Universe

The universe is commonly defined as the totality of everything that exists,^[1] including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space,^[2][3] although this usage may differ with the context (see definitions, below). The term universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or nature. Observations of earlier stages in the development of the universe, which can be seen at great distances, suggest that the universe has been governed by the same physical laws and constants throughout most of its extent and history.

History

Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the universe. The earliest quantitative geocentric models were developed by theancient Greeks, who proposed that the universe possesses infinite space and has existed eternally, but contains a single set of concentric spheres of finite size – corresponding to the fixed stars, theSun and various planets – rotating about a spherical but unmoving Earth. Over the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and theNewtonian model of the Solar System, respectively. Further improvements in astronomy led to the realization that the Solar System is embedded in a galaxy composed of billions of stars, the Milky Way, and that other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. Discovery of the red shift and cosmic microwave background radiation revealed that the universe is expanding and apparently had a beginning.

According to the prevailing scientific model of the universe, known as the Big Bang, the universe expanded from an extremely hot, dense phase called the Planck epoch, in which all the matter and energy of the observable universe was concentrated. Since the Planck epoch, the universe has been expanding to its present form, possibly with a brief period (less than 10⁻³² seconds) of cosmic inflation. Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of dark energy, and that most of the matter in the universe may be in a form which cannot be detected by present instruments, and so is not accounted for in the present models of the universe; this has been named dark matter. The imprecision of current observations has hindered predictions of the ultimate fate of the universe.

Current interpretations of astronomical observations indicate that the age of the universe is 13.75 ±0.17 billion years,^[4] and that the diameter of the observable universe is at least 93 billion light years or 8.80×1026
 metres.^[5] According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the universe is finite or infinite.

Etymology, synonyms and definitions

The word universe derives from the Old French word Univers, which in turn derives from the Latin word universum.^[6] The Latin word was used byCicero and later Latin authors in many of the same senses as the modern English word is used.^[7] The Latin word derives from the poetic contraction Unvorsum — first used by Lucretius in Book IV (line 262) of his De rerum natura (On the Nature of Things) — which connects un, uni(the combining form of unus', or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning "something rotated, rolled, changed").^[7] Lucretius used the word in the sense "everything rolled into one, everything combined into one".

An alternative interpretation of unvorsum is "everything rotated as one" or "everything rotated by one". In this sense, it may be considered a translation of an earlier Greek word for the universe, περιφορά, "something transported in a circle", originally used to describe a course of a meal, the food being carried around the circle of dinner guests.^[8] This Greek word refers to an early Greek model of the universe, in which all matter was contained within rotating spheres centered on the Earth; according to Aristotle, the rotation of the outermost sphere was responsible for the motion and change of everything within. It was natural for the Greeks to assume that the Earth was stationary and that the heavens rotated about the Earth, because careful astronomical and physical measurements (such as the Foucault pendulum) are required to prove otherwise.

The most common term for "universe" among the ancient Greek philosophers from Pythagoras onwards was τὸ πᾶν (The All), defined as all matter (τὸ ὅλον) and all space (τὸ κενόν).^[9][10] Other synonyms for the universe among the ancient Greek philosophers included κόσμος (meaning the world, the cosmos) and φύσις(meaning Nature, from which we derive the word physics).^[11] The same synonyms are found in Latin authors (totum, mundus, natura)^[12] and survive in modern languages, e.g., the German words Das All, Weltall, and Natur for universe. The same synonyms are found in English, such as everything (as in the theory of everything), the cosmos (as in cosmology), the world (as in the many-worlds hypothesis), and Nature (as in natural laws or natural philosophy).

Broadest definition: reality and probability

The broadest definition of the universe can be found in De divisione naturae by the medieval philosopher and theologian Johannes Scotus Eriugena, who defined it as simply everything: everything that is created and everything that is not created. Time is not considered in Eriugena's definition; thus, his definition includes everything that exists, has existed and will exist, as well as everything that does not exist, has never existed and will never exist. This all-embracing definition was not adopted by most later philosophers, but something not entirely dissimilar reappears in quantum physics, perhaps most obviously in the path-integral formulation ofFeynman.^[14] According to that formulation, the probability amplitudes for the various outcomes of an experiment given a perfectly defined initial state of the system are determined by summing over all possible paths by which the system could progress from the initial to final state. Naturally, an experiment can have only one outcome; in other words, only one possible outcome is made real in this universe, via the mysterious process of quantum measurement, also known as the collapse of the wavefunction (but see themany-worlds hypothesis below in the Multiverse section). In this well-defined mathematical sense, even that which does not exist (all possible paths) can influence that which does finally exist (the experimental measurement). As a specific example, every electron is intrinsically identical to every other; therefore, probability amplitudes must be computed allowing for the possibility that they exchange positions, something known as exchange symmetry. This conception of the universe embracing both the existent and the non-existent loosely parallels the Buddhist doctrines of shunyataand interdependent development of reality, and Gottfried Leibniz's more modern concepts of contingency and the identity of indiscernibles.