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End of the trail for Polynesia's star snails NS 15 apr 2003

THE onoe mumrlous Partula snails of French Polynesia - as important to the study of evolution as Darwin's finches - have a bleaker futum than arryone mallsed. A misguided attempt at biological control has wiped out 56 of the original 61 species lbund In the v,Aid, and the fate of the remaining five hangs by a thread. The unusually diverse Partuia spades endemic to the Sodety Wands, part of French Polynesia, aired biologists a ram chance to research evolution In action. But the snails' death knell was sounded in 1974 when a predatory snail called Euglandina rosen was intmduced to Tahiti to contain another Immigrant, the African land snail, which was proving a determined pest. E rosea had little lmpact on its intended target. Instead ft spread to other islands, feeding coff native species of Partula. 'A IMng lab was decimated," says Paul Pearce-Kelly at London Zoo, curator of the world's largest Partula breeding programme. The last comprehensive suniey in 1994 revealed that E rusea had only just reached the island of Huahine. But a new survey bytmwr Coote at the Zoological Sodety of London and ftic Lobve, president of Fenua Animalia, a nature welfare organisation In French Polynesia, has shown that the predator has already wiped out the t6vo Partula species lmng there (P varia and R arguto). In 1995, the last R arguta died in caput, so the species is now thought to be extinct. Two othefs, P tabrusca and P. tuteda, went extinct on Ralatea in 1994. Captive populations cyf both went eKdnct In 2002 and 1996 respectively. TWelve species now e)dst only in captft. Hopes have been raised by the chance discovery last year of a small population of as yet unidentified Partula in a remote area on the end of Moorea. Partuto snails were thought to have become ekdnd there in 1987. lt is undear, however, whether this population will survive the attentions of the predatory Invader, which reached the island in the l970s. James Randerson *

CELL Fusion claims rock stem cell research ANDY C0GHLAN

RENEWED controversy has broken out over the abilities of adult stem cells, whose apparent potential has generated much excitement. It has long been known that there are different kinds of adult stem cells in the body that give rise to particular tissues: new blood cells, say. But recent studies have shown that a few adult stem cells can transform themselves into a much wider range of specialised cells. The way suddenly appeared open for doctors to tum a patient's aduft stem cells Into matched tissues for treating diseases, avoiding the ethical and practical problems of using embryonic stem cells. Now two independent groups are challenging these claims. Blood stem cells from bone marrow can only form liver tissue by fusing with existing liver cells, they say.

"If this is truefora ll adult stem cells, it means thatturningthem into specialised tissues in the lab will be far more difficult"

If this is true for all adult stem cells, it means that tuming them directly into specialised tissues in the lab, prior to re-implanting them, will be far more difficult than thought. That could rule out some therapies. "Rather than bone marrow cells morphing themselves into liver cells, they fuse first with liver cells, then change into liver cells," says Markus Grompe, whose team at Oregon Health and Science University published its results this week (Nature, DOI: io.1038/ natureOI531). David Russell's group at the University of Washington in Seattle also reported similar results (DOI: 10-1038/natureOl539). Both teams studied mice destined to die because they lacked a liver enzyme for breaking down the amino acid tyrosine. But the mice survived when injected with normal bone marrow stem cells, which rapidly colonised their livers. Under the microscope, though, the team saw that the transplanted cells were fusing with existing liver cells instead of forming pristine new liver cells. Grompe thinks fusion reprograms stem cells to act as liver cells - and that the same mechanism is responsible for reports of blood stem cells forming other specialised cells. "I think this will tum out to be the dominant mechanism.' But many others disagree. They say such fusion might be peculiar to the liver. By fusing with each other, healthy liver cells can ramp up production of enzymes when facing overloads of a particular toxin or compensate for damaged genes in individual cells, suggests Eva Mezey of the National Institute of Neurological Disorders and Stroke in Maryland. "There's absolutely no evidence of fusion in vivo in anything other than the Iiver," says Mezey. So while she does not dispute Grompe's findings, she thinks fusion might not occur in other organs. For instance, Mezey has found that between 2 and 12 per cent of the cheek cells from women who had received bone marrow transplants from men years earlier were of male origin, and so must have come from the transplanted cells. Only 2 Out Of the 9700 cells examined showed signs of having fused, she reports in this week's Lancet.

