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    Scientists mapped dark matter around galaxies in the early universe

    Scientists have mapped out the dark matter around some of the earliest, most distant galaxies yet.

    The 1.5 million galaxies appear as they were 12 billion years ago, or less than 2 billion years after the Big Bang. Those galaxies distort the cosmic microwave background — light emitted during an even earlier era of the universe — as seen from Earth. That distortion, called gravitational lensing, reveals the distribution of dark matter around those galaxies, scientists report in the Aug. 5 Physical Review Letters.

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    Understanding how dark matter collects around galaxies early in the universe’s history could tell scientists more about the mysterious substance. And in the future, this lensing technique could also help scientists unravel a mystery about how matter clumps together in the universe.

    Dark matter is an unknown, massive substance that surrounds galaxies. Scientists have never directly detected dark matter, but they can observe its gravitational effects on the cosmos (SN: 7/22/22). One of those effects is gravitational lensing: When light passes by a galaxy, its mass bends the light like a lens. How much the light bends reveals the mass of the galaxy, including its dark matter.

    It’s difficult to map dark matter around such distant galaxies, says cosmologist Hironao Miyatake of Nagoya University in Japan. That’s because scientists need a source of light that is farther away than the galaxy acting as the lens. Typically, scientists use even more distant galaxies as the source of that light. But when peering this deep into space, those galaxies are difficult to come by.

    So instead, Miyatake and colleagues turned to the cosmic microwave background, the oldest light in the universe. The team used measurements of lensing of the cosmic microwave background from the Planck satellite, combined with a multitude of distant galaxies observed by the Subaru Telescope in Hawaii (SN: 7/24/18). “The gravitational lensing effect is very small, so we need a lot of lens galaxies,” Miyatake says. The distribution of dark matter around the galaxies matched expectations, the researchers report.

    The researchers also estimated a quantity called sigma-8, a measure of how “clumpy” matter is in the cosmos. For years, scientists have found hints that different measurements of sigma-8 disagree with one another (SN: 8/10/20). That could be a hint that something is wrong with scientists’ theories of the universe. But the evidence isn’t conclusive.

    “One of the most interesting things in cosmology right now is whether that tension is real or not,” says cosmologist Risa Wechsler of Stanford University, who was not involved with the study. “This is a really nice example of one of the techniques that will help shed light on that.”

    Measuring sigma-8 using early, distant galaxies could help reveal what’s going on. “You want to measure this quantity, this sigma-8, from as many perspectives as possible,” says cosmologist Hendrik Hildebrandt of Ruhr University Bochum in Germany, who was not involved with the study.

    If estimates from different eras of the universe disagree with one another, that might help physicists craft a new theory that could better explain the cosmos. While the new measurement of sigma-8 isn’t precise enough to settle the debate, future projects, such as the Rubin Observatory in Chile, could improve the estimate (SN: 1/10/20). More

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    The Windchime experiment could use gravity to hunt for dark matter ‘wind’

    The secret to directly detecting dark matter might be blowin’ in the wind.

    The mysterious substance continues to elude scientists even though it outweighs visible matter in the universe by about 8 to 1. All laboratory attempts to directly detect dark matter — seen only indirectly by the effect its gravity has on the motions of stars and galaxies — have gone unfulfilled.

    Those attempts have relied on the hope that dark matter has at least some other interaction with ordinary matter in addition to gravity (SN: 10/25/16). But a proposed experiment called Windchime, though decades from being realized, will try something new: It will search for dark matter using the only force it is guaranteed to feel — gravity.

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    “The core idea is extremely simple,” says theoretical physicist Daniel Carney, who described the scheme in May at a meeting of the American Physical Society’s Division of Atomic Molecular and Optical Physics in Orlando, Fla. Like a wind chime on a porch rattling in a breeze, the Windchime detector would try to sense a dark matter “wind” blowing past Earth as the solar system whips around the galaxy.  

    If the Milky Way is mostly a cloud of dark matter, as astronomical measurements suggest, then we should be sailing through it at about 200 kilometers per second. This creates a dark matter wind, for the same reason you feel a wind when you stick your hand out the window of a moving car.

