The
universe is commonly defined as the totality of everything that
exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space, 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.
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 the
ancient 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, the
Sun 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 the
Newtonian 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, and that the diameter of the
observable universe is at least 93 billion
light years, or
8.80 × 1026 metres. 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.
The word
universe derives from the
Old French word
Univers, which in turn derives from the
Latin word
universum. The Latin word was used by
Cicero and later Latin authors in many of the same senses as the modern
English word is used. 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"). 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. 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). 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). The same synonyms are found in Latin authors (
totum,
mundus,
natura) 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 of Feynman. 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 the many-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 shunyata and interdependent development of reality, and Gottfried Leibniz's more modern concepts of contingency and the identity of indiscernibles.
More customarily, the universe is defined as everything that exists, has existed, and will exist[citation needed]. According to this definition and our present understanding, the universe consists of three elements: space and time, collectively known as space-time or the vacuum; matter and various forms of energy and momentum occupying space-time; and the physical laws that govern the first two. These elements will be discussed in greater detail below. A related definition of the term universe is everything that exists at a single moment of cosmological time, such as the present, as in the sentence "The universe is now bathed uniformly in microwave radiation".
The three elements of the universe (spacetime, matter-energy, and physical law) correspond roughly to the ideas of Aristotle. In his book The Physics (Φυσικῆς, from which we derive the word "physics"), Aristotle divided τὸ πᾶν (everything) into three roughly analogous elements: matter (the stuff of which the universe is made), form (the arrangement of that matter in space) and change (how matter is created, destroyed or altered in its properties, and similarly, how form is altered). Physical laws were conceived as the rules governing the properties of matter, form and their changes. Later philosophers such as Lucretius, Averroes, Avicenna and Baruch Spinoza altered or refined these divisions; for example, Averroes and Spinoza discern natura naturans (the active principles governing the universe) from natura naturata, the passive elements upon which the former act.
It is possible to conceive of disconnected space-times, each existing but unable to interact with one another. An easily visualized metaphor is a group of separate soap bubbles, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle. According to one common terminology, each "soap bubble" of space-time is denoted as a universe, whereas our particular space-time is denoted as the universe, just as we call our moon the Moon. The entire collection of these separate space-times is denoted as the multiverse. In principle, the other unconnected universes may have different dimensionalities and topologies of space-time, different forms of matter and energy, and different physical laws and physical constants, although such possibilities are currently speculative.
Source : En.wikipedia.org/wiki/Universe
En.wikipedia.org/wiki/Earth (for the "About our Earth" post )
