In the general sense, a cosmos is an orderly or harmonious system. The word derives from the Greek term κόσμος (kosmos), literally meaning "order" or "ornament" and metaphorically "world", and is antithetical to the concept of chaos. Today, the word is generally used as a synonym of the Latin loanword "Universe" (considered in its orderly aspect). The word cosmetics originates from the same root. In many Slavic languages such as Russian, Bulgarian, and Serbian, the word Космос (cosmos) also means "outer space". In Mandarin Chinese, cosmos and universe are both translated as 宇宙 (yǔzhòu), which literally translated means space-time (宇 yǔ, "space" and 宙 zhòu "time")
Pythagoras is said to have been the first philosopher to apply the term cosmos to the Universe, perhaps referring to the starry firmament.
Russian cosmism is a cosmocentric philosophical and cultural movement that emerged in Russia in the early 20th century.
Cosmicism is a philosophical position that mankind is an insignificant aspect of a universe that is at best indifferent and perhaps even hostile. This philosophy, explored by writers such as H.P. Lovecraft (who some say is the original proponent of the philosophy) and later writers, is seen in books such as Hitchhiker's Guide to the Galaxy.
The philosopher Ken Wilber uses the term kosmos to refer to all of manifest existence, including various realms of consciousness. The term kosmos so used distinguishes a nondual Universe (which, in his view, includes both noetic and physical aspects) from the strictly physical Universe that is the concern of the traditional sciences. Wilber's nephew, Cosmo Iacavazzi, is said to have been named after the scientific term.
Ancient Greek conception of the cosmos
The Ancient Greek natural philosopher Archimedes, in his essay "The Sand Reckoner", estimated the diameter of the cosmos to be equivalent in stadia to what we call two light years.
See also: Religious cosmology
In theology, the term can be used to denote the created Universe, not including the creator. In Christian theology, the word is also used synonymously with aion to refer to "worldly life" or "this world" as opposed to the afterlife or World to Come.
The cosmos as originated by Pythagoras is parallel to the Zoroastrian term aša, the concept of a divine order, or divinely ordered creation.
Science fiction about the cosmos
Olaf Stapledon, in his science fiction novel Star Maker (1937), describes how God (the Star Maker) evolves by creating ever more complex cosmoses across multicosmic hypertime.
Isaac Asimov, in his science fiction story The Last Question (1955), examines the question of how a new cosmos could be formed after the present cosmos has degenerated into heat death due to entropy.
Cosmology is the study of the cosmos in several of the above meanings, depending on context. 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.
In physical cosmology, the term cosmos is often used in a technical way, referring to a particular spacetime continuum within the (postulated) multiverse. Our particular cosmos is generally capitalized as the Cosmos.
Age and size of the cosmos
According to current scientific theory, the cosmos began 13.81 billion years ago (short scale) in the Big Bang. The current diameter of the observable cosmos is thought to be about 93 billion light years.
The diameter of the entire cosmos is unknown. However, according to Alan Guth's inflation theory, the actual size of the cosmos is at least fifteen orders of magnitude larger than the observable universe. This means that, if the inflation theory is correct, the 93 billion light year diameter of the observable universe is approximately as much smaller than the diameter of the entire universe as the diameter of a helium atom is compared to the diameter of the Sun. This is equivalent to a minimum diameter of the entire cosmos of 1026 light years (100 septillion light years short scale).
Nature of the cosmos
Space is a fabric that can bend or twist and is the fabric of the cosmos. With time, it forms a four-dimensional fabric. The fact that the speed of light—the space traveled by light per second—is the same for everyone whether moving or not suggests that space-time adjusts itself in a way that light seems to travel at the same speed no matter what.
Space is a flexible fabric that contains the galaxies made of stars and planets as heavy objects. Their weight bends the fabric of space-time which creates a curvature that makes it possible for lighter objects to go around heavier objects, as is the case for planet Earth and the lighter weight moon, and also for the planets orbiting the sun in our solar system. This phenomenon is known as gravity which is one of the four forces of the cosmos.
Since space and time are unified, the motion through space impacts time: time slows down for the person who is moving but goes faster for the one who stands still. This implies that the passage of time as we experience it may be just an illusion. In that case, every moment in time from the beginning until far in the future coexists together, but in a different region of the cosmos. That leads to the concept of time travel. Because space and time are a unified physical entity, it is possible that there are some shortcuts in the fabric of space time that can lead us to another period of time different from our present time. Despite the possibility of time travel, there is no proof that we can change the past or even the future. This is because the different periods of time coexist and have a fixed state. Still, the exact nature of time is not fully understood.
Forces of the cosmos
General relativity: Einstein's theory of gravity, which causes the curvature of space and time
Electromagnetic force: One of nature's forces that acts on particles that have electric charge
Strong nuclear force: Force of nature that influences quarks by holding quarks together inside protons and neutrons
Weak nuclear force: Force of nature that acts on subatomic scales and is responsible for phenomena such as radioactive decay
The last three forces exist at the quantum level, which is the atomic scale. The problem for physicists is that gravity acts at the astronomic level, and the other three forces act at the quantum level, causing atoms to behave differently than expected by general relativity. With gravity, the nature of the cosmos is very predictable and ordered. However, at the quantum level, particle energy and position are unpredictable. For physicists, it is hard to fit order into the chaos that is happening at atomic level. At the quantum level, gravitational force may be explained by the presence of a particle named graviton. According to this explanation, a graviton is a particle of energy released by the effect of gravity on the fabric of space-time. However, no experiment has yet proven the existence of such a particle. At the surface space is flexible and ordered, but at quantum level it is active, unpredictable, and multidimensional.
Quest for unification
The goal is to combine all the laws in physics into one that can explain the whole cosmos from the outer space down to the quantum scale. Today, there is a popular theory named String theory that provides a possible explanation of the fundamental nature of the Cosmos.
Superstring theory, or string theory, says that fundamental ingredients are one-dimensional loops (closed strings) or snippets (open strings) of vibrating energy, which unites general relativity and quantum mechanics.
The fact that the particles behave like waves suggests that they are made of strands shaped like string at the most fundamental level. The shape of the energy strand determines what is a proton and what is a neutron. In other words, different shapes produce different particles. At present, string theory is not considered a theory in the scientific sense because no scientific experiments either support or disprove it. (For the different uses of the term, see Theory.)
Taking the nature of the cosmos into consideration, cosmos can be defined as the ordered system of all that exists from outer space down to the atomic scale.