Sperm get their kicks from lily of the valley

PERFUMES may not always do much fbr the male of the species. But there's at least one that drives sperm wild, setting them racing crff towards the source of the smell. The work is the best evidence yet for a long-held suspicion: that human sperm, like those af sea urchins, fbilow a scent trail towards the egg. However, the actual chemical attractants released in women have sdil to be identified. The vast majority of the body's protein "sensors" that detect such chemicals are fidund in the nose, of course. But a few olfactory receptors have been found in the brain, the skin and one in sperm. By looking to see if sperm expressed ariy genes similar to those for known olfactory receptors, Marc Spehr's team at Ruhr University in Bochum, Germany, managed to identify a second receptor called hOR17-4 (Science, vol 299, p 2054). The team then tested a range of chemicals to find out what hOR17-4 can "smell". Asubstance called bourgeonal, which is found in a few perfumes, produced the strongest response. "it smells like lilies of the valley,"saysSpehr.

"When bourgeonal is released nearthe sperm, they head straightfor it, swimming twice as fast as normal"

lt Is not yet clear how sperm steer towards the scent. But when bourgeonal is released near them, they head straight fbr it, swimming twice as fast as normal. The hOR17-4 receptor triggers the opening cyf calcium ion channels in the sperm, thought to make them wag their "tails" harder. The findings could have practical applications. Substances like bourgeonal could be used to select the most vigorous sperm fbr IVF, fbr instance. Or they could lead to new contraceprtim. The team has already shown that a chemical called undecanal blocks theefrectscrf bourgeonal. But human sperm are thought to have dozens of different olfactory receptors, all of which would have to be blocked to create an effectpje contraceptive. Nicola Jones

The results are in... and now it's time to party

Cosmologists celebrated their new-found status as "proper" scientists. But are they any closerto working outwhythe Universe is the way it is?

