The cosmology is the branch of the Astrophysique which studies the Univers as a physical system.
Since the 18th century, cosmology émancipée Métaphysique and Théologie: the distinction between scientific cosmology , the object of this article, and religious Cosmologie is approached in the following paragraph. The other paragraphs treat modern angle of scientific cosmology.
The concept of Univers evolved/moved during the Histoire. Whereas the Ground was generally regarded as a fixed point (theory Géocentrique, defended inter alia by Aristote and Ptolémée), the Renaissance saw emerging the idea that our planet did not occupy of privileged position in the universe (Principe copernician), and was certainly not in the center. It is as from the 17th century that the first instruments of astronomical observation made their appearance (Telescope of Galileo, then Télescope of Isaac Newton), making it possible to survey cosmos, and to think of its structure, then, as from the 20th century, with its history and its evolution.
Scientific cosmologies and religious cosmologies
See also: Metaphysical, religious Cosmology, Cosmogony
Cosmologies are divided into two groups: one distinguishes scientific cosmologies from the religious cosmologies.
Until one has the optical proof of the orbitation of the Earth, at the 18th century (with the discovery of the aberration of the stars by James Bradley), cosmology still formed part of the Métaphysique. In the classification of Christian Wolf (1729), it was indeed one of the three disciplines of the “ metaphysics spéciale ”, with the Theology (God), and the Psychology (the Heart).
By nature, scientific cosmologies confront with the scientific Méthode, and are erected scaffolding in order to be the satisfactory theories most compatible with the Observation S at a given time. The quality of the observations being improving, the theories are regularly refined, in order to take account of those, with the liking of scientific progresses and technological. In certain cases, they can be abandoned with the profit of other theories if the observations prove to be impossible to reconcile with them. The great changes of paradigm remain relatively rare in the history of cosmology (abandonment of the geocentrism to the profit of the heliocentrism, discovered interstellar scales of distance, structure of the Milky Way, and Expansion of the universe). The less drastic modifications of a given theory are more frequent (additions of the cosmic Inflation, the black Matière and the black energy with the standard Modèle of cosmology, for example).
Religious cosmologies, as for them, are the fruit of the fundamental texts of a Religion given. These cosmologies in general present a deep coherence with the other basic principles of this religion, and in particular the Morale, the ethical , and the Métaphysique. The question of knowing if the Univers is the fruit of a divine creation is the object of other discussions. Religious cosmologies do not claim today to erect scaffolding of the scientific theories, or to impose some assertion concerning the physical structure of the Univers. They are rather seen as a social Représentation which is shareable by the community of the Fidèle S of a given religion. Consequently, and in order to avoid confusion, they must present a certain character of stability in the time, which avoids inopportune rehandlings of the fundamental texts.
It is considered today that the Interprétation (Herméneutique) of the biblical texts must be considered from a point of view symbolic system and nonliteral. Also, the fundamental texts can be adjusted in time. It is the object of the Exégèse to find the words right to pass from the old texts to their Traduction and Interprétation in the contemporary context. Thus, one can note that certain cosmological passages of the Bible, if discussed at the 17th century (see the business Galileo and the question of the Héliocentrisme), were reformulated in a version which less seems to lend to confusion. Remainder, the biblical passages are generally interested in the Ground and its inhabitants, or the “skies”, whereas scientific cosmology studies cosmos as a whole, our Galaxie, the Milky Way being only one galaxy among others.
Certain radical monks dispute interpretation only symbolic system and claim that veracity of the religious texts being an established fact, the cosmology which they describe is also true. These monks thus criticize the aspects of scientific cosmology which are in disagreement with their religious cosmology. In certain cases, they can deny the relevance of any scientific study which does not go in the direction of their religious cosmology, at the point to claim the teaching of with equality with the other (see Créationnisme). This attitude is unanimously regarded as nonscientist in the physicists, and generally repudiated by the large majority of the moderate practitioners of these religions.
The question of the origin of the world is also tackled in Philosophie by the Cosmogonie. It develops several cosmological concepts, for example in the philosophy of Kant.
