The Universe on a Large Scale
To know the distribution of matter in the universe has been one of astronomers’ constant quests. It is pursued by studying more and more distant galaxies. The idea of a remote universe had been held since the middle of the eighteenth century. In the nineteenth century Herschel and Dreyer catalogued the many bright nebulae and noted how they were grouped. Recognition of the extragalactic realm of nature, mainly due to Shapley and Hubble in the years 1920–1930 (by the identification of Cepheids in Andromeda), like the discovery of the universal recession of galaxies by Hubble, opened the way to the development of extragalactic astronomy and to what would become observational cosmology. On extragalactic scales the galaxies, owing to the contrast in density that they represent (the mean density of our Galaxy within 10 kpc is approximately 2 × 10−24 g cm−3, while the mean density of the universe lies (probably) between 10−29 and 10−31g CM−3), are the most immediately discernible entities. Until recently they were thought to provide a reliable indicator of matter on a large scale, and their study led to the idea of a globally homogeneous and isotropic universe.
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