快猫短视频

As far as most of the universe is concerned, you鈥檙e inconsequential. The everyday stuff that constitutes you and everything you care about makes up just 4 per cent of the cosmos; the rest we call dark matter and dark energy. What they actually are, though, is anyone鈥檚 guess. Now we may be on the verge of enlightenment. In this article, we report how experiments are getting ready to identify dark matter, while on page 32 we consider why dark energy may be an illusion created by our place in space. Be prepared for a new cosmic order鈥�

THIS YEAR, there鈥檚 a good chance that a sizeable chunk of our universe will turn up. A fair bit of the cosmos 鈥� 22 per cent of it, in fact 鈥� seems to be made of invisible dark matter, whose extra gravity helps to bind stars together in galaxies, and galaxies together in clusters. While we have seen dark matter鈥檚 effects in space, no one has actually detected a particle of the stuff. All that may be about to change, however.

For decades, researchers have been planning and building experiments sensitive enough to capture fragments of dark matter. According to our best cosmological theories, dark matter is made of hypothetical particles called WIMPs (weakly interacting massive particles). Now the detectors are ready for action and WIMPs are finally within our grasp.

So is it time to put the champagne on ice? Well, not so fast. Catching WIMPs is all well and good, but whether they actually turn out to be dark matter is another question. If the new experiments see nothing, or show that WIMPs do not have the correct properties for dark matter, we鈥檒l be back to square one. We might even have to radically change our approach to the dark-matter problem, junking decades of work in the process. Whatever the answer, it is crunch time.

The first hints of dark matter date back to the 1930s, when astronomer Fritz Zwicky studied the Coma cluster of galaxies, some 320 million light years away. He found that the galaxies in the cluster were orbiting each other far faster than our best understanding of gravity said they should, based on the masses of all their stars. Either the galaxies had to contain much more matter than could be seen in their stars, or else Newton鈥檚 law of gravity was wrong. Zwicky opted for vast swathes of unseen gas to provide an extra gravitational tug.

When observations in the 1970s revealed that individual galaxies were themselves spinning so fast that they should rip themselves apart, astronomers initially plumped for the same explanation. Then they ran into trouble. If the unseen stuff was normal matter made of protons, neutrons and electrons, it would never have collapsed quickly enough to form the first stars and galaxies. So they began to think that something else was out there, a mysterious form of matter that interacted primarily through gravity. They called it dark matter.

Cosmologists now believe that dark matter is a vital ingredient of the universe. Without its extra gravitational glue, galaxies would not form quickly enough, nor form the galaxy clusters and superclusters we observe today. Acceptance of dark matter is now widespread, fuelled by an ever-increasing number of observations that show Newton鈥檚 laws do not work across the universe at large. Even though no one has yet detected a particle of the stuff, new results are automatically interpreted according to the tenets of dark matter.

The uncomfortable truth is that the more detailed our observations of the universe, the more confusing the dark matter picture becomes. Sometimes there is too much dark matter, as in the case of the dwarf galaxies that orbit our Milky Way. These rotate so quickly that they must be chock-full of it. But this is exactly the opposite of what we understand from our standard theory of galaxy formation, which says we should expect the amount of dark matter in galaxies to be roughly proportional to their size.

Other times, we see too little dark matter. Across the universe, there are 10 to 100 times fewer small galaxies than our theory of galaxy formation predicts. Then there are times when what we see just doesn鈥檛 make sense, as in the galaxy NGC 3379. Measurements of the orbital velocity of gas clouds in NGC 3379 suggest it contains no dark matter at all. Yet star clusters circling further out do seem to be experiencing an extra gravitational pull.

The bottom line is that it is all very confusing. What we badly need to know is what constitutes dark matter. Once we know that, we can properly simulate the way it behaves and see if this solves the problems. The trouble is, deducing the nature of dark matter is the one thing astronomers have been unable to do.

鈥淎stronomers can never tell us what the dark matter is,鈥� says Gordon Kane, a theoretical physicist at the University of Michigan, Ann Arbor. This is because they are interested in objects on a celestial scale and design their computer models based on the behaviour of giant lumps of dark matter containing some 10,000 times the mass of the sun. Clearly, such models are quite useless for predicting the behaviour of dark matter particles themselves.