    The Windchime detector is based on the notion that a collection of pendulums will swing in a breeze. In the case of backyard wind chimes, it might be metal rods or dangling bells that jingle in moving air. For the dark matter detector, the pendulums are arrays of minute, ultrasensitive detectors that will be jostled by the gravitational forces they feel from passing bits of dark matter. Instead of air molecules bouncing off metal chimes, the gravitational attraction of the particles that make up the dark matter wind would cause distinctive ripples as it blows through a billion or so sensors in a box measuring about a meter per side.

    Within the Windchime detector (illustrated as an array of small pendulums), a passing dark matter particle (red dot) would gravitationally tug on sensors (blue squares) and cause a detectable ripple, much like wind blowing through a backyard wind chime.D. Carney et al/Physical Review D 2020

    While it may seem logical to search for dark matter using gravity, no one has tried it in the nearly 40 years that scientists have been pursuing dark matter in the lab. That’s because gravity is, comparatively, a very weak force and difficult to isolate in experiments. 

    “You’re looking for dark matter to [cause] a gravitational signal in the sensor,” says Carney, of Lawrence Berkeley National Laboratory in California. “And you just ask . . . could I possibly see this gravitational signal? When you first make the estimate, the answer is no. It’s actually going to be infeasibly difficult.”

    That didn’t stop Carney and a small group of colleagues from exploring the idea anyway in 2020. “Thirty years ago, this would have been totally nuts to propose,” he says. “It’s still kind of nuts, but it’s like borderline insanity.”

    The Windchime Project collaboration has since grown to include 20 physicists. They have a prototype Windchime built of commercial accelerometers and are using it to develop the software and analysis that will lead to the final version of the detector, but it’s a far cry from the ultimate design. Carney estimates that it could take another few decades to develop sensors good enough to measure gravity even from heavy dark matter.

    Carney bases the timeline on the development of the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which was designed to look for gravitational ripples coming from black holes colliding (SN: 2/11/16). When LIGO was first conceived, he says, it was clear that the technology would need to be improved by a hundred million times. Decades of development resulted in an observatory that views the sky in gravitational waves. With Windchime, “we’re in the exact same boat,” he says.

    Even in its final form, Windchime will be sensitive only to dark matter bits that are roughly the mass of a fine speck of dust. That’s enormous on the spectrum of known particles — more than a million trillion times the mass of a proton.

    “There is a variety of very interesting dark matter candidates at [that scale] that are definitely worth looking for … including primordial black holes from the early universe,” says Katherine Freese, a physicist at the University of Michigan in Ann Arbor who is not part of the Windchime collaboration. Black holes slowly evaporate, leaking mass back into space, she notes, which could leave many relics formed shortly after the Big Bang at the mass Windchime could detect.

    But if it never detects anything at all, the experiment still stands out from other dark matter detection schemes, says Dan Hooper, a physicist at Fermilab in Batavia, Ill., also not affiliated with the project. That’s because it would be the first experiment that could entirely rule out some types of dark matter.

    Even if the experiment turns up nothing, Hooper says, “the amazing thing about [Windchime] … is that, independent of anything else you know about dark matter particles, they aren’t in this mass range.” With existing experiments, a failure to detect anything could instead be due to flawed guesses about the forces that affect dark matter (SN: 7/7/22).  

    Windchime will be the only experiment yet imagined where seeing nothing would definitively tell researchers what dark matter isn’t. With a little luck, though, it could uncover a wind of tiny black holes, or even more exotic dark matter bits, blowing past as we careen around the Milky Way. More

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    A century ago, Alexander Friedmann envisioned the universe’s expansion

    For millennia, the universe did a pretty good job of keeping its secrets from science.

    Ancient Greeks thought the universe was a sphere of fixed stars surrounding smaller spheres carrying planets around the central Earth. Even Copernicus, who in the 16th century correctly replaced the Earth with the sun, viewed the universe as a single solar system encased by the star-studded outer sphere.

    But in the centuries that followed, the universe revealed some of its vastness. It contained countless stars agglomerated in huge clusters, now called galaxies.

    Then, at the end of the 1920s, the cosmos disclosed its most closely held secret of all: It was getting bigger. Rather than static and stable, an everlasting and ever-the-same entity encompassing all of reality, the universe continually expanded. Observations of distant galaxies showed them flying apart from each other, suggesting the current cosmos to be just the adult phase of a universe born long ago in the burst of a tiny blotch of energy.

