological constant” that would be a property of the vacuum and would stretch space. Or, in a radically different alternative, dark energy could be a new type of force field that occupies space, much as air fills a balloon. That second alternative is known as “quintessence.” Finally, dark energy could be an illusion, a sign that scientists' understanding of gravity as encapsulated in general relativity isn't quite right.

    To sort through the possibilities, scientists hope to answer one key question: How does the density of dark energy vary as space expands? If dark energy is a cosmological constant, then—true to that name—its density should remain constant. If dark energy is something within space, then it should grow more dilute as space expands. It all comes down to measuring the ultrasimple “equation of state” of dark energy and whether a single parameter, denoted w, equals −1, for a cosmological constant, or something like −0.9 for a quintessence.

    To probe dark energy, astronomers can generally make two types of measurements. The first seeks to reconstruct more precisely the history of the universe's expansion. For example, type Ia supernovae all pump out the same amount of light, so observers can tell how far away one is by how bright it appears in the sky. At the same time, the universe's expansion causes each supernova to speed away from Earth and stretches its light to longer, redder wavelengths. That “redshift” reveals when a supernova went off. So by measuring the distances to many supernovae at various redshifts, scientists should be able to deduce how space stretched over cosmic time, enabling them to measure the equation of state and whether that equation has changed. Researchers can play a similar game with traces of sound waves from the infant universe, called baryon acoustic oscillations, that imprinted themselves on the afterglow of the big bang and the distribution of the galaxies.

    The second type of measurement seeks to detect dark energy's effects on the formation of the largest structures in the universe. For example, galaxies gather in huge clusters as their gravity pulls them together. But space-stretching dark energy should counteract gravity and slow such clustering. So researchers can probe dark energy and its equation of state by simply counting clusters of various sizes, as they have already begun to do. Similar clues may come from charting the vast web of mysterious dark matter thought to span intergalactic space (see p. 1091). The strands in the web form as gravity draws dark matter together, but dark energy's space-stretching effect should slow that process. Astronomers can trace the evolution of the web by measuring how the gravity from the unseen strands bends light rays from more-distant galaxies and distorts their images—an effect known as weak lensing.

    Wow …

    The Blanco Telescope in Chile, which will conduct the Dark Energy Survey, silhouetted against the Milky Way.


    To maximize their progress, astronomers hope to play these experimental tools off of one another. For example, starting later this year, 120 researchers working with the Dark Energy Survey (DES) plan to use the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile to study 200 million to 300 million galaxies, catalog 100,000 galaxy clusters, and spot 4000 supernovae. That information should enable them to use all four techniques to measure dark energy's equation of state.

    Such a multipronged attack might enable scientists to answer the most basic question: Does dark energy really exist, or is the theory of gravity lacking? For example, if studies of supernovae and galaxy clusters yield inconsistent results, then that would be a signal that the real problem lies with the theory of gravity, says Josh Frieman, an astrophysicist at Fermi National Accelerator Laboratory in Batavia, Illinois, and the nearby University of Chicago and a founder of the $50 million DES. “I don't know how far [this strategy] would get us down the road, but at least it would get us down the road,” Frieman says. “Right now, we're stuck at the crossroads.”

    Astronomers are planning even bigger efforts to probe dark energy. Late this decade, the European Space Agency plans to launch a $570 million space telescope called Euclid to study dark energy primarily through measurements of baryon acoustic oscillations and weak lensing. On the ground, astronomers in the United States hope to build the Large Synoptic Survey Telescope, a proposed $465 million, 8.4-meter telescope that would take as one of its primary missions studying dark energy using all four techniques.

    Dark energy might never reveal its nature, however. Data from supernovae, galaxy clusters, and baryon acoustic oscillations currently fix w at −0.98, give or take 10%, a result consistent with either a cosmological constant or quintessence. But theorists can't predict exactly how far from −1 w should be if dark energy is in fact a form of quintessence. So if, for example, more data yield a result for w of −0.99 plus or minus 1%, that number would still be ambiguous: consistent with either a cosmological constant or quintessence. And some scientists think such a march toward eternal uncertainty is likely. “I think it's a safe prediction that w will turn out to be −1 to within 1%, to within 0.1%, etc.,” says David Gross, a theoretical physicist at the University of California, Santa Barbara.

    Still, scientists remain optimistic that nature will cooperate and that they can determine the origins of dark energy. “I tell my observer friends that it's worth spending a decade of their careers trying to do it,” says Rocky Kolb, a cosmologist at the University of Chicago. “There's a chance to take a big step forward.”

  6. Astronomy

    How Hot Is Dark Matter?

    1. Adrian Cho

    Astronomers still don't know what the unseen "dark matter" is that provides the gravity that holds the galaxies together, but they may soon be able to narrow in on its most basic properties.


    For decades, astronomers have thought that some unseen “dark matter” provides the gravity that holds the galaxies together. Scientists still don't know what dark matter is, but that could soon change. Within years, physicists might be able to detect particles of the stuff. Even if they can't, astronomers and astrophysicists may be able to narrow in on dark matter's most basic properties by astronomical means alone.