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MICHAEL BROOKS, DAVIS

THERE were no conga lines or high fives, but this gathering of cosmologists and astronomers was about as close to a celebration as physics conferences get. Seven weeks ago, when the team behind NASA's Wilkinson Microwave Anisotropy Probe (WMAP) released its first set of results, the landscape of cosmology changed. One of the subject's wilder ideas - that shortly after the big bang the Universe went through a brief period of extremely rapid expansion called inflation - became a proper science. Last week's meeting was the first chance for cosmologists to get together and discuss the results, and the researchers who have spent decades working towards this moment were jubilant. "Inflation is now part of experimental science," announced David Spergel of Princeton University. Inflation was the brainchild of Alan Guth at the Massachusetts Institute of Technology. He didn't have a mechanism for the expansion, just an argument that showed it would solve a few cosmological puzzles. Other theorists then took up the idea and made predictions about what effects inflation ought to have on the Universe. Now those predictions have been bome out in style by the WMAP data (New Scientist, 15 February, p 12). The probe measured the way the temperature of the microwave background radiation, left over from the big bang, varies across the sky (see image, top right). much of the data is summarised in just one innocuous- looking graph (see right) on which the temperature difference between pairs of points is plotted against their angular separation, producing what Is known as a "power spectrum". The temperature variations reflect the way structure is thought to have emerged in the early Universe, and theoretical predictions for the details of this spectrum closely match WMAP's observations. Inflation, it tums out, was rather a good idea. No one is happier about that than Guth. "I am very pleased and very surprised," he told New Scientist. "It was a pretty radical idea and I was pretty much a novice in cosmology. I was quite nervous that the whole thing could just blow up and fall apart. But now it's just a matter of filling in the details." That is no small task. For a start, researchers are still in the dark about what actually caused inflation. Theorists ascribe it to a hypothetical particle called the "inflaton", but no one is really any the wiser. And there are many different versions of inflation, each of which produces a Universe with a slightly different power spectrum. So far, the data is not precise enough to distinguish between them. The key to making that distinction may lie in the "spectral index", a parameter that emerged as the star of the Davis show. It is roughly a measure of the overall slope of the power spectrum curve. Basic inflation theory suggests it is almost, but not quite, 1. Other variations of the theory give slightly different values. WMAP measured the spectral index at o.99 Ò o.o4, but as the data improves and pins down the figure further it should be possible to start ruling some of these variations in or out. It may even tell us whether there are hidden extra dimensions. One of the more surprising possibilities to emerge at the conference, presented by Lisa Randall of Harvard University, was that the inflaton might live in an extra dimension. This scenario gives a spectral index of o.96 - within the range allowed by the W@ data. It also predicts significant gravitational wave radiation during inflation, a key ingredient of many versions of the theory. Indeed, proof of the existence of gravitywavesisthenextbigtestfor inflation. This would be the smoking gun, Michael Tumer of the University of Chicago told the conference: "Gravity waves are the real holy grail in all this:' They would also be a conclusive means of settling the arguments with inflation's chief remaining rival, the cyclic Universe, which predicts there will be no gravity waves (see Box). Another hot topic at Davis was the anthropic principle", which is used to explain why the Universe seems so finely tuned to support life. This controversial argument postulates the existence of many universes with different properties, and that we just happen to be in one that allows enough complexity to create life. Physicists have started taking the anthropic principle seriously because inflation provides a mechanism by which multiple universes could have arisen. If a tiny part of space blew up to become the whole of the Universe that we see today, it is likely the same thing also happened in other regions that we can't see, causing any number of other universes to emerge, all with different properties and too far away for us to ever reach. As astronomer Martin Rees of the University of Cambridge put it, the fundamental laws of physics may be nothing more than "parochial by-laws in our cosmic patch" ' Leonard Susskind of Stanford University told the assembly that this anthropic argument is also emerging as an overwhelming prediction of string theory, which allows a range of different scenarios to develop. Guth says he now strongly believes that inflation is producing new and different universes all the time, an idea called "eternal inflation". "Any inflationary scenario that cannot @temally reproduce would seem as implausible as discovering a species of rabbits incapable of reproduction," he told the conference. But there is no way to test the idea, and University of Cambridge cosmologist Stephen Hawking says he believes it "has serious flaws". Indeed, Hawking burst the party balloons early in the Davis meeting. Clearly in a combative mood - despite the "Stop the war" badge on his lapel - he claimed inflationary models are hopelessly inconsistent and possibly irrelevant. "Even if inflation works, it won't tell us why the Universe is as it is," Hawking told the assembly. Because inflation has no defined link to what kicked off the Universe, he believes it has nothing to say about the fundamentals of physics. "It simply shifts the problem from 13.7 billion years ago to the infinite past He suggested that only "top-down" cosmology - combing through our current observations without prejudice - will yield any final answers about the nature of the Universe. But Hawking's onslaught didn't dampen spirits. The only moment of self-doubt came from Spergel, who admitted he spent most of last year worrying that one particular region of the power spectrum wouldn't fit with inflation theory. He was right: it didn't. When the temperature differences are compared across the largest distance scales, the WMAP data falls below the theoretical prediction (see Graph). "There's no explanation for it," he told NewScientist. The mismatch is disturbing, he says, because it suggests there could be something ftindamentally different about the Universe on the largest size scales. It may behave differently over huge cosmological distances, or it may simply be finite. Spergel sees no way to solve the problem in the near future. "Normally, faced with such an anomaly in physics, you do another experiment. But there's not much we can do' ' The general feeling, though, was that the WMAP team has done enough: they even worked out the error on their errors - less than two per cent. Licia Verde of Princeton, another member of the WMAP team, put the success down to an obsessive concem over minimising sources of experimental and systematic error in the equipment. She requested similar care from the cosmologists. "The rest of the analysis should have the same level of obsession," she wamed. There were repeated promises to comply, as cosmologists rejoiced in their new status as proper scientists with data on which to base their theories. Max Tegmark of Pennsylvania State University summed up the euphoria best. During his talk he repeatedly flicked his slides back and forth between data that cosmologists had at their disposal on 11 February and what was available to them the day before. "I just can't see it enough times," he said.

THE SPECRE AT THE FEW

AS INFLATION theory loops from success to suam, there remains a radical afternative that no one can rule out. According to Paul Steinhardt of Princeton University In New Jersey, space and time did not erupt out of nothingness 13.7 millon years ago and lnftb wildly to produce our Universe. Inspired by the many dlmensions of string theory, Steinhardt and his colleape Nell Tumk argue that the universe was instead formed by the collision of two 'branes' - three- dimensional worlds that oscillate back and forth along one of these extra dimensions. They say today's Universe is simply part of an endless cycle of expansion and contraction. Such cydes have been suggsted before, but the problem has been that between each cycle, space and time are squeezed Into an infinitely small point called a singularity. When that happens, the laws of nature break down, and no Information pass from one cycle to the next. But Steinhardt says in his brane-world theory, the cycling medium slips through the crunch without forming a singularity. So Information - the density fluctuations needed to seed the subsequent Universe with stars and galaxies - can survive. Steinhardt's predictions match those of inflation theory almost exactly. Steinhardt's most vocal critic Is Andrei Linde or Stanford University In California, who didnl pull any punches as he challened the assertion that the cyclical model dodges the big bang singularity. Ift sinplarity problem was not @, and remains unsound,' he says. He @ OW there is sOl no way to prove whether Information passes through each crunch or not, rendering in about past or future cycles pointlm. 'Ws like life after death,' he says. Tentativ support for Steinhardt come from Ruth Durrer of the Unversity of Geneva In Switzedend. She told the meeting that her calculations of colliding brane worlds do create scenarlos where enough Information passes thmugh to a Universe like ours, though scenarios where this doesn't happen also exist. Robert Adler, Davis 0

