宇宙学(cosmology)
把整个自然科学，尤其是天文学和物理学，结合起来，对整个宇宙进行探索的一门学科。分为三个伟大时期。第一个时期始於西元前6世纪的希腊，当时毕达哥拉斯提出了地球是一个球体的观念，与巴比伦人、埃及人不同，他设想这样一个宇宙，其中天体的运动都受自然规律的和谐关系所支配；接着又有留基伯（Leucippus）和德谟克利特（Democritus）提出无穷原子宇宙，最後归结为2世纪的托勒密宇宙模型（参阅Ptolemy）。到16世纪，哥白尼提出一个以太阳为中心的宇宙，标志着第二个伟大时期的开始；到17世纪又演变成为在18、19世纪盛极一时的牛顿无限宇宙，随後许多科学家在研究恒星和银河系都取得了迅速的进展。第三个伟大时期是20世纪初随着爱因斯坦发现狭义相对论并发展成广义相对论而揭开序幕的，天文学出现许多重大进展︰河外星系的红移、河外星云是同银河系相当的星系、宇宙正依哈伯定律膨胀等。现代宇宙学的基本假设是：宇宙在空间上是同质的，即就平均而言，在任何地方、任何时间都是一样的，因此物理规律也到处适用。
English version：
cosmology
Field of study that brings together the natural sciences, especially astronomy and physics, in an effort to understand the physical universe as a unified whole. The first great age of scientific cosmology began in Greece in the 6th century BC, when the Pythagoreans introduced the concept of a spherical earth and, unlike the Babylonians and Egyptians, hypothesized that the heavenly bodies moved according to the harmonious relations of natural laws. Their thought culminated in the Ptolemaic model (see Ptolemy) of the universe (2nd century AD). The Copernican revolution (see Copernican system) of the 16th century ushered in the second great age. The third began in the early 20th century, with the discovery of special relativity and its development into general relativity by Albert Einstein. The basic assumptions of modern cosmology are that the universe is homogeneous in space (on the average, all places are alike at any time) and that the laws of physics are the same everywhere.