    It was a surprise that shook science at its foundations, undercutting philosophical preconceptions about existence and launching a new era in cosmology, the study of the universe. But even more surprising, in retrospect, is that such a deep secret had already been suspected by a mathematician whose specialty was predicting the weather.

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    A century ago this month (May 1922), Russian mathematician-meteorologist Alexander Friedmann composed a paper, based on Einstein’s general theory of relativity, that outlined multiple possible histories of the universe. One such possibility described cosmic expansion, starting from a singular point. In essence, even without considering any astronomical evidence, Friedmann had anticipated the modern Big Bang theory of the birth and evolution of the universe.

    “The new vision of the universe opened by Friedmann,” writes Russian physicist Vladimir Soloviev in a recent paper, “has become a foundation of modern cosmology.”

    Friedmann was not well known at the time. He had graduated in 1910 from St. Petersburg University in Russia, having studied math along with some physics. In graduate school he investigated the use of math in meteorology and atmospheric dynamics. He applied that expertise in aiding the Russian air force during World War I, using math to predict the optimum release point for dropping bombs on enemy targets.

    After the war, Friedmann learned of Einstein’s general theory of relativity, which describes gravity as a manifestation of the geometry of space (or more accurately, spacetime). In Einstein’s theory, mass distorts spacetime, producing spacetime “curvature,” which makes masses appear to attract each other.

    Friedmann was especially intrigued by Einstein’s 1917 paper (and a similar paper by Willem de Sitter) applying general relativity to the universe as a whole. Einstein found that his original equations allowed the universe to grow or shrink. But he considered that unthinkable, so he added a term representing a repulsive force that (he thought) would keep the size of the cosmos constant. Einstein concluded that space had a positive spatial curvature (like the surface of a ball), implying a “closed,” or finite universe.

    Friedmann accepted the new term, called the cosmological constant, but pointed out that for various values of that constant, along with other assumptions, the universe might exhibit very different behaviors. Einstein’s static universe was a special case; the universe might also expand forever, or expand for a while, then contract to a point and then begin expanding again.

    Friedmann’s paper describing dynamic universes, titled “On the Curvature of Space,” was accepted for publication in the prestigious Zeitschrift für Physik on June 29, 1922.

    Einstein objected. He wrote a note to the journal contending that Friedmann had committed a mathematical error. But the error was Einstein’s. He later acknowledged that Friedmann’s math was correct, while still denying that it had any physical validity.

    Friedmann insisted otherwise.

    He was not just a pure mathematician, oblivious to the physical meanings of his symbols on paper. His in-depth appreciation of the relationship between equations and the atmosphere persuaded him that the math meant something physical. He even wrote a book (The World as Space and Time) delving deeply into the connection between the math of spatial geometry and the motion of physical bodies. Physical bodies “interpret” the “geometrical world,” he declared, enabling scientists to test which of the various possible geometrical worlds humans actually inhabit. Because of the physics-math connection, he averred, “it becomes possible to determine the geometry of the geometrical world through experimental studies of the physical world.”

    So when Friedmann derived solutions to Einstein’s equations, he translated them into the possible physical meanings for the universe. Depending on various factors, the universe could be expanding from a point, or from a finite but smaller initial state, for instance. In one case he envisioned, the universe began to expand at a decelerating rate, but then reached an inflection point, whereupon it began expanding at a faster and faster rate. At the end of the 20th century, astronomers measuring the brightness of distant supernovas concluded that the universe had taken just such a course, a shock almost as surprising as the expansion of the universe itself. But Friedmann’s math had already forecast such a possibility.

    In 1929, Edwin Hubble (shown) reported that distant galaxies appear to be flying away from us faster than nearby galaxies, key evidence that the universe is expanding.PICTORIAL PRESS LTD/ALAMY STOCK PHOTO

    No doubt Friedmann’s deep appreciation for the synergy of abstract math and concrete physics prepared his mind to consider the notion that the universe could be expanding. But maybe he had some additional help. Although he was the first scientist to seriously propose an expanding universe, he wasn’t the first person. Almost 75 years before Friedmann’s paper, the poet Edgar Allan Poe had published an essay (or “prose poem”) called Eureka. In that essay Poe described the history of the universe as expanding from the explosion of a “primordial particle.” Poe even described the universe as growing and then contracting back to a point again, just as envisioned in one of Friedmann’s scenarios.

