What We Don't Know in Physics

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What Is Dark Energy?

Even if dark matter is detected and its nature revealed, another curious phenome­non that physicists have called dark energy raises plenty of other questions. ­"Galaxies are being pushed apart by some repulsive force," explains Edmund ­Bertschinger, head of the physics department. "The measurements of the last decade tell us that something very much like gravitational repulsion has taken over the universe." That is, the universe is expanding at an accelerating rate, but physicists don't know why. Is it because of dark energy? Or is dark energy just a concept that's patching over a misunderstanding of the laws of physics?

Despite the similarity in their names, dark energy is probably completely unrelated to dark matter--and is a much greater mystery. "There are plausible explanations of dark matter," says Bertschinger. "We do not have plausible models of dark energy that make sense in the context of high-energy physics." Work done by Bertschinger and many others has shown that tests to distinguish between dark energy and a modified form of gravity will be very difficult to develop. However, Bertschinger is doing theoretical work that he hopes will lead to such tests over the next decade.

"This is a big time in physics," says Sciolla. "Everything will get an answer in the next few years, we hope." And then what? Well, then she and her colleagues might be out of a job, Sciolla jokes. "But," she adds, "I'm sure there will be plenty of new questions that will be unanswered."