As we know that the universe is expanding | Earth, Solar System and Universe.

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The discovery of the expansion of the universe began in 1912, with the works of the American astronomer Vesto M. Slipher. While studying the spectra of galaxies observed that, except in the nearest, spectrum lines are displaced toward the red.
This means that the majority of galaxies move away from the milky way since correcting this effect in the spectra of galaxies, demonstrates that the stars that comprise them are composed of known chemical elements. This shift to the Red is due to the Doppler effect.
If we measure the redshift in the spectrum of a star, we can know if it approaches or moves away from us. Mostly this shift is towards the red, which indicates that the focus of the radiation away. This is interpreted as a confirmation of the expansion of the universe.
At first it seems that galaxies move away from the milky way in all directions, giving the feeling that our Galaxy is the center of the universe. This effect is a consequence of the way in which the universe expands. It is as if the milky way and other galaxies were points on the surface of a balloon. Inflating the balloon, all points move away from us. If we were to change our position to any of the other points and perform the same operation, we would observe exactly the same.

The Hubble Law

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The American astronomer Edwin Powell Hubble related, in 1929, the shift towards red observed in the spectra of galaxies with the expansion of the universe. He suggested that this shift toward red, called the cosmological red shifting, is caused by the Doppler effect, and therefore indicates the speed of receding galaxies.
Hubble also noted that the speed of recession of the galaxies was larger and farther away were. This discovery led him to enunciate his law of the speed of recession of galaxies, known as the "Hubble's law", which States that the speed of a Galaxy is proportional to their distance.
The Hubble or proportionality constant is the ratio between the distance of a Galaxy to the Earth and the speed with which moves away from it. It is estimated that this constant is between 50 and 100 Km/s per megaparsec.
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