    In particular, studies of runty “dwarf galaxies” might test whether dark matter is icy cold as standard theory assumes, or somewhat warmer—essentially a question of how massive particles of dark matter are. “Maybe the mass is in a range in which we can measure it by astronomical means,” says Marc Davis, a cosmologist at the University of California, Berkeley. “That would be astounding.”

    The first signs of dark matter came in 1933, when Swiss astronomer Fritz Zwicky found that the galaxies in the Coma Cluster zing through space so fast that their own gravity shouldn't hold them together. Some invisible matter must provide the extra gravity that keeps the cluster intact, he reasoned. His wild idea got a big boost in the 1970s when American astronomer Vera Rubin and others found that the stars in individual galaxies also whirl around too fast to be held in by their mutual gravity.

    However, the strongest evidence for dark matter has come from recent cosmological measurements of the universe's origins. For example, the afterglow of the big bang, or cosmic microwave background, varies in temperature across the sky, and the tiny variations reflect the sloshing of ordinary matter and dark matter in the infant universe. In 2003, NASA's spaceborne Wilkinson Microwave Anisotropy Probe measured those variations and showed that 83% of all matter in the universe is dark matter.

    Cosmologists bolstered the case for dark matter by mapping the galaxies to determine the universe's large-scale structure. For example, starting in 1997, astronomers with the Two-Degree-Field Galaxy Redshift Survey, based at the Siding Spring Observatory near Coonabarabran, Australia, mapped 220,000 galaxies and found that they were arrayed in vast threads and sheets.


    The galaxy cluster Abell 1689 with its inferred dark matter distribution (blue haze).


    Those observations jibe with the results from another key advance: high-precision computer simulations of how the universe evolved. Those simulations show that a vast web of filaments and clumps of dark matter formed as the stuff coalesced under its own gravity. The galaxies formed as the clumps, or “halos,” drew in ordinary matter that formed stars. The simulations reproduce the statistical distributions of the galaxies so well that the scenario is now cosmology's standard model.

    Now, however, some researchers have begun to wonder if the model is not quite right. To reproduce the weblike structure, theorists assume that dark matter is “cold”—in other words, composed of slow-moving, heavy particles between one and 1000 times as massive as a proton. But cold dark matter simulations run into their own problems.

    For example, they produce myriad tiny halos. If all these halos were to collect enough gas to form galaxies, then the Milky Way galaxy should be surrounded by thousands of dwarf galaxies. Instead, astronomers have spotted about 20 dwarf galaxies, suggesting that the little halos don't exist. That discrepancy is known as the “missing satellites problem,” and it challenges the prevailing theory. “If we can prove these 100,000 halos are not there, then we can rule out cold dark matter,” says Julio Navarro, an astrophysicist at the University of Victoria in Canada.

    Just like home.

    An image from the Aquarius simulation of the dark matter halo of a galaxy the size of the Milky Way. The myriad clumps suggest our galaxy should have many more satellite galaxies than it does.


    Cold dark matter simulations also predict that the density of a dark matter halo should peak in a “cusp” at its center. Instead, observations suggest that galaxies have broader cores in which dark matter is distributed more homogeneously. Known as the “core-cusp problem,” this discrepancy also challenges the prevailing cold dark matter theory. However, in a real galaxy, gravitational tugging between dark and ordinary matter could blur the cusps.

    If dark matter were slightly warmer than assumed, however, the smallest halos wouldn't form and the cusps would smooth out. So even leaders in the field say that dark matter might be warmer than presumed and its particles lighter, each with a mass a few millionths that of a proton. “I have a huge investment in cold dark matter—my whole career and something like 300 papers—but it could be wrong,” says Carlos Frenk, a cosmologist at the University of Durham in the United Kingdom.

    Scientists have various ideas about how to take dark matter's temperature. Frenk says the real issue with the missing satellites is not the tiniest, most numerous halos, as it's easy to think of reasons they might accumulate too little ordinary matter to light up as galaxies and so remain hidden. The real problem, he says, is that the Milky Way has only three larger satellite galaxies with masses a billion times that of the sun, whereas cold dark matter simulations predict there should be about 10. That prediction depends on the total weight of the Milky Way, Frenk says, so the best way to put cold dark matter to the test is to find a way to precisely weigh the whole galaxy.

    In contrast, Navarro says the key to probing the temperature of dark matter is to tackle the core-cusp problem by studying the smallest, faintest dwarf galaxies surrounding the Milky Way. Such galaxies can have fewer than 100,000 stars and are nearly pure dark matter. So by tracing the motions of their stars, astronomers might deduce the structure of nearly pure dark matter halos and compare it with the prediction of simulations.

    Meanwhile, particle physicists might soon detect dark matter directly. A theory called supersymmetry predicts the existence of weakly interacting massive particles (WIMPs) that weigh a few hundred times as much as a proton and would be ideal candidates for cold dark matter. Detectors deep underground could spot WIMPs floating around. Or the world's largest atom smasher, the Large Hadron Collider (LHC) in Switzerland, could blast them into existence.

