# Universe

The word universe ( " turned into one" of Latin Universus " Total ", from unus and versus ), also: the cosmos, called in physics the found at a given time arrangement of all organized according to physical laws of matter and energy, from the elementary particles to larger structures such as galaxies and clusters of galaxies.

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## Space and Space - Delimitation

The word universe was Germanized in the 17th century by Philip of Zesen by the word universe. While the concept of the universe everything, including the stars and planets including the earth as is, with space often only the space outside the earth's atmosphere, or in general the space and away from designated planetary and lunar surfaces.

Since the transition from the atmosphere to space is fluid, there are several established limits. International most common is the definition of the Fédération Aéronautique Internationale, after the space begins at an altitude of 100 km ( the Kármán line). There is the speed that is required to receive lift to fly just as high as the rotational speed of a spacecraft, which is supported by the gravity of the earth in a circular orbit. According to the definition of NASA and the U.S. Air Force Space begins at an altitude of about 80 kilometers ( 50 miles) above the ground. An internationally binding height limit does not exist.

## General

The now generally accepted theory to describe the large-scale structure of the universe is the general theory of relativity by Albert Einstein. Even quantum physics has made important contributions to the understanding of the early universe specifically provided, in which the density and temperature were very high and many processes involving elementary expired ( astroparticle physics ). Probably a broader understanding of the universe is only achieved when the physics established a theory that unites general relativity with quantum physics. This is T.O.E. ( Theory Of Everything) or also called world formula. In this theory of quantum gravity, the four fundamental forces of physics ( electromagnetic force, gravity, strong and weak nuclear force) should be explained consistently. Even Albert Einstein has many years endeavored to establish such an all-encompassing theory - without success. Addition, in its concept, the strong and weak interactions were not included so that his search for the Ultimate Theory and therefore - because incomplete - was doomed to failure. Only in the 1960s were the mathematical prerequisites for the development of an association theory available, so that the search of the physicists began after this big unified picture of the world.

Cosmology, a branch of both the physics and the modern philosophy of science, is concerned with the study of the universe and attempts to answer characteristics of the universe such as the question of the fine-tuning of the fundamental constants.

## Age and composition

The classic and now widely accepted Big Bang theory assumes that the universe is created in a certain moment, the Big Bang, from a singularity and expands out since then (see expansion of the universe ). Time, space and matter are thus created in the Big Bang. Times "before" the Big Bang and places " outside " of the universe are not physically definable. Therefore, " there " there in physics neither a spatial " Outside ", nor a temporal "before" nor a cause of the universe.

Since the scientific laws for the extreme conditions for about the first 10-43 seconds ( Planck time ) after the Big Bang are not known, the theory describes the actual process is not strictly speaking. Only after the Planck time, the other processes can be physically traced. This allows the early universe, for example, a temperature of 1.4 × 1032 K ( Planck temperature) to assign.

The age of the universe is very accurately measured due to precision measurements by the Planck space telescope: 13.80 ± 0.04 billion years. A previous determination of age by the satellite WMAP showed the slightly less accurate result of 13.7 billion years. Age can also be calculated by extrapolation of the current expansion rate of the universe at the time when the universe was compressed into a point. This calculation but strongly depends on the composition of the universe, because matter and energy by gravitational slowing the expansion. The only indirectly been proven dark energy, the expansion but also speed. Thus, different assumptions about the composition of the universe can lead to various ages. By the age of the oldest stars, a lower limit can be given for the age of the universe. In the current standard model both methods agree very well.

All calculations for the age of the universe assume that the Big Bang can actually be considered as a temporal beginning of the universe, which is not secured immediately due to lack of knowledge of the physical laws of the state after the beginning of the Big Bang. Although a static universe is infinitely old and infinitely large, be excluded, but not a dynamic infinitely large universe. This is due, among other things, by the observed expansion of the universe. Furthermore, already had the astronomer Heinrich Wilhelm Olbers out that at infinite expansion and infinite age of a static universe, the night sky would shine brightly ( Olberssches paradox), as each view one directed to heaven, would automatically fall on a star. Is the universe infinite, however, but only has a finite age, so us the light of certain stars simply has not yet been reached.

The space between galaxies is not completely empty. It is filled with a weak solution of hydrogen gas. This intergalactic medium consists of about one atom per cubic meter. Within galaxies, however, the density of matter is much higher. Similarly, the space of fields and radiation is penetrated. The temperature of the background radiation is currently 2.7 Kelvin ( ie about -270 ° C). It was formed 380,000 years after the Big Bang and is also referred to as the birth cry of the universe. The universe is only a small part of us known matter and energy ( 4%), in turn, emits only 10% of the light and thus is visible. A larger part (23 %) makes "dark matter ", which is responsible for the gravitational cohesion of galaxies. Dark matter is indirectly detected by a variety of observations, but their composition is still poorly understood. The largest part is " dark energy " ( 73%), which is responsible for the accelerated expansion. On the dark energy was concluded from the data of distant supernova explosions, their existence is confirmed by satellites such as COBE, WMAP and Planck, balloon experiments such as BOOMERanG as well as gravitational lensing and the galaxy distribution in the universe.