Enter particle physicists, for whom the hunt for dark matter has become intricately connected with the search for new particles that could provide important clues about the nature of the universe shortly after the big bang. Our best cosmological models predict that if WIMPs are what make up dark matter, then trillions of them must pass through the planet every second. So far, researchers have largely concentrated on building experiments in the hope of catching a passing WIMP. However, no one has yet detected one, and this is thought to be because their interactions with ordinary matter are so puny.

The lack of detectable WIMPs has inspired a completely different approach: if you cannot find a passing WIMP, why not produce them in the laboratory instead? And there is no better place to do this than inside the most powerful particle accelerator ever built 鈥� the (LHC) near Geneva, Switzerland. Previous attempts have failed because the accelerators have simply not been powerful enough to produce these particles, which theory suggests are at least 100 times more massive than the proton.

The LHC is set to change that when it switches on later this year and starts producing over a billion particles every second. Among them might be particles that stem from an idea called supersymmetry, which posits that for every known particle, there is a far heavier counterpart. All of the supersymmetric particles are massive and interact very weakly with matter. But the lightest one is widely regarded as the best contender for a dark matter WIMP, because it cannot decay to any of the other, heavier superpartners and so is stable.

The LHC will achieve the kind of energies present in the very early universe, allowing us to test theoretical descriptions of that era such as string theory, which is often used in conjunction with supersymmetry. 鈥淚t will very quickly identify whether there is a lightest superpartner 鈥� optimists say within a few months of operation,鈥� says Kane.

Such optimists would do well to temper their excitement, however. Spotting the lightest superpartner among the billions of other particles produced every second at the LHC will not be easy. 鈥淧articles do not come out wearing labels that say 鈥業鈥檓 supersymmetric鈥�,鈥� says Andy Parker, a high-energy physicist at the University of Cambridge. Instead, the superpartners will slip unseen through the detector, leaving a deficit of energy and momentum as telltale footprints. To untangle the properties of these unseen ghosts, we need to collide particles over and over to build up enough evidence. Only then can we decide whether a particular particle is capable of being dark matter.

After several years of analysis, researchers expect to pin down the individual masses of the WIMPs produced at the LHC and the strengths of their interactions with normal matter. 鈥淭hat will tell you whether there is a reasonable dark-matter candidate among the WIMPs,鈥� says Kane. 鈥淏ut just because you find a new particle at the LHC doesn鈥檛 mean that it is stable for the age of the universe.鈥� That鈥檚 because we observe the particles immediately after they are born, and they leave the LHC鈥檚 cathedral-sized detectors after only a tiny fraction of a second.

So the LHC won鈥檛 be able to tell us for sure if the particles it finds are what constitutes dark matter or not. Thankfully, other experiments starting up this year will plug the gaps.

About half a dozen of them are more sensitive versions of earlier experiments designed to detect the passage of a WIMP. Some, such as the deep in the Soudan Mine in Minnesota, aim to detect the heat generated when a WIMP strikes an atom in ultracold germanium and silicon crystals. Others, such as the experiment in the Gran Sasso National Laboratory near L鈥橝quila in Italy, use noble gases in their detectors. Here, the idea is to use large amounts of xenon as a detector. On the rare occasions when a WIMP strikes a xenon nucleus, it can impart enough energy to produce a flash of light and loosen an electron, which leads to a measurable electric current.

Last April, the Xenon team published results from a prototype detector containing 15 kilograms of liquid and gaseous xenon. Although they failed to find any WIMPs, the results showed that, in principle, the experiment was sensitive enough to detect them. A bigger detector and more time will boost their chances, and the team is about to start again, this time using 150 kilograms of xenon. For every hundred trillion WIMPs passing through the detector, they hope to catch at least one. 鈥淚鈥檓 optimistic that we will find between 1 and 10 detection events per year,鈥� says team member Laura Baudis of the University of Zurich in Switzerland.