Knockout broccoli fights cancer

EATING your greens could be even better for you than anyone thought. Macerated raw broccoli tums out to contain small amounts of a potent chemical that inhibits the oxidising enzymes that damage DNA and potentially cause cancer. When you chew broccoli, its cells rupture, releasing an enzyme that produces a class of chemicals called sulphoraphanes. Nathan Matusheski, a researcher at the University of Illinois at Urbana-Champaign, crushed raw broccoli in the lab to mimic chewing, and tested the resufting mush. Matusheski told ACS delegates that in common supermarket broccoli, 20 per cent of the sulphoraphanes are the anti-carcinogenic kind, which have an extra sulphur atom in each molecule. The rest lacks this crucial sulphur and has no cancer-fighting capability. But when he tested broccoli that had been heated to 6o 'C, he found the relative levels were reversed, favouring the anti-cancer compound. A protein in broccoli called ESP plays a role in pushing the balance towards the sulphur-poor sulphoraphane. Matusheski confirmed that heating the broccoli destroys the ESP, tipping the balance in favour of the beneficial sulphoraphane. But cooking broccoli conventionally does not help, as the enzyme that produces sulphoraphanes in the first placed is also destroyed. One way to ensure high levels of the beneficial compound may be to eliminate the genes that code for the ESP protein. This could be done by making hybrids with wfld strains, says Matusheski, who prefers this approach on an ethical basis. Another method would be gene silencing. "This is one of many studies that will build our knowledge" of immune-enhancing foods, says Sara Risch, a food technology adviser with the Chicago-based food science consultancy Science by Design. At the ACS,.chemists also probed the alleged immune effects of cranberries and soybeans, with less conclusive results. "But this broccoli research identifies something that could be taken to the plant breeders," says Risch.

Quintessence and Supernovae

LAST year, Chris Smith saw 25 stars explode. He is a lucky man. In the three millennia before the 2oth century, we recorded far fewer supernovae. Now these stellar cataclysms are being collected like new species of beetles: over the next decade, astronomers should spot thousands. But this catalogue of stellar disasters is only a means to an end. In a project called ESSENCE, based at the Cerro Tololo Inter-American Observatory in Chile, Smith and his colleagues are hunting something far more important than explosions. They are on the trail of arguably the deepest mystery in physics: dark energy. This cosmic puzzle was first encountered in 1998, when two groups of astronomers reported that dozens of distant supernovae appeared surprisingly faint (New Scientist, 11 April 1998, p 26). They concluded that the expansion of the Universe must be accelerating, dimming these distant explosions. It came as rather a shock, because although we have known for more than 70 years that the Universe is expanding, people had always assumed that the expansion must be slowing down. The gravity of all the matter in the Universe should be putting the brakes on the expansion.

But the supemovae showed that about 10 billion years ago something began to overpower gravity, making the expansion accelerate. It's as if gravity were working in reverse, or as if a cosmic poltergeist were grabbing galaxies and flinging them outwards.

So far, we know almost nothing about dark energy. But that's about to change. Astronomers like Smith are beginning to try and pin down its nature. The plan is to trace exactly how the expansion is speeding up, and therefore just how repulsive dark energy is. "it is like trying to figure out how many cylinders a car engine has by watching the car accelerate," says ESSENCE team member Peter Garnavich from the University of Notre Dame in Indiana. "Twelve cylinders are going to get the car zooming better than four. We are watching the Universe start to accelerate after the matter-dominated era. How fast it takes off is a clue to what is driving it."