## Shape and volume

Intuitively, it seems likely that from the big bang theory a " spherical shape " of the universe follows; However, this is only one of several possibilities. So in addition to a flat infinite universe, many other forms have been proposed. Among them, for example, a Hypertorusform, or even in popular science publications as " football shape " and " trumpet" shaped become known forms. Some data of the satellite WMAP also talk that the Universe is an ellipsoid.

In the CDM standard model (CDM of Engl. Cold Dark Matter, "cold dark matter" ) and the more recent Lambda - CDM standard model, which takes into account the measured acceleration of the expansion of the universe, the universe is flat, that is, the space is defined by Euclidean geometry described. Such a universe must have an infinite volume is not mandatory, as well as compact topologies for the space are possible. Based on the currently available observations can be given for the expansion of the universe only a rough lower limit. After Neil J. Cornish of Montana State University Data from the WMAP satellite, that the universe according to most models must have a minimum diameter of 78 billion light years. In the Lambda -CDM model, a standard flat geometry is considered with infinite extent, therefore, mostly.

Important is the difference between infinity and infinity: Even if the universe would have a finite volume, so it could still be unlimited. Light can graphically represent this model as follows: a sphere ( sphere ) is finite but has no center and is unlimited ( you can move on her, without ever reaching an edge ). Just as a two-dimensional spherical surface encloses a three-dimensional ball, you can, if the universe is not flat, but curved, imagine the three-dimensional space as the " edge " of a higher-dimensional space. Mind you, this is for illustration only, because the universe is not embedded in the classical cosmology in a higher-dimensional space.

### Relationship between mass density, the local geometry and shape

Although the local geometry is very close to a flat Euclidean geometry, a spherical or hyperbolic geometry is not excluded. Since the local geometry is linked to the global shape (topology ) and the volume of the universe is ultimately also unknown whether the volume is finite (expressed mathematically: a compact topological space ) or whether the universe has an infinite capacity. What geometries and shapes for the universe are possible depends according to the Friedmann equations which describe the evolution of the universe in the standard Big Bang model, again much of the energy density and the mass density in the Universe from:

- This density is less than a certain specified value as a critical density, the overall geometry is referred to as a hyperbolic, since they can be considered as the three-dimensional analog of a two-dimensional hyperbolic surface. A hyperbolic universe is open, that is, a given volume element within the universe expands more and more, without ever coming to a standstill. The total volume of a hyperbolic universe can be both infinite and finite.
- If the energy density exactly equal to the critical density, the geometry of the universe flat ( Euclidean ). The total volume of a flat universe is the simplest case, if we assume a Euclidean space as the simplest topology infinite. But there are also topologies with a finite volume compatible with a Euclidean universe. For example, a Hypertorus is possible as form. A flat universe is like the hyperbolic universe open, so a given volume element expands ever further. However, its expansion is slowing down rapidly, so that after infinite time, a finite extension is reached.
- The energy density is greater than the critical density, it is referred to as " spherical". The volume of a spherical universe is finite. In contrast to the Euclidean and hyperbolic universe for the expansion of the universe eventually comes to a stop and reverses it. The universe that is " crashes " back in on itself.

Current astronomical observational data do not allow to distinguish the universe from a Euclidean universe. The previously measured power density of the universe is thus as close to the critical density, that the experimental errors do not allow to differentiate between the three basic cases.

Dark energy continues to affect the expansion properties of the universe. To lead a large proportion of dark energy to the fact that a spherical universe does not collapses into itself, or flat universe constantly accelerated. Certain forms of dark energy can even cause the universe is expanding locally faster than the speed of light and is so torn apart in a Big Rip, there can be no interactions between particles more.

### Consequences of an infinite space-time volume

The assumption of a universe with an infinite space-time volume raises some questions about the epistemological consequences of this assumption. Here particularly the Anthropic principle plays a role, as formulated eg by Brandon Carter. Then you have - are taken into account at least the necessity of the existence of an observer in the interpretation of astronomical data - in the most cautious interpretation; ie observation data are not necessarily representative of the entire universe.

Examples of consequences which were variously concluded are about that a locally seemingly hospitable universe as a whole can be extremely hostile to life, or that even extremely unlikely but possible events would be infinitely often occur in such a universe. More recently, the physicist Max Tegmark has pointed out that from the current standard model of the universe follow along with the quantum theory that, on average, all meters must a "twin world " exist. Some of the consequences, however, would arise even at universes with finite, but sufficiently large volume.

## Structures in the universe

On the currently largest observable scale one finds clusters of galaxies that come together to even larger superclusters. These in turn form span ( engl. empty voids, void =) thread-like filaments, the huge, bubble-like, practically galaxies free cavities. One speaks sometimes of the honeycomb-like structure (so called cosmic web) of the universe. This results in the following ranking of the largest to the smallest structures of the observable universe:

Note: The size scales are greatly going into each other, so for example, there Monde, exceed the planet 's size, asteroids, which are substantially larger than some moons, etc. In fact, the classification of celestial objects is due to their size in astronomy currently very controversial, for example, the question of which solar satellites are to be counted among the planets and which are not ( Plutinos, Trans Neptune, etc.). Pluto was a planet since its discovery in 1930, is considered one of the dwarf planet since August 24, 2006, by definition, the International Astronomical Union ( IAU).