Jump to navigation Jump to search
Revision as of 01:52, 18 February 2021 by Netfreak (Created page with "<pre> UNRAVELING UNIVERSE Is the cosmos younger than the stars it contains? Was Einstein's biggest blunder not a mistake? Here's why cosmology is in c...")
UNRAVELING UNIVERSE Is the cosmos younger than the stars it contains? Was Einstein's biggest blunder not a mistake? Here's why cosmology is in chaos Time Magazine (Mar 6) Tod Lauer is starting to feel more than a little fed up with his fellow astronomers. Not long ago, Lauer and his close friend and collaborator Marc Postman, of the Space Telescope Science Institute, in Baltimore, Maryland, announced the results of a telescopic study they had been working on for more than a year. The young scientists reached the astonishing conclusion that rather than expanding outward in a stately fashion like the rest of the universe, a collection of many thousands of galaxies, including our own and spanning a billion light-years or so, may be speeding en masse toward a point somewhere in the direction of the constellation Virgo. Yet rather than try to assimilate this new finding, most of their colleagues are proclaiming that it must be a mistake. No one can explain what Lauer and Postman might have done wrong, despite strenuous efforts to do so. The analysis is incorrect, they say, simply because it doesn't fit in with any existing theory of how the cosmos works. ''Listen,'' fumes Lauer, who is stationed at the National Optical Astronomy Observatories in Tucson, Arizona, ''we knew this was a shocking result. That's why we spent over a year trying to debunk it ourselves before we went public. If anyone can present a good argument why it's wrong, we'll listen.'' Allan Sandage is angry at his astronomical brethren too, but his beef is just the opposite of Lauer's. The Carnegie Observatories astronomer has spent much of his nearly 40-year career trying to measure the age of the universe; it's a task he inherited from his mentor Edwin Hubble, the legendary scientist who discovered that the universe is expanding and that galaxies exist beyond the Milky Way. For decades, Sandage's results have suggested that the cosmos is 15 billion to 20 billion years old or thereabouts. That fits beautifully with cosmological theories -- but almost nobody believes him anymore. Instead they're listening to a young whippersnapper named Wendy Freedman, who happens to work just down the hall from Sandage at the Carnegie's center in Pasadena, California. Freedman and a group of colleagues have lately used the Hubble Space Telescope to peg the age at somewhere between 8 billion and 12 billion years -- which would make the cosmos 2 billion years younger than some of the stars it contains. ''Our opponents,'' says Sandage bitterly, ''are so wonderfully kind. They say we don't have anything to stand on.'' Tension between theory and observation is part of the normal course of science. It keeps both sides honest, and, at those rare times in history when the two lock horns irreconcilably, it can lead to nothing less than a full-fledged scientific revolution. Without such clashes, in fact, we'd still believe that the sun orbits Earth and that disease is caused by evil spirits. But what's happening these days in cosmology -- the study of the universe -- verges on the bizarre. Astronomers have come up with one theory-busting discovery after another, hinting that a scientific revolution may be close at hand. At stake are answers to some of the most fundamental questions facing humanity: What is the origin of the universe? What is it made of? And what is its ultimate destiny? Nobody can say what the turmoil means -- whether the intellectual edifice of modern cosmology is tottering on the edge of collapse or merely feeling growing pains as it works out a few kinks. ''If you ask me,'' says astrophysicist Michael Turner of the Fermi National Accelerator Laboratory, near Chicago, ''either we're close to a breakthrough, or we're at our wits' end.'' WEIRD DATA, WEIRD THEORIES: The bewildering discoveries by Lauer, Postman, Freedman and company are only the latest in a barrage of bafflements that stargazers have had to absorb lately. Over the past few years, astronomers have uncovered the existence of the Great Wall, a huge conglomeration of galaxies stretching across 500 million light-years of space; the Great Attractor, a mysterious concentration of mass hauling much of the local universe off in the direction of the constellations Hydra and Centaurus; Great Voids, where few galaxies can be found; and galaxies caught in the throes of formation a mere billion years after the Big Bang, when they should not yet exist. ''If we really trust the data,'' exclaims Stanford astrophysicist Andrei Linde, ''then we are in disaster, and we must do something absolutely crazy.'' That's a big ''if.'' Observes David Schramm, a theoretical astrophysicist at the University of Chicago: ''Whenever you're at the forefront of science, one-third of the observational results always turn out to be wrong.'' But this hasn't stopped the theorists from doing crazy things anyway; they've proposed one mind-stretching idea after another to explain what's going on. One of these was inflation theory, which says the universe expanded like a balloon on amphetamines before the cosmos was one second old. Then there was cold dark matter, hypothetical