The acceleration of the Universe is too gentle to feel, so instead astronomers detect it using a particular kind of stellar explosion called a type la supernova. These come from binary star systems where a small white dwarf, the ember of a star like our Sun, is orbiting a red giant. If the white dwarf is close enough, it pulls material off the red giant until eventually it reaches a critical mass and begins to collapse. That heats up the white dwarf so much that the carbon and oxygen nuclei in it suddenly fuse, releasing enough heat to blast the star apart. Because these stars all have the same critical mass, and therefore the same amount of nuclear fuel, the explosions are all of similar brightness. So if astronomers compare that known value with their apparent brightness as seen from Earth, they can work out how much the light has spread out during its joumey, and therefore how long ago the star went bang, They then look at the red shift of the supernova light. As the light travels through an expanding Universe, its wavelength gets stretched, as if the light wave were drawn on the surface of an expanding balloon. The increase in wavelength moves the light towards the red end of the spectrum. So by measuring the wavelength of light from, say, hydrogen atoms in a supemova and comparing it with the light from hydrogen in the laboratory, astronomers can work out the red shift, and therefore find out how much cosmic expansion took place during its journey to Earth. Measure enough supemovae at different red shifts, and astronomers can tell whether the light has travelled through space that is expanding at a constant rate, or ever more slowly, or ever faster. Because red shift tells you how much space has stretched, and brightness tells you how much time has elapsed, you can plot the size of space against time - the history of the cosmos reduced to a line on a graph (see "Stretching space", page 32). This was how teams led by Saul Perlmutter of the Lawrence Berkeley Laboratory in Califomia and Brian Schmidt of Mount Stromlo Siding Springs Observatories originally discovered the acceleration: the graph turned up instead of down. By looking even more closely at how thegraphtakesoff,astronomers might just identify the culprit behind dark energy. Or that is the plan: the suspects are still rather shadowy figures. At the moment we have only one clue. Einstein's general theory of relativity, which describes gravity as a distortion of space and time, says that gravitational forces are generated not only by mass and energy, but also by pressure. If you have stuff under high pressure, that adds to the gravity it generates. Conversely, if you have stuff under tension, it produces negative, repulsive gravity. Pick up a rubber band or a spring and stretch it, and you have just set up an antigravity field. But it is not up to much. In fact, the positive gravity produced by the rubber band's mass will overwhelm the tiny antigravity created by its tension. No ordinary substance can be stretched hard enough to produce a net antigravity effect. So what kind of extraordinary substance could do it?

Educated guesswork Physicists have made several guesses. The first thing they thought of was empty space - or rather, the not-quite empty space of the quantum vacuum, where fluctuations in energy allow short-lived virtual particles to constantly appear and disappear. Forces between these particles could give space-time a tension. Unfortunately, when you calculate how much antigravity the vacuum ought to have, using quantum theory plus a bit of guesswork, you get a number loll' times too big. With a vacuum energy like that, every molecule in the Universe would explode. "If that's your leading candidate," says Michael Tumer of the University of Chicago, "boy, you haven't made much progress." This vacuum energy (sometimes called the cosmological constant) also leaves us with a puzzling cofhcidence: we seem to be living at a special time, when the vacuum energy is roughly equal to the energy density of matter. The latter is decreasing all the time, but the vacuum energy is a constant. When it was set, at the time of the big bang, it would have been many orders of magnitude smaller than the matter energy density; in the distant future, it will be many orders of magnitude bigger. Why should the two values be about the same now? Uncomfortable with the cosmological constant, physicists have devised other tense substances. "Quintessence", for example, was developed in 1998 by three cosmologists, Robert Caldwell at Dartmouth College in New Hampshire, Paul Steinhardt of Princeton University and Rahul Dave at the University of Pennsylvania. They imagined a kind of dark energy a bit like a thin fluid filling space. Its energy density can change over time and it has always been relatively close to that of matter so the coincidence is not such a problem. But there are many versions of quintessence, all with different properties.

And it is not clear what the stuff actually is. Even stranger is the idea of "topological defects". These are mismatches in space-time left behind by critical events in the Universe's history - when the so-called electroweak force split into separate weak and electromagnetic forces, for example. Topological defects would be like giant walls, stretched across the Universe and under high tension. Or maybe there is no such thing as dark energy, it is just that we've got gravity wrong. Perhaps, over very large distances, the force of gravity emanating from ordinary matter changes from attractive to repulsive. Possibly, some kind of new theory of gravity could even account for both dark energy and dark matter - that other nagging problem, of the Universe's missing mass. It is all guesswork, though. Any of these things might be true -or none of them. "I strongly suspect that the correct answer is not on this list," says Turner. But there is only one way to find out: pin down the expansion history of the Universe and see if it matches any of the theories. Cosmologists label the repulsiveness of dark energy with a single number, w, the ratio of pressure to energy density. Quantum vacuum energy has a w of -i, so the stuff has a lot of tension, and it is very repulsive. In some versions of quintessence, w is -0.5, which is, only fairly repulsive (see "Cosmic antigravity", below). The value of w corresponds to the number of cylinders in Gamavich's car, so measuring how fast the acceleration kicks in could tell us what kind of dark energy surrounds us. But the effect is pretty slight, so in order to tell what is really going on we need a lot of supernova data.

