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The 'Dark' Universe May Be Full of Strange Interactions

Most of the universe may not be as constant as physicists think it is. Dark energy and dark matter—the two main ingredients of the universe, and two of the great mysteries of science—can turn into each other over time, according to cosmologist Elisa Ferreira at McGill University in Montreal and her colleagues. If further data supports this scenario, this discovery might controversially suggest that, as is largely thought now, dark energy is not an immutable force of nature—a cosmological constant that evenly controls the expansion of the universe—but instead changes over time. This subtle ring of dark matter runs through galaxy cluster Cl 0024+17. If true, it could upend our understanding of the universe itself. Ever since the Big Bang, <a href="http://skyserver.sdss.org/dr1/en/astro/universe/universe.asp"> the universe has been expanding </a> , a fact that astronomers first discovered nearly a century ago when they found that galaxies were hurtling away from us. Scientists had initially assumed that the attractive force of gravity would slow down the universe’s expansion over time to eventually either stop it or even collapse everything back together in a “ <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2014/08/when-the-end-is-just-the-beginning-exploring-cosmic-cycles/"> Big Crunch </a> .” However, about 25 years ago, researchers unexpectedly discovered that cosmic expansion was not only not slowing down, but that it was speeding up. <h2> Shedding Light on Dark Energy </h2> Scientists call the force driving this mysterious acceleration dark energy and suggest that it could make up roughly 70% of all matter and energy in universe. In comparison, they estimate that matter only makes up about 30% of the universe. However, physicists don’t really have a clue as to what dark energy is—so much remains unknown about dark energy that some researchers wonder <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/04/whos-afraid-of-the-dark-alternatives-to-dark-energy/"> if it even exists </a> . Dark matter, on the other hand, is thought to be an invisible material that makes up roughly five-sixths of all matter in the universe. This means that dark matter composes about 25% of the universe, while ordinary matter only makes up about 5%. Currently, the consensus among scientists is that dark matter is made of unknown particles that lie outside the Standard Model, which is the best description we have to date of how subatomic particles behave. The new model suggests that dark energy changes over time, decaying to become dark matter. Ferreira’s new research that suggests a link between dark energy and dark matter relies on data from the <a href="http://www.sdss.org/surveys/boss/"> Baryon Oscillation Spectroscopic Survey (BOSS) </a> , which was designed to map the history of the universe’s expansion. Baryons are particles such as protons and neutrons, the building blocks of atoms, and astronomers use the term as a shorthand way of referring to the ordinary matter that makes up stars, planets, and people. When the universe was very young and small, all of its matter was very hot and densely packed together, and sound waves zipping through it led to ripples of density, causing material to clump together in spots. These initial baryon acoustic oscillations are visible today as clusters of galaxies. As the largest program in the third Sloan Digital Sky Survey (SDSS-III)—which seeks to create one of the most detailed maps of objects in the sky—BOSS has mapped the positions of roughly 1.5 million galaxies using the Sloan Foundation Telescope at the Apache Point Observatory in New Mexico. By pinpointing the locations of galaxies of different ages, scientists can deduce the rate at which the universe has expanded over time. This in turn helps explain the effects of dark energy over the course of cosmic history. The Sloan Foundation Telescope has helped map the position of 1.5 million galaxies. Beyond a distance of about 6 billion light years, galaxies become fainter and more difficult to see. To map baryon acoustic oscillations beyond this distance, BOSS relied on roughly 160,000 quasars, the brightest objects in the universe, whose light probably comes from supermassive black holes feeding on surrounding matter. As light from distant quasars passes through hydrogen gas in the interstellar and intergalactic void, pockets of greater density absorb more light. The absorption lines that hydrogen leaves behind in the spectrum of light from these quasars are known as Lyman-alpha lines, which are so numerous that they resemble a forest. Astronomers can use this so-called “Lyman-alpha forest” to determine the locations of these pockets of hydrogen gas and help create a 3D map of the universe. Ferreira and her colleagues say there are hints from BOSS’s data that undermines the current leading theory of how dark energy and dark matter should have forced the universe to evolve, technically known as <a href="https://en.wikipedia.org/wiki/Lambda-CDM_model"> Lambda-CDM </a> . One potential explanation is that dark energy possesses a bizarre quality known as negative energy density, where dark energy stores more and more energy as it stretches out. However, Ferreira notes that both normal matter and dark matter, the other major components of the universe, have positive energy density, so it would be strange if dark energy did not. Instead, the researchers suggest a simpler model to explain these BOSS findings—that dark energy and dark matter are linked. “The possibility of interaction between the two largest components of the universe is allowed and even favorable in some contexts in physics,” Ferreira says. “Interacting dark energy models, in which the dark components interact with each other and their evolution is linked, are possible and should be considered.” Specifically, this