    Such a discovery would greatly bolster the case for cold dark matter. But it might not change the way astronomers probe the stuff's behavior. “If the LHC announced tomorrow that it had found something that could be the dark matter, what I would do on that day would be pretty much the same,” says Kristine Spekkens, an astronomer at the Royal Military College of Canada in Kingston, who is trying to deduce the structure of galaxy halos. If physicists don't detect WIMPs, then astronomical observations would remain the only way to study dark matter. Apparently, that wouldn't necessarily stop progress.

  7. Astronomy

    Where Are the Missing Baryons?

    1. Yudhijit Bhattacharjee

    Not only are astronomers unable to pin down dark energy and dark matter, but more than half of the "baryonic matter"—ordinary atoms and ions—remains unaccounted for.


    To describe the universe, you need to know what's in it and where the components reside. So far, astronomers are a long way from completing that inventory. It's not just that they can't pin down dark energy and dark matter, the two invisible components that make up 95% of the cosmos (see pp. 1090 and 1091). More than half of the remaining 5%—“baryonic matter,” the ordinary atoms and ions that make up stars, planets, dust, and gas—remains unaccounted for as well.

    Baryonic matter is so called because its main ingredients, protons and neutrons, belong to a class of particles called baryons. Cosmologists have calculated the density of baryons in the primordial universe from measurements of the cosmic microwave background—the faint afterglow of the big bang. The universe has changed a lot in the 13.7 billion years since the big bang, but its original complement of baryons should still be with us today.

    As astronomers count baryons from the early universe to the present day, however, the number drops mysteriously, as if baryons were steadily vanishing through cosmic history. By analyzing light from distant quasars to measure the amount of the hydrogen isotope deuterium in ancient baryonic clouds, astronomers can infer that nearly all of the universe's original baryons were still around 10 billion years ago. By contrast, when they take stock of the nearby (and thus more recent) universe—adding up the mass of stars, gas, and everything else they can detect—the inventory barely reaches half of what it should be.

    Although galaxies seem like the weightiest things in the universe, they account for only 10% of its baryonic mass. Another 10% comes from warm gas in the space between galaxies. A further 30% or so is present in blobs of cold gas in intergalactic space.

    Thin fog.

    Artist's conception shows how astronomers have detected warm intergalactic gas by studying its effect on x-ray light from distant quasars.

    Too hot to be seen.

    The yellow regions in this simulation of a pocket of the universe represent the warm-hot intergalactic medium, which may make up the bulk of missing baryons.


    Astrophysicists suspect that the missing 50% lies between galaxies in the form of a diffuse, hot plasma just a millionth as dense as the gas found between stars. Astronomers call this material the warm-hot intergalactic medium (WHIM). The medium's temperatures are so high, ranging from 100,000 to 10 million K, that the material in it is highly ionized and can absorb and emit radiation only at far-ultraviolet or low-energy x-ray wavelengths. Because of that finickiness and WHIM's low density, light passing through the medium does not produce spectral lines of the kind that astronomers rely on to spot and study interstellar gas. As a result, detecting WHIM remains a challenge.

    To find WHIM, astronomers have been searching spectra of light passing through intergalactic space for lines corresponding to ions that are thought to survive in the WHIM and attest to its presence. The markers include Neon VIII (neon atoms stripped of eight electrons) and Oxygen VI (oxygen stripped of six electrons). In a paper submitted recently to The Astrophysical Journal, J. Michael Shull and others at the University of Colorado, Boulder, claim to have detected WHIM through Oxygen VI. But other astronomers, including Romeel Davé of the University of Arizona in Tucson, are skeptical that the Oxygen VI Shull's group has seen resides in WHIM. “The bottom line is that a rock-solid, unambiguous, fully accepted detection of the WHIM has yet to be made,” says Davé, adding that next-generation x-ray telescopes could help find the stuff.

    Meanwhile, researchers have been improving the baryon inventory through other measurements. A team led by Jason Tumlinson of the Space Telescope Science Institute in Baltimore, Maryland, has used the Oxygen VI tracer to make better estimates of the number of baryons floating on the edges of galaxies. Surveying the outskirts of 42 nearby galaxies using the Cosmic Origins Spectrograph on the Hubble Space Telescope, Tumlinson and his colleagues discovered that this circumgalactic medium (CGM) contained almost as much baryonic mass as the stars inside the galaxies did. That helps account for some missing baryons, but CGM makes only a small dent in the overall missing-baryon problem.

    A related mystery is a deficit of baryons in the dark matter halos within which galaxies are nestled. Here, too, astronomers find fewer baryons than expected. The deficit is more severe in small galaxies than in large galaxy clusters, says James Bullock, an astrophysicist at the University of California, Irvine. Davé says that might be because smaller galaxies have less gravitational pull to hold on to their gas when stellar explosions and other violent events hurl it toward intergalactic space.

    Accounting for missing baryons on the cosmic scale as well as in galactic halos should help astronomers understand how cosmic structure and galaxies have evolved. “This low-density material is the basic reservoir for new star formation, and the inflow and outflow of this material from galaxies plays a very important role in how galaxies change,” says Todd Tripp, an astronomer at the University of Massachusetts, Amherst. In other words, looking for missing baryons is not just a bean-counting exercise. It's a key to understanding how the universe came to be what it is.

