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The Sciences

Dispatches from AAS: The Not-There Universe

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One of my favorite songs from the 1960s is The Zombies’ “She’s Not There,” a blast of anxious pop music about a girl whose most fascinating quality is that she is nowhere to be found. One of my favorite themes from the January meeting of the American Astronomical Society in Long Beach, California, might be called “Discoveries That Aren’t There”: Several major teams of researchers presented studies that definitively did not find what they were looking for, and that were all the more fascinating for it.

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The Zombies' first U.S. album, featuring "She's Not There." Photo courtesy of Amazon.com. Finding nothing might sound like a recipe for boredom but—as in a great pop song—these “not-there” findings replace one mystery with another, deeper and more intriguing one. Robert Nemiroff of Michigan Technological University (the man behind the beautiful Astronomy Picture of the Day web site) kicked things off with an experiment designed to map the fundamental structure of space. His quest was to examine the fundamental structure of space at extremely fine scales, much smaller even than a single atom. Albert Einstein’s general theory of relativity implies that space should be smooth at very small distances, just as it is smooth at the distances we experience. Some newer theories, which attempt to go beyond relativity, suggest otherwise: They predict that sub-subatomic space is a froth of unseen particles and energy. Nemiroff figured out a way to see who is right. He tracked gamma rays—radiation that is like light but much more energetic—from an exploding star roughly 7 billion light years from Earth, and looked for signs that they had scattered off any frothy space along the way. He found none. For the umpteenth time, a challenge to Einstein has failed. OK, so the universe is smooth. But is it steady? Two other researchers at Long Beach tried that tack instead, looking for evidence that the laws of nature change over time. Scientists (and all of us, really) take for granted that nature works the exact same way today as it did yesterday, but there’s no reason why that must be true. In fact, some much-heralded theories that attempt to extend the known laws of physics say that it should not be true. As a result, some basic measure of physical reality, like the rules of electromagnetism or the relative masses of key particles like protons and electrons, might look different in the distant universe. Jonathan Whitmore of Swinburne University tackled the former possibility, while Roger Thompson at the University of Arizona went after the latter. In a sense the results were obvious before Whitmore and Thompson opened their mouths. If either man had found a breakdown in the laws of physics, the news would have made global headlines. So I already had anticipated what you undoubtedly anticipated as well: Nothing. Nada. Zero. Zilch. Dark Matter Perhaps the most intriguing not-there result at the Long Beach meeting came from Douglas Finkbeiner at the Harvard-Smithsonian Center for Astrophysics. Over the past five years, a number of credible astrophysicists have claimed to find anomalous radiation emanating from the center of our galaxy. And not just any kind of radiation; these rays (another form of gamma rays, coincidentally) seemed to be the visible byproduct of the otherwise invisible material known as dark matter. Dark matter seems to be everywhere, spinning galaxies and pulling them together, but nobody has ever seen it; the evidence for dark matter is totally circumstantial, based on the gravitational pull of this unseen stuff. This is the kind of mystery that drives astronomers mad, and that makes everyone else wonder if they are just making things up. According to one theory, dark matter particles might come into view on rare occasions when they bump into each other and disintegrate in a flash of radiation. That is the reported sighting that Finkbeiner determined to track down.

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Results from NASA's Galaxy Evolution Explorer and the Anglo-Australian Telescope atop Siding Spring Mountain in Australia confirm that dark energy (represented by purple grid) is a smooth, uniform force that now dominates over the effects of gravity (green grid). Yet dark energy has yet to be seen. Illustration courtesy of NASA/JPL-Caltech. Sing it again: She’s Not There. After eight months of watching for gamma rays using NASA’s ultra-sensitive Fermi space observatory, Finkbeiner found very little supporting evidence of disintegrating dark matter—or really, of anything strange at all. The lack of strangeness being reported at the American Astronomical Society meeting, though, was itself very strange. Nobody thinks that the current models of physics are complete, because they cannot explain some fundamental things such as how gravity works over very short distances. And the universe appears to be full of dark matter and enigmatic types of energy that are not explained by current physics theory. As great of a genius as Einstein was, his equations cannot—do not—account for all of the mysteries of the universe. “We should all be lying awake at night wondering what dark matter is,” Finkbeiner reflected after his presentation. “What gets me out of bed in the morning is the prospect of finding brand new physics.” Nemiroff echoed a similar sentiment, saying that “perhaps the golden age of cosmology is not over just yet. There may be more discoveries out there.” Or put another way: Just because we haven’t seen her yet doesn’t mean she’s not there. --

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