This is a confusing scenario for astronomers. The James Webb Space Telescope has observed, in the remote ages of the Universe, a population of very massive galaxies that appear to have formed at a much faster rate than predicted, according to a study published Wednesday in Nature.

This phenomenon, which further analysis will have to confirm, occurred between 500 and 700 million years only after the Big Bang, which occurred 13.8 billion years ago. Either in the Universe very young, so very distant.

Six much more massive news

The James Webb Telescope (JWST), operational since July 2022, was able to explore this little-known region thanks to its NIRCam instrument and its powerful vision in the infrared, a wavelength invisible to the human eye and whose observation makes it possible to go far back in time. There have been six galaxies much more massive than expected in this early Universe. According to the interpretation of the new JWST images, these galaxies - called "candidates" at this stage because the discovery will have to be confirmed by spectroscopic measurements - contain many more stars than expected.

One of them would contain up to 100 billion. "That's about the size of the Milky Way, which is crazy," said Ivo Labbé, first author of the study. Because it took our galaxy 13.8 billion years to form this amount of stars, when this young galaxy would have done the same in just 700 million years. "That's 20 times faster," says the researcher from Swinburne University of Technology in Australia. Such distant galaxies of this size have no place in the current cosmological model that tries to understand the structure of the Universe. "The theory tells us that in these remote ages, galaxies are all small and grow very slowly. We could typically expect them to be 10 to 100 times smaller in terms of the number of stars," says the astrophysicist.

A model questioned?

So how to explain this scenario? The suspect could well be dark matter, mysterious invisible matter that populates the Universe. If scientists can't detect it, they know its behavior well enough and know that it plays a key role in galaxy formation. "Dark matter must 'fit together' to form a halo that attracts the gas from which the stars will be born," explains Ivo Labbé. However, this "coagulation" process is supposed to take a long time.

It would therefore seem that "things have particularly accelerated" in this early Universe, which would have been "more efficient than we thought" to make stars, comments David Elbaz, astrophysicist at the French Atomic Energy Commission (CEA), who did not take part in the study. The subject stirs up debate among cosmologists and this discovery is "all the more exciting because it is one more clue that the model is cracking," analyzes the scientist. He said Europe's Euclid space telescope, which is due to be launched into orbit this summer to try to unlock the secrets of dark matter, should help unravel the mystery.

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