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Darkness Apparent: The Hidden Stuff of the Cosmos

The bright suns I see and the dark suns I cannot see are in their place.

Walt Whitman (Song of Myself)

“I wish you wouldn’t keep appearing and vanishing so suddenly: you make one quite giddy!” said Alice.

“All right,” said the Cat; and this time vanished quite slowly, beginning with the end of the tail and ending with the grin, which remained some time after the rest of it had gone.

Lewis Carroll (Alice’s Adventures in Wonderland)

THE CHESHIRE GALAXY

In Cheshire, cats tend to fade in and out of view—at least according to Lewis Carroll. Recently, astronomers working in that English county discovered that galaxies can similarly hide from sight. In the case of VIRGOHI21, an invisible galaxy detected at the Jodrell Bank Observatory in Macclesfield, Cheshire, only a grin of radio waves reveals its stealthy presence.

The shadowy creature first flashed its smile in a 2004 radio survey of the Virgo cluster. A team led by Cardiff University researchers Robert Minchin and Jonathan Davies found a rotating



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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos 4 Darkness Apparent: The Hidden Stuff of the Cosmos The bright suns I see and the dark suns I cannot see are in their place. Walt Whitman (Song of Myself) “I wish you wouldn’t keep appearing and vanishing so suddenly: you make one quite giddy!” said Alice. “All right,” said the Cat; and this time vanished quite slowly, beginning with the end of the tail and ending with the grin, which remained some time after the rest of it had gone. Lewis Carroll (Alice’s Adventures in Wonderland) THE CHESHIRE GALAXY In Cheshire, cats tend to fade in and out of view—at least according to Lewis Carroll. Recently, astronomers working in that English county discovered that galaxies can similarly hide from sight. In the case of VIRGOHI21, an invisible galaxy detected at the Jodrell Bank Observatory in Macclesfield, Cheshire, only a grin of radio waves reveals its stealthy presence. The shadowy creature first flashed its smile in a 2004 radio survey of the Virgo cluster. A team led by Cardiff University researchers Robert Minchin and Jonathan Davies found a rotating

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos disk of hydrogen atoms approximately 100 million times the mass of the Sun. When they gauged the rotational speed of the pancake-shaped entity, it presented itself as if it were actually 1,000 times heftier—that is, 100 billion times the Sun’s mass. Consequently, the astronomers concluded that more than 99.9 percent of the body is composed of unseen material. Observations shortly thereafter at the Arecibo radio observatory in Puerto Rico confirmed the Jodrell Bank picture. A close examination of the dark colossus found absolutely no trace of stellar objects. Hence, VIRGOHI21 is the first-known completely starless galaxy. The Cardiff team has speculated about the invisible galaxy’s origins. One possibility the team investigated was that it consists of material wrested from other galaxies in a cosmic tug-of-war. However, no nearby galaxies stood in the proper positions for exerting such tidal forces. “If it is tidal debris,” Minchin and his colleagues concluded, “then the putative parents have vanished.” Another possibility the team investigated is that VIRGOHI21 is a gravitationally bound system whose hydrogen is too dispersed to clump into stars. Its scattered pockets of hydrogen may lack the critical density to fuse together and burn. Given the data the researchers found, this seemed to them the most likely option. Using their discovery as a model, they launched a concerted effort to find other invisible galaxies in space, for although VIRGOHI21 is the first-known completely dark galaxy, other galaxies brim with unseen material. This matter forms dark halos around the shining stars, protruding far beyond the visible bounds of these objects. The nature of this dark substance is largely unknown—a long-standing astronomical mystery. Astronomers have suspected for many decades that the visible content of the universe falls far short of the amount that is apparently exerting gravitational force. Hints of this missing mass conundrum date as far back as the early 1930s, when (as we discussed) astronomers Jan Oort and Fritz Zwicky found unexplained behavior

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos in galaxies, and clusters respectively. These ideas sat in the literature for years, however, until the pioneering work of Carnegie Institute astronomer Vera Rubin catapulted the issue to prominence. THE MISSING MASS Vera Rubin seems an unlikely revolutionary. The thick-spectacled, soft-spoken astronomer appears more the patient teacher than a radical firebrand. She has a methodical way of speaking that conveys her impressive attention to detail. Her research advisor, George Gamow, was, in contrast, quite a showman. He loved to make bold claims and was not always so careful in his statements. He could be loud, funny, and boastful. A prolific amateur cartoonist, he relished teasing his colleagues through clever sketches and verse. Despite the difference in styles, they shared an exceptional interest in pedagogy. She found him a brilliant lecturer with wonderful intuition, who even in his bravado often turned out to be prescient. This had a profound influence on her own career, which was geared toward education as well as discovery. (Perhaps there is no stronger statement of Rubin’s ability to project enthusiasm for learning than the fact that all four of her children have Ph.D.s.) Indeed, Rubin came upon dark matter while developing an assignment for her students. Eight years after she received her doctorate from Georgetown under Gamow’s tutelage, she was experimenting with creative ways of conveying information to her classes. As she recalled: In 1962, with my students at Georgetown University, we were looking at the [astronomical] literature. I was teaching at the graduate school. Most of my students worked at the Naval Observatory. I decided to have a class project in which we scanned stars beyond the solar system.