  8. Astronomy

    How Do Stars Explode?

    1. Yudhijit Bhattacharjee

    Many details of what goes on inside a star when its fuel has been spent and it explodes into a giant fireball known as a supernova, as well as how that explosion unfolds, remain a mystery.


    Stars live a glamorous life, shining for millions or even billions of years. For many stars, however, death is far more glamorous. When their fuel has been spent, they explode into a giant fireball known as a supernova, producing the brilliance of multiple suns and sometimes outshining entire galaxies.

    How these explosions occur has been a subject of observational and theoretical study for decades. In recent years, advances in supercomputing have enabled astronomers to simulate the internal conditions of stars with increasing sophistication, helping them to better understand the mechanics of stellar explosions. Yet, many details of what goes on inside a star leading up to an explosion, as well as how that explosion unfolds, remain a mystery.

    All stars are powered by the same basic process: the fusion of hydrogen into helium, as well as the fusion of light atoms such as hydrogen and helium into progressively heavier elements such as carbon, oxygen, and eventually iron. However, what happens when all the fuel in the stellar core has run out depends on the mass of the star and other factors. That's why astronomers see different kinds of supernovae across the universe.

    One kind, known as a Type II supernova, occurs in stars that are at least eight times as massive as the sun. After such a star has used up the fuel in its core—which turns into iron at this point—it ceases to emit radiation. As a result, the core can no longer generate the radiation pressure necessary to counterbalance the weight of the outer layers. All the material in the core collapses under this weight and gets compacted to form a neutron star. Powerful shocks emanate from the collapse, blasting away the outer layers of stellar material in a fiery explosion.

    You might think that bigger stars would make bigger supernovae. But that is not the case: Stars with masses greater than 20 or 25 suns don't result in type II supernovae. “More massive stars have dense layers of oxygen and silicon just outside their iron cores—and their iron cores are more massive to start with,” says Stanford Woosley, an astrophysicist at the University of California, Santa Cruz. “As the explosion tries to develop, these dense layers fall in and bottle it up.” Instead of blowing up, the star becomes a black hole.

    Doomed splendor.

    Astronomers seek to understand exactly how shock waves from the core of a star blow out its outer shells in a brilliant supernova, as shown in this computer-generated visualization.


    Another intensely studied category of explosions is type Ia supernovae, which have helped cosmologists measure distances in space and led to the discovery that the expansion of the universe is accelerating. Astronomers think type 1a supernovae occur in binary star systems after nuclear fusion ceases in the core of one of the two stars, turning it into a white dwarf. As the two objects move around each other, gas from the second star starts getting pulled onto the white dwarf until the white dwarf 's mass approaches 1.38 solar masses. At that point, the white dwarf collapses and erupts in an explosion of a standard brightness.

    Although this general picture of how type Ia supernovae work is widely accepted, many questions remain. For instance, astronomers aren't sure how massive the second star needs to be for the explosion to occur. Astronomers would also like to understand the mechanics of the explosions in greater detail, such as how long it takes for the white dwarf to strip enough material from the other star onto itself, as well as the sequence of events in the final moments before the explosion.

    To figure out the blow-by-blow action in these and other supernova explosions, astronomers such as Daniel Kasen of the University of California, Berkeley, have been studying the postexplosion signature of the blasts. “The explosion happens on such a short time scale that typically what is observed is really the aftermath,” Kasen says. In recent years, Kasen and others have matched observations with models of this aftermath, detailing aspects that include “what the debris is composed of, how fast the debris is moving, and how light is produced in it,” he says.

    Astronomers are gaining further insights into supernovae from observations of gamma ray bursts (GRBs): short, intense flashes of gamma radiation emitted by stars. A GRB that lasts 2 seconds or longer is thought to be emitted by a massive, rapidly spinning star as its core collapses into a black hole. A whirling disk of stellar material forms around the black hole, and as this material gets sucked in, two conelike jets of gamma ray particles shoot out perpendicular to the disk.

    In several cases, astronomers studying the afterglow of GRBs have been treated to the sight of highly energetic supernova explosions occurring within 2 to 4 days after the burst. As a result, detecting long-duration GRBs has become a “nice trigger for multiwavelength observations of the coming supernova,” says Neil Gehrels, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and principal investigator of the Swift orbiting telescope. In recent years, Swift observations have enabled astronomers to study three supernovae in the nearby universe from the very first moments of the explosion.

    Even though an analysis of these unusually well documented stellar deaths has helped firm up the link between GRBs and explosions, researchers still don't know precisely how these kinds of supernovae build and explode. The formation of the black hole, followed by the development of the whirling accretion disk that triggers the emission of gamma ray flashes, appears to be “an evolutionary path that can lead to very powerful supernovae,” Gehrels says. In the view of Woosley and others, the rapid rotation of the disk—thought to be responsible for the jets—could be playing a role in the development of the explosion.

  9. Astronomy

    What Reionized the Universe?

