Hubble trouble

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Science  10 Mar 2017:
Vol. 355, Issue 6329, pp. 1010-1014
DOI: 10.1126/science.355.6329.1010

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There is a vigorous new debate over the Hubble constant, the expansion rate of the universe. New techniques suggest that the Hubble constant is 8% lower than a leading number that astronomers had mostly settled on. For nearly a century, they have calculated it by meticulously measuring distances in the nearby universe and moving ever farther out. But lately, astrophysicists have measured the constant from the outside in, based on maps of the cosmic microwave background, the dappled afterglow of the big bang that is a backdrop to the rest of the visible universe. By making assumptions about how the push and pull of energy and matter in the universe have changed the rate of cosmic expansion since the microwave background was formed, the astrophysicists can take their map and adjust the Hubble constant to the present-day, local universe. The numbers should match. But they don't. It could be that one approach has it wrong. But if the disagreement holds, it will be a crack in the firmament of modern cosmology that points to new physics.