If Xenon or any of the other detectors finds dark-matter particles arriving from space, it would mean that WIMPs are definitely stable. But with so few detections per year, it will take a long time to clock up the kind of numbers that the LHC will be able to furnish about other WIMP properties, such as mass. In other words, we need both approaches.

This is not the whole story, though. Even if we can establish that WIMPs exist and are stable, we still have to find out if there are enough of them in the universe to account for all the dark matter. That鈥檚 where NASA鈥檚 latest space telescope enters the scene. At present named , though NASA is planning to rename it before its launch in October, the telescope will probe the universe in detail at gamma-ray wavelengths and provide yet another way to detect WIMPs.

Even though the WIMPs react only sparingly with normal matter, they can react much more readily with each other. According to supersymmetry theory, a pair of identical supersymmetric particles will annihilate one another when they collide, releasing a pair of gamma rays. Gamma rays from such annihilations should all carry similar energies, leading to a noticeable spike within that energy range in gamma-ray measurements of the cosmos. This is what GLAST will be looking for, surveying the whole sky once every few days and building up an unprecedented picture of the universe in exactly the energy range expected from WIMP annihilations.

Cosmic cocktail

The first signs of WIMPs could show up in the data GLAST collects in its very first year, according to Edward Baltz, formerly of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford, California. GLAST might discover heightened readings originating from the centre of our galaxy, where WIMPs are expected to cluster, or from nearby dwarf galaxies, which appear to hold more dark matter than expected. The more gamma rays that GLAST detects, the more WIMPs there must be. So we鈥檒l be able to tell if there are enough of them to be dark matter.

Put all the experiments together 鈥� LHC, Xenon, GLAST 鈥� and the prospects for discovering the true identity of dark matter look promising. Yet there could be a sting in the tail. 鈥淢ost scenarios assume a single dark-matter candidate,鈥� says Scott Watson at the University of Michigan, Ann Arbor. 鈥淭hat seems over-optimistic.鈥�

鈥淯nmasking the true identity of dark matter might also reveal a sting in the tail鈥�

He believes that while WIMPs could very well be part of the solution, we will need to identify or rule out other possible ingredients before we can say that the mystery of dark matter is truly solved.

Astronomers and physicists have never been at a loss when proposing possible dark-matter candidates. Myriad faint or invisible celestial objects have been suggested in the past, including black holes, red and white dwarfs, and even planets. Yet more hypothetical particles have been touted as dark-matter candidates, including axions (快猫短视频, 18 July 2006, p 35), which, having drifted in and out of fashion, have recently enjoyed a resurgence in popularity. 鈥淎ll good string theories have axions and superparticles,鈥� Kane says.

Then there are neutrinos, subatomic particles produced in copious numbers shortly after the big bang and during the birth, collision, and death of stars. We used to think neutrinos were massless until 1998 when the Super-Kamiokande experiment in Japan discovered that neutrinos weigh something, although we still don鈥檛 know how much. 鈥淣eutrinos have to be part of dark matter, even if not a substantial part,鈥� says Kane.

There is a growing realisation that we may end up with a cocktail of dark-matter particles. After all, normal matter is made of a rich variety of particles such as protons, neutrons and electrons. Sorting out all the dark matter constituents will be a tricky problem, and the only way to make progress is to have all these different experiments working together. 鈥淚n the past, astronomers and particle physicists have not really talked together,鈥� says Watson. 鈥淣ow that is beginning to change.鈥� The problem is just too big for one group to solve on its own.

What if no one sees any WIMPs at all? Parker is quite sure what this would mean. 鈥淚f supersymmetric particles are too heavy to be created in LHC, they could not have been created by the big bang in sufficient numbers to account for dark matter,鈥� he says.

What then? Turn to another dark-matter candidate and start the whole weary process all over again? Or should researchers take a more radical approach? If the universe is not filled with dark matter, then perhaps the laws of gravity are wrong. A single answer is unlikely; there could be a component of both solutions. 鈥淲e just don鈥檛 know what nature has dealt us,鈥� says Baudis. By this time next year, we just might have a better idea.

Read the companion feature on dark energy

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