new model suggests that dark energy changes over time, decaying to become dark matter. “There is less dark energy in the past than we have today,” Ferreira says. In contrast, the current leading of model of dark energy and dark matter suggests that dark energy is a cosmological constant, meaning its strength has remained the same over time, just like other constants such as the speed of light. <h2> Einstein’s ‘Biggest Blunder’ </h2> Albert Einstein first proposed the existence of a cosmological constant in his equations for general relativity to support the accepted view at the time that the universe was static—while gravity would cause the universe to contract, a cosmological constant would push the universe apart and keep such a collapse from occurring. However, after Hubble discovered that the universe was expanding and not static, physicist George Gamow said that Einstein called the cosmological constant his “ <a href="http://www.theatlantic.com/technology/archive/2013/08/einstein-likely-never-said-one-of-his-most-oft-quoted-phrases/278508/%20target="> biggest blunder </a> .” Still, interest in the concept of the cosmological constant revived when evidence began accumulating that dark energy might be one of the cosmological constants. Ferreira notes that their interacting dark energy model is consistent with past data from the Wilkinson Microwave Anisotropy Probe (WMAP) launched in 2001 and the Planck satellite launched in 2009. These two spacecraft analyzed the cosmic microwave background, the heat left over from the Big Bang, to provide some of the most accurate measurements yet of a number of key cosmological parameters, such as what the universe is made of. “The consistency of this result with other observations, like Planck, WMAP, and others is very encouraging to me,” she says. Hydrogen gas in the intergalactic void, seen here scattered throughout this portrait of the Milky Way by the Plank Observatory, helps physicists map the universe in 3D. “I would say that the evidence now is overwhelming that dark energy cannot be a cosmological constant,” says theoretical astrophysicist <a href="http://www.physics.arizona.edu/~melia"> Fulvio Melia </a> at the University of Arizona at Tucson, who did not participate in this study. However, the notion that dark energy is not a cosmological constant is a controversial one, and many disagree with it. “I think overall that the BOSS data is very nicely consistent with a cosmological constant model,” says cosmologist Daniel Eisenstein at Harvard University, director of SDSS-III. “We do find mild discrepancies, but I don’t see them as being statistically significant enough to argue against a cosmological constant.” If dark energy is not a cosmological constant, “it’s a very important discovery,” Eisenstein says. “It would mean that whatever is causing the large-scale acceleration of the expansion is actually changing on cosmological measurable time scales. That could signal a breakdown in general relativity or the presence of a very pervasive low-energy component of the universe that is still evolving.” Dark energy as a cosmological constant is “very problematic.” In addition, if dark energy’s properties do change over time, that means that dark energy has to have particle-like characteristics, Melia says. “Dark energy would presumably be made up of particles like electrons or quarks or the Higgs,” Melia says. Though he does point out that dark energy particles, if they exist, would definitely lie beyond the Standard Model. Such dark energy particles might interact with other particles in ways besides gravity, perhaps through as-yet unknown forces, he adds. Melia says that while the idea that dark energy is not a cosmological constant is controversial, the converse—that dark energy is a cosmological constant—is “very problematic. Its measured value is some 10 120 times different from what it should be in the context of quantum mechanics. That’s 120 orders of magnitude. So it would not disappoint anyone if it turns out that there is no cosmological constant and that dark energy is instead something more understandable in the context of particle physics.” Ferreira stresses that their conclusions are not definitive—the BOSS data that suggests a departure from the current leading model of dark energy and dark matter falls short of the five-sigma level of confidence that physicists often rely on to confirm a result. Plus, while the paper has been submitted to the journal Physical Review D, it has not yet been accepted. “We need other experiments to confirm this effect, and also, we need a better precision in the measurements so that we can say more precisely that such deviations from Lambda-CDM are real,” Ferreira says. Future models and data could also examine the strength and duration of these interactions, she says. Ferreira notes there are now many projects investigating dark energy that could support or refute their interacting dark energy model, such as the Dark Energy Survey operating in Chile, the Javalambre Physics of the Accelerating Universe Survey operating in Spain, and the proposed BINGO telescope in Uruguay. “I think that the prospects of this area of research for the future are very good and promising,” Ferreira says. While Eisenstein says that current astronomical observations suggest that dark energy is in fact a cosmological constant, he nevertheless supports research to rigorously test that idea. “It’s very reasonable to keep exploring all the possibilities about the dark sector,” Eisenstein says. “We’re all very interested in testing the very simple model of the cosmological constant at higher precision. It’d be great if we could find something to disprove it.” Image credits: NASA/ESA/M.J. Jee and H. Ford/Johns Hopkins University, Sloan Digital Sky Survey, ESA/NASA/JPL-Caltech

Dark matter may morph into dark energy, potentially altering our understanding of how the universe is bound together.

