# universe

universe
The Hubble Ultra Deep Field image offers a very deep view of the universe. (The photo encompasses a solid angle that corresponds approximately to the 150th part of the average lunar disk .)
Physical properties (related to the observable universe )
Mass (visible) approx. 10 53  kg
Medium density approx. 4.7 · 10 −30  g / cm 3
Age 13.81 ± 0.04 billion  years
Number of galaxies approx. 2 trillion
Background radiation temperature 2.725 ± 0.002  K
Structure of the universe

The universe (from the Latin universus 'total' ), also called the cosmos or the universe , is the totality of space, time and all matter and energy in it. The observable universe , however, is limited to the found arrangement of all matter and energy , starting with the elementary particles up to the large-scale structures such as galaxies and galaxy clusters .

The Cosmology , a branch of both the physics and the current philosophy of science , deals with the study of the universe and tries properties, such as the question of the universe fine-tuned universe to answer.

The theory generally accepted today for describing the large-scale structure of the universe is the standard model of cosmology . It is based on the general theory of relativity in combination with astronomical observations. The quantum physics has to understand important contributions especially in the early universe the time shortly after the Big Bang come in which the density and temperature were very high. An expanded understanding of the universe will probably not be achieved until physics develops a theory that combines general relativity with quantum physics. This theory of quantum gravity called the “Theory Of Everything” or the world formula is intended to explain the four basic forces of physics in a uniform manner.

## Origin of the designations

The word "Universum" was Germanized by Philipp von Zesen in the 17th century with the word "Weltall". While the universe or space encompasses everything, the term space only refers to the space outside the earth's atmosphere and outside the atmospheres of other celestial bodies, in which there is almost a vacuum . Colloquially, “Weltall” or “All” is also used with the meaning of “Space”.

The term “cosmos” is borrowed from ancient Greek κόσμος “order” and expresses in addition to the term “universe” that the universe is in an “orderly” state, as a counter-term to chaos . It has been attested since the 19th century and is the root word for cosmonaut , the name for Soviet or Russian spacemen.

## Age and composition

The Andromeda Galaxy , the closest larger galaxy to us

The classic and now widely recognized Big Bang theory assumes that the universe emerged from a singularity at a certain moment, the Big Bang , and has since expanded ( see expansion of the universe ). Time, space and matter were created with the Big Bang. Times “before” the Big Bang and places “outside” the universe cannot be physically defined. Therefore, in physics there is neither a spatial “outside” nor a temporal “before” nor a cause of the universe.

Strictly speaking, the theory does not describe the actual process because the scientific laws for the extreme conditions are not known during the first 10 −43  seconds ( Planck time ) after the Big Bang. Only after the Planck time has elapsed can the further processes be physically traced. So the early universe z. B. assign a temperature of 1.4 · 10 32  K ( Planck temperature ).

The age of the universe is measured very precisely thanks to precision measurements by the Planck space telescope : 13.81 ± 0.04 billion years. An earlier determination of the age by the satellite WMAP gave the somewhat less precise result of 13.7 billion years. The age can also be calculated by extrapolating from the current speed of expansion of the universe to the point in time at which the universe was compressed in one point. This calculation depends heavily on the composition of the universe, since matter or energy slows down the expansion through gravity. The dark energy , which has so far only been indirectly proven, can, however, also accelerate the expansion. Different assumptions about the composition of the universe can lead to different ages. A lower limit for the age of the universe can be given by the age of the oldest stars. In the current standard model, the results of these methods agree very well.

All calculations for the age of the universe assume that the big bang can actually be regarded as the beginning of the universe in time, which is not certain for the state immediately after the beginning of the big bang due to ignorance of the physical laws. A static universe that is infinitely old and infinitely large can be excluded, but not a dynamic, infinitely large universe. This is due, among other things, to the observed expansion of the universe . Furthermore, the astronomer Heinrich Wilhelm Olbers already pointed out that with an infinite expansion and infinite age of a static universe, the night sky would have to shine brightly ( Olbers paradox ), since every glance one looks at the sky must automatically fall on a star. However, if the universe is infinitely large, but only has a finite age, then the light from certain stars simply has not yet reached us.

The space between galaxies is not completely empty, but contains not only stars and dust clouds, but also hydrogen gas. This intergalactic medium has a density of about one atom per cubic meter. Within galaxies, however, the density of matter is much higher. Likewise, the space is interspersed with fields and radiation . The temperature of the background radiation is 2.7  Kelvin (about −270 ° C). It was created 380,000 years after the Big Bang. The universe consists only to a small extent of known matter and energy (5%), of which only 10% emits light and is therefore visible. A larger part (27%) is made up of dark matter . Dark matter has been indirectly detected by a large number of observations, but its composition is still largely not understood. Most of it is dark energy (68%) which is responsible for the accelerated expansion. The dark energy was inferred from the data of distant supernova explosions, its existence is confirmed by satellites such as COBE , WMAP and Planck , balloon experiments such as BOOMERanG as well as gravitational lens effects and the distribution of galaxies in the universe.

