Upon its birth in the big bang, nearly 14 billion years ago, the cosmos began expanding. And for most of the 20th century, scientists assumed gravity would gradually slow down that expansion, with all the universe’s matter acting as drag.
But observations in the late 1990s showed that’s far from the case. Careful, high-precision estimates of cosmic distances using a special class of exploding stars called type Ia supernovae revealed that, against all expectations, the universe’s expansion is actually speeding up. This is akin, on cosmic scales, to tossing a ball overhead only to see it fly away at an ever increasing speed rather than fall back down. The matter of “why” this can happen remains one of the most pressing mysteries in physics.
“The theorists are having a field day,” says Tamara Davis, an astrophysicist at the University of Queensland in Australia. “There are hundreds upon hundreds of theories about what’s driving cosmic acceleration.”
Scientists generally call the culprit dark energy, but no one knows what it is. Its behavior, however, may offer a potent clue to its identity: if dark energy’s expansion-accelerating effect holds steady over time, this would fit quite comfortably within what’s known as the standard model of cosmology, the best overarching explanation of the universe’s evolution that scientists have yet devised. The trouble is that no one has been able to say with certainty whether dark energy is actually so fixed—and if dark energy’s strength can change over time, reconciling this with the rest of physics may require rethinking our understanding of gravity.
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