Testing times Smith and his team use the BlancO 4-metre telescope at Cerro Tololo. In October, November and December of last year, they spent every other night searching the same small patch of sky, sitting up to analyse the images. "It was pretty gruelling," Smith says. They were looking to see if any of the galaxies in that patch of sky had suddenly acquired a bright spot - a candidate supemova. Follow-up observations with other telescopes checked the kind of explosion and measured the red shifts of the 15 newly discovered type la supernovae, giving 15 new points on the expansion graph. But that still does not pin down the graph tightly enough, because there are slight uncertainties in the measurements. We know that type Ia supemovae are not all exactly the same brightness, for example, and although most of the variation can be corrected for by observing other characteristics of the explosions, a small uncertainty remains. Later this year, ESSENCE should collect a few dozen more supernovae. And so on for another five years. It will take at least three more years and a couple of hundred supernovae before the data are good enough to really narrow down the possible range of w. By the end of the project, Smith hopes to get it to within io per cent of its true value. That will certainly narrow down the possibilities. Topological defects would have a w of -%, so ESSENCE will either rule them out, or, more excitingly, rule out vacuum energy. "If we show that w is not -i, it means the dark energy is something even weirder than quantum vacuum energy," says Tumer. But either way, ESSENCE will not nde out an the models - there are too many of them. Some versions of quintessence say that w changes, and that it should have homed in close to -1 during the past few billion years. That would make quintessence look very like vacuum energy, and the only way to tell them apart would be to trace the expansion back lo billion years - to when there was last a significant difference between them. You cannot do this with the ground-based telescopes that ESSENCE uses because the light from these very distant supemovae has been stretched tO 2.5 times its original length, red-shifted right out of the visible band into the infrared. At these wavelengths, the atmosphere itself is so bright it messes up the measurement.

To see that far back in time, you need to go into space. And that is just what the Supemova/Acceleration Probe (SNAP) will do. As it orbits the Earth, it will search for these ancient explosions in a patch of sky - actually, 20 separate patches of sky, each about five to six times the size of the full Moon. The aim is to catch about 6ooo supernovae, and monitor their light as they each brighten and fade over a period of a few weeks. If the plan is approved by Congress at the end of this year, says Perlmutter, SNAP could launch as soon aS 2009. Within three more years it could pin down w to within 5 per cent, and pick up a change as small as 15 per cent over the io billion years. It is an impressive prospect: SNAP could finally provide the clues needed to work out the nature of dark energy. That does not mean we will understand the source of dark energy straight away, though. "Here's one doomsday scenario," Tumer says. "We build SNAP and find that w is -o.88, and that it varies with time - but we still don't understand it for a hundred years." It is a fair point. Other puzzling aspects of the cosmos, such as dark matter, have resisted explanation for many decades, even though we have known more about them than we do about dark energy. But Perlmutter is more optimistic. By plotting the past expansion in fine detail, he hopes we can do more than just weed out some of the rival models. Perhaps it will indicate a new explanation of dark energy, one that fits in neatly with other mysteries of cosmology, such as the nature of inflation, or even dark matter. "I hope we'll get that'aha!' moment and see a fundamental unifying principle that will make sense," he says. Even if there is no such epiphany, measuring the change in w could at least tell us the distant future of the Universe. If the value of w is fixed, then dark energy will always be repulsive. It will keep accelerating the Universe outwards, stretching it into bland emptiness. But if it can change, we might be in for a more interesting future. In February, Renata KaBosh and Andrei Linde of Stanford University pointed out that dark energy could eventually move from being repulsive to being attractive Uoumal ofcosmology andastroparticle Physics, DOI: io.lo88/1475-7516/2003/02/002). Then the Universe would stop accelerating, put on the brakes, and collapse inwards. Time would end in a big crunch. The supemova data already gathered shows that no such turnaround is happening just yet. According to Kallosh and Linde's calculations, that means a collapse is unlikely to happen within the next 18 billion years. But the far more detailed measurements from ESSENCE and SNAP should let us look very much further ahead, picking up advance seaming of a big crunch a trillion years from now. Even Tumer might agree that, given so much time, we could figure out what dark energy is.