The cosmos (, ) is the universe. Using the word cosmos rather than the word universe implies viewing the universe as a complex and orderly system or entity; the opposite of chaos. The cosmos, and our understanding of the reasons for its existence and significance, are studied in cosmology - a very broad discipline covering any scientific, religious, or philosophical contemplation of the cosmos and its nature, or reasons for existing. Religious and philosophical approaches may include in their concepts of the cosmos various spiritual entities or other matters deemed to exist outside our physical universe.
The philosopher Pythagoras first used the term cosmos (Ancient Greek: ) for the order of the universe. The term became part of modern language in the 19th century when geographer-polymath Alexander von Humboldt resurrected the use of the word from the ancient Greek, assigned it to his five-volume treatise, Kosmos, which influenced modern and somewhat holistic perception of the universe as one interacting entity.
Cosmology is the study of the cosmos, and in its broadest sense covers a variety of very different approaches: scientific, religious and philosophical. All cosmologies have in common an attempt to understand the implicit order within the whole of being. In this way, most religions and philosophical systems have a cosmology.
When cosmology is used without a qualifier, it often signifies physical cosmology, unless the context makes clear that a different meaning is intended.
Physical cosmology (often simply described as 'cosmology') is the scientific study of the universe, from the beginning of its physical existence. It includes speculative concepts such as a multiverse, when these are being discussed. In physical cosmology, the term cosmos is often used in a technical way, referring to a particular spacetime continuum within a (postulated) multiverse. Our particular cosmos, the observable universe, is generally capitalized as the Cosmos.
In physical cosmology, the uncapitalized term cosmic signifies a subject with a relationship to the universe, such as 'cosmic time' (time since the Big Bang), 'cosmic rays' (high energy particles or radiation detected from space), and 'cosmic microwave background' (microwave radiation detectable from all directions in space).
According to Charles Peter Mason in Sir William Smith Dictionary of Greek and Roman Biography and Mythology (1870, see book screenshot for full quote), Pythagoreans described the universe.
It appears, in fact, from this, as well as from the extant fragments, that the first book (from Philolaus) of the work contained a general account of the origin and arrangement of the universe. The second book appears to have been an exposition of the nature of numbers, which in the Pythagorean theory are the essence and source of all things. (p. 305)
Cosmology is a branch of metaphysics that deals with the nature of the universe, a theory or doctrine describing the natural order of the universe. The basic definition of Cosmology is the science of the origin and development of the universe. In modern astronomy the Big Bang theory is the dominant postulation.
In theology, the cosmos is the created heavenly bodies (sun, moon, planets, and fixed stars). In Christian theology, the word is also used synonymously with aion to refer to "worldly life" or "this world" or "this age" as opposed to the afterlife or world to come.
The 1870 book Dictionary of Greek and Roman Biography and Mythology noted
The book The Works of Aristotle (1908, p. 80 Fragments) mentioned
Eastern and Western thought differed greatly in their understanding of space and the organization of the cosmos. The Chinese saw the Cosmos as empty, infinite, and intertwined with the Earth. Western ideas, based on the ancient Greeks' understanding of the cosmos, believed in a multi-planar divided cosmos that was finite and filled with air.
Early Europeans viewed the cosmos as a divinely created, spatially finite, bifurcated cosmos, divided into sublunary and superlunary realms. Objects above the lunar sphere were believed to be stable, with heavenly bodies believed to be made out of a refined substance called "quintessence". This was understood to be a crystalline, completely transparent substance that held all of the superlunary spheres in perfect order. After their creation by God, these spheres did not change except for their rotation above the Earth. Objects below the lunar sphere were subject to constant combination, separation, and recombination. This was because they consisted of the chaotic elements of earth, air, fire, and water.
The idea of celestial spheres was developed in the cosmological models of Plato, Eudoxus, Aristotle, Ptolemy, Copernicus, and others. They believed in a stable cosmos created by God, where distinct realms were subject to different kinds of order. Europeans maintained the Aristotelian view that infinity could only be seen as an attribute of God, with the cosmos being finite. Furthermore, following the Aristotelian view that "nature abhors a vacuum", Europeans believed that the space between the spheres were filled with air. This theory persisted until the Scientific Revolution, when the discovery that the Sun was in the center of the planetary system rocked cosmological understanding to its core.
The Chinese had multiple theories of the processes and components of the cosmos. The most popular of these beliefs was the Xuan Ye theory, the astronomical view of the cosmos as an infinite space with floating pieces of condensed vapor. The Chinese believed that the Earth consisted of condensed yin and the heavens of yang; and that these properties coexisted in constant relation to each other, with yin and yang being used together to explain processes on Earth as well of those relating the Earth in conjunction with the heavens. This idea was described by Joseph Needham as a cosmos that functioned similarly to a complex organism, with discernible patterns in an ever-changing structure. There was both a pattern and a randomness to the cosmos. Because of this, the Chinese believed that earthly phenomena could affect heavenly bodies.
The Chinese believed that qi was the substance of all things in the cosmos and Earth, including inanimate matter, humans, ideas, emotions, celestial bodies and everything that exists or has existed; and that it was qi condensing that created all the matter within the cosmos. This is relatively consistent with our modern understanding of the congregation of matter through gravitational fields.
The Chinese held a belief associated with the Xuan Ye theory, which held space as both empty and infinite. This was inconsistent with the Aristotelian concepts that nature would not contain a vacuum, and that infinity could only be a divine attribute. The idea of the nothingness of space was later recognized as one of the most important discoveries of modern science.
There is one way that both the Chinese and the Europeans, along with countless other ancient societies, related to the cosmos. This was through meaning, placed on celestial bodies, that were observed moving above the Earth. The Chinese had a very complex astronomical understanding of the stars and the cosmos that influenced everything from their art and architecture to their myths and science. This was also true of the Greeks and Romans, whose 48 constellations, including the zodiac signs and the constellation of Orion, have been passed down to modern Western cultures. These were likely passed down to them from ancient Babylonian and Egyptian astronomers. Copernicus is said to have been inspired by the fecund sun deity of neoplatonic thought, which may have initially inspired his vision of a heliocentric universe.