subatomic particles that may account for 99% of the mass of the universe and may relegate ordinary atoms -- and the stars, planets and people they make up -- to the status of a cosmic afterthought. Another notion described distortions in the very fabric of space and time, going by the name cosmic strings and cosmic textures. And lately theorists have revived an old idea known as hot dark matter, and an even older one called the cosmological constant. The latter is a kind of cosmic antigravity that gives the expanding universe an extra outward push; it was first conceived by Albert Einstein himself, who then rejected it as ''the greatest blunder of my life.'' Each of these ideas is still floating around, championed by its own corps of diehards. In fairness, it must be acknowledged that cosmologists have had very little information to go on, at least until very recently. The distant galaxies that bear witness to the universe's origin, evolution and structure are excruciatingly faint, and it takes every bit of skill observers have to tease out their secrets. It hasn't been until the past decade, in fact, that astronomers have had powerful telescopes like the Hubble out in space and the Keck atop Hawaii's Mauna Kea, ultrafast supercomputers and super-sensitive electronic light detectors to give them the data they hunger for. In a very real sense, cosmology has only lately crossed the dividing line from theology into true science. Cosmologists can now say with some confidence that the universe started out in a very hot and very dense state somewhere between 8 billion and 25 billion years ago, and that it has been expanding outward ever since -- the Big Bang in a nutshell. They believe galaxies are strewn around the cosmos not randomly but according to a pattern that includes some patches with lots of galaxies and others with very few. They believe the universe is pervaded by mysterious dark matter, whose gravity has dominated cosmic history from the start. But beyond that, things get murky. The experts don't know for sure how old or how big the universe is. They don't know what most of it is made of. They don't know in any detail how it began or how it will end. And, beyond the local cosmic neighborhood, they don't know much about what it looks like. Each of these questions is now under study; each bears directly on the others; and each could yield within the next few years to the intellectual and instrumental firepower now being brought to bear on it. Assuming, that is, that the universe cooperates. THE AGE CRISIS: ''You can't be older than your ma,'' quips Christopher Impey of the University of Arizona's Steward Observatory. Sounds obvious, maybe, but if Freedman and her colleagues are right about their space-telescope observations, it would seem that the universe hasn't caught on to this bit of common sense. The most straightforward interpretation of their data implies that the cosmos is 12 billion years old, max. But experts insist that the oldest stars in the Milky Way have been around for at least 14 billion years. ''They could quite easily be several billion years older than that,'' says Yale's Pierre Demarque. Demarque and his fellow stellar astronomers make a good case. The life and death of stars is something the scientists think they understand pretty well. They know about the nuclear reactions that power starshine; they know about what chemical elements the stars contain, and in what proportions; and they have created detailed, accurate computer simulations of stellar life cycles. When they say 14 billion years, it probably pays to listen. But it also pays to listen to Freedman. She's a highly respected observational astronomer, and so are the 13 others on her space-telescope team. Moreover, theirs is only the latest in a series of measurements that point to a relatively young universe. Just a month before these results appeared in the journal Nature, two other sets of astronomers came out with their own young-universe observations. And while a handful of studies have emerged over the past few years arguing instead for an older cosmos, many more have converged on a younger age. The Freedman team's observations are considered by far the most definitive because they are based on the Hubble's extraordinarily clear vision. Moreover, the concept that underlies their calculations is utterly straightforward. Astronomers have known since Hubble's heyday in the 1920s that you need only two pieces of information to deduce the age of the universe: how fast the galaxies are flying apart and how far away they are. The ratio of these two numbers tells you how fast the cosmos is expanding (a rate known as the Hubble Constant; it's expressed, for those who insist on the proper terminology, in units of kilometers per second of recessional speed per megaparsec of distance). A simple calculation then tells you how long it's been since the expansion started. ''There are these two loopholes, though,'' notes University of Oklahoma astrophysicist David Branch. ''What's the right distance, and what's the right speed?'' These loopholes are big enough to drive the Starship Enterprise through. It's terrifically hard to measure how far away galaxies are. If they came in a standard brightness, like 100-W light bulbs, the astronomers could just figure that a dimmer galaxy was more distant than a bright one. Unfortunately, they don't. Edwin