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Exactly how fast is the universe expanding? Astrophysicists are closing in on the Hubble constant

Scientists are still not completely sure, but a Princeton-led team of astrophysicists has used the neutron star merger detected in 2017 to come up with a more precise value for this figure, known as the Hubble constant. Their work appears in the current issue of the journal Nature Astronomy.

"The Hubble constant is one of the most fundamental pieces of information that describes the state of the universe in the past, present and future," said Kenta Hotokezaka, the Lyman Spitzer, Jr. Postdoctoral Fellow in Princeton's Department of Astrophysical Sciences. "So we'd like to know what its value is."

Currently, the two most successful techniques for estimating the Hubble constant are based on observations of either the cosmic microwave background or stars blowing themselves to pieces in the distant universe.

But those figures disagree: Measurements of exploding stars -- Type Ia supernovae -- suggest that the universe is expanding faster than is predicted by Planck observations of the cosmic microwave background.

"So either one of them is incorrect, or the models of the physics which underpin them are wrong," said Hotokezaka. "We'd like to know what is really happening in the universe, so we need a third, independent check."

He and his colleagues -- Princeton's NASA Sagan Postdoctoral Fellow Kento Masuda, Ore Gottlieb and Ehud Nakar from Tel Aviv University in Israel, Samaya Nissanke from the University of Amsterdam, Gregg Hallinan and Kunal Mooley from the California Institute of Technology, and Adam Deller from Swinburne University of Technology in Australia -- found that independent check by using the merger of two neutron stars.

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