Wrongly considered a primordial explosion, the Big Bang does not represent a starting point, but the horizon line beyond which we cannot go, and from which were born the notions of space and time. What do we know about this primitive era?
The discovery in 1929 by Edwin Hubble that the Universe was expanding had very serious consequences for cosmology. Indeed, if the universe is expanding, then it was once much more concentrated. Taking time backwards is possible, but up to a certain limit. Reaching zero time is indeed impossible for us (the equations do not allow it). On the other hand, we can get closer.
13.77 billion years ago , we know that our universe was incredibly hot (more than a quadrillion degrees). He was also incredibly small (the size of an orange). Astronomers suspect that when it was less than a second, the cosmos went through an incredibly rapid period of expansion known as inflation. In less than the blink of an eye, our universe then expanded by a factor of at least 10^52 .
Once this expansion phase was over, what generated this inflation in the first place (we don't know what it is) decayed, flooding the universe of matter and radiation.
At that moment, the universe was too hot and too dense for anything stable to form . The cosmos was nothing but a soup of elementary particles of matter and antimatter born of pure energy composed of quarks and antiquarks, electrons and positrons, neutrinos and antineutrinos which annihilated as soon as that they met.
Still during that first second, the nuclear force then caused the quarks to come together to form protons or neutrons. These will in turn form the nuclei of hydrogen atoms.
The first second finally passed, the universe then continued to slowly expand and cool, leading the strong nuclear force to combine protons and neutrons. After three minutes, the first nuclei of atoms a little more complex than the simple nucleus of a hydrogen atom were then able to emerge.
Nucleosynthesis will continue naturally for several minutes, then it will be necessary to wait for the thermonuclear fusion machines that are the first stars to enrich the table of elements.
All of this is only known through computation, because none of these primordial phases will ever be observable by our instruments. Our world will indeed remain opaque until its 380,000 years, when the first stars "lit up" .
While some details are indeed inferred, we also ignore a lot, especially during the period before the first elements are formed.
The question of dark matter arises, for example. We don't know what it is made of, but we do know that it represents more than 80% of the matter in the universe . We also don't know when or how dark matter came into the picture. Did it appear during the first seconds or much later? Did it act on the primitive chemistry leading to the formation of the first elements or did it remain in the background? We don't know.
Inflation is also a problem. We don't know what provided the power source for this incredible expansion event. If we know how long this phase lasted, we do not know how and why it finally stopped.
Another problem, and not the least important, is that of matter-antimatter asymmetry. We see from experiments that matter and antimatter are perfectly symmetrical :for every particle of matter created in reactions throughout the universe, there is a corresponding particle of antimatter.
But we know that during that famous "first second", the matter of which we are made took precedence over the antimatter. These two forms of matter therefore did not behave in the same way. At present, we still do not know the reasons for this imbalance .
Thus many blank pages remain under the noses of cosmologists, but we are still trying to study this primitive time.
If we cannot directly see the state of the universe when it was only a few seconds old, we can in particular try to recreate these conditions in our powerful accelerators of particles.
This chaotic inferno also must have unleashed a torrent of ripples into the fabric of spacetime. Also, with the right technical means (which we don't have yet), we could try to capture gravitational waves from this famous Big Bang.