The 100-Year-Old Idea That Could Change Flight

Since at least the myth of Icarus, wings seen in nature have inspired humanity’s dreams of flight. In the more than a century since the Wright brothers flew the first airplane, Flyer 1, people have begun making these fantasies a reality, flying high enough above the Earth to make it all the way to the moon.However, in many ways, nature still has much to teach aircraft designers. For instance, while airplanes now often break the sound barrier, their stiff wings limit their performance. In contrast, the wings of bats and birds can alter their shapes, helping them fly more efficiently and granting them the versatility to cruise, maneuver, and dive as needed. Modern aircraft rely on hinged flaps such as ailerons, rudders, slats, and tabs on their wings for controlled flight. Now, aerospace engineers from a number of different institutions are developing morphing wings, including ones that can bend and flex much like the wings of bats and birds and are covered by skins of overlapping pieces that resemble scales and feathers. “I would say there are at least 10 groups worldwide working on different approaches to morphing,” says Daniel Inman, chair of aerospace engineering at the University of Michigan at Ann Arbor and one of the many engineers investigating morphing wings. This biological mimicry extends into the very airframes of these wings—they are made of modules, a bit like how animals are made of cells. This new approach could lead to a wide range of new aircraft and other morphing robots.Nowadays, conventional aircraft rely on hinged flaps such as ailerons, rudders, slats, and tabs on their wings to help them control the way they roll, pitch, and yaw as they fly. While these new morphing wings rely on cutting edge materials science and computer-aided control systems, the way they work has not changed for decades.The world’s very first airplane, the Wright brothers’ Flyer 1, did not have hinged flaps like modern aircraft. Instead, it used wires and pulleys that bent and twisted the aircraft’s pliant wood-and-canvas wings. Essentially, Flyer 1 had primitive morphing wings. Wilbur and Orville Wright's first flight, seen here, was controlled using primitive morphing wings. “They were trying to create as lightweight a structure as they could, and given that it was flexible, they utilized that flexibility. That was an amazing innovation to see,” says Kenneth Cheung, a research scientist at NASA Ames Research Center at Moffett Field, California, and another engineer developing morphing wings.Though the new wings draw inspiration from the Wright brothers’, they are far more sophisticated, promising to improve the efficiency, agility, and stealth of future aircraft. What’s more, they could be far easier—and cheaper—to produce. <h2>Bioinspiration</h2>The Wright brothers’ warping wings went out of style once airplanes got faster. Wood-and-canvas wings fare poorly at high speeds, so engineers substituted stiffer materials, which required wings with hinged flaps.Still, the kind of flexible wings seen in nature have long tantalized scientists for the benefits they can provide. Birds and bats can greatly vary the shape of their wings, helping them rapidly switch between a variety of tasks in complex environments. For instance, falcons can save energy loitering aloft until they see prey, at which point they can adopt a strike configuration to swoop downwards, adjusting their wings at the last second to help them deftly capture desperately jinking prey. A closeup of a morphing wing Although stiff aircraft wings are each designed for optimal flight at one speed and one angle of attack—the angle between the wing and the direction of the airflow—they do not behave as well at other speeds or angles of attack, Inman explained. For example, commercial airliners have wings that sweep moderately toward the rear, while the wings of many jet fighters sweep much farther back to enable supersonic speeds.The hinged flaps needed to control flight also create gaps at the edge of wings. This can lead to turbulent airflow, and “turbulence tends to increases drag on the wing, reducing fuel efficiency,” Inman says. “A morphing or shape-changing wing has the potential for being optimal across all flight conditions, thus reducing fuel consumption,” Inman says.In addition, the absence of tilted surfaces and sharp edges on morphing aircraft could reduce their visibility and radar signature, thus enhancing their stealth properties.For decades, scientists have sought to develop aircraft with varying levels of ability to change the shape of their wings. For instance, the Grumman F-14 Tomcat, first flown in 1970, could alter the angle at which its wings swept back. At lower speeds, the wings swing forward to help provide lift, and as velocity increases, the wings swing backward to allow a high-speed cruise that consumes less fuel.Most previous attempts to develop morphing wings that can flex relied on adding mechanical control structures within the wings. These add weight, complexity and cost, usually offsetting any benefits that morphing could provide. A tube running the length of the wing twists the tips to control the aircraft. The main advance underlying the morphing aircraft from Cheung and his colleagues involves not adding new control mechanisms into the wings, as has often been tried with previous morphing aircraft. Instead, “we’re turning the wing into the control mechanism,” says physicist Neil Gershenfeld, director of the Center for Bits and Atoms at MIT and Cheung’s former advisor.The new morphing wings came about when Spirit AeroSystems, a major supplier to aerospace titans such as Boeing and Airbus, approached Cheung, Gershenfeld, and their colleagues and “asked us if we could 3D-print an entire aircraft,” Cheung says. Instead of 3D printing, Gershenfeld, Cheung, and their colleagues had long explored building objects from Lego-like parts. “Just as electronic hardware in essence is built from a few kinds of components, so too were we trying to think about how physical and even mechanical systems could be constructed using the building-block method,” Cheung says. Just as birds and bats are made of cells, “so, too, is this a cellular approach,” he adds.