    Although Poe had studied math during his brief time as a student at West Point, he had used no equations in Eureka, and his essay was not recognized as a contribution to science. At least not directly. It turns out, though, that Friedmann was an avid reader, and among his favorite authors were Dostoevsky and Poe. So perhaps that’s why Friedmann was more receptive to an expanding universe than other scientists of his day.

    Today Friedmann’s math remains at the core of modern cosmological theory. “The fundamental equations he derived still provide the basis for the current cosmological theories of the Big Bang and the accelerating universe,” Israeli mathematician and historian Ari Belenkiy noted in a 2013 paper. “He introduced the fundamental idea of modern cosmology — that the universe is dynamic and may evolve in different manners.”

    Friedmann emphasized that astronomical knowledge in his day was insufficient to reveal which of the possible mathematical histories the universe has chosen. Now scientists have much more data, and have narrowed the possibilities in a way that confirms the prescience of Friedmann’s math.

    Friedmann did not live to see the triumphs of his insights, though, or even the early evidence that the universe really does expand. He died in 1925 from typhoid fever, at the age of 37. But he died knowing that he had deciphered a secret about the universe deeper than any suspected by any scientist before him. As his wife remembered, he liked to quote a passage from Dante: “The waters I am entering, no one yet has crossed.” More

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    Astronomers may have seen a star gulp down a black hole and explode

    For the first time, astronomers have captured solid evidence of a rare double cosmic cannibalism — a star swallowing a compact object such as a black hole or neutron star. In turn, that object gobbled the star’s core, causing it to explode and leave behind only a black hole.

    The first hints of the gruesome event, described in the Sept. 3 Science, came from the Very Large Array (VLA), a radio telescope consisting of 27 enormous dishes in the New Mexican desert near Socorro. During the observatory’s scans of the night sky in 2017, a burst of radio energy as bright as the brightest exploding star — or supernova — as seen from Earth appeared in a dwarf star–forming galaxy approximately 500 million light-years away.

    “We thought, ‘Whoa, this is interesting,’” says Dillon Dong, an astronomer at Caltech.

    He and his colleagues made follow-up observations of the galaxy using the VLA and one of the telescopes at the W.M. Keck Observatory in Hawaii, which sees in the same optical light as our eyes. The Keck telescope caught a luminous outflow of material spewing in all directions at 3.2 million kilometers per hour from a central location, suggesting that an energetic explosion had occurred there in the past.

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    The team then found an extremely bright X-ray source in archival data from the Monitor of All Sky X-ray Image (MAXI) telescope, a Japanese instrument that sits on the International Space Station. This X-ray burst was in the same place as the radio one but had been observed back in 2014.  

    Piecing the data together, Dong and his colleagues think this is what happened: Long ago, a binary pair of stars were born orbiting each other; one died in a spectacular supernova and became either a neutron star or a black hole. As gravity brought the two objects closer together, the dead star actually entered the outer layers of its larger stellar sibling.

    The compact object spiraled inside the still-living star for hundreds of years, eventually making its way down to and then eating its partner’s core. During this time, the larger star shed huge amounts of gas and dust, forming a shell of material around the duo.

    In the living star’s center, gravitational forces and complex magnetic interactions from the dead star’s munching launched enormous jets of energy — picked up as an X-ray flash in 2014 — as well as causing the larger star to explode. Debris from the detonation smashed with colossal speed into the surrounding shell of material, generating the optical and radio light.

    While theorists have previously envisioned such a scenario, dubbed a merger-triggered core collapse supernova, this appears to represent the first direct observation of this phenomenon, Dong says.

    “They’ve done some pretty good detective work using these observations,” says Adam Burrows, an astrophysicist at Princeton University who was not involved in the new study. He says the findings should help constrain the timing of a process called common envelope evolution, in which one star becomes immersed inside another. Such stages in stars’ lives are relatively short-lived in cosmic time and difficult to both observe and simulate. Most of the time, the engulfing partner dies before its core is consumed, leading to two compact objects like white dwarfs, neutron stars or black holes orbiting one another.

