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Big Bang Big primordial explosion from which the Universe would be formed from an initial state of high density and temperature, which would be followed by a rapid expansion. Proposal towards 1946 by G. Gamow, originates from the solution of the equations of general relativity for a homogeneous and isotropic universe, known as the Friedmann-Lemaitre model (cosmology). By solving the above equations backward in time, under the assumption that the mass of the Universe remains constant, and using the observation relative to the current expansion of the Universe, is in fact arrive at an initial instant in which the size of the ‘ universe tend to zero and the density of matter tends to infinity. In this singular condition follows, according to the original model Gamow, expansion, during which the raw material, originally consisting of only neutrons can be considered as a more or less uniform fluid, in thermodynamic equilibrium with the radiation, characterized by a absolute temperature T. This decreases in inverse proportion to the dimension of the Universe. The quantum physics of subsequent developments in 1950 have led to modify the description of the first seconds of the Universe keeping for the next evolution of the Gamow theory. Quantum physics does not allow you to define the geometry of the Universe during a first short stage of its evolution, as long as its dimensions were lower than the Planck length. Immediately after, the primordial fluid was ruled by a unified force and gravity, and the energy of the radiation significantly exceeded that of the particles. In the next microsecond before the strong force separated electroweak interaction and then the weak interaction she separated from the electromagnetic; at this point the fluid was made up of quarks, leptons and bosons (photons). About 3 minutes later, it began a phase, which lasted less than one minute, in which protons and neutrons combined to form deuterium nuclei, and the latter reacted by further synthesizing helium nuclei (so it enters the so-called nuclear age). The photons interacted frequently with charged particles; so they were in thermodynamic equilibrium with matter, and the Universe was opaque. After about 300,000 years, when the temperature dropped to about 10,000 K, electrons and nuclei combined to form atoms of hydrogen, deuterium and helium (atomic age). He declined so sharply the interaction between photons and matter, radiation is uncouple from matter, not prejudice more in balance with it, and the Universe became transparent to its own light. The cosmic background radiation was the father at the time and continued to expand and then cool down, according to the law described above. Since then the continued expansion has subtracted energy radiation and matter became dominant on the radiation itself: had begun the era of matter, which is still going.