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos Rubin and her students plotted the velocities of stars in the outer reaches of the Milky Way versus distances from its central bulge. They fully expected velocity curves that would drop off with radial distance, like the foothills that surround a lofty mountain. Newton’s laws, applied to concentrated systems, mandate such behavior. They proscribe slower speeds for the outermost orbits, where gravity tugs weakly. For example, distant Neptune revolves around the Sun at a far more languid pace than does Earth. By similar reasoning, the Georgetown class anticipated that remote stars in the Milky Way, far from its dense hub, would lag closer objects. Wholly expecting velocity “foothills,” the group was astounded when the velocity curves turned out to be “plateaus.” They found no dropoff in speed, no matter how far from the galactic center they looked. Hence, the stars in the periphery of the galaxy moved much faster than they ought to, given the known matter distribution in the Milky Way. Some hidden substance seemed to exert an extra pull. As Rubin described this result, “The interesting thing about dark matter is that in order to have a flat rotation curve we have to have matter that we do not see, but [that is] spread farther out.” Rubin and the students wrote up their results in an article and submitted it to the prestigious Astronomical Journal. Shortly thereafter, Rubin received a disturbing call from the journal’s editor. He was adamant that he wouldn’t publish a paper with the names of students on it, which would break the journal’s strict policy. Rubin insisted on keeping the names. The paper was published anyway, setting a marvelous precedent. It took virtually another decade for the astronomical community to recognize the magnitude of the problem. By then numerous studies by Rubin and others had demonstrated that other galaxies similarly possess flat rotation curves; over vast distances from their centers, the velocities of their stars do not taper off but rather remain fairly level. Astronomers came to realize that something was drastically wrong. From one galaxy to another, and even in the spaces between, a huge portion of the cosmic bulk was simply invisible.

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos Rubin encountered controversy about whether or not her discoveries applied to less prominent galaxies. “Many astronomers said to me, ‘If you look at faint galaxies, there’s no dark matter,’ but actually the opposite is true.” The difference between VIRGOHI21 and the galaxies examined by Rubin is that, while the latter have dark halos comprising approximately 90 percent of galactic mass, the former appears wholly dark. Nevertheless, masked operators seem to drive all these cosmic carousels. What are their secret identities? GRAVITY’S LENS One prominent category of dark-matter candidates has a name befitting its supposedly Herculean strength. Called MACHOs (Massive Astronomical Compact Halo Objects), it consists of bodies (in the peripheries of galaxies) thought to be gravitationally powerful en masse but each too lightweight to shine. Like an army of tiny ants dragging fruit off a picnic table, these diminutive orbs would tug vigorously on visible stars. Conceivable MACHO types include brown dwarfs (nonshining stars), red dwarfs (very dim stars), white dwarfs, large planets, neutron stars, and black holes—in short, anything lacking a supply of hydrogen sufficiently concentrated to light its nuclear furnace. In 1986, Princeton astrophysicist Bohdan Paczynski proposed a clever way of searching for and classifying potential MACHOs. Using the Einsteinian result that gravity bends light, he developed the technique of gravitational microlensing. Ordinary gravitational lensing occurs when an extremely massive object, such as a galaxy, stands directly in the path of emissions from a more distant body, such as a quasar. Rays from the latter bend around either side of the former, resulting in a double image (and sometimes even a multiple image). Astronomers literally see “twins” of the remote system they’re observing. If the lensed object and the lens lie directly on the same line of sight, then instead of multiple copies, a circular image appears called