History of scientific cosmologies
See also: History of scientific cosmologies pre-relativists
The scientific cosmology established at a given moment depends in a crucial way of what one knows of the universe. Before the 19th century, the known universe was primarily tiny room to only the Solar system, and cosmology thus related only to the training of this one. It is only starting from first half of the 19th century that the distance to the star S close relations could be known (starting from 1838 thanks to Friedrich Wilhelm Bessel). The study of the space distribution of stars within our galaxy was then carried out until the beginning of the 20th century. Lastly, in the Années 1920 the extragalactic nature of what one then called the “Nébuleuse S” (today galaxies) was discovered by Edwin Hubble. A little later this one also discovered the Expansion of the universe, i.e. the fact that the galaxies of the universe move away from/to each other, and this all the more quickly as they are far. Cosmology such as it today is heard is thus the study of the structure, the history and the evolution of a filled up universe of galaxies as far as the eye can see.
Modern cosmology
Some orders of magnitude
Distances
The Ground is a relatively modest Planet of size (approximately 6 300 km of ray), in Orbit around a star, the Sun. The distance Ground-Sun is worth a astronomical Unité, that is to say approximately 150 million kilometers. Another planets orbit around the Sun. The planet furthest away from the Sun (by not counting dwarf planets) is Neptune, distant of approximately 4 billion and half of kilometers of the Sun, is thirty times the distance Ground-Sun.
The solar system itself is plunged in a structure, the Galaxie (in the case of our galaxy: the Milky Way), including/understanding several hundreds of billion stars. The star nearest to the Sun, Proxima of the Centaur, is located at a little more than 4 light-years, that is to say 45 000 billion kilometers of this one, is 10 000 times more than the distance Sun-Pluto. The majority of stars visible with the naked eye in the night sky are with distances of several tens, even a few hundred light-years. The Sun is not at all located at the center of the Galaxy. It is approximately with 25 000 years light; the Galaxy has a ray approximately 2 times larger than this distance, for a diameter of approximately 100 000 light-years. These dimensions make a typical galaxy of the universe of it.
If one excludes the dwarf galaxies which exist of number in the vicinity of our galaxy, the massive galaxy nearest to us is the Galaxie of Andromède, whose distance is slightly higher than 2 million light-years. Our galaxy and that of Andromède are the two most massive representatives of a group of galaxies called local Groupe, broad of a few million light-years. There exist other larger structures in the universe, called Galaxy cluster and Superamas. The cluster nearest to the local Group is the Virgo Cluster (or cluster of Virgo, of the Latin name of the Constellation), even located to him close to the center of the Superamas of the Virgin. The superclusters are the largest structures existing in the universe. Their size does not exceed 200 to 300 million light-years.
The size of the observable universe, it, is estimated at 43 billion light-years.
One estimates at:
- 10 million the number of superclusters located in the observable universe;
- 25 billion the number of galaxy cluster located in the observable universe;
- 350 billion the number of massive galaxies (higher than or about that of our galaxy) located in the observable universe;
- 30 000 billion billion (3×10 22 ) the number of stars located in the observable universe.
Masses
The density of the Earth is from approximately 5 ton S by Cubic meter. Being given its size, its mass is of approximately 6×10 24 kg. The Sun, which is a typical star, is approximately 300 000 times more massive, are 2×10 30 kg. For the larger objects (galaxies, galaxy cluster), it is of habit to use the solar Masse like unit of mass, the kilogram becoming a too small unit within sight of the numbers concerned.
The observations indicate that the galaxies are significantly more massive than the stars which compose it. One about certain that in addition to the ordinary matter of which we are made, there exists another matter shape, currently unknown in laboratory, is today called black Matière. Contrary to the ordinary matter, this black matter does not interact with the light and is thus invisible. Moreover, it does not form compact structures like stars, planets or asteroids, but has a distribution definitely more diffuse within the galaxies. The mass of the black matter within the galaxies (and in the entire universe) is approximately six times higher than that of the ordinary matter. The mass of our galaxy is thus of a little more than thousand billion solar masses.
The estimated mass of the superclusters is approximately some 10 15 solar masses. Paid to their size, the superclusters are objects extremely not very dense: a few tens of Atom S per cubic meter only. The mass of the observable universe is estimated at 1,4×10 24 solar masses.
Durations
The period of revolution of the Earth around the Sun is one year (in fact a Tropical year). The further away the planets are from the Sun, the more their period of revolution is large, consequence of the third law of Kepler. Thus, Neptune has one 165 years period.
The orders of magnitude increase considerably if one looks at the period of revolution of the Sun around the galactic center: it is approximately 200 million years. The stars are not immutable objects. They are formed, start to shine, then die out nuclear fuel fault in their center. The age of the Sun is approximately 4,5 billion years. The oldest stars of our galaxy have approximately 10 billion years. It is also the age of our galaxy. The galaxies also are born starting from immense clouds from gas. The universe itself such as we know it is not eternal. It is resulting from a phase extremely dense and hot, the Big Bang, which occurred there is approximately 13,7 billion years.