“We’re turning the wing into the control mechanism.”The interior of each new wing that Cheung, Gershenfeld, and their colleagues developed consists of a system of tiny, strong, lightweight modules made of black carbon-fiber composites. “We take sheets of solid material and use water-jets to cut shapes out of it that can snap together into little diamond or square shapes that are hollow in the middle,” Cheung says. “Imagine each of these shapes as Legos stacked together as 3D structures, but with excess material taken out of each Lego. Put together, it looks like foam.”Some of the modules in these structures are rigid, while others are relatively flexible. For instance, the 12-sided dodecahedral modules are easier to deform than eight-sided octohedral ones, and modules consisting of thinner struts are naturally easier to squash than ones made of thicker struts, Cheung says.The rigidity or flexibility of each module influences how a wing flexes when force is applied to it. Through a tube reaching across the wing to each wingtip, a small motor located in the center of an aircraft can apply twisting pressure to change the shape of each wing uniformly along its length.To accommodate that flexing, the exteriors of these new wings are covered in orange artificial “skins” made of overlapping strips of flexible resin that resemble fish scales or bird feathers. These strips are attached to the wings via steel retaining pins and slide across each other as the wings change shape, providing a smooth outer surface.In wind-tunnel tests, a radio-controlled aircraft equipped with these new morphing wings matched the aerodynamic properties of a comparable standard aircraft, but at just roughly one-tenth the weight. “The test pilot said it handled just like a good aircraft — it flew straight and level when he wanted, turned when he wanted, went where he wanted it to go,” Cheung says. “He even did some acrobatics with it.”<h2>Simplify, Simplify</h2>Ultimately the goal is to not just build morphing wings, “but to progress to a whole aircraft whose shape you can deform,” Gershenfeld says. Cheung adds that a virtue of building an entire airplane using their approach is potentially much greater simplicity.“A Boeing 747 has about 6 million parts,” Cheung says. “About 3 million of those are rivets, and let’s say they’re all the same rivet, although they are not. Of the remaining 3 million parts, on the order of 1 million are unique parts. In contrast, we’ve estimated that, to build an aircraft with our method that serves the same function, you’d probably need several times more parts, maybe 30 million, but instead of 1 million unique parts, we’re talking about fewer than 1,000 unique parts as a rough estimate. Having that much fewer unique parts is a huge benefit since they are easier to mass manufacture and replace or repair.”A number of major companies have expressed interest in this new work. “Airbus is actively involved, as is Moog in Buffalo, which works on control systems for all major aircraft companies from cockpit to tail,” Cheung says.Several other groups around the world are developing their own style of morphing aircraft. For example, Inman and his colleagues have received roughly $6 million in funding from the Air Force Office of Scientific Research to spend five years investigating bird-inspired morphing vehicles.“We have three bird biologists in our group, and we’re looking at many different kinds of birds to understand their aerodynamics and integrate what we learn into small aircraft and unmanned aerial vehicles, from radio-controlled planes up to the Predator drone,” Inman says. Cheung and Gershenfeld's morphing wing is built with small, repeated modules and covered with flexible resin scales. Instead of using simple tugs to control the wings, Inman and his colleagues are investigating piezoelectric materials that convert electricity into physical movement as well as shape memory alloys that change shape when supplied with electricity. “We also have funding from some automotive companies to look at morphing as a way to increase fuel economy of road vehicles,” Inman says.All morphing aircraft research currently underway will take a long time before it helps build passenger jets, due to the highly risk-averse nature of the passenger aircraft industry. “The best-case scenario that anyone in commercial aviation will ever give you is that it will take 30 years before you see something new in an airport,” Cheung says.Instead, in the near-term, he says they are focusing on creating new unmanned morphing aircraft. If they succeed, morphing-wing drones may be able to stay aloft efficiently for long times to deliver medicine or wireless phone or internet access to far-off areas, Gershenfeld says.In the long run, Gershenfeld says that such research extends well beyond aviation, possibly leading to wind turbines driven by giant morphing rotors that are better at collecting wind power than conventional machines. It could also inspire to robotic arms, legs, and bodies whose shapes can bend continuously across their entire length, instead of at fixed joints. “You could one day have boats or submarines that swim more like fish, as well as planes that fly more like birds,” Cheung says.In a way, any future menagerie of animal-inspired morphing robots will owe much of its success to the flexing wings of the Wright brothers. “All aviation is following in the footsteps of the Wright brothers,” Cheung says. “They were the first to show that flexing wings worked.” Photo credits: <a href="https://commons.wikimedia.org/wiki/File:A319_starboard_wing,_from_aft_(2715672497).jpg">Bill Abbott/Wikimedia Commons (CC BY-SA)</a> , Kenneth Cheung/NASA

Inspired by birds, bats, and the Wright brothers, engineers are building the next breakthrough in aviation.