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos an “Einstein ring.” The radius of the ring depends on the gravitational distortion of the matter bending the light. Therefore, it provides a means of gauging how much mass lies in the light rays’ path. Pacyznski adapted this method to account for minute concentrations of mass, showing that a customized form of gravitational lensing would be a suitable way of discerning small, otherwise invisible, bodies in galactic halos or elsewhere. The microlensing effect would be much more subtle than large-scale lensing—involving a slight brightening and dimming as the MACHO passed by—but still potentially discernible by computer-aided instruments. Using gravitational microlensing to identify intervening MACHOs is akin to employing an eye chart to find the best-fitting glasses. In optometry, if an image looks distorted, the cause of such blurriness can be inferred and then corrected. Similarly, in astronomy any change in the appearance of a star could indicate the fleeting distortion caused by an intervening object. Working backward from the image, we can deduce the properties of the unseen agent, particularly its mass and size. Armed with this powerful technique, several teams of astronomical detectives set out to sleuth for MACHOs in the early 1990s. One collaboration, headed by Charles Alcock and simply called the “MACHO group,” conducted an extensive scan of millions of stars in the Large Magellanic Cloud (LMC)—a small satellite galaxy of the Milky Way. Another team, called EROS (Expérience de Recherche d’Objets Sombres—French for “The Experimental Search for Dark Objects”) focused on both the LMC and the Small Magellanic Cloud—scanning a similarly vast array of stars. Each group looked for evidence of brightness variations, caused by invisible objects passing between the stars and Earth. The immense numbers of stars needed for these surveys derived from the tremendously low odds that any one of them would be lensed by a MACHO. Both the star and the MACHO would need to line up almost perfectly (within one milliarcsecond of angle, to be

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos specific)—an extremely rare coincidence. Observing millions of stars offered the teams much higher chances for success. Also, the greater the number of observations, the easier it was for the researchers to eliminate false calls. Many stellar bodies naturally vary in brightness—for example, the Cepheid variable stars employed by astronomers to gauge distances in the universe. The scientists needed to identity all these events (according to their characteristic profiles) and exclude them from their data. Only then could they be reasonably certain they were witnessing true MACHO microlensing. In 1993 the teams were excited to report the first candidate objects. The solution to the dark-matter problem, at least for galaxies, began to seem close at hand. As the decade wore on, however, a glaring discrepancy opened up between the two groups’ reports. The MACHO collaboration collected a bevy of positive results, pointing to an abundance of halo objects in the LMC. The typical mass of these bodies, about half that of the Sun, suggested they were stellar dwarfs (of some sort) radiating too weakly to detect. EROS, at first, found very few such objects. Thus, while the MACHO group’s statistics portrayed galactic dark matter as consisting mainly of dim halo objects of half-solar size, the EROS team explicitly ruled out that possibility. Gradually, though, the MACHO collaboration began to downsize its estimates—drawing closer to what EROS found. Today, astronomers believe that only about a fifth of galactic dark matter is comprised of such dim stellar bodies—the composition of the remainder is still unknown. What’s more, identifying the missing material in galaxies is only a small facet of a much greater mystery. Unseen gravitational influences occur on all known astronomical scales. Therefore, it is clear that considerable quantities of dark matter lie beyond galaxies—in the spaces between them in clusters and in the voids between clusters themselves. Furthermore, evidence indicates that most of the invisible substance is non-baryonic in nature. Baryons are the stuff of atomic nuclei—protons, neutrons, and such. If the major part of the

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos unseen material excludes atoms, it can’t possibly be anything like stars. It must be something much more slippery to have thus far escaped detection. CAPTURING THE NEUTRINO Scientists often classify dark matter into two distinct categories based on its thermal properties: hot and cold. Hot dark matter involves fast-moving particles, such as neutrinos. Individually, these particles have negligible mass. They are so abundant, however, that even with small nonzero masses they’d collectively produce a significant gravitational effect. Cold dark matter, in contrast, consists of slower-moving materials. MACHOs are often placed in this category. Non-baryonic examples include various classes of hypothetical particles, called axions and WIMPs (weakly interacting massive particles). The latter is a play on words—contrasting these diminutive constituents with “manly,” stellar-sized MACHOs. Axions, WIMPS, and neutrinos are thought to be largely unseen because they interact so rarely with ordinary matter. Neutrinos, for instance, pass straight through Earth all the time. They are extraordinarily common particles, but because they are lightweight, electrically neutral, and impervious to the strong nuclear force, they have few opportunities for contact with other matter. For the most part they are oblivious to their surroundings, like someone in a sensory deprivation chamber. Their main mode of interaction lies in the weak nuclear force—through, for instance, the process of beta decay. Identified but not fully understood in the late 19th century, beta decay is the name given to a process wherein neutrons break down into protons, electrons, and neutrinos. (Until the time of Pauli, physicists didn’t know about neutrinos; he inferred their existence through conservation principles.) It is a common process—occurring, for example, during the transformation of radioactive isotopes.

