The grav-wave seed heads are getting closer to nailing Constant, the speed of rapid expansion of the universe • The Register

Scientists believe they have taken a big step forward in determining the rate of expansion of the universe, called the Hubble constant, thanks to gravitational wave data.

The astroboffins, based in Princeton in the United States, estimate this constant between 65.3 and 75.6 km per second per megaparsec, according to an article published this week in Nature Astronomy. This means that the universe has been observed at a speed of up to 75.6 kilometers per second on an observed megaparsec, which spans 3.2 million light-years.

The rate of expansion has been the subject of intense scientific debate, as the two main methods used to calculate the constant do not seem to agree. One model, called the cosmological model, analyzes cosmic background noise (CMB) over large distances, while the cosmic distance scale method studies local supernova explosions to estimate the Hubble constant.

"One or the other of them is incorrect, or the models of the physics that underlie them are wrong. We would like to know what is really going on in the universe, so we need a third independent control, "said Kenta Hotokezaka, lead author of the journal and postdoctoral fellow at the Princeton Department of Astrophysics.

The Princeton team decided to look at a different source to measure the Hubble constant: the gravitational waves emitted by two merging neutron stars. The waves of the event detected two years ago (codename GW170817) provide researchers with a distinctive pattern that encodes detailed information about the size of stars and their distance from the Earth when they crashed.

Since gravitational waves wave in space at the speed of light, scientists can estimate the extent of their redshift or elongation over long distances to determine the rate of expansion. The results indicate an average measurement of about 70.3 kilometers per second per megaparsec, which is faster than the estimates calculated from the CMB but slower than the other figures obtained by looking at the stars.

"The resolution of the radio images that we took was so high that if it was an optical camera, it could see hair forming on a person's head three miles away," he said. Adam Deller, co-author of the journal and associate professor of physics at Swinburne University of Technology in Australia.

Unfortunately, the number does not quite solve the mystery of how quickly the universe is expanding. Hotokezaka said that the accuracy of the Hubble constant calculated from gravitational waves was at best "pretty good". The team estimates that it will need to study at least 15 other neutron fusion events, such as GW170817, to further refine the Hubble constant.

"This is the first time that astronomers have been able to measure the Hubble constant using a joint analysis of gravitational wave signals and radio images," said Hotokezaka. "It is remarkable that a single fusion event allows us to measure the Hubble constant with great precision – and this approach is neither based on the cosmological model nor on the cosmic distance scale." ®

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