The entire astronomical community undoubtedly had high hopes when the James Webb Space Telescope (JWST) launched on December 25, 2021. It reached the Lagrange point L2, its destination, that is, the region of Earth's orbit where the Planck satellite carried out its spectacular study of fossil radiation, the oldest observable light in the cosmos, telling us about its age, curvature, shape and its content informed in dark matter and energy.
The fossil radiation was emitted within a few thousand years, about 380,000 years after the Big Bang. The James-Webb does not make such early observations in the history of the observable universe, but it could allow us to go back at least 250 million years after the Big Bang and at least better understand the layers of light. Let's say between 400 million and a billion years, which was already accessible with Hubble, but much more imperfect.
The universe has continuously evolved for 13.8 billion years. Contrary to what our eyes tell us when we look at the sky, what makes it up is anything but static. Physicists make observations at different ages of the universe and run simulations in which they recreate its formation and evolution. It appears that dark matter played a large role from the beginning of the universe to the formation of the large structures observed today. © CEA Research
Primitive galaxies that should be invisible in Lyman-α emission
Nevertheless, a paper published in Nature Astronomy, available open access on arXiv, reports JWST observations that solve a puzzle that has puzzled cosmologists for some time. According to the standard cosmological model, which is based on dark matter and dark energy, the most distant galaxies should not glow much due to the so-called Lyman-α emission from hydrogen atoms. They would glow even less if we observed them in ancient layers of light, to the point that they would be invisible only less than a billion years after the Big Bang.
That's not the case, why? Would this point to another problem in standard cosmology like that of the value of the famous Hubble-Lemaître constant?
To understand what it really is, we have to turn to the emission of fossil radiation. In a few thousand years, the temperature of the universe's plasma has fallen so much due to its expansion that during this period the first hydrogen and helium atoms are formed, whose nuclei capture the free electrons and give rise to neutral atoms. This is the beginning of the famous Dark Age, because it will take about a hundred million years before a large number of stars begin to appear.
The phenomenon of reionization occurred very early in the history of the universe, making direct observation difficult. A few minutes after the Big Bang, the universe was still too hot for electrons to be captured by atomic nuclei: it was then completely ionized. The universe then continued to expand and cool until its temperature was low enough for electrons to bind to nuclei and form the first atoms. This so-called “recombination” took place about 380,000 years after the Big Bang. This moment also marks another important event in the history of the universe: While light is scattered very easily by electrons when they are free, this is much less so when they are bound to nuclei. . Recombination also marks the moment when the universe became transparent and light could spread freely there. © HFI Planck
These stars are very hot and emit radiation in the ultraviolet region, exactly corresponding to Lyman-α emission. However, even at this point, there is still masses of neutral hydrogen, especially near forming galaxies, and it will take hundreds of millions of years for the radiation from stars in these young growing galaxies and perhaps the first giant black holes to accumulate matter heats up and radiates accordingly, ionizing this neutral hydrogen between galaxies, which is rather opaque to Lyman-α emissions. The observable cosmos should therefore only slowly become transparent during the so-called reionization period, which, as we know, will end at the latest about a billion years after the Big Bang.
“Galaxies” made up of multiple colliding galaxies?
Astrophysicists believe they now have the key to the mystery of the abnormal luminosity of young galaxies while reionization is not yet sufficient. It therefore comes to us from JWST and its NIRCam, one of its instruments that observes in the near infrared and is able to see the light shifted towards these frequencies for distant galaxies.
NIRCam resolved images of galaxies that were actually large galaxies, but were surrounded by small nearby galaxies that were interacting or even colliding with each other.
Did you know ?
Lyman-α emission is light emitted at a wavelength of 121.567 nanometers when the electron in an excited hydrogen atom moves from an excited state in the n=2 orbital to its ground state n=1 (the lowest energy state that the atom has can). Quantum physics dictates that electrons can only exist in very specific energy states, meaning that certain energy transitions – such as when the electron of a hydrogen atom moves from the n=2 to the n=1 orbital – can be identified based on the wavelength of the hydrogen atom. the light emitted during this transition. The emission of Lyman-α is important in many areas of astronomy, in part because hydrogen is so abundant in the universe, and also because hydrogen is often excited by energetic processes such as active formation in the progression of stars. Therefore, Lyman-α emission can be used as a sign of active star formation. © ESA
The team behind the Nature Astronomy paper then used computer simulations to reproduce the phenomena occurring in these galaxies and, as an ESA press release states, “its members discovered that the rapid accumulation of stellar mass due to galaxy mergers leads to powerful emission of hydrogen and facilitated the escape of this radiation through channels that were free of the abundant neutral gas. Thus, the high rate of mergers of smaller, previously unobserved galaxies provided a compelling solution to the long-standing mystery of the unexplained early emission of hydrogen.
The team plans follow-up observations with galaxies in different stages of merger to further develop their understanding of how hydrogen emissions are emitted from these evolving systems. Ultimately, they will be able to improve our understanding of the evolution of galaxies.”
Even more clearly, the close collisions between several dwarf galaxies and large galaxies, originally surrounded by a halo of neutral hydrogen, led to ionization of this halo, allowing the formation of an ionized bubble responsible for the alpha emissions from hydrogen the frenzy of young stars forming in these transparent galaxies.