Some of the universe's gamma rays, seen here in five years of data from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions.

Dark Matter’s Invisible Hand

Dark matter is currently one of the greatest mysteries in the universe. It’s thought to be an invisible substance that makes up roughly five-sixths of all matter in the cosmos, a dark fog suffusing the universe that rarely interacts with ordinary matter. But when it does, according to an unexpected finding by theoretical physicist Lisa Randall, the consequences could be momentous. Astronomers first detected dark matter through its gravitational pull, which apparently keeps the Milky Way and other galaxies from ripping themselves apart given the speeds at which they spin. Scientists have mostly ruled out all known ordinary materials as candidates for dark matter. The current <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/04/whos-afraid-of-the-dark-alternatives-to-dark-matter/"> consensus </a> is that dark matter lies outside the Standard Model of particle physics, currently the best description of how all known subatomic particles behave. Specifically, physicists have suggested that dark matter is composed of new kinds of particles that have very weak interactions—not just with ordinary matter but also with themselves. However, Randall and other scientists have suggested that dark matter might interact more strongly with itself than we suspect, experiencing as-yet undetected “dark forces” that would influence dark matter particles alone. Just as electromagnetism can make particles of ordinary matter attract or repel each other and emit and absorb light, so too might “dark electromagnetism” cause similar interactions between dark matter particles and cause them to emit “dark light” that’s invisible to ordinary matter. <h2> Differentiated Dark Matter </h2> The evidence for this theory can be seen in potential discrepancies between predictions and observations of the way matter is distributed in the universe on relatively modest scales, such as that of dwarf galaxies, says Randall, a professor at Harvard University. For example, repulsive interactions between dark matter particles might keep these particles apart and reduce their overall density, explaining why current estimates of the density of the innermost portion of galaxies are higher than what is actually seen. “There’s no reason to think that dark matter is composed of all the same type of particle.” Most dark matter models suggest that dark matter particles are all of one type—they either all interact with each other or they all do not. However, Randall and her colleagues propose a more complex version that they call “partially-interacting dark matter,” where dark matter has both a non-interacting component and a self-interacting one. A similar example in real particles can be seen with protons, electrons, and neutrons—positively charged protons and negatively charged electrons attract one another, while neutrally charged neutrons are not attracted to either protons or electrons. “There’s no reason to think that dark matter is composed of all the same type of particle,” Randall says. “We certainly see a diversity of particles in the one sector of matter we do know about, ordinary matter. Why shouldn’t we think the same of dark matter?” In this model, Randall and her colleagues suggest that only a small portion of dark matter—maybe about 5%—experiences interactions reminiscent of those seen in ordinary matter. However, this fraction of dark matter could influence not only the evolution of the Milky Way, but of life on Earth as well, an idea Randall explores in her latest book, Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe . Standard dark matter models predict that dwarf galaxies orbiting larger galaxies should be scattered in spherical patterns around their parents. However, astronomical data suggest that many dwarf galaxies orbiting the Milky Way and Andromeda lie roughly in the same plane as each other. Randall and her colleagues suggest that if dark matter particles can interact with each other, they can shed energy, potentially creating a structure that could not only solve this dwarf galaxy mystery, but also have triggered the cosmic disruption that doomed the dinosaurs. <h2> Dark Disks </h2> In the partially interacting dark matter scenario, the non-interacting component would still form spherical clouds around galaxies, consistent with what astronomers know of their general structure. However, self-interacting dark matter particles would lose energy and cool as they jostled with each other. Cooling would slow these particles down, and gravity would make them cluster together. If these clouds were relatively immobile, they would simply shrink into smaller balls. However, since they likely rotate—just like the rest of the matter in their galaxies—this rotation would make these clouds of self-interacting dark matter collapse into flat disks, in much the same way as spherical clouds of ordinary matter collapsed to form the spiral disks of the Milky Way and many other galaxies. Conservation of angular momentum causes these would-be