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Dark energy is apparently even more mysterious than astronomers had thought.

Scientists first proposed the existence of this invisible force two decades ago, to explain the surprising discovery that the universe's expansion is accelerating. (Surprising and incredibly important; the find netted three researchers the Nobel Prize in physics in 2011.)

The most-used astrophysical model of the universe's structure and evolution regards dark energy as a constant. Indeed, many astronomers believe it to be the cosmological constant, which Einstein posited in 1917 as part of his theory of general relativity. [The History & Structure of the Universe in Pictures]

But a new study of enormous, superbright black holes known as quasars suggests that dark energy could be miscast as the cosmological constant, or any kind of constant; the force may have varied since the universe's birth 13.8 billion years ago, research team members said.

"We observed quasars back to just a billion years after the Big Bang, and found that the universe's expansion rate up to the present day was faster than we expected," study lead author Guido Risaliti, of the University of Florence in Italy, said in a statement. "This could mean dark energy is getting stronger as the cosmos grows older."

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Category: Science