Big Bang, definition and study. A-Z index of Cognitio.

The Big Bang is a cosmological model based on the idea that the universe began expanding at a very high speed at a time in the past from a bending, temperature, and extreme density state and that this process continues today.

Cosmology, definition and study. A-Z index of Cognitio.


The Big Bang theory has been deduced from the equations of general relativity, solving them in special conditions designed to simplify the problem, but of a hypothetical nature. The most important of these is the hypothesis of homogeneity and the isotropic of the universe, known as the cosmological principle. It generalizes the Copernican principle to the whole universe. The Big Bang theory was immediately in accord with the new conception of the structure of the universe that in the very same decades was emerging from the astronomical observation of the nebulae. The extrapolation of the expansion of the universe backward over time by using general relativity leads to such a high density and temperature condition numerically to tend infinitely; this condition has been maintained in a time of infinitesimal, so short that it is difficult to study with current physics. This singularity indicates the point where general relativity loses validity. You can continue with this extrapolation up to Planck’s time, which is the smallest timetable time interval with current physical laws. The hot and dense initial phase called “Big Bang” is considered the birth of the universe. Depending on the expansion measurements of type the supernovae, measurements of temperature fluctuations in the underlying cosmic radiation, measurements of the galaxy correlation function, and the latest and most reliable data provided by the Planck Surveyor Space Telescope-Probe European Space Agency, the universe has a calculated age of 13,798 billion years.

Galaxy, definition and study. A-Z index of Cognitio.


An important feature of Big Bang’s space-time is the presence of a cosmological horizon. Because the universe has a finite age and light travels at a finite speed, there may be events that happened in the past whose light did not have enough time to reach the Earth. This involves a limit or horizon in the past on the most distant events that can be observed. Instead, since space is expanding and distant objects are moving ever faster, the light emitted today from a point on Earth might never be received by farthest objects.

Cosmic space, definition and study. A-Z index of Cognitio.

Cosmic space

This defines a horizon in the future, which limits future events that we can influence. The observations of galaxies and quasars show that these objects present the phenomenon of red-shift, that is, their emitted light is shifted to greater wavelengths.

Astronomical observation, definition and study. A-Z index of Cognitio.

Astronomical observation

These red-shifts are homogeneous, isotropic and uniformly distributed among objects observed in all directions. For some galaxies it is possible to calculate their distance from Earth through the scale of cosmic distances. When distances are compared with these distances, a linear relationship is known, known as Hubble’s law. In the days following the Big Bang, the universe was in a thermodynamic equilibrium state, with photons that were continuously emitted and absorbed, giving radiation a shape similar to the spectrum of a black body.

As it expanded, the universe cooled to a temperature that would no longer allow the creation and destruction of photons.

The temperature was, however, still high enough not to allow the electrons to bond with the nuclei to form atoms and the photons were constantly reflected by these free electrons through a process called Thomson scattering. Because of this repeated scattering, the universe was initially “opaque”. Detailed observations on morphology and distribution of galaxies and quasars provide compelling evidence of the Big Bang theory. The combination of observations and theories suggests that the first quasars and early galaxies formed about one billion years after the Big Bang, and since then the largest structures, such as galactic clusters and super masters, were formed. Stellar populations have evolved over time, so the farthest galaxies (seen as they were in the young universe) appear very different from the galaxies closest to us, as the latter are observed in a more recent state. After some controversy, the age of the universe, as estimated by the Hubble constant and the background radiation, is substantial (ie slightly larger) with the ages of older stars, measured by applying the theory of starving evolution to clusters globular and through the radiometric dating of single stars. 

Cosmic radiation background, definition and study. A-Z index of Cognitio.

Cosmic radiation background

While few researchers now question the fact that the Big Bang occurred, the scientific community was previously divided between those who supported that theory and who considered other cosmological models possible. In this context of heated debate, many problems related to Big Bang theory and its ability to reproduce cosmological observations were raised. By now these problems are mostly mentioned mainly because of their historical interest; solutions to them have been obtained either through changes to the theory or as a result of better observations.

Other issues, such as the problem of the cusp of galactic alone, the great presence of dwarf galaxies, and the nature of cold dark matter, are not considered unsolvable and are supposed to come to the fore through further refinements of that theory.

Big Bang, definition and study. A-Z index of Cognitio.



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Big Bang, definition and study. A-Z index of Cognitio.

Big Bang