    The final stages of these systems are exactly what observatories like the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, detect when capturing spacetime’s ripples, Dong says (SN: 8/4/21). Now that astronomers know to look for these multiple lines of evidence, he expects them to find more examples this strange phenomenon. More

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    ‘Flashes of Creation’ recounts the Big Bang theory’s origin story

    Flashes of CreationPaul HalpernBasic Books, $30

    The Big Bang wasn’t always a sure bet. For several decades in the 20th century, researchers wrestled with interpreting cosmic origins, or if there even was a beginning at all. At the forefront of that debate stood physicists George Gamow and Fred Hoyle: One advocated for an expanding universe that sprouted from a hot, dense state; the other for a cosmos that is eternal and unchanging. Both pioneered contemporary cosmology, laid the groundwork for our understanding of where atoms come from and brought science to the masses.

    In Flashes of Creation, physicist Paul Halpern recounts Gamow’s and Hoyle’s interwoven stories. The book bills itself as a “joint biography,” but that is a disservice. While Gamow and Hoyle are the central characters, the book is a meticulously researched history of the Big Bang as an idea: from theoretical predictions in the 1920s, to the discovery of its microwave afterglow in 1964, and beyond to the realization in the late 1990s that the expansion of the universe is accelerating.

    Although the development of cosmology was the work of far more than just two scientists, Halpern would be hard-pressed to pick two better mascots. George Gamow was an aficionado of puns and pranks and had a keen sense of how to explain science with charm and whimsy (SN: 8/28/18). The fiercely stubborn Fred Hoyle had a darker, more cynical wit, with an artistic side that showed through in science fiction novels and even the libretto of an opera. Both wrote popular science books — Gamow’s Mr Tompkins series, which explores modern physics through the titular character’s dreams, are a milestone of the genre — and took to the airwaves to broadcast the latest scientific thinking into people’s homes.

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    “Gamow and Hoyle were adventurous loners who cared far more about cosmic mysteries than social conventions,” Halpern writes. “Each, in his own way, was a polymath, a rebel, and a master of science communication.”

    While the Big Bang is now entrenched in the modern zeitgeist, it wasn’t always so. The idea can be traced to Georges Lemaître, a physicist and priest who proposed in 1927 that the universe is expanding. A few years later, he suggested that perhaps the cosmos began with all of its matter in a single point — the “primeval atom,” he called it. In the 1940s, Gamow latched on to the idea as way to explain how all the atomic elements came to be, forged in the “fireball” that would have filled the cosmos in its earliest moments. Hoyle balked at the notion of a moment of creation, convinced that the universe has always existed — and always will exist — in pretty much the same state we find it today. He even coined the term “Big Bang” as a put-down during a 1949 BBC radio broadcast. The elements, Hoyle argued, were forged in stars.

    As far as the elements go, both were right. “One wrote the beginning of the story of element creation,” Halpern writes, “and the other wrote the ending.” We now know that hydrogen and helium nuclei emerged in overwhelming abundance during the first few minutes following the Big Bang. Stars took care of the rest.

    Halpern treats Gamow and Hoyle with reverence and compassion. Re-created scenes provide insight into how both approached science and life. We learn how Gamow, ever the scientist, roped in physicist Niels Bohr to test ideas about why movie heroes always drew their gun faster than villains — a test that involved staging a mock attack with toy pistols. We sit in with Hoyle and colleagues while they discuss a horror film, Dead of Night, whose circular timeline inspired their ideas about an eternal universe.

    In the mid-20th century, two astronomers emerged as spokesmen for dueling ideas about the origin of the cosmos. George Gamow (left) was a passionate defender of the Big Bang theory, arguing that the universe evolved from a hot, dense state. Fred Hoyle (right) upheld the rival “steady state model,” insisting that the universe is eternal and unchanging.From left: AIP Emilio Segrè Visual Archives, George Gamow Collection; AIP Emilio Segrè Visual Archives, Clayton Collection

    And Halpern doesn’t shy away from darker moments, inviting readers to know these scientists as flawed human beings. Gamow’s devil-may-care attitude wore on his colleagues, and his excessive drinking took its toll. Hoyle, in his waning decades, embraced outlandish ideas, suggesting that epidemics come from space and that a dinosaur fossil had been tampered with to show an evolutionary link to birds. And he went to his grave in 2001 still railing against the Big Bang.

