At the end of the Big Bang, about 13.8 billion years ago, the primordial universe was plunged into darkness. Matter forms a “soup” so dense that it constitutes a veritable prison for photons, which therefore cannot yet travel freely. Then, 380,000 years later, the state of matter changes and light can finally escape. But what really happened?
13.8 billion years ago, the early universe was very different from our current universe. Indeed, the Standard Model of cosmology tells us that at that time the latter was extremely hot and dense. Following the Big Bang (about 10 -12 seconds later), the universe enters the "quark epoch", matter then exists as a primitive "soup" of free quarks and gluons called "quark-gluon plasma", in which there are also leptons (electrons, neutrinos, etc.). During this period, the temperature and density of the universe are so high that the energy of the different particles and their collisions prevent quarks from binding together to form hadrons.
Then, the universe gradually cooled, the energy per particle decreasing correspondingly until it was no longer sufficient for the quarks to remain free. So, about 10 -6 seconds after the Big Bang, quarks bind and combine with each other via the mechanism of color confinement to form hadrons.
At the end of the quark epoch, the quarks therefore bind together to form hadrons, more particularly protons and neutrons (nucleons). Subsequently, the nucleons in turn combine to form the first atomic nuclei of hydrogen, helium and lithium:this is primordial nucleosynthesis. However, the temperature of the universe is still too high for these atomic nuclei to capture electrons and form atoms. The electrons then circulate freely in the universe.
In the middle of atomic nuclei and free electrons, photons (light) try to make their way. Nevertheless, since the universe is in thermal equilibrium, the average energy of electrons and photons being substantially identical, the interaction between matter and radiation is maximal:the photons being continuously absorbed, emitted and diffused by the surrounding electrons via the mechanism Thomson scattering (scattering of a photon by a free electron). The mean free path, that is to say the average distance traveled by a particle between two collisions of photons is then very short, giving a dark, dark and opaque universe, like a dense fog.
The photons are thus truly trapped by the electrons which prevent them from circulating freely in the universe.
Gradually, the temperature of the universe decreases until the energy per particle is lower than the ionization energy of the atomic nuclei, the universe is then no longer in thermal equilibrium. Then begins the era of recombination, 380,000 years after the Big Bang.
The temperature of the universe is no longer sufficient to keep atomic nuclei and electrons separate, so that these combine to form the first atoms. Recombination begins first with helium and lithium atoms. Then, when the universe reaches a temperature of around 2700°C (~3000 K), it is hydrogen, representing more than 90% of atomic nuclei, which recombines.
The recombination of hydrogen leads to the almost total disappearance of all the free electrons that have been captured by the hydrogen nuclei.
Once the majority of electrons have been captured by the atomic nuclei to form atoms, the photons cease to be continuously absorbed and scattered, and can finally travel freely in the universe. This is radiation decoupling (light literally decouples from matter). The first photons escaping at that time from the area of the universe called the "last scattering surface" form the cosmic microwave background observed today.
