History of the "big bang" theory

Studying physical space properties in the presence of matter, Einstein, in 1917, found that this space was curved (similar to a spherical surface) and which could not be stable, but it had to expand and diminish. This consequence, mathematically derived from the equations of general relativity, was regarded as unacceptable by Einstein, who modified therefore their equations, introducing a term, defined “cosmological”, capable of ensuring a stationary solution for the configuration of the Universe. Then the term “cosmological” was abandoned and it was made a mathematical theory, by means of A.A. Friedmann in 1922, which provided a Universe expanding. According to this new model, the “sphere-Universe” stems from a single point, as originated from a huge explosion (from here the term “big bang”) and it is then intended to expand in the relativistic space-time, folded by the substance and no longer Euclidean.

It is not simple that you can imagine a tridimensional curved space, and therefore we must get us the general idea of using a two-dimensional model. Now, Euclidean two-dimensional space is practically a plan unlimited and without borders. But also a spherical surface has the same properties of two-dimensional space, although in this case the geometry to use is a non-Euclidean geometry, namely that it respects not the parallel postulate. The tridimensional curved Universe, reduced to two dimensions, it resembles a spherical surface in uniform expansion. Now, this sphere has some “strange” properties, or:

1)      you can define it unlimited, because we can proceed indefinitely on it, without ever reaching its borders, and yet it is not unlimited;

2)      it is a perfectly homogeneous Universe without a centre or a periphery: each his point, and no point may be considered to be the centre of the Universe;

3)      all the points on the sphere go away themselves with speed proportional to their distance;

4)      Progressing us on the two-dimensional surface of the sphere always in the same direction,  we can return to the starting point.

We then are ourselves about something similar to a spherical surface in expansion, but with one more dimension. You can also see an important fact, namely that the problem of the centre of the Universe here is “strangely” resolved: there is not the Centre of the Universe!

Indeed, the Hubble’s law, set out from our Galaxy,  would be identical from anywhere point in the Universe. Then seeing us all galaxies away from our galaxy with speed proportional to the distance, we could consider ourselves to be at the Centre of the Universe, but the same thing would happen looking from any other Galaxy. We can conclude that the spherical surface expansion model therefore allows you easily understand how there may be those “strange” conditions that make compatible, at least in part, the Olbers’ cosmological principles with the Hubble’s law. That is a “finite” Universe, but “without borders”, a Universe “homogeneous” and similar “for each observer” located somewhere within.

Among its features, the big bang model includes one which is likely to experimental control. According to this model, the Universe behaves itself own a gas that is initially very hot, and then it cools expanding himself. The universe will have to give out the “black body” radiation (1), with a spectral matter dependent on its temperature and perfectly “isotropic” (or “with the same characteristics” in each direction of observation), to be in agreement with the idea of a “homogeneous” Universe. This “essential radiation” would be a kind of “big bang” initial "echo", or “present testimony” of that very ancient event: so it is called “fossil radiation”.

The widespread disbelief of scientists in respect of the big bang was not stimulated any experimental research concerning the existence of this radiation. In the early 1960s, however, two physicists, A. Penzias and R. Wilson, who were building an antenna very sensitive for space communications, acquired the presence of a radiation (a “noise”) in the field of the microwaves. This radiation appeared not directional and it could not be deleted in any way. In an attempt to find the cause of this noise, they said that the thing has been originate by a pair of pigeons that had done the nest near the antenna. The nest was removed, but the noise remained unchanged. In 1965 they began to think that this radiation should be thermal radiation of "black body” corresponding to the current temperature of the universe, about 3 K, expected from the big bang model. All experimental controls confirmed this assumption, showing that it was a thermal radiation with maximum length waveforms of approximately 1 mm, corresponding to a temperature of 2.7 K5. After this discovery, the model of the big bang, although defined in a rough shape (still remain today many unresolved questions), became the landmark for each following cosmological research (2).

1)      “Black-body” radiation is the theoretical model (G.R. Kirchhoff, 1859) of a system capable of absorbing all the radiation which invests him.

2)      For the theoretical issues presented in this article, see Larsen K. (2005) “S. Hawking: A Biography (Greenwood Biographies)”: Hardcover: 12 sgg.; and National Academy of Sciences and Addison Greenwood (1992), “Science at the Frontier”: Hardcover: 76 sgg.

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