    Capturing the history of the Big Bang theory is no easy task, but Halpern pulls it off. The biggest mark against the book, in fact, may be its scope. To pull in all the other characters and side plots that drove 20th century cosmology, Gamow and Hoyle sometimes get forgotten about for long stretches. A bit more editing could have sharpened the book’s focus.

    But to anyone interested in how the idea of the Big Bang grew — or how any scientific paradigm changes — Flashes of Creation is a treat and a worthy tribute to two scientific mavericks.

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    Physicists’ devotion to symmetry has led them astray before

    Second of two parts

    Physicists have a lot in common with Ponce de León and U2’s Bono. After decades of searching, they aren’t getting any younger. And they still haven’t found what they’re looking for.

    In this case, the object of the physicists’ quest is SUSY. SUSY is not a real person or even a fountain relevant to aging in any way. It’s a mathematical framework based on principles of symmetry that could help physicists better explain the mysteries of the universe. Many experts believe that particles predicted by SUSY are the weakly interacting massive particles, or WIMPs, that supposedly make up the invisible “dark matter” lurking throughout the cosmos.

    So far, though, SUSY has been something of a disappointment. Despite multiple heroic searches, SUSY has remained concealed from view. Maybe it is a mathematical mirage.

    If SUSY does turn out to be a myth, it won’t be the first time that symmetry has led science on a wild WIMP chase. Reasoning from the symmetry of circular motion originally suggested the existence of a new form of matter out in space more than two millennia ago. Devotion to that symmetry blinded science to the true nature of the solar system and planetary motion for the next 19 centuries.

    You can blame Plato and Aristotle. In their day, ordinary matter supposedly consisted of four elements: earth, air, fire and water. Aristotle built an elaborate theory of motion based on those elements. He insisted that they naturally moved in straight lines; earth and water moving straight down (toward the center of the world), air and fire moving straight up. In the heavens, though, Aristotle noticed that motion appeared to be circular, as the stars rotated around the nighttime sky. “Our eyes tell us that the heavens revolve in a circle,” he wrote in On the Heavens. Since the known four elements all moved in a straight line, Aristotle deduced that the heavens must consist of a fifth element, called aether — absent on Earth but predominant in space.

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    Plato, on theoretical rather than observational grounds, had already insisted that circularity’s symmetry signified perfection, and therefore circular motion should be required in the heavens. And so for centuries, the assumption that celestial motion must be circular held a stranglehold on natural philosophers attempting to understand of the universe. As late as the 16th century, Copernicus was willing to depose Aristotle’s Earth from the middle of everything but still believed that the Earth and other planets revolved around the sun with a combination of circular motions. Another half century passed before Kepler established that planetary orbits are elliptical, not circular.

    Aristotle’s belief in an exotic form of matter in space is not so different from the picture scientists paint of the heavens today, albeit in a rather more rigorous and sophisticated theoretical way. Dark matter predominates in space, astronomers believe; it is inferred to exist from gravitational effects altering the motions of stars and galaxies. And physicists have determined that the dark matter cannot (for various noncircular reasons) be made of the same ordinary matter found on Earth.

    SUSY particles have long been one of the most popular proposals for the identity of this cosmic dark matter, based on more complicated notions of symmetry than those available to Plato and Aristotle. And since the onset of the 20th century, symmetry math has generated an astounding string of scientific successes. From Einstein’s relativity to the theory of elementary particles and forces, symmetry considerations now form the core of science’s understanding of nature.  

    These mathematical forms of symmetry are more elaborate examples of symmetry as commonly understood: a change that leaves things looking like they did before. A perfectly symmetric face looks the same when a mirror swaps left with right. A perfect sphere’s appearance is not altered when you rotate it to see the other side. Rotate a snowflake by any multiple of 60 degrees and you see the same snowflake.

    In a similar way, more sophisticated mathematical frameworks, known as symmetry groups, describe aspects of the physical world, such as time and space or the families of subatomic particles that make up matter or transmit forces. Symmetries in the equations of such math can even predict previously unknown phenomena. Symmetry in the equations describing subatomic particles, for instance, revealed that for each particle nature allowed an antimatter particle, with opposite electric charge.

    In fact, all the known ordinary matter and force particles fit neatly into the mathematical patterns described by symmetry groups. But none of those particles can explain the dark matter.

