The assumption that the universe is homogeneous and isotropic on large scales, meaning it looks the same in all directions and at all locations when averaged over sufficiently large volumes. This principle underlies modern cosmological models.
From Greek 'kosmos' (order/universe) and Latin 'principium' (foundation). The principle was formalized in the 1930s by cosmologists like Howard Robertson and Arthur Walker as they developed mathematical descriptions of expanding universe models.
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