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A Cosmic Crisis

How a Dispute over a Single Number Became a Cosmological Crisis

Two divergent measurements of how fast the universe is expanding cannot both be right. Something must give—but what?

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  • Astronomers have repeatedly calculated the rate of the universe’s expansion—the Hubble constant—with two different techniques. These measurements have produced a seemingly intractable conflict.
  • One method, which involves measuring supernovae and stars in the relatively recent universe, arrives at one value. The other strategy, which uses light left over from shortly after the big bang, finds another.
  • Experimental problems could cause the discrepancy, but no one is sure what those problems would be. Another possibility is that the conflict points to undiscovered phenomena—“new physics.”

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