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos The main source of neutrinos passing through Earth, though, is not radioactivity but rather the Sun, through its cycles of fusion. Inside the Sun’s churning cauldron, the wispy particles are produced in great abundance. Passing though the Sun’s outer layers and released into space, they rain down continuously on Earth and the other planets. Originally, physicists thought these particles were completely massless. Special relativity implied, therefore, that like massless photons they moved at the speed of light. Furthermore, they were thought to consist of only a single variety—as uniform as the desert sands; but puzzling results from neutrino detectors capturing emissions from the Sun would eventually challenge these assumptions. In 1967, Raymond Davis of the University of Pennsylvania inaugurated the first observatory designed for collecting solar neutrinos. The apparatus was very simple—a mammoth tank filled to the brim with 100,000 gallons of chlorine-based cleaning fluid. As endless droves of neutrinos swarmed through the tank, each had a slim chance of colliding with one of the chlorine atoms and transforming it into radioactive argon. The new isotope would then announce itself through a characteristic emission. Based on the average spacing between atoms, the size and geometry of the tank, Earth’s distance from the Sun, the dynamics of solar processes, and other factors, Davis and his assistants could then compare the expected influx of neutrinos with the actual number of events. If the tank had been exposed to the atmosphere, the experiment would have been hopeless. Cosmic rays of all sorts bombard our planet all the time. It would be impossible to know which collisions were the “real deal” and which were phonies. Some kind of “bouncer” was required to keep the hoi polloi out of the club and let in only well-pedigreed neutrinos. Fortunately, the experimenters found a natural solution. They realized that, if they placed the apparatus deep underground, Earth’s mighty strata of rocks and soil would keep a firm guard against unwanted intruders. Only neutrinos would have

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos the slinky skills required to penetrate the rocky layers. Thus, the team lowered the tank into the everlasting darkness of the Homestake gold mine, almost a mile beneath the Black Hills of South Dakota. After the experiment was running for some time, Davis began to notice a severe discrepancy between the expected and actual numbers of neutrinos. Something like two-thirds of the anticipated crowd simply wasn’t showing up. There was either a fatal flaw in the apparatus or a gross misunderstanding of how solar dynamics worked. When subsequent experiments at other subterranean detectors around the world confirmed the deficit found at Homestake, researchers began to rethink their suppositions. Theorists dusted off certain alternative neutrino theories that imagined them as more versatile characters. Could the neutrinos produced along with electrons in beta decay represent only one of several different types? Might there be a special kind associated with muons (particles similar to electrons but considerably heavier) as well? Later, with the discovery of tauons (even heavier than muons), this scheme was augmented to include three different types: electron neutrinos, muon neutrinos, and tau neutrinos. Perhaps under particularly intense conditions, such as those of the Sun’s nuclear furnace, neutrinos “oscillated,” or changed from one breed into another. If that were the case, it would explain why the bulk of solar neutrinos could not be detected—they were simply wearing different guises. Furthermore, neutrino oscillation models imply that they must possess different masses. In transforming, they shift from one rung of a ladder of masses to another. Thus, they cannot be absolutely massless. This realization, combined with a fervent interest in resolving the dark-matter quandary, set off a race to pin down the masses of the neutrino varieties. Recognizing that these values would be extremely small, researchers hoped that delicate statistical measures could help distinguish them from zero. Three neutrinos, with various degrees of heft, would offer a handy trio of dark-matter components. If the combined weights of electron neutrinos turned out to

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos be insufficient to address the missing-matter dilemma, could muon and tauon neutrinos offer enough bulk to do the job? These burning questions motivated the construction of the largest neutrino detector to date, the Sudbury Neutrino Observatory (SNO) in Ontario. A converted nickel mine, more than a mile and a quarter beneath the Canadian soil, houses a gargantuan acrylic vessel filled with 1,000 tons of ultrapure heavy water (with deuterium instead of hydrogen) surrounded, in turn, by a reservoir of ordinary water. Thousands of photomultiplier tubes (high-precision light sensors), arranged like sentries around the tank, stand guard for the unique flashes of neutrino collisions. Each type of neutrino, as it slams into a deuterium atom, produces a characteristic signature. These signals are collected and statistically analyzed, offering a sample of the Sun’s varied output. In 2001 the SNO collaboration—a team of Canadian, American, and British scientists headed by Art McDonald of Queen’s University—announced the first results. In a stunning breakthrough, they found enough events to resolve the solar neutrino problem and prove that these particles come in three varieties. This finding, along with results by the Liquid Scintillator Neutrino Detector experiment at Los Alamos, helped establish the mass differences between each of the types. Based on these and other critical results from around the world, today scientists believe that the neutrino trio constitutes a segment, but not a major component, of the dark matter in the universe. Even tallying neutrinos along with MACHOS yields far too little mass to fill the gap. Attention has shifted to some of the other candidates— particularly axions and WIMPs. MASKED MARAUDERS Like the Mixmaster universe, axions are whimsically named after a commercial product—in this case a brand of laundry detergent. Particle physicist Frank Wilczek couldn’t resist the opportunity to