spheres to flatten out. While cooling would still cause them to collapse vertically, they would not collapse along the same plane as their rotation. If dark matter in large galaxies was concentrated in disks, it’s likely that at least some of the orbiting dwarf galaxies would be concentrated in flat planes because of the gravitational pull of dark matter on the dwarf galaxies, Randall and her colleagues say. The researchers suggest these “dark disks” should be embedded in the visible disk of larger galaxies. But here’s where dark matter begins to exert its influence. The relationship between the dark disk and the stars in the galaxy is not entirely stable. The Sun, for example, completes a circuit around the Milky Way’s core roughly every 240 million years. During its orbit, it bobs up and down in a wavy motion through the galactic plane about every 32 million years. Coincidentally, some researchers previously suggested that meteor impacts on Earth rise and fall in cycles about 30 million to 35 million years long, leading to regular mass extinctions. Earlier researchers proposed a cosmic trigger for this deadly cycle, such as a potential companion star for the Sun dubbed “Nemesis” that would ensnare meteoroids and send them hurling toward Earth. Instead, Randall and her colleagues suggest that the Sun’s regular passage through the Milky Way’s dark disk might have warped the orbits of comets in the outer solar system, flinging them inward. Such disruption may have then led to disastrous cosmic impacts on Earth, including the collision about 67 million years ago that likely caused the Cretaceous-Tertiary extinction event, the most recent and most familiar mass extinction which killed off all dinosaurs (except those which would evolve into birds). “I did not set out to explain the extinction of the dinosaurs.” The main suspect behind this disaster is an impact from an asteroid or comet that left behind a gargantuan crater more than 110 miles wide near the town of Chicxulub in Mexico. The collision, likely caused by a meteor about 6 miles across, would have released as much energy as 100 trillion tons of TNT, more than a billion times more than the atomic bombs that destroyed Hiroshima and Nagasaki, killing off at least 75% of life on Earth. In research detailed in 2014 in the journal <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.161301"> Physical Review Letters </a> , Randall and her colleague Matthew Reece analyzed craters that are more than 12.4 miles in diameter and were created in the past 250 million years. When they compared the ages of these craters against the 35-million-year cycle they proposed, they discovered that it was three times more likely that the craters matched the dark matter cycle instead of simply occurring randomly. “I want to be clear that I did not set out to explain the extinction of the dinosaurs,” Randall says. “This work was about exploring the story of how our universe came about, to explore one possible connection between many different levels of the universe, from the universe down to the Milky Way, the solar system, Earth, and life on Earth.” Geologist Michael Rampino at New York University, who did not participate in this study, finds a potential link between a dark disk and mass extinctions “an interesting idea,” he says. “If true, it ties together events that happened on Earth to large-scale cycles in the rest of the solar system and even the galaxy in general.” <h2> Dark Life? </h2> However, not everyone agrees that Randall and her colleagues present a convincing case. “They tie mass extinctions to the cratering record, but there are all kinds of estimates one can make with the cratering record depending on what craters one think makes the cut—do you accept all craters above a certain size or craters out to a certain age?” says astrobiophysicist Adrian Melott at the University of Kansas, who did not take part in this research. “You can get all different kind of answers, from cycles 20 million to 37 million years long.” Moreover, other research suggests that the cycle of mass extinctions on Earth is actually roughly 27 million years long, Melott says. “That’s way too short a duration for motion oscillating back and forth through the disk.” Randall notes that data from the European Space Agency’s satellite Hipparcos, launched in 1989 to precisely measure the positions and velocities of stars, allowed for the theoretical existence of a dark disk. She adds that ESA’s Gaia mission, launched in 2013 to create a precise 3D map of matter throughout the Milky Way, could reveal or refute the dark disk’s existence. One intriguing possibility raised by interacting dark matter models is the existence of dark atoms that might have given rise to dark life, neither of which would be easily detected, Randall says. Although she admits that the concept of dark life might be far-fetched, “life is complicated, and we have yet to understand life and what’s necessary for it.” Image credit: Tim De Chant/NASA/DOE/Fermi LAT Collaboration

If dark matter interacts with regular matter as predicted, it could cause unexpected and momentous consequences.

