One of the greatest discoveries of the twentieth century in the field of physics was that of the expansion of the Universe. Cosmology and cosmogony were particularly benefited by the work of the American astronomer Edwin Hubble (1889-1953).
While working at the Mount Wilson Observatory, California, Hubble managed to discover a relationship between the speed with which the galaxies travel radially and the distance they are.
The radial velocity of the galaxies is determined by studying its spectrum and seeing if it is displaced towards blue or red and by how much. In physics this phenomenon is called the Doppler effect and it is that an observer who receives light from an approaching source will see the lines of its spectrum displaced towards the blue. On the other hand, if the light source moves away from it, its spectrum will run red.
Using this property, Hubble established that the spectrum of most galaxies is shifted to red, and that the magnitude of the displacement is proportional to distance: the farthest galaxies have a spectrum that is more red.
All the above information coincides with the theory of the expanding Universe. The space between the galaxies expands, so they all move away from all. Even our galaxy, the Milky Way.
For a better understanding of the problem it is necessary to refer to a better known image. Suppose, for example, that the radius of the Earth began to increase. At the same time, the surface would begin to grow and all distances would increase.
All cities would move away from Madrid, the farther away, the faster. But also all cities would move away from Barcelona, Bilbao, Seville or Valencia. All distances would increase, so if we were located anywhere on Earth, we would see that all other points move away from us and that they do so at a higher speed the farther they are. As dots drawn on the surface of a balloon that swells.
The fact that the Universe is expanding implies that in the past the distances were smaller. By calculating the current rate of expansion and projecting it into the past, it is concluded that 14 billion years ago the entire Universe was concentrated at one point; all galaxies gathered; all matter confined to a small volume of high density and high temperature.
It is necessary to reiterate that this calculation is made assuming that the current rate of expansion is the same as in the past. However, astronomers know that this is not real, since the journey of the galaxies is gradually slowed by the force of gravity exerted by matter.
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