"Hubble Ultra Deep Field" by NASA and the European Space Agency / Public Domain

How fast is the Universe expanding? Cosmologists just got more confused

Jul 17, 2019

By Davide Castelvecchi

For much of this decade, the two most precise gauges of the Universe’s rate of expansion have been in glaring disagreement. Now, a highly anticipated independent technique that cosmologists hoped would solve the conundrum is instead adding to the confusion.

In results unveiled1 on 16 July and due to appear in the Astrophysical Journal, a team led by astronomer Wendy Freedman of the University of Chicago in Illinois presents a technique that measures the expansion using red-giant stars. It had promised to replace a method that astronomers have been using for more than a century — but for now, the speed measurement has failed to resolve the dispute because it falls half way between the two contentious values.

“The Universe is just messing with us at this point, right?” tweeted one astrophysicist about the paper.

“Right now, we’re trying to understand how it all fits together,” Freedman told Nature. If the cosmic-speed discrepancy is not resolved, it could mean that some of the basic theories cosmologists use to interpret their data — such as assumptions about the nature of dark matter — could be wrong. “Fundamental physics hangs in the balance,” Freedman says.

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