Posted on Thursday, March 13, 2008 - 8:23 am:

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Scott:

Don, this is an argument for downloading consciousness - just don't do it on a PC for hell's sake!

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Posted on Thursday, March 13, 2008 - 9:15 pm:

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Dark energy may just be a cosmic illusion

* 07 March 2008
* Amanda Gefter



WHEN two teams of astronomers set out in 1998 to measure the expansion rate of the universe, it was a routine sort of mission. The cosmic backstory had already been written: the universe began with the big bang, surging rapidly outward and then continuing to expand more and more slowly, held back by the relentless pull of gravity. The astronomers were searching for supernovae, exploding stars whose precious light would confirm these details.

They didn't imagine that the supernovae would have an entirely different tale to tell. Distant supernovae turned out to be much farther away than would be expected if the expansion of the universe had been slowing all along. Both teams were stunned by the inevitable conclusion: instead of slowing down, the universe's expansion was speeding up. But why?

It has become the most troubling question in astrophysics, and 10 years on we are no closer to answering it. Most physicists think the solution lies with an elusive force known as dark energy, which lurks in the emptiness of space, accounting for more than 70 per cent of the cosmos and causing space to expand at an ever-increasing rate.

What exactly is this dark energy? It might be the energy inherent in the fabric of space itself; or an exotic field called quintessence that expands space at changing rates; or something stranger still, a phantom energy that might one day tear the universe apart. Each possibility has its share of problems (New Scientist, 16 February 2007, p 28).

So a small but growing number of physicists have suggested something more radical: dark energy may not exist at all. Several recent papers argue that the universe's expansion is in fact decelerating as expected; it's just that gravitational effects arising from the distribution of galaxies create the illusion of acceleration. This is a controversial idea, to be sure, but if proven correct it would mean far more than just rethinking our models: a whopping 70 per cent of the universe will have just turned up.

The original argument for dark energy rests on a crucial assumption. To interpret the supernova observations, researchers assume the universe operates according to the standard model of cosmology. This model uses Einstein's general theory of relativity to calculate the geometry and overall behaviour of the universe. General relativity describes gravity at large scales, but gravity depends on how matter is distributed throughout space. The standard view since Einstein has been that although the density of matter in the universe varies from place to place - a galaxy here, a bit of empty space there - overall the universe is approximately smooth and uniform.

Through recent projects like the Sloan Digital Sky Survey, however, astronomers have created 3D maps of space, and they have come to realise that the universe simply isn't smooth (see Map). While matter started out evenly spread, as can be inferred from the nearly uniform temperature of the cosmic microwave background radiation - relic heat from the nascent universe - over billions of years it has coalesced into larger and larger structures. Hundreds of billions of stars aggregate in galaxies, galaxies form clusters, clusters amass into superclusters, and superclusters string together in filaments that encircle vast voids of empty space. "Now that we have precise observations, the theoretical treatment should catch up," says Syksy Rasanen, a physicist at the University of Geneva in Switzerland.

Despite the observed lumpiness, most cosmologists still think the universe behaves, on average, as though it were uniform. The problem, first raised by George Ellis in the early 1980s, has been that they couldn't test this idea because there was no way to take an average of space-time geometry using Einstein's equations. That changed in 2000, though, when Thomas Buchert of the University of Lyon in France published a set of equations based on general relativity that allowed cosmologists to average the universe's behaviour while including the effects of an uneven matter distribution. This paved the way for physicists to try to explain the observed expansion history of the universe using models based on the lumpy distribution of matter. "There has been an explosion of research in this direction," says Buchert (see "Living in a void"). "Before one invents exotic solutions like dark energy, this is the more natural approach."

But how can the distribution of matter account for the apparent accelerated expansion? The most promising model so far has been put forward by David Wiltshire, a physicist at the University of Canterbury in New Zealand (Physical Review Letters, vol 99, p 251101). Wiltshire has shown that by combining Buchert's equations with some strange quirks of general relativity he can explain the supernova observations without resorting to dark energy (New Journal of Physics, vol 9, p 377).

