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Debate accelerates on universe’s expansion speed

Discrepancy between different ways of measuring Hubble constant refuses to fade

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12:00pm, July 22, 2016
spiral galaxy UGC 9391

UNRULY UNIVERSE  Different methods for measuring the universe’s expansion rate disagree. To make one such measurement, scientists observed Cepheid variable stars and a type 1a supernova in the spiral galaxy UGC 9391 (shown), located 130 million light-years away in the constellation Draco.

A puzzling mismatch is plaguing two methods for measuring how fast the universe is expanding. When the discrepancy arose a few years ago, scientists suspected it would fade away, a symptom of measurement errors. But the latest, more precise measurements of the expansion rate — a number known as the Hubble constant — have only deepened the mystery.

“There’s nothing obvious in the measurements or analyses that have been done that can easily explain this away, which is why I think we are paying attention,” says theoretical physicist Marc Kamionkowski of Johns Hopkins University.

If the mismatch persists, it could reveal the existence of stealthy new subatomic particles or illuminate details of the mysterious dark energy that pushes the universe to expand faster and faster.

Measurements based on observations of supernovas, massive stellar explosions, indicate that distantly separated galaxies are

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