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Instead, the astronomers were astonished to find that cosmic expansion is not slowing down at all: It is speeding up. What's more, this unheralded acceleration has been going on for the past five billion years. It is as if a ball, thrown into the air, at first slowed but then sped up and simply flew away. No natural force on Earth can do such a thing—and none in the known universe could be accelerating the cosmic expansion rate. Nor is the newly discovered force particularly subtle: Taking to heart Einstein's E=mc²—that energy and matter are two sides of the same coin—scientists calculate that the new force comprises 70 percent of all the matter and energy in the universe.

Physicists have taken to calling this unknown force dark energy. But as University of Chicago cosmologist Rocky Kolb says, "Naming is not explaining," and nobody yet knows what dark energy actually is.

It may well be that dark energy is inherent to space itself. Physicists had long suspected that such a "vacuum energy" must exist, since quantum fields, which contain energy, permeate even the emptiest voids out between the galaxies. Yet when physicists calculate the amount of energy in the vacuum, they get absurdly large results, ranging from infinite ("That can't be right," ex Nobel laureate physicist Steven Weinberg mused) to zillions of times more than is required even to account for the mighty force of dark energy. The disparity troubles them; Weinberg calls it "the worst failure of an order-of-magnitude estimate in the history of science." Clearly, something is wrong with either the observations (but no error has yet been found, in many ongoing studies with Hubble and other telescopes) or with the consensus models of physics and cosmology, which for all their flaws stand as one of the grandest attainments of modern science.

Fortunately, knotty problems often prove to be gateways to scientific breakthroughs. Knowing this, the cleverest scientists are more attracted to vexing questions than to comforting answers. (Physicist Niels Bohr, confronted with one such puzzler, reportedly exclaimed, "How wonderful that we have met with a paradox. Now we have some hope of making progress.") But where might the dark-energy riddle lead?

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