## Shape and volume

Intuitively, the assumption is made that a “spherical shape” of the universe follows from the Big Bang theory ; however, this is only one of several possibilities. In addition to a flat, infinite universe, many other shapes have been proposed, including, for example, a hypertorus shape , or the shapes that have become known in popular scientific publications as the “football shape” and “trumpet shape”. Some data from the WMAP satellite also suggest that the universe is an ellipsoid .

In the CDM standard model (CDM from C old D ark M atter, "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, space is described by Euclidean geometry . Such a universe does not necessarily have to have an infinite volume, since compact topologies are also possible for the space. Based on the observations available, only a rough lower limit for the extent of the universe can be given. According to Neil J. Cornish of Montana State University , data from the WMAP satellite show that, according to most models, the universe must be at least 78 billion light years in diameter . In the Lambda CDM standard model, therefore, a flat geometry with infinite dimensions is usually considered.

Television signals transmitted from the earth never reach the edge of this picture.

The background to the calculated minimum size is that a curvature of the universe could not be measured. The measurement inaccuracy is relatively large at 2%. If one assumes that this measurement inaccuracy leads to a curvature of the universe of a maximum of 2%, then the universe could be curved back into itself. However, the curvature could actually be zero or it could assume a value between zero and the maximum conceivable curvature. In the first case the universe would be infinitely large, in the latter it would be larger than 78 billion light years.

Since the universe is 13.8 billion years old, only objects can be perceived whose light was emitted a maximum of 13.8 billion years ago. This is the observable universe . Since space has expanded significantly over the past 13.8 billion years, the places from which objects emitted light 13.8 billion years ago are now more than 45 billion light-years away. The objects themselves can have moved further away from these locations over a period of 13.8 billion years due to their own movement within space.

The important thing is the difference between infinity and infinity: Even if the universe had a finite volume, it could be unlimited. This model can be illustrated clearly as follows: A spherical surface (sphere) is finite, but has no center on this surface and is unlimited (one can move on it without ever reaching an edge). Just as a two-dimensional spherical surface envelops a three-dimensional sphere, one can imagine the three-dimensional space as the surface of a higher-dimensional space if the universe is not flat but curved. Mind you, this is only used for illustration purposes, because in classical cosmology the universe is not embedded in a higher-dimensional space.

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

Although the local geometry is very close to a flat, Euclidean geometry , a spherical or hyperbolic geometry is also not excluded. Since the local geometry is linked to the global shape ( topology ) and the volume of the universe, it is ultimately also unknown whether the volume is finite (in mathematical terms: a compact topological space) or whether the universe has an infinite volume. According to the Friedmann equations , which describe the development of the universe in the standard big bang model, which geometries and shapes are possible for the universe depends essentially on the energy density or the mass density in the universe:

• If this density is smaller than a certain value called the critical density, then the global geometry is called hyperbolic, since it can be viewed 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 continues to expand without ever coming to a standstill. The total volume of a hyperbolic universe can be both infinite and finite.
• If the energy density is exactly the same as the critical density, the geometry of the universe is flat (Euclidean). The total volume of a flat universe is in the simplest case, if one takes a Euclidean space as the simplest topology, infinite. However, topologies with a finite volume of space can also be reconciled with a Euclidean universe. For example, a hyper torus is possible as a shape. A flat universe, like the hyperbolic universe, is open, so a given volume element continues to expand. Its expansion slows down noticeably, so that after infinite time a finite expansion is reached.
• If the energy density is greater than the critical density, the universe is referred to as "spherical". The volume of a spherical universe is finite. In contrast to the Euclidean and hyperbolic universes, the expansion of the universe comes to a standstill at some point and then reverses. The universe "collapses" again.

Current astronomical observation data do not allow the universe to be distinguished from a Euclidean universe. The energy density of the universe measured so far is so close to the critical density that the experimental errors do not make it possible to differentiate between the three fundamental cases.

Dark energy continues to affect the expansion properties of the universe. A large proportion of dark energy means that a spherical universe does not collapse, or a flat universe continues to accelerate. Certain forms of dark energy can even cause the universe to expand locally faster than the speed of light and thus be torn apart in a big rip , since interactions between particles can no longer take place.

### 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. The anthropic principle plays a particularly important role here, as it is e.g. B. was formulated by Brandon Carter . According to this - in the most careful interpretation - at least the necessity of the existence of an observer must be taken into account when interpreting astronomical data; d. that is, observational data are not necessarily representative of the entire universe.

