Science

Scientists still cant agree on the universes expansion rate

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SANTA BARBARA, Calif. — Its one of the most talked-about issues in physics: Two measurements of the universes 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 were 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).

In the new study, cosmologist Wendy Freedman of the University of Chicago and colleagues made another measurement of the Hubble constant that also relies on supernovas. Scientists determine how fast the universe is expanding by measuring how the supernovas light is stretched to redder wavelengths by that expansion. But that requires an estimate of how far away those supernovas are from Earth.

 Freedman and colleagues used a different, independent technique to help gauge the supernovas distances. Supernovas of a variety known as type 1a supernovas have a known brightness. Using a “distance ladder,” that relies on other, nearby objects with known brightness, scientists can work out the distances of the supernovas. Previous distance ladders had relied on variable stars called Cepheids, but the new work used red giant stars, which reach a uniform peak brightness. 

The new Hubble constant measurement is a bit lower than the previous supernova estimates, about 70 kilRead More – Source
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