Gravitational Waves May Permanently Alter Spacetime

For decades, physicists searched in vain for evidence of gravitational waves, the stretches and squeezes in spacetime that were first predicted by Albert Einstein’s theory of general relativity a century ago. Even Einstein himself was uncertain that they existed. But then, in February and June of this year, scientists detected two events that produced gravitational waves. Now that gravitational-wave detection is likely becoming a regular occurrence—we’ll probably find evidence of many more in the next few years—physicists are again pondering an obscure detail about gravitational waves that was once also thought virtually impossible to observe—gravitational-wave memory, which involves permanent changes in the distance between two objects. “For so many years, people were simply concentrating on making that first detection of gravitational waves,” says Paul Lasky, and astrophysicist at Monash University in Australia. “Once that first detection happened, our minds have become focused on the vast potential of this new field.” Physicists are turning again to the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) project that first discovered gravitational waves in the past year since it might also be able to detect the permanent distortions in spacetime that such waves may leave behind as they zip across the universe. If scientists can find signs of this gravitational-wave memory, such a discovery could help support research that famed theoretical physicist Stephen Hawking and his colleagues are conducting to solve a major puzzle—whether black holes violate the laws of physics by destroying information or not. To understand what gravitational-wave memory entails, imagine a binary system in outer space consisting of two black holes. There are two astronauts drifting side by side in orbit around this black hole binary, who “are initially separated from one another by some distance, 10 meters say,” Lasky says. As the black holes spiral towards one another, they will release gravitational waves that will travel at the speed of light and cause the distance between the astronauts and the black holes to fluctuate, and they will also cause the distance between the astronauts to oscillate between a little more and a little less than 10 meters. Once the black holes have collided and merged into a single black hole, these oscillations will stop, and the astronauts will once again be separated by a constant distance. However, “this constant distance is not the original 10 meters, but will be slightly more or slightly less,” Lasky says. The gravitational waves will have permanently stretched or squeezed the fabric of spacetime. “In other words, spacetime has a permanent memory of the collision of these two black holes,” he says. “Our minds have become focused on the vast potential of this new field.” This memory effect was first proposed by Russian scientists in 1974, but it has largely remained obscure since. “Although it’s a strange phenomenon that certainly has public appeal, our ability or potential to detect it has always been very remote,” Lasky says. “The effect is so weak that people did not believe we would be able to measure it with LIGO.” Gravitational waves are generated when anything with mass moves. However, gravitational waves are extraordinarily weak, making them extremely difficult to detect. Only those from the most violent events in the universe—the collisions of black holes or neutron stars—can produce gravitational waves strong enough for LIGO to pick up. Now, though, Lasky and his colleagues think they have discovered a way for LIGO to detect gravitational-wave memory. They recently detailed <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.061102"> their findings </a> in the journal Physical Review Letters . “This is a very clever way of measuring gravitational-wave memory and exploring it observationally,” says LIGO co-founder Kip Thorne, a physicist at the California Institute of Technology, who did not participate in this study. “I never thought it’d be possible with LIGO.” <h2> Weak Waves </h2> LIGO uses a pair of widely separated detectors in the United States—one in Hanford, Washington, the other in Livingston, Louisiana—to sense the warping that gravitational waves cause as they move through matter. Two detectors are used so each could be used to confirm the results of its partner. Both LIGO detectors are shaped like gigantic Ls, with arms 2.5 miles long. The arms of each detector are normally the same length, so the laser beams that shoot down their arms take the same amount of time to traverse each. However, when gravitational waves pass through the Earth, if they make the arms expand and contract in size by within one-ten-thousandth the diameter of a proton, the detector can detect the split-second difference that it takes for laser beams to travel down one arm versus the other. That sensitivity was on full display when LIGO gravitational waves were first picked up. They emanated from an event known as GW150914 located nearly 1.3 billion light years away, which likely involved black holes each roughly 29 and 36 times the mass of the Sun slamming into one another near the speed of light. In a fraction of a second, the catastrophic merger converted an amount of mass equal to three times the Sun’s into gravitational-wave energy, with a peak power output about 50 times that of the entire visible universe. Nevertheless, these gravitational waves only caused the LIGO arms to wobble by about one-five-hundredth of a femtometer, Lasky says. To put that into perspective, a femtometer is millionth of a billionth of a meter, and a proton is only about 1.68 femtometers wide. In June, LIGO detected gravitational waves a second time, from an event named GW151226. Suddenly, scientists weren’t just looking at the possibility of detecting one event, but potentially dozens. They’d likely end up with so much data that they could begin to probe other theories involving gravitational waves, including gravitational-wave memory. <h2> Fractionally