Sunday, September 27, 2009

Why do we need dark energy to explain the observable universe? Two mathematicians propose an alternate solution that, while beautiful, may raise even more questions than it answers.

Wide Angle / by Veronique Greenwood /
An alternative theory eliminates dark energy by placing Earth at the center of expansion


Expanding Universe What dark materials lie behind universal expansion? Maybe none NASA
Against all reason, the universe is accelerating its expansion. When two prominent research teams dropped this bombshell in 1998, cosmologists had to revise their models of the universe to include an enormous and deeply mysterious placeholder they called “dark energy.” For dark energy to explain the accelerating expansion, it had to constitute more than 70 percent of the universe. It joined another placeholder, “dark matter,” constituting 20 percent, in overshadowing the meager 4 percent that make up everything else—things like stars, planets, and people.

That a huge fraction of the universe could be composed of this enigmatic stuff was unnerving, to say the least. But what was most disturbing to cosmologists was that the discovery required adding a term to Einstein’s equations of general relativity. These equations were derived from pure mathematics and had already beautifully predicted the expansion of the universe, discovered by Edwin Hubble in 1929. To many, even those who accepted its usefulness in explaining the data, dark energy was an inelegant addition. Over the last decade, some researchers have been working to describe what dark energy might be, but others have gone back to see if the equations of general relativity can be tweaked to avoid having to use such a troublesome piece of math.

Building from the Einstein equations, mathematicians Blake Temple and Joel Smoller have now found a way to explain the observations that led researchers to propose dark energy. If their solution, published in the Proceedings of the National Academy of Science, fits the data, it could provide a way out of the unpalatable notion of a dark-energy-dominated universe.

The concept of dark energy—which is simply that a phenomenal amount of energy exists in the vacuum of space—emerged from a discrepancy between how far away supernovae were supposed to be and how bright they appeared. In the 1990s, two teams, one led by cosmologists at the Lawrence National Labs (LBNL) and the other by Australia’s Mount Stromlo Observatory, were in the midst of a massive survey of Type Ia supernovae. Measuring the brightness of these huge stellar explosions lets scientists deduce how much the universe has expanded since the light began its journey.

The two teams were hoping to observe that the brightness of the supernovae, which grows increasingly dim the farther they are from Earth, would plateau at the farthest edge of the observable universe. This would reflect the theory that the universe’s expansion was slowing down due to the gravitational pull of matter. But what they observed was the exact opposite. In fact, the supernovae were receding at a rate that would only be possible in a universe with no matter at all. After considering and ultimately rejecting alternative explanations for the dimming, both teams came to the same conclusion: The supernovae were dim because they were being pushed away by a wave of universal, accelerating expansion. And that could only happen if a huge amount of energy was counteracting the force of gravity. Thus, dark energy was conceived.

To Temple and Smoller, mathematicians at the University of California–Davis and the University of Michigan, respectively, dark energy seemed an ad hoc addition to cosmology. While performing mathematical research on shockwaves, Temple and Smoller realized that an expanding wave with its epicenter near the Earth could produce the dimming effects the two teams had observed. The two started talking to astrophysicists and other mathematicians to flesh out the idea. What they discovered was as utterly unexpected to them as dark energy was to cosmologists: An accelerating wave of expansion following the Big Bang could push what later became matter out across the universe, spreading galaxies farther apart the more distant they got from the wave’s center. If this did happen, it would account for the fact that supernovae were dim—they were in fact shoved far away at the very beginning of the universe. But this would’ve been an isolated event, not a constant accelerating force. Their explanation of the 1998 observations does away with the need for dark energy.

The theory is attractive because it describes the effect astronomers observed using only general relativity. It also provides a mechanism for a scenario that’s been discussed in cosmology for some time, the “bubble of underdensity”—the idea that the Earth might be in an area with a low mass density compared to the rest of the universe, which would account for the distance of the supernovae. And Smoller and Temple say that once they have worked out a further version of their solutions, they should have a testable prediction that they can use to see if the theory fits observations.

But as an explanation to replace dark energy, there are at least two serious problems with Temple and Smoller’s wave of expansion, cosmologists say. One is that the supernovae are not the only clues we have that indicate dark energy is real; various characteristics of the cosmic microwave background (the “afterglow” of the big bang, produced when the universe was extremely young) also suggest the presence of dark energy, says Philip Hughes, an astrophysicist at University of Michigan who worked with Temple and Smoller while they were developing their hypothesis. Simulations of the growth of the universe reflect our observations with breathtaking fidelity when dark energy is included, adds Michael Wood-Vasey, a cosmologist at University of Pittsburgh who studied at LBNL. As ad hoc as dark energy may be, it nevertheless reflects reality.

But perhaps the largest objection voiced is that this model would require Earth to be at the center of the universe. In other words, it would violate the Copernican principle, which states that the Earth does not have a special, favored place and that the universe is essentially homogeneous.

Smoller and Temple readily acknowledge this issue. “If you want to preserve the Copernican principle and explain anomalous acceleration, you have to use dark energy,”
Smoller says. “But it is not a law of physics—it’s just a simplifier.” Galaxies and stars are accepted violations of the Copernican principle, of universal homogeneity, on a very local scale. After all, a point where a star has formed is not homogenous with the empty space surrounding it. They argue that their expanding wave could be a similarly legitimate violation on a larger scale. Hughes points out, however, that surveys have found the universe to be homogenous even above the level of our cluster of galaxies. And beyond the empirical level, violations of the Copernican principle make cosmologists uneasy. “It’s very philosophically disconcerting,” Wood-Vasey says. “It’s not very satisfying.”

More than anything, an aesthetic sense of what’s natural or elegant seems to divide the mathematicians and cosmologists. Temple and Smoller emphasize that acceleration comes naturally from their equations, with no need for extra factors. The addition of dark energy is, in the words of Jim Glimm, former president of the American Mathematical Society, “a little bit ugly.” But Wood-Vasey and other cosmologists balk at a theory that arises purely from equations instead of observations. “We all want a fundamentally deeper, more beautiful explanation,” he says, a simple rule that explains a variety of observations. “Dark energy isn’t satisfying because we don’t know what it is,” he concedes. But it manages to bring our models of the universe in accord with our observations, not an insignificant feat, and not devoid of beauty in its own right.

Some cosmologists are dismissive of Smoller and Temple’s theory: “It’s a piece of mathematics,” says Arizona State’s Lawrence Krauss. “I think that these mathematicians might have chosen the beautiful over the true.” But Hughes, who calls it “a tour de force of mathematical analysis,” argues that though it presents a radical philosophical shift, the wave theory could nevertheless be useful to cosmologists.
“The concept of ‘dark energy’ is a way of parameterizing our ignorance,” he said in an email. “Given our shaky understanding of the physics behind it, I would hope that people are open-minded enough to see what might be learned from this work. We have for practical purposes no understanding of ‘dark energy’; there isn’t even a glimmer of consensus.”

While the debate continues on Earth, NASA’s Joint Dark Energy Mission, headed by the LBNL cosmologists, is planning a space telescope to gather more detailed data than ever before about what dark energy might be. As the mathematicians work towards testing their theory, a process that might need JDEM data, the project moves slowly through conceptual shifts, funding cuts, and changing timelines. The launch date, as of now, is 2016.

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