Hubble himself didn't realize this and triggered an earlier ''age crisis'' in the 1940s when he announced that the universe was 2 billion years old. Geologists already knew that Earth was older than that. Astronomy's most reliable light bulb, or, to use the preferred and quainter term, standard candle, is a type of star called a Cepheid variable, whose inherent brightness can be easily calculated. But Cepheids can't be spotted more than a few galaxies away. And these nearby galaxies are virtually useless in filling in the other half of the equation -- the expansion rate. Reason: in a universe that's expanding overall, neighboring galaxies are flying apart much more slowly than distantly spaced ones. Nearby galaxies are also subject to their neighbors' gravity. The Andromeda galaxy, for example, is being pulled closer to the Milky Way, despite the overall cosmic expansion. Since accurate distances can be measured only nearby, while useful galaxies are found only deep in space, astronomers do the best they can to bridge the gap. They use the close galaxies to estimate distances to the faraway ones. But the method is inexact, which is why they haven't been able to agree on what the age actually is. It's also why the Hubble Space Telescope was explicitly designed, at least in part, to find Cepheid-variable stars at greater distances than ground-based telescopes could. nasa and its scientific advisers figured that the deeper they could go into the universe before they had to switch from more to less accurate ways of gauging distance, the better. The Hubble's misshapen mirror delayed things for a while, but shortly after the spectacularly successful repair mission in December 1993, Freedman and her team focused the space telescope on a faraway galaxy called M100. ''We could see right away that we'd be able to find Cepheids,'' she recalls. The observations were moved to the head of the Hubble schedule, and by July, Freedman was looking at a pattern on her computer screen that was as familiar as the face of an old friend. ''Boom!'' she remembers. ''All of a sudden there was this glorious Cepheid light curve, as beautiful as any that have ever been measured.'' By the end of the observing run, Freedman and her colleagues found 19 more, enough to peg M100's distance at some 56 million light-years from Earth. That still isn't far enough out to give a direct measure of the Hubble -- the cosmic rate of expansion. But M100 is part of a huge group of galaxies known as the Virgo cluster. The M100 calculation gave the astronomers the distance to Virgo, and they used that number in turn to estimate the distance to the Coma cluster of galaxies, about five times as far away. Coma, finally, is far enough out that it's a reliable indicator of the Hubble Constant. Based on Freedman's analysis, the Constant comes in at 80, indicating a universe between 8 billion and 12 billion years old. While most astronomers take these numbers very seriously -- along with the cosmic paradox they imply -- Allan Sandage, Freedman's grumpy colleague down the hall, is having none of it. He doesn't quibble with her measurement of the distance to M100, but insists that the analysis breaks down after that. Like most astronomers, Sandage has his favorite method of gauging the relative distance of galaxies. He finds a type of supernova -- an exploding star -- and compares supernova brightnesses from one galaxy to another. He claims, as he has done for more than 20 years, that the Hubble Constant is lower, which means the age of the universe goes up considerably. Says Oklahoma's David Branch, his close collaborator: ''We're very happy to be in this controversy because we think we're right.'' But most astronomers don't -- partly because just about everyone else gets different results, partly because they suspect Sandage is guilty of a cardinal sin of science: having a preferred answer in mind before making observations. Freedman is the first to admit her team's age figures could be off 20% in either direction. The reason: no one knows whether M100 lies inside the Virgo cluster or whether it is more in the foreground or background. Astronomers have to check out other galaxies in the area before they are sure that M100 fairly measures the distance of the cluster as a whole. They're also checking galaxies outside Virgo, and while Freedman won't say what they have found so far, she told Time that the results are ''consistent'' with the preliminary figures. If she's right and Sandage is wrong, as many cosmic handicappers are betting, then the age crisis won't go away without some fundamental change in the way astronomers understand the cosmos. That means at least some scientists will have to give up their cherished beliefs about how stars work or how the universe is organized or what it's made of -- or maybe even all of the above. THE DARK-MATTER PROBLEM: High on the list of concepts that astronomical theorists would hate to lose is cosmic inflation. It sounds nutty, but the universe actually makes a lot more sense if you assume that just after it was born all of space went into overdrive, exploding outward for the briefest fraction of a second. Inflation explains, among other things, such mysteries as why the universe looks pretty much the same in all directions and how a peanut-butter-smooth distribution of matter in the young cosmos evolved into today's lumpy