    SUSY particles as a dark matter possibility emerged in the 1970s and 1980s, when theorists proposed an even more advanced symmetry system. That math, called supersymmetry (hence SUSY), suggested the existence of a “super” partner particle for each known particle: a force-particle partner for every matter particle, and a matter-particle partner for every force particle. It was an elegant concept mathematically, and it solved (or at least ameliorated) some other vexing theoretical problems. Plus, of the super partner particles it predicted, the lightest one (whichever one that was) seemed likely to be a perfect dark matter WIMP.

    Alas, efforts to detect WIMPs (which should be hitting the Earth all the time) have almost all failed to find any. One experiment that did claim a WIMP detection seems to be on shaky ground — a new experiment, using the same method and materials, reports no such WIMP evidence. And attempts to produce SUSY particles in the world’s most powerful particle accelerator, the Large Hadron Collider, have also come up empty.

    Some physicists have therefore given up on SUSY. And perhaps supersymmetry has been as misleading as the Greek infatuation with circular motion. But the truth is that SUSY is not a theory that can be slain by a single experiment. It is a more nebulous mathematical notion, a framework within which many specific theories can be constructed.   

    “You can’t really kill SUSY because it’s not a thing,” physicist Patrick Stengel of the International Higher School of Advanced Studies in Trieste, Italy, said at a conference in Washington, D.C., in 2019. “It’s not an idea that you can kill. It’s basically just a framework for a bunch of ideas.”

    At the same conference, University of Texas at Austin physicist Katherine Freese pointed out that there was never any guarantee that the Large Hadron Collider would discover SUSY. “Even before the LHC got built, there were a lot of people who said, well, it might not go to a high enough energy,” she said.

    So SUSY may yet turn out to be an example of symmetry that leads physics to success. But just in case, physicists have pursued other dark matter possibilities. One old suggestion that has recently received renewed interest is a lightweight hypothetical particle called an axion (SN: 3/24/20).

    Of course, if axions do exist, symmetry fans could still rejoice — the motivation for proposing the axion to begin with was resolving an issue with yet another form of symmetry. More

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    The dark matter mystery deepens with the demise of a reported detection

    First of two parts

    In mystery stories, the chief suspect almost always gets exonerated before the end of the book. Typically because a key piece of evidence turned out to be wrong.

    In science, key evidence is supposed to be right. But sometimes it’s not. In the mystery of the invisible “dark matter” in space, evidence implicating one chief suspect has now been directly debunked. WIMPs, tiny particles widely regarded as prime dark matter candidates, have failed to appear in an experiment designed specifically to test the lone previous study claiming to detect them.

    For decades, physicists have realized that most of the universe’s matter is nothing like earthly matter, which is made mostly from protons and neutrons. Gravitational influences on visible matter (stars and galaxies) indicate that some dark stuff of unknown identity pervades the cosmos. Ordinary matter accounts for less than 20 percent of the cosmic matter abundance.

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    For unrelated reasons, theorists have also long suggested that nature possesses mysterious types of tiny particles predicted by a theoretical mathematical framework known as supersymmetry, or SUSY for short. Those particles would be massive by subatomic standards but would interact only weakly with other matter, and so are known as Weakly Interacting Massive particles, hence WIMPs.

    Of the many possible species of WIMPs, one (presumably the lightest one) should have the properties necessary to explain the dark matter messing with the motion of stars and galaxies (SN: 12/27/12). Way back in the last century, searches began for WIMPs in an effort to demonstrate their existence and identify which species made up the dark matter.

    In1998, one research team announced apparent success. An experiment called DAMA (for DArk MAtter, get it?), consisting of a particle detector buried under the Italian Alps, seemingly did detect particles with properties matching some physicists’ expectations for a dark matter signal.

    It was a tricky experiment to perform, relying on the premise that space is full of swarms of WIMPs. A detector containing chunks of sodium iodide should give off a flash of light when hit by a WIMP. But other particles from natural radioactive substances would also produce flashes of light even if WIMPs are a myth.

    So the experimenters adopted a clever suggestion proposed earlier by physicists Katherine Freese, David Spergel and Andrzej Drukier, known formally as an annual modulation test. But let’s just call it the June-December approach.