Examples of conclusions that have been drawn from this on various occasions are, for example, that a locally apparently life-friendly universe can on the whole be extremely hostile to life, or that even extremely improbable but possible events would have to occur infinitely often in such a universe. In recent times u. a. The physicist Max Tegmark claims that the current standard model of the universe, together with quantum theory, means that on average a “twin world” must exist every meter. The arguments mentioned by Tegmark also apply to a universe with finite, but sufficiently large, volume. However, these arguments as well as the conclusions are controversial. B. in the publication About the Infinite Repetition of Histories in Space by the sentence " these scenarios remain no more than literary tales ". ${\ displaystyle {10} ^ {{10} ^ {29}}}$

## Structures within the universe

On the largest observable scale, one can find galaxy clusters that come together to form even larger superclusters . These in turn form thread-like filaments that span huge, bubble-like, practically galaxy-free cavities ( Voids , void = empty). Sometimes one speaks of the honeycomb-like structure ( cosmic web ) of the universe. The following ranking results from the largest to the smallest structures of the observable universe :

The Milky Way
1. Large Quasar Group (LQG) (e.g .: U1.27 , diameter: about 4 billion light years)
2. Filaments and voids (e.g. Great Wall , diameter: about 1 billion light years)
3. Superclusters (e.g. Virgo superclusters , diameter: about 200 million light years)
4. Galaxy clusters (e.g. local group , diameter: about 10 million light years)
5. Galaxies (ex .: Milky Way , diameter: about 100,000 light years)
6. Star clusters ( globular star clusters , open star clusters , diameter: tens to hundreds of light years)
7. Planetary systems (e.g. our solar system , diameter: about 300  AU = 41 light hours)
8. Stars ( e.g. sun , diameter: 1,392,500 km)
9. Exoplanets and planets (ex .: Earth , diameter: 12,756.2 km)
10. Moons (e.g. Earth's moon diameter: 3,476 km)
11. Asteroids , comets (diameter: a few kilometers to several 100 km)
12. Meteoroids (diameter: from the meter down to the millimeter range)
13. Dust particles
14. Molecules
15. Atoms
16. Protons
17. Electrons

Note: The size scales are partially overlapping. For example, there are moons that are larger than planets or asteroids that are significantly larger than some moons.

## Map of the astronomical objects

Illustration of the distance relation of various astronomical objects in a representation not to scale - the celestial bodies appear too large, the distances are scaled logarithmically.

## literature

Commons : Universe  - collection of images, videos and audio files
Wiktionary: Universe  - explanations of meanings, word origins, synonyms, translations
Wikibooks: Wikijunior Solar System / Space Research  - Learning and teaching materials

Videos:

## Individual evidence

1. J. Richard Gott III a. a .: A Map of the Universe. In: The Astrophysical Journal. Edition 624, No. 2, arxiv : astro-ph / 0310571 .
2. Planck Collaboration u. a: Planck 2015 results. XIII. Cosmological parameters In: Astronomy & Astrophysics 594, A13 (2016), arxiv : 1502.01589v3 , page 32.
3. Hubble Reveals Observable Universe Contains 10 Times More Galaxies Than Previously Thought. In: NASA. Retrieved January 22, 2018 .
4. Kenneth R. Lang: A Companion to Astronomy and Astrophysics. Chronology and Glossary with Data Tables. Springer, 2006, p. 242 .
5. Christa Pöppelmann: 1000 errors in general education . Compact-Verlag, January 2009, ISBN 978-3-8174-6689-4 , p. 191.
6. Astronomy - planets, stars, galaxies. GEO Bibliographical Institute & F. A. Brockhaus AG. GEO subject lexicon. Vol. 5. GEO, Gruner + Jahr, Mannheim 2007. ISBN 3-7653-9425-4 .
7. The Universe - An Ellipsoid? At: Astronews.com. September 27, 2006, accessed June 23, 2008.
8. ^ Neil J. Cornish, Ph.D. Professor. ( Memento of February 4, 2012 in the Internet Archive ).
9. How big is our flat universe? At: welt.de. January 21, 2015, accessed March 1, 2020.
10. ^ B. Carter: Large Number Coincidences and the Anthropic Principle in Cosmology. In: Confrontation of Cosmological Theories with Observational Data. Copernicus Symposium 2nd IAU Symposium. Vol. 63. Reidel, Dordrecht 1974, 291-298. ISBN 90-277-0456-2 .
11. Nick Bostrom: Anthropic Bias Observation Selection Effects in Science and Philosophy. Routledge, New York 2002. ISBN 0-415-93858-9 .
12. Max Tegmark : Parallel Universes. 2003; Published abridged in Scientific American , May 2003.
13. Francisco José Soler Gil , Manuel Alfonseca : About the Infinite Repetition of Histories in Space (2013) , accessed May 31, 2020.