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos searches would be inundated with a flood of extraneous particles from space. Countless superfluous events would need to be screened out. Therefore, subterranean locales, such as caverns and mines, offer greater protection against cases of mistaken identity and improved chances for genuine results. For example, one leading search for WIMPs is based under the Oroville Dam (California)—the tallest in the United States. Another is set more than two-thirds of a mile underground in the Boulby salt mine (North Yorkshire, England)— the deepest in Europe. Even in the perpetual darkness, however, unwanted particles can intrude, so researchers try to choose locales far away from known veins of radioactive ore. As an added precaution, they often house their apparatus in thick layers of either lead or copper. Finally, some experimenters have dramatically reduced thermal noise by cooling their detectors close to absolute zero. The main method for detecting WIMPs is called nuclear recoil, which involves rare collisions that cause atomic nuclei to shift slightly back and forth, giving off photons in the process. The minute quantities of energy released in such jolts can be detected in certain standard ways, including ionization and scintillation. The former method, used at Oroville, entails the release of outer electrons from atoms. Germanium, a hard, gray-white material, has been found particularly effective for this purpose. Scintillation, on the other hand, involves special types of material that absorb energy and release detectable flashes of light. The Boulby team, for example, has used two types of scintillating materials: sodium iodide and liquid xenon. Photomultipliers placed around these substances can pick up their characteristic flashes. Computers then analyze these signals to discern the possible signatures of WIMP collisions. Currently, most researchers working in the field are still waiting for the first signs of WIMPs. However, a group from Italy, called DAMA (particle DArk MAtter searches with highly radiopure scintillators at Gran Sasso), has already claimed success. Led by Rita

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos Bernabei of the University of Rome, the team first announced positive preliminary findings in 1996. They have since updated their methods and have continued to publish affirmative results. The DAMA technique focuses on the possibility of a WIMP wind from the galactic halo. Our planet’s annual revolution around the Sun and the motion of the solar system through the Milky Way would make this wind periodic. Certain times of the year we’d be speeding into it, and other times we’d be heading away from it. Thus, it would manifest itself as a cyclic flux (amount per area) of weakly interacting particles pounding down on Earth at a variable rate. It is this fluctuating storm of WIMPs that the Italian group claims to have detected. But are the DAMA detectives on the right track? Close on their tail has been another group, conducting the Cryogenic Dark Matter Search (CDMS) experiment, based originally in a tunnel under Stanford University and now running in the Soudan mine in Minnesota. That team has conducted a comparable analysis with its own detectors and has reached a diametrically opposite conclusion. Not only do the CDMS researchers see no evidence of an annual modulation, they have also conjectured that the particles found by DAMA could be ordinary neutrons, rather than exotic particles. Both groups are currently gearing up for the next stages of the battle, showing that WIMP research is not for wimps. Regardless of whether or not WIMPs have actually been observed, theorists have lined up an ample selection of other candidates, including bloated massive neutrinos of an unknown sort, bosons beyond the current radar screen of detection, and especially sparticles, the massive supersymmetric companions of ordinary particles. SPARTICLES AND SHADOW WORLDS Supersymmetry is a theoretical link between the two great kingdoms of particles: fermions and bosons. An example of the former is an

