Cosmology seems to be heading for a showdown on one of its most basic questions: how fast is the Universe expanding?
For more than a decade, two types of measurement have been in disagreement. Observations of the current Universe typically find the rate of expansion — called the Hubble constant — to be about 9% faster than predictions based on early-Universe data.
Researchers hoped that the James Webb Space Telescope (JWST), which launched in late 2021, would help to settle the question once and for all. But consensus has so far failed to materialise. Instead, two teams of cosmologists have calculated different values for the Hubble constant — despite both observing the recent Universe using the JWST.
Wendy Freedman, an astronomer at the University of Chicago in Illinois, and her collaborators presented preliminary results from their JWST observations today at a conference at the Royal Society in London. The Hubble constant they measured was 69.1 kilometers per second per megaparsec, meaning that galaxies separated by one million parsec (around 3 million light years) are receding from each other at a rate of 69.1 km/s.
This is only slightly larger than the 67 km/s per megaparsec predicted using early-universe data from Europe’s Planck satellite. But it is at odds with recent work by Adam Riess, an astrophysicist at Johns Hopkins University in Baltimore, Maryland, and his collaborators, who calculated a substantially higher Hubble constant, of at least 73 km/s per Mpc1,2,3.
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