Because the universe is not smooth, says Wiltshire, observers need to take into account their own position in order to properly interpret cosmological measurements. In relativity, distance and time measurements are made in terms of an observer's rods and clocks. Cosmologists usually assume that rods and clocks across the universe are all identically calibrated, but for Wiltshire this is where they have gone wrong. Clocks that were in sync in the smooth, early universe become mismatched as the matter distribution grows increasingly lumpy. That's because gravity slows time, a proven relativistic effect. So a clock in a galaxy will tick more slowly than a clock in empty space. By now, Wiltshire says, the time told by a clock in our galaxy and the time told by one floating in a void could differ by as much as 38 per cent.

It is this mismatch that can explain the supernova data, he says. Back in 1998, the two teams, led by Saul Perlmutter of the Lawrence Berkeley National Laboratory in California and Brian Schmidt of the Australian National University in Canberra, looked at type 1a supernovae, which burn with a known brightness. Comparing a supernova's apparent brightness with its intrinsic brightness reveals its distance. Its red shift - the stretching of the light's wavelength - reveals how much the intervening space has expanded from the time the light left the supernova to the time it reached our telescopes. When the teams looked at distant supernovae, they found that they were much farther away, for their measured red shift, than they would be if the universe's expansion had always been decelerating.

This interpretation, however, assumes the standard cosmological model is correct. The standard model, because it is based on a uniform space with no distinct physical structures, describes us observers as floating in a freely expanding space, rather than confined to a galaxy. If our rods measure smaller volumes and our clocks are ticking more slowly than those of an observer in a void, as Wiltshire contends, then the simplification can lead to wrong conclusions.

For instance, the calculated expansion rate of the space between the Earth and the supernovae depends in part on the density of the intervening matter, because gravity slows expansion. Density is the amount of mass in a given volume, but volume depends on the way in which space is curved. In voids, space is negatively curved, so the volume for a given radius is larger than in the relatively flat space in which we live. Buchert had realised that taking the changing volumes into account alters what we calculate to be the universe's expansion history. This change alone is not enough, however, to account for the apparent acceleration.

Wiltshire's key realisation was that in addition to these volume corrections, the lumpiness of matter also requires corrections to clocks. Because we live in a gravitationally bound system - our galaxy - our clocks run more slowly than they would in a void. This means our calculations of how fast space is expanding will be wrong too. Together, Wiltshire says, the corrections to volumes and times do away with the apparent acceleration. "It is not really that the expansion of space is accelerating," he says. "Rather, our estimates of volume are too small and our estimates of time are too slow." Wiltshire's conclusion? The universe's expansion is slowing down, as originally thought (see Diagram).

His model challenges other basics of standard cosmology as well. According to Wiltshire's calculations, the age of the universe from our point of view should be 14.7 billion years, rather than the standard 13.7 billion. For hypothetical observers in voids, the situation is even more dramatic: for them the universe is 18.6 billion years old. Wiltshire thinks this can help account for some of the advanced structures that appear to have existed far earlier in the history of the universe than they should. "A return to thinking about observers and measurements in the manner Einstein taught us is what is going to solve a lot of puzzles in cosmology," he says.

Other researchers think it is a step in the right direction, but that the assumption of dense regions and voids, with nothing in between, may be unrealistic. "This is already very interesting," says physicist Luciano Pietronero of the University of Rome in Italy, "but I wonder what would be the result for a more realistic model." Nevertheless, he says that Wiltshire's work could ultimately lead to new insights into how to devise more realistic cosmological models.

Physicist Yurij Baryshev of St Petersburg State University in Russia agrees that the uneven matter distribution requires a new cosmological model, but he thinks Wiltshire's is too simplistic. "It is interesting, but only as the beginning of a discussion of the problem," Baryshev says.

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Is dark matter mystery about to be solved?

* 10 March 2008

As far as most of the universe is concerned, you're 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's 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...

THIS YEAR, there's a good chance that a sizeable chunk of our universe will turn up. A fair bit of the cosmos - 22 per cent of it, in fact - 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's 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'll 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's 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's 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't 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.

"Astronomers can never tell us what the dark matter is," 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.

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