Erasing Dark Energy

Wide Angle / by Veronique Greenwood /

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.

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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.

Originally published September 24, 2009

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