distribution, with clusters of galaxies surrounded by empty space. Inflation theory doesn't just explain things; it makes predictions. Chief among them: the blackness of space is only seemingly empty. In fact, it probably abounds with vast amounts of matter -- matter that cannot be directly detected because it doesn't shine. If this theory is correct, then there must be precisely enough of this dark matter so that gravity will forever slow the expansion of the universe without ever quite stopping it, balancing space on a gravitational knife edge between eternal growth and eventual collapse. Dark matter is more than merely theoretical. The first hint that the cosmos contains more than meets the eye came back in the 1930s, when Caltech astronomer Fritz Zwicky pointed his telescope at the Coma cluster of galaxies and realized that it shouldn't exist. Individual galaxies in the cluster were orbiting each other so fast that they should long since have flown out into deep space -- unless gravity from some unseen matter was keeping them together. Nobody took Zwicky too seriously; the idea was crazy, first of all, and besides, the measurements of orbital speeds were difficult to make and prone to error. Nor did anybody take Vera Rubin seriously when in 1970 she and a colleague at the Carnegie Institution of Washington discovered that some galaxies were rotating too fast on their own axes -- again, evidence of extra gravity from unseen matter. Not until a little more than a decade ago was dark matter finally accepted as a huge problem rather than a nagging anomaly. Observation after observation showed that galaxies moved as if they were embedded in clouds of invisible matter containing 10 times as much mass as was accounted for by visible gas and stars. Clusters of galaxies behaved as if there was 30 times as much dark matter as visible matter exerting its gravitational pull. To satisfy inflation theory, the ratio would have to be even greater: 100 times as much dark matter as visible. Leaving aside theory, the challenge of identifying and understanding the stuff that makes up most of the universe has become one of the most irresistible -- and frustrating -- quests in science. For more than a decade, the campaign has proceeded on two fronts: attempts to directly observe the missing matter, and attempts to identify it via computer simulations. Those who do the latter assume that dark matter is made of a given particle or substance, then create a computer model of the cosmos based on that assumption, let it evolve in cyberspace, and see if the result looks like the real universe. An early theory was that the missing matter is composed of commonplace particles called neutrinos. One problem with this is that dark matter is massive, and no one knows if neutrinos have mass. Even if they have, in computer simulations they do a poor job of making a recognizable universe. Cold dark matter was another possibility (''cold,'' in physics jargon, means slow-moving; neutrinos, by contrast, are ''hot''). Also known as wimps, for weakly interacting massive particles, these are purely hypothetical particles derived from speculative theories. They perform somewhat better in computer models, but wimps can't account for such newly discovered features of the cosmos as Great Walls, Great Voids and Great Attractors. Physicists hoping to observe dark matter directly have searched for objects both large and subatomic. On the theory that the dark stuff is made of some as yet undiscovered particle, they have built all manner of sensitive detectors. On the chance that it is composed of very dim stars or large planet-like objects (known collectively as machos, or massive compact halo objects), they have studied stars for telltale flickers that might indicate a macho has passed by. Physicists and astronomers have looked for all of the above and more, but results have been inconclusive. wimp searches are barely getting under way; macho hunts have turned up disappointingly few flickers at the outer edges of the Milky Way (but surprisingly many toward the galaxy's core). The latest teaser came last month when news leaked out that researchers at Los Alamos National Laboratory had seen evidence for what could be a slightly massive neutrino. If it's true, the finding is of literally cosmic significance: there are so many neutrinos in the universe that they alone could account for some 20% of the dark matter that inflation theory requires. Just add in another 80% worth of wimps and you've got it, says Joel Primack of the University of California, Santa Cruz. With this recipe, Primack has used supercomputers to produce synthetic universes that look almost identical to the data gathered by real-life astronomical observers. But some theorists think Primack is grasping too quickly at a ''discovery'' that is still controversial. Neutrinos with mass might help solve the dark-matter problem and thus provide support for the inflation theory. But in some ways that would just make the crisis in cosmology worse. The more dark matter there is in the universe, the harder it is to explain the new findings made by Freedman's group about the age of the cosmos. When they say the universe is between 8 billion and 12 billion years old, their vagueness reflects uncertainty about how much matter the cosmos contains. If there's a lot, as inflation suggests, its gravity would be