The Big Bang and the birth of galaxies

If the universe shows us an expansive motion, it is natural to suppose that, if we would be able to rewind at the same speed the tape of this expansion, all the matter that composes the Universe would come back to form the first cluster, very dense and really hot. This experiment of thought encouraged, in the Forties, the physicist G. Gamow to elaborate the Big Bang or standard model theory. According to his theory, about 15-20 billions of years ago, the Universe was characterized by an incredibly high temperature and density state, concentrated in a very small place.
In an infinitesimal time, it started to expand itself to an enormous speed, decreasing temperature and density, until it reached the current dimensions and aspect.
This model allows to explain several observations, like the universal abundance of lighter atomic nuclei (hydrogen, helium, deuterium, lithium) and the existence of a cosmic microwave background radiation. But let’s try to examine the evolution of our Universe dividing it into phases, to make easier our comprehension of this phenomenon.
Phase 1
This first phase is extended from the instant t=0 to the instant t=5,39 x 10-44 (also called Time of Planck). In this phase, probably, the four interactions of the physics, strong and weak nuclear, electromagnetic and gravitational,  were probably unified. In this instant, temperature was incredibly elevated (T= 1032 K) while the Universe itself was just a mathematical singularity.
Phase 2
The gravitational interaction breaks apart from the other three fundamental interactions, still unified according to the Great Unification Theory or GUT.
In the present moment, the Universe is full of radiation of mutual interaction, that means in thermal balance, with electrons and neutrinos.
Phase 3
Little by little, with the decreasing of temperature (T=1027 K), the bariogenesis process starts, which causes the prevail of the matter on the antimatter. The Universe is made of quarks, leptons and corresponding particles, gluons and bosons.
Phase 4
We observe the separation of the electroweak interaction in weak and electromagnetic. The Universe is dominated by quarks, leptons, photons, neutrinos and dark matter.
Phase 5
Only at 10-4 s after the Big Bang, protons and neutrons are generated, which remain in thermodynamic balance with electrones and neutrinos.
Phase 6
About 0,7 s after the Big Bang, neutrinos separate from the remaining part of the matter, creating a neutrinos fossil reaction that we still can see.
Phase 7
When the Universe has got about 3 minutes of life, the formation of light nuclei, like 2H, 3He, 4He and 7Li is completed.
At the end of these first three minutes, the Universe is dominated by the presence of protons, neutrons, light nuclei, neutrinos and dark matter.
Phase 8
When the Universe was 300.000 years old, the radiation separated from the matter. This radiation arrived till us, actually named cosmic microwave background radiation. From this moment, it is possible to make direct observations, because the Universe becomes transparent to this radiation.
Phase 9
After some hundreds billions of years, temperature decreases until 4000 K. Small fluctuations of density may start to attract gravitationally the surrounding matter, creating protogalaxies (giant clouds of extremely cold gas), and then of galaxies and cluster of galaxies. After 4 billions years, the first stars appeared.

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From the Multimedia section

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    Virgo cluster

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    The Hercules Globular Cluster (M13)

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