Dwarf galaxies have made the universe transparent

It was a big question: how did the universe emerge, at the very beginning, from its hydrogen fog? We even suspected black holes, but no. Answer today thanks to the James Webb telescope: these are small galaxies, a hundred times smaller than ours, through their radiation, which excited the gases and made the universe transparent.

An important discovery in astronomy: we understood how the universe became transparent. Mathilde Fontez, editor-in-chief of the scientific magazine Epsiloon sToday, we are interested in a new result from the James Webb Space Telescope, which observes the distant universe from its orbit, 1.5 million kilometers from Earth.

franceinfo: The James Webb Space Telescope saw one of the key stages in the formation of the Universe?

Mathilde Fontez: Yes, this principle is always dizzying: by looking very far into space, telescopes go back in time, since the light from these distant stars took time to travel to us: we photograph them as they were. were billions of years ago. More than 12 billion years ago, for these small galaxies which were captured by the Webb telescope last summer, and whose measurements have just been published.

The universe is 13.7 billion years old. These are therefore galaxies from the very beginning of the universe, 8 small packets of primordial stars, dwarf galaxies, 100 times smaller than our Milky Way. And they bear witness to one of the key stages in the history of the universe.

At this distance, can we study them?

Their number already provides information. The fact that we found eight like this, in this observation campaign, shows that they were numerous, very early after the Big Bang. And the researchers were able to collect information on the amount of ultraviolet radiation emitted by these galaxies.

It is higher than the simulations predicted: four times higher. And this is the data that astrophysicists have been waiting for. This radiation is powerful enough, we now see, to have dissipated the fog that filled the universe at the very beginning. It is these dwarf galaxies that have made the cosmos transparent.

Until now, we didn’t understand this step?

It was one of the big questions, yes: we know that the universe was born in a Big Bang, in the form of a very hot soup of particles. We know that the universe then cooled, diluted, the particles assembled into gases, the gases collapsed to form stars and galaxies.

But there was still gas everywhere: the universe was plunged into an opaque fog of hydrogen. How did it dissipate? Astrophysicists have talked about the action of large galaxies, or the powerful jets of matter emitted by black holes. There were different hypotheses.

There, today, we see that these are the small galaxies. Their UV radiation was powerful enough to excite the hydrogen gas and make it transparent. It’s a bit as if they had emitted bubbles of transparency around them, which gradually filled the entire universe, et released the light of the stars, so that it reaches us.