    1. Edwin Cartlidge*

    A few hundred million years after the big bang, most of the universe's matter turned into the light-transmitting ionized plasma that it remains today. What caused this cosmic reionization? No one is sure.


    “Cosmologists are often wrong but never in doubt,” the Russian physicist Lev Landau once quipped. That might have been true for much of the 20th century, when scientists pondering how the universe worked had many ideas but few data. Over the past couple of decades, however, increasingly precise observations by ground-based telescopes and space probes have shored up cosmologists' standard model, which shows that the universe started in a fiery big bang 13.7 billion years ago and then expanded and cooled. As it did so, tiny initial irregularities in the density of the cosmos attracted matter via gravity to form stars, galaxies, and clusters of galaxies.

    But a major gap in our knowledge remains. Some 400,000 years after the big bang, protons and electrons had cooled off enough for their mutual attraction to pull them together into atoms of neutral hydrogen. Suddenly photons, which previously scattered off the electrons, could travel freely through the universe. A few hundred million years later, something stripped the electrons off the atoms again. This time, however, the expansion of the universe had dispersed the protons and electrons enough so that the new energy source kept them from recombining. The “particle soup” was also dilute enough so that most photons could pass through it unimpeded. As a result, most of the universe's matter turned into the light-transmitting ionized plasma that it remains today.

    Way-back machine.

    The Low Frequency Array in the Netherlands is searching for signs of the neutral hydrogen that vanished from the universe almost 13 billion years ago.


    What caused this cosmic reionization? No one is sure. Astronomers can see the faint cosmic microwave background (CMB), the first radiation liberated when the formation of atoms set light free. Meanwhile, the Hubble Space Telescope and large ground-based observatories have spotted galaxies as far back as about 800 million years after the big bang. Reionization, however, took place between these times, during the “dark ages” in which the first stars and galaxies started to form—a period that astronomers have trouble seeing.

    Dark times.

    After particles cooled enough to form atoms, something blew them apart again. Finding the culprit may help researchers reconstruct the history of the first stars.


    Astrophysicists' best bet is that the energy came from ultraviolet radiation emitted by stars in the first galaxies. In this picture, the galaxies blew bubbles of ionized hydrogen that grew and merged until virtually all of the neutral hydrogen had disappeared. Unfortunately, astronomers—extrapolating from the number of bright, early galaxies Hubble sees far off and the relative abundance of brighter and dimmer galaxies closer to Earth—have worked out that there probably weren't enough galaxies to supply the necessary energy.

    But George Becker of the Kavli Institute for Cosmology at the University of Cambridge in the United Kingdom says the model may still be correct. Astronomers may have underestimated either the number of bright stars in early galaxies or the galaxies' porosity to ultraviolet light, he says. Or faint early galaxies may have been more numerous than the population in our part of the universe suggests. Alternatively, other astrophysical objects—such as supermassive black holes or even annihilating particles of dark matter—may have done much of the hydrogen-demolition work.

    Answers may come from a larger census of early galaxies and from measurements of their chemical composition and star-formation rates. One or more 20- to 40-meter ground-based telescopes planned for the next decade should see galaxies dating back to when the universe was about 300 million years old, says Richard Ellis, an astronomer at the California Institute of Technology in Pasadena. NASA's $9 billion infrared James Webb Space Telescope (JWST), due for launch in 2018, should be able to look back about 100 million years earlier still.

    Even the JWST, however, has its limits. “There will always be galaxies too faint to see,” says Abraham Loeb, a theoretical astrophysicist at Harvard University. Instead of looking for the objects that caused the ionization, Loeb favors what he calls a more “elegant” approach: looking at the hydrogen itself.

    Neutral hydrogen has a spectral feature that the ionized variety does not: a line at 21 centimeters caused by a flip in the “spin” of the electron relative to the spin of the proton. As the 21-centimeter signal travels through space on its way to Earth, the expanding universe stretches its wavelength to several meters—a phenomenon known as “redshift.” That puts it out of range of traditional radio dishes, so astronomers are building telescopes made of many simple “dipole” antennas spread out across large areas and synchronized by computer. By measuring the strength of the 21-centimeter signal at different redshifts, researchers hope to chart the growth and merger of ionized bubbles around distant galaxies. The information should help identify the ionizing source. For example, because black holes emit x-rays as well as ultraviolet radiation, they would tend to ionize the hydrogen gas more uniformly than some other sources, leading to a less distinct bubble structure.

    Among the several 21-centimeter telescopes worldwide, the Low Frequency Array in the Netherlands should start taking data this winter and might see the line within about 2 years. But detailed observations will probably have to wait for the more powerful $2 billion Square Kilometre Array, which Australia and South Africa are competing to build over the next 10 years.

    One other approach, which might have more to offer in the short term, is to track how hydrogen cooled after heating up in the process of reionization. Hot gas that cooled relatively slowly, for example, would suggest that the energy for reionization came from energy sources called quasars, which tend to produce more energetic photons than ordinary stars do.

