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The Big Bang model

The standard Big Bang model is a generally adopted evolutionary theory of the Universe. According to this theory, about 13.7 billion years ago the Universe was filled with very dense and very hot plasma, in which there was a thermodynamic equilibrium between the elementary constituents of matter and radiation (photons). As a result of the Big Bang the entire Universe began to expand. As a consequence of the expansion space, the temperature and density of both matter and radiation decreased. This enabled, in the first fractions of a second, the emergence of elementary particles which we know today, such as protons and neutrons. In the time between more or less the first second and the first three minutes after the Big Bang, the nuclei of isotopes of light elements, such as hydrogen, helium and lithium were formed. A fast decrease in the density and temperature prevented the formation of heavier elements (they could be formed only in stars). After about 300 thousand years a recombination of space plasma was possible - electrons were attached to the nuclei of light isotopes, neutral atoms were formed, and the Universe became transparent to photons. It is from this era that thermal radiation comes which is observed today as the cosmic microwave background radiation of temperature around 2.73 K. Only after a further few hundred million years, as a result of a rise of the initial, slight unevenness in the distribution of matter, first galaxies and stars appeared. A large scale structure of the distribution of matter was also formed: clusters of galaxies, superclusters, the great vacuum. And, as a consequence of nucleosynthesis in stars, the chemical evolution of galaxies progressed.

The general relativity theory is a theoretical basis for the Big Bang model. Solutions to Einstein equations where they describe the entire Universe were given by Alexander Friedman (1888-1925) in the early 1920s.

Many predictions of an experimentum crucis (conclusive test) character can be derived from the Big Bang model. A lack of observational confirmation of these projections would mean an overthrow of this theory. Such predictions include, for example:

  1. The Universe as a whole expands according to Hubble's law, saying that the galaxies are moving away from the observer with a speed proportional to the distance from him.
  2. The chemical composition of the Universe is (by weight) nearly 74% hydrogen (out of this one in a hundred thousand atoms is in the form of deuterium, an isotope of hydrogen), approximately 24% helium, and lithium in proportions roughly one atom per billion, and other elements, which account for 2%.
  3. The entire cosmos is filled with thermal radiation (a blackbody spectrum) at a temperature of a few degrees Kelvin.
  4. Cosmic objects are evolving - their number per unit volume, chemical composition, clarity, etc. are changing.
  5. The temperature of the cosmic background radiation in space has changed over the cosmic epochs in a precise manner.
  6. In the past there was an era with no galaxies.
  7. The large-scale structure of matter distribution developed gradually.

The first of these predictions has been confirmed observationally in 1929 by Edwin Hubble who observed the phenomenon of receding galaxies. The second prediction, derived in detail in the 1960s by P. J. E. Peebles (born in 1935), and R. Wagner (born in 1938) was gradually confirmed by several teams of observers. Similarly, the third prediction, concerning the chemical evolution, as well as changes in the number of objects of that type as a result of cosmic evolution, has now been fully confirmed observationally. At the turn of the 20th century, several teams of observers were able to determine the temperature of the cosmic background radiation in remote past epochs - the results are fully consistent with the predictions of the Big Bang model. In the first decade of the twenty-first century, thanks to observations carried out from the orbit of the Earth, it was observationally confirmed that the first galaxies and stars originated several hundred million years after the Big Bang. In that decade, observational evidence was also obtained on the evolution of the structure of matter distribution in large scales.

The name of the theory “Big Bang” comes from astronomer Fred Hoyle (1915-2001) who used it in 1949 in a BBC radio broadcast.

Stanisław Bajtlik
Nicolaus Copernicus Astronomical Centre
Polish Academy of Sciences