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.

Cluster and supercluster of galaxies
The structure of our Universe seems like a kind of sponge. As a matter of fact, the measurement of the placements of thousands of galaxies showed us that they are not equally arranged. On large-scale, the Universe is composed by groups of galaxies, called clusters, creating all together giant and flat thickenings. They are divided by immense and empty regions. Moreover, many galaxy clusters are involved in overall motions towards other large clusters, called attractors, because of their gravitational force. The birthplace of this large-scale structure seems to be hidden in very small unhomogenities in the initial matter distribution.

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The Hubble’s Law
In 1929, Hubble saw 18 galaxies, also estimating their distance. He also discovered that all the galaxies seem to move away from us. In fact, the radiation emitted by these galaxies moved towards the red, in the electromagnetic spectrum: this is called redshift. This phenomenon has a simple explanation: whenever a source moves away from us, the number of oscillation per second decreases, so the wavelength seems to raise and we are used to say that the light turns to red. While a source comes towards us, the number of oscillations per seconds increases, and so the wavelength decreases and the light seems moved to the blue (blueshift).

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Quasar
The Quasars (Quasi-stellar Radio Source) are really far galaxies, the furthest we saw, able to emit an enormous quantity of energy, above all on the radio frequencies. Looking with a telescope, they appear like bright dots, with a stellar aspect (from which it derives the definition, Quasi-stellar Radio Sources), while their spectrum shows bands considerably inclined to the red color (redshift). If we suppose that this redshift is due to the Doppler effect, this would mean that these objects are coming away from us with high speeds, superior than 35000 km/s, too elevated for a normal star. In conclusion, we are talking about really far extragalactic sources.

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At present, we still don’t know the real composition of our Universe! Researchers found several evidences about the existence of an invisible matter, able to connect with its gravitational attaction galaxies and cluster of galaxies. It constitutes about the 90% of the whole universal matter. Its secret nature is still misterious, anyway its presence is revealed by some indirect effects in the surrounding space:

Spiral galaxies rotation. If we observe the trend of rotation speed in a spiral galaxy, in operation to the distance of the galactic centre, we will discover that, in large distances, this speed remains elevated, instead to decrease. We can explicate this phenomenon just admitting the existence of other matter, not visible, able to produce a strong gravitational field, increasing the speed.

Speed distribution in the galactic clusters. A single cluster is stable if it owns mass enough to connect all the included galaxies. These galaxies should have an inferior speed, in comparison to a precise stability limit. Scientists discovered that in many galactic clusters, the galaxies are able to reach speeds really superior to this limit of stability, but they still maintain a stable structure. This implies the presence of a really intense gravitational field, able to tie together these galaxies. We are not able to explain this field with the visible matter. The cluster should be tied together by an invisible matter, really more abundant than the bright one.

Gravitational lenses. The gravitational lenses are objects, or groups of objects, endowed of an extremely large mass, whose gravitational field is intense enough to deflect the course of the nearest beams. As a result, if one of these “lenses” would be placed between us and a far light source (for instance, a quasar), we will be able to admire more images for the same object. This kind of optical illusion allows us to discover the potential dark object which deviated the light.

Anyway, which is the composition of this dark matter? It may be normal matter, for instance, planets or “brown dwarfs”, so small that they are unable to trigger the nuclear fusion reactions on their inside. Nevertheless, these small objects are really too few to be the real cause of the effects observed in the dark matter.

A different hypothesis concerns mass-fitted neutrinos. Scientists believe, in fact, that neutrinos are particles without mass, but recent experiments seem to indicate that they hidden a mass, even if very small (1/5000 of an electron mass). Being very diffused in the Universe, neutrinos would be enough to justify all the observed effects of the dark matter. An other type of possible obscure matter is made of particles, whose existence has still not been proved, called WIMPS (Weakly Interacting Massive Particles).

Dark energy
Recent observations indicate that the Universe, contrary to what we supposed, is not braking like we would imagine in the case of a Universe full of matter. Instead, it is accelerating. Scientists tried to explain these observations supposing a new type of energy, with negative pressure, the so-called dark energy. According to the theory of relativity, this negative pressure produces an antigravitational force (push) on a large scale, allowing to bridge the important portion of missing mass.

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The inflationary model
Two distant objects are causally connected when they are able to communicate with a signal, provoking an effect the one to the other. The effects of this signal won’t be perceived immediately, but after some times, the higher the more distant they are because the speed of these signals, in the space, is finished. The causal horizon exactly represents the spacetime region where the two objects are connected in cause and effect. Our Universe is made of galaxies thickenings and relatively empty regions, but in the main it appears homogeneous and isotropic isotrope (equal in every directions). How is it possible that regions so far from each other, out of the causal horizon, own such properties?

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