Contributions of general relativity
See also: General relativity
The purpose of cosmology is to describe the universe, which one can initially represent by a relatively uniform distribution (with large scales) of matter. It is that Newtonian mechanics proves to be unable to describe a uniform and infinite distribution of matter. To describe the universe, it is essential to call upon the General relativity, discovered by Albert Einstein in 1915. Einstein is besides the first to publish a cosmological Modèle modern, solution of its theory coldly discovered, but describing a finished and static Univers homogeneous. This model primarily as much is justified by philosophical considerations than physical, but is introduced an idea extremely clever (and slightly hazardous at the time), the cosmological Principe.
The discovery a few years later of the Expansion of the universe by Edwin Hubble calls into question the static model of universe of Einstein and finishes providing the foundations of modern cosmology: the Univers (or in any case the area accessible to the observations) is expanding, and describes by the General relativity. Its evolution is determined by this theory, like by the physical properties of the forms of Matière present in the Univers. It is primarily according to these last that the various cosmological theories will emerge.
The observations indicate that the universe is expanding. It was denser and more heat in the past. It is there the idea founder of the Big Bang, whose model emerged in the middle of the 20th century. It indicates that universe such as we know it is resulting from phase dense and hot (without claiming to know what it occurred to the whole beginning from this phase), at the conclusion which it was in an extremely homogeneous state, i.e. without astrophysical objects (stars, galaxies…). These objects were formed thereafter by a mechanism called gravitational Instabilité. As astrophysical objects are formed, the physical conditions which reign in the universe change, for finally producing the universe such as we know it. The detail of these processes depends on many parameters, such as for example the age of the universe, its density, and the properties of the various matter shapes which coexist in the universe.
In practice, the researchers work out model cosmological, i.e. a kind of scenario describing here the various phases by which the universe passed since and possibly during Big Bang. In the Années 1990 finally emerged the standard Modèle of the cosmology, which represents the capable simplest model to explain the whole of the cosmological observations.
See also: cosmological Model
The standard model of cosmology
See also: standard Model of cosmology
General relativity, the quantum Mechanical and the theory of the fields, coupled to many astronomical observations make it possible today to outline a relatively reliable scenario of the history of the universe on 13 or the last 14 billion years. It is of habit from now on of speaking about a standard Modèle of cosmology, following the example standard Modèle in physics of the particles, although this last is quantitatively tested better and better constrained. The standard model of cosmology is based on the concept of the Expansion of the universe, and the fact that this one was denser and more heat in the past (from where the hot term of Big Bang). Its description rests on the use of general relativity to describe the dynamics of its expansion, and the data of its given material contents to some extent by the direct observation, to some extent by a unit of theoretical and observational elements. One considers today that the universe is homogeneous and isotropic (i.e. it always has the same aspect whatever the place from where it is observed and the direction in whom one observes it), whom his space Courbure is null (i.e. the geometry with large scales corresponds to the Solid geometry usual), and that it is filled up of a certain number of the matter shapes, namely:
- Of the ordinary matter (Atome S, Molécule S, electron S, etc), so called baryon matter, returning for approximately 5% of the composition of the universe
- Another matter shape called black Matter (or matter sinks), of nonbaryon origin, composed of massive particles not detected to date, entering for approximately 25% of the total composition
- Another form of energy whose nature is badly known, but which could be a cosmological Constante, and called génériquement black energy, entering for 70% the composition of the material contents of the universe
It is probable that in the past the material contents were different. For example, there does not exist or only very few Antimatière in the universe, however it is thought that in the past matter and antimatter existed in equal quantities, but that a surplus of matter ordinary was formed during a process, still badly known, called Baryogenèse. At present, only the most moved back times of the phase of expansion of the universe are badly known. One of the reasons to that is that it is not possible to observe these times directly, the detectable most remote radiation at present (the cosmological diffuse Fond) having been emitted approximately 380 000 years later. A certain number of scenarios describing part of the former times exist, among which most popular of the cosmic Inflation is that.
The Destin of the Universe is not either known with certainty at present, but a great number of elements let think that the expansion of the universe will continue indefinitely (see Accélération of the expansion of the universe). Another unsolved question is that of the Topologie of the universe, i.e. its structure with very large scales, where various ideas were proposed (see the article Topologie of the Universe).
References and sources
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