    Because cooling changes the way hydrogen absorbs light, astronomers can gauge the temperature of the gas by measuring how it affects the light from more-distant quasars. The technique is less technically demanding than 21-centimeter observations, Becker says. Its main drawback is that it works only for hydrogen that is almost entirely ionized; as a result, the technique can reveal only what happened immediately after reionization, not during the process. “It's just so difficult to make observations of this early period in cosmic history that you need as many different methods as possible,” Becker adds.

    • * Edwin Cartlidge is a science writer in Rome.

  10. Astronomy

    What's the Source of the Most Energetic Cosmic Rays?

    1. Daniel Clery

    Data taken from detectors in the past few years have provided some clues to the origin of the highest energy cosmic rays but, as yet, no smoking gun.


    Fifty years ago in New Mexico, scientists saw a particle that shouldn't exist. A cosmic ray—an atomic nucleus hurtling through space—struck a detector in an array called the Volcano Ranch experiment with an energy of 1020 electronvolts, or 100 exaelectronvolts (EeV)—so high that no known process could have produced it.

    Nearly 30 years later, at a cosmic ray detector in Utah called Fly's Eye, another one hit. Clocked at 300 EeV, the particle—probably a proton traveling at just 0.0000000000000000000005% under the speed of light—packed as much kinetic energy as a baseball hurled at 100 kilometers per hour. Physicists called it the “Oh my God” particle.

    Where had the intruders come from? Astrophysicists were stumped. Fly's Eye gave an approximate direction for the particle it spotted, but no obvious culprit appeared in that part of the sky. The data stream picked up during the 1990s, when the largest cosmic ray detector at the time, the Akeno Giant Air Shower Array (AGASA) near Tokyo, bagged about a dozen more particles with energies of about 200 EeV. The AGASA results galvanized astrophysicists into building a much bigger detector optimized for ultrahigh-energy cosmic rays. In the past few years, the first results from this facility—the Pierre Auger Observatory in western Argentina—have provided some clues to their origin but, as yet, no smoking gun.

    In the 100 years since balloon-borne experiments first showed that a mysterious all-pervading radiation was coming from space, researchers have found out a lot about cosmic rays. They know that 89% of them are simple protons, or hydrogen nuclei. Most of the rest are helium nuclei with a smattering of heavier nuclei, electrons, and antimatter. Astrophysicists think most of the lower-energy particles—up to 1010 eV—are from the sun and that those from 1010 eV up to 1015 or even 1018 eV (1 EeV) are from elsewhere in our galaxy. Because cosmic rays with even higher energies seem to come equally from all parts of the sky rather than bunching in the plane of our galaxy, they probably originate outside the Milky Way.

    Researchers think midrange cosmic rays get their energy from the shock waves of supernovae. But that mechanism can't explain energies above 1015 eV. Theorists have suggested a variety of sources for such ultrahigh-energy monsters: hot spots in energetic radio galaxies, in gamma ray bursts, and in jets streaming from supermassive black holes at the heart of active galactic nuclei (AGNs). Cosmologists have come up with more fanciful ideas, such as the decay of exotic superheavy elementary particles created in the big bang or the collapse of hypothetical “topological defects” in the universe such as cosmic strings, domain walls, and monopoles.

    Whatever their cause, the highest-energy rays probably come from our own neighborhood, cosmically speaking. In the 1960s, theorists calculated that ultrahigh-energy cosmic rays traveling through space lose energy as they interact with photons of the all-pervasive cosmic microwave background radiation. As a result, particles that travel more than about 160 million light-years probably wind up with energy below 50 EeV, a limit called the Greisen-Zatsepin-Kuzmin (GZK) cutoff. In that case, cosmic rays with energies above the cutoff energies must originate in the few thousand galaxies closest to the Milky Way out of hundreds of billions of galaxies in the universe.

    The source?

    There are many candidates for the origin of ultrahigh-energy cosmic rays, but recent results from the Pierre Auger Observatory point to the jets emanating from active galactic nuclei such as Centaurus A.


    If the GZK cutoff is real, astrophysicists ought to see a sharp drop in the number of cosmic rays at energies above 50 EeV. Do they? For a while, the evidence was contradictory: Between 1990 and 2004, AGASA spotted 11 particles with energies greater than 100 EeV—more than GZK would suggest—while an upgraded version of Fly's Eye found just two.

    Cosmic downpour.

    A simulation of an air shower caused by a 1012 eV proton hitting the atmosphere 20 kilometers above Chicago's lakefront.


    The Pierre Auger Observatory aims to settle this dispute and narrow down the list of candidate sources. The observatory consists of four banks of telescopes and 1600 particle detectors spread out over 3000 km2 of Argentine pampas. It hunts cosmic rays indirectly by spotting air showers: cascades of other particles that erupt into being when cosmic rays from space plow into the nuclei of atoms in Earth's upper atmosphere. As the path of the air shower follows the track of the incoming cosmic ray, the detectors can reconstruct the ray's original direction.

    Auger began taking data in 2005 and 2 years later announced its first result: The GZK cutoff appears to be valid. If AGASA's measurements were correct, the Auger Observatory would have seen about 30 rays above 100 EeV; in fact it saw just two, consistent with the GZK theory.

