Expansion of the universe

In Cosmology, the expansion of the universe is the phenomenon according to which the distant Galaxie S seem to move away from/to each other. This distance, which one can naively interpret like a movement of the galaxies in space, is interpreted in a more exact way by a dilation of the space itself, in which the celestial objects are thus brought to move away from/to each other (in any case as soon as their gravitational interactions are not important enough). This movement of expansion is not accompanied by a variation of size of the objects of the universe.

The expansion of the universe results from the near total of the cosmological model resulting from the General relativity, which predict all (except for the first of them, the Univers of Einstein) that the universe is not static, but that it is described by a dynamics whose temporal evolution is determined by the properties of the matter which fills up it (see Équations of Friedmann).

The observational discovery of the expansion of the universe coincides about with the prediction of the phenomenon, also it is sometimes presented according to the authors like a checking of general relativity, or a prediction of this one. One of the natural consequences of the expansion of the universe is that the universe was denser, and thus hotter in the past. The concept of the Big Bang (which affirms that such a dense and hot time actually existed in the past of the universe) is the most known consequence.

Manifestation of the expansion of the universe

See also: Law of Hubble

The expansion of the universe appears by the observation a speed of recession of the remote astrophysical objects compared to the Galaxie. If one directly does not observe a displacement of those, one observes a shift of their emission lines and their absorption lines which one can interpret in term of Doppler effect. This shift being almost systematically towards the red, one concludes from it that the objects move away from us. Moreover, this movement of relative distance is homogeneous and isotropic in the universe: a galaxy located at a given distance moves away from ours at the same speed, whatever the direction where it is, and it is the same for any observer which would be located in another galaxy. There thus exists a relation between the speed of recession of the galaxies and the distance which separates us from them, known under the name of Loi of Hubble , of the name of its discoverer, Edwin Hubble, in 1929.

Movements in space or expansion of space?

In Mechanical traditional or restricted Relativity, the observation of a shift towards the red is interpreted in term of displacement in space and Doppler effect. In General relativity, such an interpretation is significantly more delicate: there does not exist indeed concept of absolute space as in traditional mechanics, or all at least having a certain rigid structure as in restricted relativity. The space of general relativity is, in a certain “elastic” direction, the distance between the points being for example function of the structure of the gravitational Champ in their vicinity. It does not remain about it less than general relativity stipulates that locally , space is identified with that of restricted relativity, and that locally it is completely possible to compare the expansion to movements of objects in space. If this interpretation were generalized with more large scales, that could raise a paradox, because that would like consequently to say that the object located beyond the observable Univers would move away at high speeds to that from the light and, occasion, enfreindraient the laws of restricted relativity. It in is nothing because these object do not move in the universe but with him. As this movement is only due to the expansion of the universe, there is no inconsistency with restricted relativity.

Expansion of the universe, but not of the astrophysical objects

Contrary to an generally accepted idea, the expansion of the universe does not mean only the astrophysical objects, whatever it are, see their size varying: it is only the distance between those which varies during time, and this only for sufficiently distant objects. This is caused by the fact that the forces necessary to counter the movement of expansion on a star or planet, atom scale extremely weak are compared with the or not gravitational gravitational forces which structure these objects. It is thus very easy with the force of Gravitation, the electromagnetic forces or the strong Nuclear force to counter the distance due to the expansion of the universe.

An intuitive way to include/understand this is to take again the analogy of the elastic fabric which one stretches in all the directions. If one is satisfied to draw reasons on the fabric, then those grow bigger at the same time as they seem to move away from/to each other when the fabric is stretched. On the other hand, so instead of drawing reasons, one sticks on the fabric a rigid object (a coin for example), then while stretching the fabric, one still will move away the objects from/to each other, but this time they will keep a constant size. It is primarily a process of this type which is with work with the expansion of the universe.

History of the discovery

The discovery of the expansion of the universe dates from first half of the 20th century and was done in several stages.

• At the beginning of the 20th century, of many diffuse objects was seen in the telescopes. These objects were all called under the generic term of “nebulas”. These “nebulas” were in fact the Nébuleuse S which we know today, as well as Galaxie S, i.e. objects external with the Milky Way. At the time, neither true nature, nor their distance were known.
• the Spectroscopy, still stammering at that time made it possible to bring a first tangible element starting from 1914, year when the American Astronome Vesto Slipher showed that a certain class of these “nebulas” (makes the galaxies of them) showed a systematic tendency to move away from us. It thus seemed possible that these objects can be located apart from our Galaxy because in the contrary case one would have expected that a part equalizes these objects approach and move away from us.
• In 1920 took place what was thereafter called the Great Debate, on the nature of “nebulas”, opponent Harlow Shapley with Heber Doust Curtis on the extragalactic nature or not of certain nebulas, in particular the Galaxie of Andromède. The debate could not at the time showing a final decision for lack of sufficient data.
• Starting from 1925, Edwin Hubble could prove the extragalactic nature of certain nebulas, by observing there Céphéide S thanks to the Télescope Hooker of 2,5 meters of the Observatoire of the Mount Wilson.
• After several years of observations, Edwin Hubble, seems it preceded by Georges Lemaître, establishes the relation between speed of recession and distance from certain renamed nebulas galaxies, thus proving the expansion of the universe. If Hubble discovered the phenomenon, there remained perplexed as for its interpretation.

Other attempts at interpretation

See also: tired Light

The expansion of the universe was in contradiction with the static model of universe proposed by Albert Einstein in 1917 (see Univers of Einstein). Alternative, known explanations under the name of tired Light (term suggested by Richard Tolman in 1930), were proposed to reconcile static universe and shift towards the red as of the discovery of the expansion of the universe in 1929, and this until the Années 1970. No satisfactory explanation not having been able to be found, these theories are for a long time given up by the vast majority of the scientific community.

History of the expansion of the universe

See also: Equations of Friedmann

The expansion of the universe is a generic consequence of the laws of the General relativity. Those stipulate indeed that the universe as a whole is subjected to forces imposed by the various matter shapes which compose it, and which except particular case, this one cannot remain static. Moreover, the expansion of the universe influences the density and the pressure of the various matter shapes which exist in the universe. Thus, it is the knowledge of certain physical properties of all these matter shapes (in particular them equation of state) which makes it possible to predict the exact behavior of the expansion. The equations which describe it are known under the name of equations of Friedmann. The current observations not only make it possible to know the Rate growth of the current universe (the Constante of Hubble), but also that of the universe in the past, making it possible to obtain information on the matter shapes which fill up the universe. In 1998 two teams of astronomers arrived at the unexpected result that the expansion of the universe accelerated. This result was surprising because it requires the existence of the unknown matter shape whose Pression is negative, having a repulsive and not gravitational behavior with respect to the gravitation. This matter shape, commonly called black energy or sometimes Constant cosmological whose true nature at present represents one of the mysteries of modern cosmology.

See too

• Constant of Hubble

• Law of Hubble
• Equations of Friedmann
• standard Model of cosmology
• Acceleration of the expansion of the universe