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos electron and of the latter a photon. Basically, fermions comprise the constituents of ordinary matter, while bosons relate to the forces between them. The main difference between the two types is spin. In this context, spin relates to a particle’s set of possible transformations in an abstract space. Some particles, such as the Higgs, are always in the same spin state, called spin zero. Others, such as electrons, have two possible spin states, called up and down (also known as +1/2 and –1/2). Yet others have three (–1, 0, +1) or four (–3/2, –1/2, +1/2, +3/2) possibilities. Fermions are defined as those particles with half-integer spins—granting them an even number of possible spin states. Bosons, in contrast, possess integer spins—generally allowing them an odd number of possible states. In traditional field theory, no matter how hard you tried, you couldn’t transform a half-integer spin state into an integer state. Quantum physicists accommodated to this principle long ago and developed separate statistical methods for each category. This seemed satisfactory until the advent of string theory. String theory arose in the early 1970s as a description of the strong force but has since been framed as a road to unifying all natural interactions. In its original incarnation, it described bosons as flexible “rubber bands” of energy and fermions as their end points. This description was intended to represent the property of the strong force to restrain nuclear particles if they move too far away from each other, but it allows them abundant freedom if they are relatively close. Mathematical analyses of bosonic strings demonstrated how their vibrations could represent various energy levels. This is similar to how various vibrations of stringed instruments produce distinct pitches. If music has harmonics, why not the fundamental constituents of nature? Given the neat mathematics of bosonic strings, physicists began to wonder if fermions could be described similarly. The problem was how to transform an integer spin theory into a half-integer spin theory. Pierre Ramond, of the University of Florida, put forth an

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos intriguing solution. He postulated a hitherto-unseen symmetry of nature, encompassing both fermions and bosons and allowing for half-integer spin transformations from one to the other. Jean-Loup Gervais and Bunji Sakita of the City College of New York independently proposed a related version around the same time, which was further developed by Julius Wess of the University of Munich and Bruno Zumino of CERN. Because it superseded known symmetries, this clever transformational scheme became known as supersymmetry. Soon, supersymmetric string theory, or “superstrings” for short, acquired acclaim in some circles and notoriety in others, when physicists John Schwarz and Joël Scherk showed that it naturally predicted a spin-two particle with particular properties. If you identify this particle as the carrier of the gravitational force, the resulting theory yields the equivalent of general relativity. Proposing superstrings as a candidate “theory of everything,” Schwarz speculated that it could unite other interactions with gravity into a single model. Meanwhile, Wess, Zumino, and other theorists applied supersymmetry to standard particle physics (without strings) to produce a generalization of general relativity called supergravity. Supergravity’s star rose in the late 1970s and early 1980s until researchers in the field encountered a host of formidable mathematical difficulties. Then in 1984, Schwarz, along with Michael Green of the University of London, demonstrated in an influential paper that superstring theory was free of many of these ailments, bolstering its status. Encouraged by Schwarz and Green’s results, a number of prominent physicists joined the superstring bandwagon. With its alluring mathematical properties, superstring theory seemed to offer a whiff of the physics of the future. Leading physicist Ed Witten proclaimed that “string theory is a piece of 21st century physics that happened to fall into the 20th century.” Nevertheless, many traditional physicists expressed strong doubts. Dismayed by the poor prospect for experimental confirma-

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos tion of the theory any time in the conceivable future, they cringed at the thought of losing a generation of theorists to such a speculative approach. As journalist John Horgan said: Unfortunately, the microrealm that superstrings allegedly inhabit is completely inaccessible to human experimenters. A superstring is supposedly as small in comparison to a proton as a proton is in comparison to the solar system. Probing this realm directly would require an accelerator 1,000 light-years around. This problem led the Nobel laureate Sheldon Glashow of Harvard University to compare superstring theorists to medieval theologians. A continuing subject of controversy concerns the strange doppelgangers predicted by any supersymmetric theory—not just strings. Applied to standard elementary particles, supersymmetric methods yield counterparts with opposite spin properties. Physicists denote these hypothetical companions by tacking on an initial “s” or a final “ino”—referring, respectively, to the bosonic equivalents of fermions or the fermionic mates of bosons. Hence, a supersymmetric twist turns quarks and electrons into squarks and selectrons and transforms photons and W particles into photinos and winos. The supersymmetric companions of particles are in general known as sparticles. None of these particle counterparts have thus far been found in nature—which some see as a failing and others as an opportunity. Does the lack of sightings signify that supersymmetry is based on a false premise? Or does it simply mean that we haven’t yet searched at high enough energies? If the latter is true, supporters argue, sparticles would be extremely massive and could well constitute the missing ingredient of the universe. Foremost among the supersymmetric WIMP candidates is a peculiar hybrid called the neutralino. Rather than the partner of a single particle, it is the supersymmetric soulmate of several different