    As the Earth orbits the sun, the sun also moves, traveling around the Milky Way galaxy, carried by a spiral arm in the direction of the constellation Cygnus. If the galaxy really is full of WIMPs, the sun should be constantly plowing through them, generating a “WIMP wind.” (It’s like the wind you feel if you stick your head out of the window of a moving car.) In June, the Earth’s orbit moves it in the same direction as the sun’s motion around the galaxy — into the wind. But in December, the Earth moves the opposite direction, away from the wind. So more WIMPs should be striking the Earth in June than in December. It’s just like the way your car windshield smashes into more raindrops when driving forward than when going in reverse.

    As the sun moves through space, it should collide with dark matter particles called WIMPs, if they exist. When the Earth’s revolution carries it in the same direction as the sun, in summer, the resulting “WIMP wind” should appear stronger, with more WIMP collisions detected in June than in December.GEOATLAS/GRAPHI-OGRE, ADAPTED BY T. DUBÉ

    At an astrophysics conference in Paris in December 1998, Pierluigi Belli of the DAMA team reported a clear signal (or at least a strong hint) that more particles arrived in June than December. (More precisely, the results showed an annual modulation in frequency of light flashes, peaking around June with a minimum in December.) The DAMA data indicated a WIMP weighing in at 59 billion electron volts, roughly 60 times the mass of a proton.

    But some experts had concerns about the DAMA team’s data analysis. And other searches for WIMPs, with different detectors and strategies, should have found WIMPs if DAMA was right — but didn’t. Still, DAMA persisted. An advanced version of the experiment, DAMA/LIBRA, continued to find the June-December disparity.

    Perhaps DAMA was more sensitive to WIMPs than other experiments. After all, the other searches did not duplicate DAMA’s methods. Some used substances other than sodium iodide as a detecting material, or watched for slight temperature increases as a sign of a WIMP collision rather than flashes of light.

    For that matter, WIMPs might not be what theorists originally thought. DAMA initially reported 60 proton-mass WIMPs based on the belief that the WIMPs collided with iodine atoms. But later data suggested that perhaps the WIMPs were hitting sodium atoms, implying a much lighter WIMP mass — lighter than other experiments had been optimally designed to detect. Yet another possibility: Maybe trace amounts of the metallic element thallium (much heavier atoms than either iodine or sodium) had been the WIMP targets. But a recent review of that proposal found once again that the DAMA results could not be reconciled with the absence of a signal in other experiments.

    And now DAMA’s hope for vindication has been further dashed by a new underground experiment, this one in Spain. Scientists with the ANAIS collaboration have repeated the June-December method with sodium iodide, in an effort to reproduce DAMA’s results with the same method and materials. After three years of operation, the ANAIS team reports no sign of WIMPs.

    To be fair, the no-WIMP conclusion relies on a lot of seriously sophisticated technical analysis. It’s not just a matter of counting light flashes. You have to collect rigorous data on the behavior of nine different sodium iodide modules. You have to correct for the presence of rare radioactive isotopes generated by cosmic ray collisions while the modules were still under construction. And then the statistical analysis needed to discern a winter-summer signal difference is not something you should try at home (unless you’re fully versed in things like the least-square periodogram or the Lomb-Scargle technique). Plus, ANAIS it still going, with plans to collect two more years of data before issuing a final analysis. So the judgment on DAMA’s WIMPs is not necessarily final.

    Nevertheless, it doesn’t look good for WIMPs, at least for the WIMPs motivated by belief in supersymmetry.   

    Sadly for SUSY fans, searches for WIMPs from space are not the only bad news. Attempts to produce WIMPs in particle accelerators have also so far failed. Dark matter might just turn out to consist of some other kind of subatomic particle.

    If so, it would be a plot twist worthy of Agatha Christie, kind of like Poirot turning out to be the killer. For symmetry has long been physicists’ most reliable friend, guiding many great successes, from Einstein’s relativity theory to the standard model of particles and forces.

    Still, failure to find SUSY particles so far does not necessarily mean they don’t exist. Supersymmetry just might be not as simple as it first seemed. And SUSY particles might just be harder to detect than scientists originally surmised. But if supersymmetry does turn out not to be so super, scientists might need to reflect on the ways that faith in symmetry can lead them astray. More