It has been observed that the distances between the large structures of the universe (clusters and superclusters of galaxies) increase progressively. This observational fact is called the expansion of the universe and was discovered by Edwin Powell Hubble and Milton Lasell Humason in 1929. If you take any two clusters of galaxies, the distance between them grows steadily, and it does so not because the galaxies or clusters are move, but because the space between them grows. Moreover, the more distant the clusters considered are from each other, the faster the increase in distance. The intensity of the expansion of the universe at any instant in its history is assessed by the Hubble parameter, H. The current value of the Hubble parameter is called the Hubble constant and is symbolized as H0. The expansion of the universe does not affect gravitationally bound systems, that is, the process does not alter the distances between the atoms of our bodies, between the Earth and the Sun or even between the stars of the Galaxy or between galaxies belonging to the same cluster. Observations indicate that the expansion of the universe is accelerating more and more, for reasons that are not yet clear.