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos bosons. (Apparently polygamy is allowed in the particle world.) It forms an amalgamated companion of the photon (photino), Z boson (zino), and Higgs boson (higgsino), mixed together in a quantum state. The lightest version of the neutralino fits the profile of a dark-matter candidate well. First, unlike numerous short-lived particles, it is believed to be stable. Second, it is thought to interact only through the weak and gravitational interactions. Finally, its mass is in the appropriate range. For these reasons, many physicists have tagged it as a likely bet. In coming years we’ll see if they win their wagers. Though WIMPs are bizarre, the minds of theorists have produced even stranger possibilities. Consider the case of shadow matter: particles that respond only to gravity. They’d be invisible not just to optical telescopes but to all light-sensing instruments. Participating in no known decays or interactions, they’d be imperceptible to standard detectors. Neither chemical transformations nor nuclear recoils would herald their passage through Earth. Only extremely sensitive gravity wave detectors, well beyond current capability, would stand a chance of revealing these ghosts. One hypothetical type of shadowy material, called “mirror matter,” was proposed as a dark-matter candidate by physicists Rabindra Mohapatra and Vigdor Teplitz in 1999. Mirror matter would consist of particles of opposite chirality (handedness) than their counterparts in the conventional particle world—for example, right-handed “mirror neutrinos” corresponding to ordinary left-handed neutrinos. Symmetry principles would preclude electroweak interactions between mirror particles and garden variety materials; only gravity could provide a connection. Thus, a particle Alice could never step foot in the looking-glass world; she could only sense its gravitational pull. Some researchers have speculated that somewhere in space— maybe even in the “Cheshire galaxy”—entire planets could be shaped from substances impervious to light. Well beyond our awareness,

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos shadow aliens could be conducting their daily business, eating their shadow food, and basking in the unseen energy of their shadow stars. They would likewise be oblivious to our own type of matter—until, someday, pulsating gravitational signals link our disparate civilizations. Such an event would be one of the most outlandish resolutions of the dark-matter dilemma. THE ESSENCE OF QUINTESSENCE The discovery of cosmic acceleration has triggered a search for yet another type of missing substance—more precisely, an unknown source of energy. In many ways, dark energy is even more mysterious than dark matter. No material with which we’re familiar exhibits an antigravitational force. We have discussed the repulsive properties of hypothetical objects with negative mass—but imagine a force with enough muscle to push all the mass in the universe apart! In H. G. Wells’s classic novella, The First Men in the Moon, he describes a substance called “Cavorite” that enables spacecraft to overcome gravity and effortlessly lift off from Earth. In the tale an inventor named Cavor “believed that he might be able to manufacture this possible substance opaque to gravitation out of a complicated alloy of metals and something new…. If one wanted to lift a weight, however enormous, one had only to get a sheet of this substance beneath it and one might lift it with a straw.” Could quintessence represent a kind of Cavorite that is able to counteract universal gravitation and accelerate the universe? Is it possible that this substance dominates certain phases of the universe but not others? Could its influence even be growing in strength? Many physicists don’t think of dark energy as a substance at all, in the traditional sense. Rather, they view it as a vacuum energy— the impact of the sea of virtual particles that pop in and out of the froth. From this perspective—rather than an independent, detectable quintessence—it is simply the lambda term, an essential quantum feature of space.

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos Indeed, long before the discoveries of Perlmutter, Schmidt, and their colleagues, the cosmological-constant issue was well familiar to field theorists. They came upon the problem from a different direction—their calculations produced far too high a value. Yet at the time most cosmologists assumed that lambda must be zero. How could these disparate values be reconciled? As Alan Chodos, associate executive officer of the American Physical Society, has remarked, “The old question was why is it zero? Now it is, why is it almost zero and incredibly tiny?” This dilemma perplexed numerous researchers, such as the young Indian physicist Raman Sundrum, currently at Johns Hopkins. Sundrum carried out his Ph.D. studies of this issue under Lawrence Krauss and Mark Solvay at Case Western University well before the discovery of universal acceleration. As he recounted: At the time there were only bounds on the acceleration, nobody had actually seen acceleration. We saw the expansion, but not the acceleration. But these bounds were already a problem in the sense that the bounds said, whatever the acceleration was it was very small, whereas theory preferred very big. And so there was already a puzzle, that got a lot more interesting when we actually saw that there was not just a bound, but actually some finite acceleration. Sundrum delved into this riddle with gusto, trying to find an explanation in the realm of field theory. Each particle model carried with it a gumbo of masses, interaction strengths, and other parameters. By stirring these ingredients, you could try to cook up the most savory stew—matching as much as possible the flavor of observed astronomical data. In particular, you might create the magic broth that yields a delectable value of lambda. As Sundrum realized, “The cosmological constant is incredibly sensitive to microscopic physics.” Standard field theories, however, generally serve up whopping plates of lambda, too large for general consumption. Finding these

