Washington (AFP)

Astrophysicists have known for 90 years that the universe is expanding, and they have been trying since to measure the exact speed of this expansion. But different methods produce different results, forcing scientists to wonder if their theory of the cosmos is solid.

In the 1920s, astrophysicists Georges Lemaître and Edwin Hubble discovered that the universe was expanding, instead of being a calm and narrow field.

In 1998, two teams of researchers discovered that the pace of this expansion accelerated with distance, and that the universe was filled with a mysterious "dark energy" causing this acceleration, since 14 billion years. They won the Nobel Prize in 2011.

The speed is calculated thanks to the "Hubble constant". This constant is estimated more and more precisely by physicists. The problem is that it is 67.4 according to one method, and 73 according to another.

The unit is in kilometers per second per megaparsec. A megaparsec equals about three million light-years. It reads as follows: galaxies three million light-years away (1 megaparsec) are 67.4 (or 73!) Kilometers apart.

Both numbers seem close, but not for cosmologists. Since each method has a small margin of error, the gap between the two results can not be explained by a simple miscalculation: there would be a more fundamental problem, something that would elude us about the universe and that the theory current is not able to explain.

Maybe the equation explaining the Big Bang and the cosmos needs to be updated.

- "Unknown physical laws" -

Current research, involving many teams around the world, is not only to refine star distance measurements, but also to find new methods to measure the escape velocity of galaxies and perhaps solve what physicists call soberly instead of a controversy or a crisis, the "tension" of the Hubble constant.

Illustrating this frenzied quest, a study describing a new method, written by researchers at the Max Planck Institute of Astrophysics in Germany and other universities, was published Thursday in the American journal Science.

They managed to measure the distance of two stars by observing how its light curves around large galaxies on its path to the Earth.

This team measured the constant at 82.4 km / s / megaparsec, but with a margin of error of plus or minus 10%, much more than the measurements made by other teams.

"If the tension is real, it means that the ancient universe was governed by unknown physical laws," says cosmologist Inh Jee AFP, coauthor of the study. The purpose of this new work is to check if there really is a fundamental problem.

She hopes that other observations made with her method will reduce the error, which she admits to be too great for the moment.

Asked by AFP, one of the 2011 Nobel Laureates Adam Riess, Johns Hopkins University, confirms that these new results are not precise enough to really weigh in the controversy, at least for the moment.

"I do not think that brings much to the current state of knowledge, but it's nice that people are looking for alternative methods, so congratulations," concludes the famous astrophysicist in an e-mail.

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