The Big Bang marked the birth of our universe. An extraordinary event that, approximately 13,800 million years ago, started the enormous expansion of space that has given way to an immense cosmos (an expansion, by the way, that is still active and accelerated today).
In the first millionths of a second of the universe, the fundamental particles of nature were forged, including quarks and electrons. After a few seconds, the quarks had already combined to make protons and neutrons, and soon after these components came together to give rise to the first atomic nuclei in the cosmos (mainly hydrogen and helium).
At that time, the temperatures of the early universe were too high for atomic nuclei and electrons to combine: each time an electron (with a negative electrical charge) was captured by an atomic nucleus (with a positive electrical charge), the intense radiation from the oven of the universe he was tearing it out again.
But a soup of free electrons is not a place where electromagnetic radiation (light) can move and travel freely: the constant interactions with the electrons prevent it.
As the universe continued to expand, the temperature dropped until, approximately 380,000 years after the Big Bang, atomic nuclei and electrons had managed to combine to form nature's first chemical elements (atomic hydrogen and helium). At that instant, light was freed, for the first time, to travel through the entire universe. It was extraordinarily energetic electromagnetic radiation, in the form of gamma rays.
Today, after 13.8 billion years of expansion and cooling of the cosmos, that radiation reaches us from all directions, but converted into microwaves (a form of light, less energetic than what we commonly call visible light, and that our eyes cannot see). perceive). The study of this cosmic microwave radiation has allowed us to know, with an exquisite level of detail, what the young universe was like and how events unfolded in its first moments of life.
But although high-energy radiation from the early universe could travel freely, no visible light sources yet existed. Our universe was still a dark place.
A few hundred million years after the Big Bang, the first stars were created, when gravity, through a slow but unstoppable process, collapsed the great concentrations of hydrogen and helium into huge spheres.
These first suns shone with powerful light, but the cosmos was still opaque. The reason is that visible light has just enough energy to interact with hydrogen atoms in space and excite their single electron. In this way, the radiation emitted by the suns was sequestered, unable to go too far without finding hydrogen atoms with which to interact. It was the phase of the life of the universe that we call the Dark Ages.
However, after approximately 1 billion years, the universe had become totally transparent. That is, visible light could already travel enormous distances without obstacles. It is the cosmos we currently see.
The process that marked the end of the Dark Ages has been called reionization. As new stars were created, and these clustered into galaxies, the light emitted by these energy sources began to separate the electrons from the hydrogen nuclei again. Without atomic hydrogen, visible light was freed to travel through space, and free electrons, which in the first sighs of the universe had prevented any type of radiation from advancing, were now not a problem thanks to the fact that the cosmos was sufficiently expanded to so that the probability of interaction was very small.
However, until now we did not have too many details about the transition process between the opaque and transparent universe. For example, at what rate did the reionization process progress?
Fortunately we have, in space, the deepest-gazing eye in history. The James Webb Space Telescope can collect and analyze very old light, and is capable of showing what our universe was like shortly after the first stars and galaxies formed.
By observing galaxies that existed in the transition stage from the Dark Ages to the transparent universe, a group of researchers, led by the Federal Institute of Technology in Zurich, has discovered that about 900 million years after the Big Bang, the radiation emitted by galaxies began to create, around it, large bubbles of ionized hydrogen, with a radius of approximately 2 million light-years (for reference, the distance that separates our Milky Way from the Andromeda galaxy is 2.5 million years- light).
And over the next hundreds of millions of years, as the ionization of hydrogen progressed, these bubbles increased in size until they overlapped, eventually making our entire universe transparent.
The data that have allowed us to understand this sequence of events have been obtained by combining the observations of the James Webb with those of the W. M. Keck telescopes in Hawaii, and the VLT and Magellan of the European Southern Observatory (ESO) in Chile. This collaboration of observatories has made it possible to study how the distant light from very old galaxies has been affected, on its way to our instruments, by ever younger regions of the universe.
The study has been published in the Astrophysical Journal.