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos theories unpalatable, theorists sought a way of balancing out this excess with a factor that contained a negative cosmological constant. Such considerations led various researchers, including Sundrum and physicist Lisa Randall, to postulate the background geometry of the cosmos as a type of manifold called an “anti-de Sitter space.” Such a space is warped (along a fifth dimension) rather than strictly flat. With deep connections to new versions of superstring theory, particularly the approach known as M-theory, the Randall-Sundrum model represents a popular new means of addressing the dark-energy dilemma. Many field-theoretical approaches (including Randall-Sundrum) postulate that gravity takes a different form on various scales, which would naturally explain why gravity seems strictly attractive on the local level (the solar system, say) but harbors a repulsive component much farther out. Physicists like Eric Adelberger of the Eöt-Wash group have engaged in high-precision torsion balance testing of this hypothesis but have found no deviation so far from the standard law of gravity. Nevertheless, theorists have pressed on with a variety of alternative gravitational models. THE BIG RIP If antigravity turns out to be a dynamic property of the universe, one of the most frightening possibilities is it snowballing beyond control. The scenario unfolds like this. In the early universe, matter dominated dark energy. As the universe expanded, dark energy caught up and eventually slightly tipped the balance. But suppose this growth is far from over, releasing new reservoirs of repulsive force. Over time, gravity would increasingly cower before its towering competitor. Large structures such as clusters, then galaxies and smaller entities, would break apart. Ultimately, this would lead to the complete decimation of every shred of material in the universe— in other words, a “Big Rip.”

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos The Big Rip joins the wailing chorus of other apocalyptic scenarios, including the “Big Crunch” and the “Big Whimper.” Such dire circumstances—though in the far, far future—attract great interest. Like drivers slowing down to view a wreck or watchers of disaster movies, most people are curious about catastrophes. The demise of everything in existence certainly fits the bill. Which will it be then—a splitting apart, a smashing together, or a more quiescent ending? Is being torn into pieces worse than being pulverized to a point? Clearly that is a matter of personal preference. Fortunately, we don’t have to worry about this for many billions of years—long after the death of the Sun, when all life on Earth will presumably be extinguished. One of the major differences between the various scenarios has to do with the prospect for communication with other galaxies before the ultimate cataclysm. What if there are alien civilizations beyond the Milky Way attempting to contact kindred beings? Suppose their beacons were somehow powerful enough to reach us, albeit in the far future. Could we ever hope to receive their signals? The answer depends on the relative motion of galaxies in space. In the Big Crunch scenario, there would eventually be a limit to galactic recession. Billions of years from now, distant galaxies would stop traveling away from us and start to move closer. From that point on, they’d gradually become more prominent in the sky, facilitating the possibility of intergalactic communication. As the universe grew smaller and smaller, perhaps proximity (and necessity) would spur an intergalactic civilization. Conceivably, the greatest minds in such a culture would join together in an attempt to circumvent the complete destruction of the universe. Combating universal catastrophe would be a less urgent matter in the Big Whimper picture, which represents a slow, continued expansion. Thus, perhaps, the need for intergalactic cooperation to address a common danger would be less immediate. That is fortunate, because as the universe continued to grow, its galaxies would be

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos increasingly dispersed. The sky would be dotted with fewer and fewer spirals, ellipticals, and other ensembles of stars. Eventually all that could be seen, even with a telescope, would be the local group of galaxies; the others would lie beyond visual range. Consequently, any alien signals sent by remote extragalactic civilizations would never reach us. We’d be cosmic hermits, separated from other systems, until all the stars in the Milky Way burned out—turning into white dwarfs, neutron stars, and black holes. When doomsday finally arrived, it would be extraordinarily dark and lonely. The road to a Big Rip would greatly exacerbate this isolation. The lambda force would pull the universe apart at breakneck speed, like a glutton attacking a bucket of chicken wings. Sooner than in the previous scenario, signals would be unable to span the increasingly formidable gaps. Hence, if we have any chance of intergalactic communication we’d best attempt it expeditiously; otherwise, we may someday wake up and find that it is too late. If it seems ambitious to talk about possible events billions of years hence, you’ve perceived correctly. With the ease of meteorologists forecasting one or two days ahead, cosmologists feel comfortable projecting eons into the future. Yet if you read the fine print, many of the predictions are based on the proposition that the known laws of nature, as measured from Earth, must hold true for all places and all times. But what if the principles of nature themselves evolve, like the stunning metamorphoses of “Darwin’s finches” on the Galapagos Islands? Unless we perfectly understood these changes, we’d be in little position to make projections. Indeed, some of the exciting new cosmological theories posited to resolve the dark-matter and dark-energy riddles are based on the astonishing notion that nature’s very “constants” could alter throughout the ages.