what we know and what we don’t

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Wrongly considered as a primordial explosion, the Big Bang does not represent a starting point, but the horizon line beyond which we cannot go, and from which the notions of space and time were born. . What do we know about this early time?

The discovery in 1929 by 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 the time back is possible, but up to a certain limit. Reaching time zero is indeed impossible for us (the equations do not allow it). On the other hand, we can get closer to it.

Expansion and first elements

There is 13.77 billion years, we know our universe was incredibly hot (over 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 a period of incredibly rapid expansion known as inflation. In less than the blink of an eye, our universe was then enlarged by a factor of at least 10 ^ 52.

Once this phase of expansion ended, what generated this inflation in the first place (we don’t know what it is) decomposed, flooding the universe with matter and radiance.

At that moment, the universe was too hot and too dense for something stable to form. The cosmos was nothing but a soup of elementary particles of matter and antimatter born from the pure energy composed of quarks and antiquarks, electrons and positrons, neutrinos and antineutrinos which annihilated each other as soon as they were that they would meet.

Also during this first second, the nuclear force then caused the quarks to come together to form protons or neutrons. These in turn will form the nuclei of the hydrogen atoms.

The first second finally elapsed, the universe then continued to expand and cool slowly, 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 hydrogen atoms could then see the light of day.

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 this is only known through calculation, 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”.

From the Big Bang to the present day. Credits: Wikipedia

Dark matter and inflation

While some details are indeed deduced, a lot is also ignored, especially in the period before the formation of the first elements.

The question of dark matter arises, for example. We don’t know what she’s made of, but we know she 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 few seconds or 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.

Matter / anti-matter

Another problem, and not the least, that of matter-antimatter asymmetry. We see from experiments that matter and antimatter are perfectly symmetrical : For every particle of matter created in reactions across the universe, there is a corresponding particle of antimatter.

However, we know that during this famous “first second”, the matter of which we are made up took precedence over antimatter. These two forms of matter did not therefore behave in the same way. At present, we still do not know the reasons for this imbalance.

telescopes stars galaxies big bang
Credit: Hubble / NASA / ESA

Thus, many blank pages remain under the noses of cosmologists, but we are still trying to study this early era.

If we cannot directly see the state of the universe when it was only a few seconds old, for example we can try to recreate these conditions in our powerful particle accelerators.

This chaotic hell must have released a torrent of ripples in the fabric of space-time. With the technical means (which we do not yet have), we can then also try to capture gravitational waves from this famous Big Bang.

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