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It’s one of the most talked-about issues in physics: Two measurements of the universe’s expansion rate disagree. Now, a technique that aimed to resolve the mismatch has produced a third estimate that falls between the previous two. So the controversy endures, scientists report in a study accepted in the Astrophysical Journal. 

One measurement of how fast the universe is expanding — a number known as the Hubble constant — comes from supernovas, or exploding stars. Another is based on the cosmic microwave background, the light released shortly after the Big Bang. Previous supernova measurements indicate that the universe is expanding at a rate of about 74 kilometers per second per megaparsec, or about 3.3 million light-years. But the cosmic microwave background pegs that number at around 67 kilometers per second per megaparsec. 

That difference has led some researchers to suggest that we’re missing something important in our understanding of the universe, such as new, unidentified subatomic particles that might inhabit the cosmos (SN: 8/6/16, p. 10).

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