    By 2007, Auger had detected 27 rays with energies greater than 57 EeV and mapped where in the sky they had come from. Although galactic and extragalactic magnetic fields bend the path of cosmic rays, ultrahigh-energy rays have so much momentum that they follow relatively straight, easy-to-trace paths. The Auger team found that all 27 rays had come from within 3° of an AGN less than 250 million light-years from Earth. The result is suggestive, but the Auger team stresses that it is not proof. At that distance, 3° encompasses so much space that the true source could be something else near the AGN or even in a nearby galaxy. Auger has now observed 113 cosmic rays above 55 EeV; their paths continue to point toward AGNs, although less strongly than the team expected.

    Cosmic-ray research suffered a setback in recent years, when shrinking budgets forced scientists to shelve plans for a northern-sky counterpart to the Auger Observatory in the United States. But relief is expected soon in the shape of the Extreme Universe Space Observatory, an orbiting telescope that will look for fluorescent flashes of air showers from above. Designed by the European Space Agency, it was dropped in 2004 only to be picked up again by Japan. The current plan is to launch it in 2016 and attach it to the Japanese laboratory of the International Space Station.

    After a century of cosmic-ray research, the most energetic visitors from space remain stubbornly enigmatic and look set on keeping their secrets for years to come.

  11. Astronomy

    Why Is the Solar System So Bizarre?

    1. Richard A. Kerr

    Enigmas such as Mercury's makeup (mostly iron core, with a thin veneer of rock) and Uranus's skewed magnetic field continue to bedevil planetary scientists, and no tidy resolution is in sight.


    All manner of planets circling other stars have been popping up of late: big ones, little ones; gassy ones, rocky ones; hot ones, cold ones. But the freakish diversity of worlds starts much closer to home. From the 1960s to the 1980s, space probes returned the first close-up looks at eight of the then-nine planets. To researchers expecting a simple story that would explain what shaped our solar system, the observations sent a sobering message: in your dreams. Today, enigmas such as Mercury's makeup (mostly iron core, with a thin veneer of rock) and Uranus's skewed magnetic field continue to bedevil planetary scientists, and no tidy resolution is in sight.

    For a long time, the planetary oddball to beat all oddballs was Pluto. A ball of dirty ice just 1/250 as massive as the next lightest planet, Mercury, Pluto circles so far from the sun that its year is 248 Earth-years long. Its orbit is oblong, not circular, and is tipped 17° to the rest of the solar system. So, many planetary scientists were relieved to realize that Pluto is not a planet at all. The 1992 discovery of other small, icy, airless bodies swarming far beyond Neptune showed that it is just one of the largest of myriad similar objects left over from the formation of the solar system.

    Family portrait.

    The rocky planets (from left: Mercury, Venus, Earth, Mars) are a diverse bunch.


    The mysteries of the remaining eight planets are proving more recalcitrant. Consider the four innermost planets: Mercury, Venus, Earth, and Mars. All have rocky outer shells and metallic cores, but the family resemblance of these terrestrial planets ends there. Earth and Venus have approximately the same mass, size, and composition, but Earth is cloaked in a mild, life-sustaining atmosphere while the venusian atmosphere is crushingly dense, acid-laced, and hot enough to melt lead. Earth is an ocean planet; there is no sign Venus ever had an ocean. Earth is encased by wandering tectonic plates bearing continents; Venus has a single, immobile shell of rock. Earth has a magnetic field generated by the churning of its liquid iron core, it has a large moon, and it rotates 365 times per orbit. Venus has neither moon nor magnetic field, and it rotates—backward—less than once per venusian year.

    The more diminutive Mercury-Mars pair isn't much better behaved. Mars has twice the mass of Mercury, but its core-generated magnetic field sputtered out early on. Little Mercury, on the other hand, is still churning out a magnetic field, albeit an oddly weak one whose center is far from the center of the planet.

    Their interiors differ too. Mars has a proportion of central metal to enclosing rock similar to that of Earth; Mercury is mostly metal. And just last year, researchers learned that Mercury seems to have been made out of rather different primordial stuff than the rest of the terrestrial planets.

    The four outermost planets—Jupiter, Saturn, Uranus, and Neptune—have their own quirks. Uranus and Neptune have about 20 times the mass of Earth but orbit 20 and 30 times as far from the sun, respectively—a region where there was precious little material for planet building when the solar system got its start. Jupiter has a nifty “mini-solar system” of four large satellites, while Saturn has only one large satellite but boasts a set of spectacular rings. And the magnetic fields of the outer planets are mostly awry. Jupiter's is slightly tilted with respect to its axis of rotation—as theory would have it—but Neptune's is tilted 47° and Uranus's a whopping 60°. Saturn's field is perfectly aligned with its rotation axis. That makes for quite the magnetic muddle.

    Planetary scientists have some ideas, of course. Orbital distance from the sun was likely a factor in many planetary histories, such as Venus having a “runaway greenhouse.” As for planets that seem out of place, lately planetary dynamicists have been exploring the idea that some may not have always orbited where they do today. Neptune, for example, might have formed closer to the sun, where agglomerating into a planet would have been easier, and then migrated outward.

