Once agriculture developed, people had to observe the sky: farming depended on the seasons and, above all, on the sun's height in the sky at a given time. After the Neolithic Revolution and the rise of the earliest civilizations, Mesopotamia already had astronomers who studied the movements of the stars and planets and made them gods. The age lent itself to religious and mystical interpretations. As a result, astronomical observation, already highly advanced among the Babylonians, could also merge with astrology, which claimed to read the future or people's character from the positions of celestial bodies relative to the mythological figures traced across the sky. These myths were so deeply rooted that the names of those figures—the Virgo Cluster, the constellation Orion and others—remain useful today, even though they have lost their symbolic meaning. But observing something is one thing; explaining it is another. That is what the Greeks did, developing the first explanations and interpretations of the movements of celestial objects. In the Middle Ages and the centuries that followed, navigators used the positions of celestial bodies to determine their own position.
Copernicus and Kepler shattered the prevailing anthropocentrism. With universal gravitation, Newton established why the planets revolved around the sun. Laplace and many others continued to formalize the observations. In his Théorie du ciel (Theory of the Heavens), Immanuel Kant laid the philosophical foundations of cosmology, envisioning an immutable, eternal Euclidean universe. Astronomy remained governed by Newton's laws until the beginning of the 20th century.
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The beginnings of scientific cosmology
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The year is 1915. Albert Einstein puts the finishing touches to the theory of general relativity, which he has been developing since the early 1910s. With it, cosmology becomes a science: as a theory of the gravitational field, it can define the properties of an entity—the universe—whose boundaries have yet to be determined. This requires accepting two principles: the laws of physics are universal, valid in the past as well as the future; and the universe is the same everywhere, with no privileged place from which to observe it (the principle of isotropy). Dispensing with these principles would mean using a different physics or assuming that the properties of our local region of space do not exist elsewhere in the universe. Thanks to general relativity, the universe as a physical object is described by an equation in a non-Euclidean geometric space.