As Powerful </h2> All kinds of events that emit gravitational waves will also generate this memory effect, “even very weak events,” Lasky says. The memory effect will look like an additional flare of gravitational waves near the end of the event that caused the main burst of gravitational waves, he adds. However, the memory effect is extremely small. “In general, we expect the size of the memory effect to be between about one-tenth and one-hundredth of that of the gravitational waves,” Lasky says. “For almost all events other than the most catastrophic collisions in spacetime, the effect cannot be measured.” For instance, while GW150914 caused the LIGO arms to wobble by only about one-five-hundredth of a femtometer, the memory effect from this collision would only be “about one-twentieth of the size of the gravitational waves that were detected by LIGO,” Lasky says, or about one-ten-thousandth of a femtometer. Whether this memory effect causes the distance between two points to permanently grow or shrink depends on their orientation with respect to the source of the gravitational waves. An event that stretches spacetime in one direction will squeeze it in the direction at right angles to it. For example, consider three astronauts arranged at equal distances around a gravitational-wave source, with two along the equator of the source and the third at one of its poles. If gravitational-wave memory causes the distance between a pair of these astronauts to shrink, it will make distance between the other pair grow, Lasky says. Previous research suggested that LIGO is not sensitive enough to detect signs of the memory effect from any single black hole merger. While this remains true, scientists have predicted that LIGO should detect the mergers of tens to hundreds of black holes in the next few years, Lasky says. “Our work has shown that the combination of all these mergers will enable us to measure the memory effect over time,” he says. “The key is being able to stack the signals from all of the events in a clever way.” In principle, this stacking boosts the detectability of the memory effect. The researchers then compare predictions of how the stacked gravitational waves should look if the memory effect is or is not there. “The key is being able to stack the signals from all of the events.” Lasky and his colleagues predict that LIGO may be able to discern the memory effect after detecting 35 to 90 mergers involving black holes with similar masses and distances from Earth as GW150914. He added that LIGO researchers are striving to make the gravitational-wave observatory even more sensitive, which would mean fewer mergers needed to detect the memory effect. “I thought it would’ve been very difficult to detect the memory effect using LIGO unless the signal came from very close by, but I didn’t consider this idea of using multiple events to build up evidence for this effect,” says theoretical physicist Marc Favata at Montclair State University in New Jersey, who has done previous work on gravitational-wave memory but did not take part in this research. “It’s very exciting work. It’s always good when one finds there is actually a likelihood of observing something in nature that you’ve been working on for years.” <h2> Information Paradox </h2> The detection of gravitational-wave memory would amount to more than just the discovery of an esoteric phenomenon. “Gravitational-wave memory is tremendously important in modern physics when it comes to the matter of whether or not information is stored on the surface of a black hole,” says Thorne, the Caltech physicist. The conventional view of black holes suggests that their gravitational pull is so strong that nothing can escape from them, not even light, which is why they are called black holes. The border past which there is supposedly no return is known as the event horizon. However, this view suggests that all information about whatever falls past an event horizon is destroyed. This contradicts quantum physics, the best explanation so far for how the universe behaves on a subatomic level, which suggests that information can never be destroyed. Steven Hawking and his colleagues Andrew Strominger at Harvard University and Malcolm Perry at the University of Cambridge recently proposed <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2014/02/stephen-hawking-serves-up-scrambled-black-holes/"> a possible answer </a> to this “black hole information paradox” that involves gravitational-wave memory. They suggest that black holes possess “ <a href="https://physics.aps.org/articles/v9/62"> soft hair </a> ”—that is, essentially zero-energy forms of electromagnetic and gravitational radiation that release information as black holes <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2015/09/how-big-a-deal-was-stephen-hawkings-big-black-hole-announcement/"> evaporate </a> . What would happen if you fell into a black hole? The answer could revolutionize the fundamental laws of nature. “In the last few years, we have discovered that ‘empty’ space, as described by Einstein’s theory of gravity, is not as empty as we once thought,” Strominger says. “It actually has a lot of hidden structure which can store information. The information is stored at the edges of space such as infinity or the horizon of a black hole. We all this structure ‘soft hair.’ ” “The gravitational-memory effect measures the soft hair,” Strominger says. “It would be most interesting to directly measure it.” “This is indeed a very exciting prospect,” Lasky says. “It is fantastic that people are thinking about the implications of such a detection, and it is especially wonderful to hear that it could have an influence on such fundamental questions as the black hole information paradox.” Video/image credit: <a href="http://www.black-holes.org/gw150914"> Simulating Extreme Spacetimes (CC-BY-SA 4.0) </a>

Gravitational waves may leave indelible marks on spacetime—and could help us solve the black hole information paradox.

Charles Choi

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