    Looming over all the attempts to explain planetary diversity, however, is the chilling specter of random chance. Computer simulations show that the chaos of caroming plane-tesimals in our still-forming planetary system could just as easily have led to three or five terrestrial planets instead of four. Mercury may have largely formed with a thick rocky shell only to have it blown away by a chance collision with a still-forming planet nearly its own size. A rare big hit to Uranus might have not only knocked it on its side, where it spins to this day, but also shaken up its rocky core. If so, the more organized churnings of a shallow fluid shell could be generating its magnetic field, producing the observed tilt.

    Ferreting out rare random events in the early days of the nascent solar system could be problematic, scientists concede. They may have to settle for working out many of the rules of the planet-making game without pinning down exactly how a particular planetary quirk came to be. Help might come from planets orbiting other stars. As exoplanet hunters get beyond stamp-collecting planets solely by orbit and mass, they will have a far larger number of planetary outcomes to consider, beyond what our local neighborhood can offer. Perhaps patterns will emerge from inchoate diversity.

  12. Astronomy

    Why Is the Sun's Corona So Hot?

    1. Richard A. Kerr

    How does heat dissipating from the sun's core out beyond the surface to its corona abruptly punch temperatures up by a factor of 200 and more?


    In all its glory.

    The sun's corona reveals itself during a total solar eclipse as the moon passes in front of the 5780 K visible surface of the sun. Here the moon is visible by Earth light. How the sun heats the wispy corona to a million K and more must involve magnetic field lines of the sun.


    Yes, the sun is hot—really hot. It's 16 million K hot at its fusion-fueled core, cooling, as the second law of thermodynamics requires, to a still-blistering 5780 K at its visible surface. But for the better part of a century, solar physicists have been mystified by the sun's ability to reheat its corona, the encircling wispy crown of light that emerges from the glare during a total solar eclipse. There, temperatures again soar to 1 million K and more. How would heat dissipating from the core out beyond the surface abruptly punch temperatures up by a factor of 200 and more?

    The basics of coronal heating are clear enough. Everyone agrees that there is plenty of energy to do the job in the churning solar interior just beneath the visible surface. And everyone agrees that the sun's magnetic field carries the required energy outward to the corona. But that's where consensus ends. Just how the magnetic field transports the energy is much debated, and how the energy gets deposited once it reaches the corona is even more mysterious. Even so, decades of hard scientific work have narrowed the field of explanations to a half-dozen variants of two main mechanisms.

    One popular candidate is heating by magnetic waves. The sun's magnetic field lines, locked to ions in its churning gases, vibrate endlessly in ways that send all manner of waves and oscillations traveling along the lines. The solar wave of the hour is the Alfvén wave, a twisting oscillation along a magnetic field line. Researchers have studied Alfvén waves in the lab for decades but have only recently detected them rising from the sun; so far, no one knows whether they carry enough energy to heat the whole corona, or—if they do—how that energy could be converted into heat there.

    The alternative to wave heating is “nanoflares” produced when magnetic field lines snap and reconnect. The forests of magnetic field lines rooted in the churning visible surface are continually getting tangled, and those tight tangles store a lot of energy. But field lines don't simply untangle themselves to release that energy. Instead, adjacent lines can reconnect, as they do in Earth's magnetic field. Two lines cross, fuse, and then break across the point where they melded to form two hybrid loops. Unleashed, the new loops snap back like slingshots. On the sun, reconnection hurls superheated plasma toward the corona as nanoflares. Whether enough of them occur and deliver enough energy to heat the corona remains unclear.

    Solar physicists have made some progress on the heating problem in recent years using observations from the Japanese Hinode spacecraft and NASA's Solar Dynamics Observatory, among others. But much of the wave action suspected of energizing the corona is too quick to be caught and quantified by current instruments. And a nanoflare is still too small to be detected. In fact, solar physicists can't yet directly measure many crucial solar properties above the visible surface, including electric fields, electrical resistance, and the level of wave turbulence; they can only infer these properties imprecisely.

    The current fleet of solar spacecraft failed to solve the coronal heating mystery, and no one is promising a solution anytime soon. Still, solar physicists are looking forward to a new crop of solar observatories. NASA's Interface Region Imaging Spectrograph (IRIS) is scheduled for launch this December. The Advanced Technology Solar Telescope is under construction on Haleakalā in Hawaii; its mirror will be more than twice the size of the world's current largest solar telescope. And the European Space Agency's Solar Orbiter, which will be launched in 2017, will get a better view of the solar poles. A common goal of these and other planned projects is to sharpen the view of the sun in both space and time.

    A particular target of the coming projects is the chromosphere, the 5000 kilometers or so between the visible surface of the sun and the millions of kilometers of corona. Whatever heats the corona—a little of this, a little of that, perhaps—must pass through the chromosphere.

    One key that may be crucial to solving the corona mystery won't involve even glancing at the sun. Three-dimensional simulations of the visible sun—which work much like weather forecasting models—have made considerable strides in recent years. They now spontaneously generate both a corona and a chromosphere, so researchers seeing ever more solar details will also be able to play with their own “sun in a box” to help figure out what makes the corona so crispy.