slowing down the universe's expansion, making the universe younger than it looks. If, on the other hand, there is relatively little matter, the slowing has been minimal, and 12 billion is more like it. If inflation is correct, then, the age crisis is as bad as it can possibly be. No amount of theory adjustment can bring stars down to 8 billion years of age. So if Freedman's initial attempt to date the universe holds up, Primack and plenty of other theorists may have to begin prying themselves away from an idea they have held dear for more than a decade -- unless they can think of some clever way out. EINSTEIN'S BIGGEST BLUNDER: Even at an optimum age of 12 billion years, the universe is too young to accommodate 14 billion-year-old stars, so even the radical step of abandoning the inflation theory might not be enough to resolve the age crisis. But there could be a solution that allows inflation to remain. All the theorists have to do is throw out another of their cherished beliefs: that Einstein was right when he repudiated his concept of a cosmological constant. Says Princeton physicist Jim Peebles: ''People hate the cosmological constant. I used to hate it too. But it's something we might grow to love.'' They might have to. The constant can be thought of as a kind of universe-wide repulsive force, a sort of antigravity. Einstein thought that he needed it in his general relativity theory to balance the pernicious influence of gravity. Without a cosmological constant, said the equations, the universe would have to be either contracting or expanding -- which it didn't seem to be. It was only when Edwin Hubble discovered, a decade later, that it was indeed expanding that Einstein dropped the constant like a hot potato. But particle physicists later found that a cosmological constant arose naturally from their own theories. And now cosmologists may need it to get out of the age crisis. If the constant has the right value, then cosmologists can keep inflation. The cosmos would have started out in a long period described by scientists as ''loitering'' or ''coasting,'' providing stars and galaxies with ample time to form. ''Then,'' says Sandra Faber, Primack's Santa Cruz colleague, ''suddenly the cosmological constant would kick in, gunning the expansion, making it faster.'' Measuring a large Hubble Constant and an apparently low age today, in other words, wouldn't be a reliable indicator of what was going on earlier in the universe's lifetime. Theorists might hate Einstein's abandoned child, but, says John Huchra, an astronomer at the Harvard-Smithsonian Center for Astrophysics, ''to an experimentalist it seems no more ad hoc than inflation.'' THE STRUCTURE PROBLEM: At least there are some models of the cosmos -- unpopular though they may be -- that accommodate Freedman's age estimates. The same cannot be said for Lauer and Postman's detection of large-scale motions across the universe. Most scientists are betting that their observation is just plain wrong, but they haven't yet been able to pinpoint why. And both Lauer and Postman admit that the effect may wash out as they collect more data from deeper in space. If it holds up, though, the theorists will have to rethink their position in a hurry. One explanation for the observation would be that galaxies are being pulled toward a concentration of mass so huge that it would make the Great Attractor look like a joke. Another might be that the Big Bang may have been lopsided, so that the universe has more energy and mass in some sectors than in others. In that case, the anomalous motion is an illusion. But both ideas are almost impossible to reconcile with any known model of the universe. Admits Postman: ''If I'd been at the receiving end of this news, I'd be skeptical too. But the modus operandi of an observer is to report what Nature is telling us.'' And that's true whether or not the news conforms to the conventional wisdom. Princeton astrophysicist David Spergel offered a telling historical anecdote in an address to colleagues at the American Astronomical Society's January meeting in Tucson, Arizona. In the 19th century, it dawned on astronomers that the orbits of Uranus and Mercury weren't exactly what theory predicted. So they proposed the existence of as-yet-undiscovered planets whose gravity was causing the anomalies -- sort of the Cold Dark Matter of the time. Sure enough, Neptune finally appeared in their telescopes. But the other planet, Vulcan, never did materialize. In the end, said Spergel, it took the theory of general relativity to explain Mercury's odd behavior. Which story is applicable today? Will a crucial new observation tie up the loose ends in cosmology? Or do theorists need a fundamentally new framework for understanding the universe? ''I don't know,'' admits Spergel, noting that it's a lot easier to add bells and whistles like cosmic strings or a cosmological constant to existing theories than to come up with something as powerful as relativity. As they search for answers, the noisy clash of egos and the confusion of conflicting claims may be taken as signs that science is alive and well and likely on the cusp of a major new insight. Says astrophysicist John Bahcall of the Institute for Advanced Study in Princeton: ''Every time we get slapped down, we can say, 'Thank you Mother Nature,' because it means we're about to learn something important.''