The James Webb telescope, in the first light of cosmic dawn – Orange news

AFP, published on Saturday, December 11, 2021 at 3:34 p.m.

The James Webb Space Telescope (JWST) will lift the veil a little more on the first light of cosmic dawn, when the first stars and galaxies to populate the Universe appeared.

It will take over from Hubble, which observes space essentially in the domain of visible light, by venturing into a wavelength that escapes the eye: the mid-infrared.

In astronomy, the further one sees, the longer one sees. Where particles of sunlight take eight minutes to reach an earth’s eye, the James Webb’s dome aims to capture light from the first galaxies more than 13.4 billion years ago.

Those that appeared in the young Universe, less than 400 million years after the Big Bang.

But with expansion, this light travels more and more ways to reach the observer, and in doing so it “blushes”.

As the sound of a moving object becomes muffled, the light wave stretches and passes from the frequency visible to the naked eye, to that of the infrared.

Hubble, launched in 1990, reached a limit at 13.4 billion years ago, with the discovery of the galaxy GN-z11, an insignificant little spot but also “a surprise, with a brightness that we did not expect in such a distance “, Swiss astrophysicist Pascal Oesch, his” discoverer “, told AFP.

The JWST “will provide even more precise images, with a 100 times greater sensitivity, which will allow us to explore this era in extraordinary detail”. He is betting that “many, many more galaxies, but much less luminous” will then be revealed.

– Turn on the light –

Using his infrared capability, James Webb will not only see older objects, but also the clouds of interstellar dust that absorb starlight and hide them from Hubble’s view.

This “invisible light makes it possible to see what is hidden in the clouds, the birth of stars and galaxies”, explains David Elbaz, an astrophysicist at the French Atomic Energy Commission.

The French Atomic Energy Commission (CEA) is the prime contractor for the Mirim imager, which, coupled with the NASA MRS spectrograph, will study these structures in the mid-infrared, and will also be able to “see the signature of the atoms in distant galaxies, ”Elbaz adds.

The stakes are high: to help explain a key stage in the evolution of the Universe, when, as Mr. Oesch says, we “turned on the light, when the very first stars began to form”. In other words, “the cosmic dawn”.

Shortly after the Big Bang flash, the Universe entered a “dark age”, a neutral gas bath composed mostly of hydrogen and helium, without light.

The theory wants that this gas is condensed in “wells” of dark matter, – a mysterious undetectable substance, whose existence is simply theorized -, to give birth to the first stars. These stars, which will multiply by a snowball effect, will begin to ionize, that is to say electrically charge, the neutral gas of the Universe. A process called “reionization”, which will bring the Universe out of its opacity to make it “transparent”.

However, “we do not know when the first galaxies are formed”, remarks astrophysicist Françoise Combes, simulations placing the action between 100 and 200 million years after the Big Bang.

“The theory is that all the small galaxies will reionize the Universe, because there are so many of them, and what we’re going to check with the James Webb is to see if there are enough of them for that”, explains Professor Combes.

However, no one imagines observing the first stars. Called “population III” stars, these giants of perhaps 100 to 1,000 solar masses, with phenomenal temperatures and short lifespans, will begin to ionize the neutral gas of the Universe.

To study this phenomenon is to study the formation of galaxies.

The hope is to see “the first galaxies, which carry the second generation of stars, which will perhaps teach us things about the first”, says Nicole Nesvadba, astronomer at the Lagrange laboratory of the Observatory of the Riviera. Before adding facetiously, “I don’t know, ask us in five years.”