The cosmology ( Greek κοσμολογία , kosmología , "the doctrine of the world") deals with the origin, development and the fundamental structure of the universe and the universe as a whole. It is a branch of astronomy that is closely related to astrophysics .
Cosmology describes the universe by means of physical laws. In this context, the “lumpy” distribution of galaxies and galaxy clusters observed today, with large voids in between, is to be understood in contrast to the spatial homogeneity on larger scales.
Cosmology also describes the overall low curvature of space , the structures that differ over time (radiation, quasars , galaxies), the cosmic background radiation , the redshift of light interpreted as an expansion of the universe , the numerical values of the natural constants and the frequency of the chemical elements in the universe.
The standard or big bang model sees the beginning of the universe in an almost infinitely dense state, from which it developed in an expansion known as the big bang to the current state, whereby the cosmos observable today changes from an almost point-like expansion to a radius of more than 45 Billion light years bloated. It is based essentially on the general theory of relativity and is supported by observations:
The density averaged over different length scales shows fluctuations of varying degrees. On the length scale of 10,000 Megaparsec (Mpc) the fluctuations are less than 1%, while on scales from 100 Mpc to 1 Mpc the structures become increasingly lumpy. The largest structures include the Sloan Great Wall with a length of a good 400 mega parsec and the Hercules – Corona Borealis Great Wall with an extension of 2000 to 3000 Mpc, which has only been marked by a good dozen gamma-ray bursts (GRB) .
The fluctuations that can be observed today are said to have developed from quantum fluctuations during inflation , i.e. shortly after the beginning of time, with the development progressing more slowly on large scales than on smaller scales.
Frequency of elements
In primordial nucleosynthesis ( Big Bang Nucleosynthesis ) shortly after the Big Bang (10 −2 s), the universe was so hot that matter was dissolved into quarks and gluons . The expansion and cooling of the universe created protons and neutrons . After a second, the nuclei of light elements ( 2 H , 3 He , 4 He , 7 Li ) merged from protons and neutrons . This process ended in about three minutes. So the relative abundances of these light elements were largely determined before the first stars were formed.
Cosmic background radiation
Postulated by George Gamow in 1946 , the English cosmic microwave background (CMB) was discovered in 1964 by Arno Penzias and Robert Woodrow Wilson - with an average temperature of 2.725 Kelvin . The background radiation comes from the period 300,000 years after the Big Bang, when the universe was about a thousandth of its current size. That is when the universe became transparent, before it was made of opaque ionized gas. Measurements for example by COBE , BOOMERanG , WMAP , Planck space telescope .
Expansion of the universe
Edwin Hubble was able to prove the expansion of the universe in 1929, since galaxies show an increasing redshift in the spectral lines with increasing distance . The proportionality factor is the Hubble constant H, the value of which is assumed to be 67.74 (± 0.46) km / s Mpc −1 (as of 2016). H is not a constant, but changes over time - inversely proportional to the age of the universe. We are not in the center of the expansion - space itself expands equally everywhere ( isotropic universe ). By calculating the expansion back, the age of the universe is determined. If the Hubble constant (see Hubble time ) is correct, it is around 13.7 billion years. Based on the data and supernova observations obtained so far by the WMAP probe , an open, accelerated expanding universe with an age of 13.7 billion years is now assumed.
Evolution of the universe
According to the standard model of cosmology, roughly the following sequence arises.
- Planck era; up to 10 −43 seconds; all four forces still united;
- Inflationary phase also GUT era; ends after 10 −33 s to 10 −30 seconds; extreme expansion by a factor between 10 30 and 10 50 ;
- Quark era; up to 10 −7 seconds; quarks, leptons and photons are formed ; the imbalance of matter and antimatter arises in baryogenesis ;
- Hadrons era; up to 10 −4 seconds; Protons, neutrons and their antiparticles are created; also muons , electrons , positrons , neutrinos and photons;
- Lepton era; up to ten seconds; Muons decay, electrons and positrons annihilate ;
- Primordial nucleosynthesis ; up to three minutes; Hydrogen , helium and lithium are produced;
- Radiation era; about 300,000 years;
- Matter era; til today; Universe becomes transparent, galaxies arise.
To explain the observed expansion and the flat geometry of the universe on a large scale, the Big Bang model is supplemented today according to the ideas of Alan Guth that a symmetry break in the early days of the universe resulted in a very strong brief expansion, which reduced the uniformity of the universe at the edge of the observable area (horizon) explained. The greatest challenge to the cosmological theory is the disproportion between observable matter and its distribution as well as the observed mean speed of propagation of the universe. The usual explanation makes dark matter (with 23%) and dark energy for the parts of the required matter density that cannot be observed by means of electromagnetic radiation (with 73%) responsible.
These proportions are time-dependent: After the radiation-dominated era in the early days of the universe, the matter era followed, in which matter made up the largest proportion. That era ended when the universe was about 10 billion years old; since then, dark energy has made up most of it. The temporal course of the expansion changed accordingly: it was slowed down until the end of the matter era, since then the expansion has been accelerated. This transition can be traced directly and independently of the model by observing supernovae over a wide range of distances.
Steady State Theory
The steady state theory was developed in 1949 by Fred Hoyle , Thomas Gold, and others as an alternative to the big bang theory . During the 1950s and well into the 1960s, this theory was accepted by most cosmologists as a possible alternative.
The "steady-state theory" was postulated on the basis of calculations that showed that a purely static universe would not be compatible with the assumptions of general relativity . In addition, observations by Edwin Hubble showed that the universe is expanding. The theory now postulates that the universe does not change its appearance, although it gets bigger. To do this, matter has to be constantly re-formed to keep the average density the same. Since the amount of new matter to be formed is very small (only a few hundred hydrogen atoms per year in the Milky Way), the new formation of matter cannot be observed directly. Although this theory violates the law of conservation of energy, it had, among other things, the "attractive" property that the universe has no beginning and questions about the before or the reason for the start of the expansion are superfluous.
The difficulties of this theory began in the late 1960s. Observations have shown that the universe actually changes over time, that is, the condition of stationarity is explicitly violated: Quasars and radio galaxies were only found in distant galaxies. Halton Arp interpreted the available data differently since the 1960s and stated that there were quasars in the nearby Virgo cluster . The decline of the steady state theory was accelerated by the discovery of cosmic background radiation, which was predicted by the big bang theory.
Since then, it is not the steady-state theory but the big bang theory that has been considered the successful standard model of cosmology by the majority of astronomers. It is implicitly assumed in most publications on astrophysics .
History of cosmology
Beginnings and the Ptolemaic worldview
Records of mythical cosmologies are known from China ( I Ching , Book of Changes), Babylon ( Enuma Elish ) and Greece ( Theogony of Hesiod ). Cosmological ideas had a high priority in Chinese culture, especially in Daoism and Neoconfucianism . The Babylonian myths - which presumably go back to older Sumerian myths and in turn are likely to be the model for the biblical Genesis - and observations of the sky probably influenced the later Greek cosmological ideas, which became the basis of medieval Western cosmology . Cosmological records were made not only by the Babylonian but also by the Egyptian priesthood. In the pyramid texts , the world of gods is associated with cosmic beings, which are mainly related to the sun, but also to the moon and numerous stars. An astronomical background becomes clear. This emerges from the relief of the Codex Hammurapi , which shows the cosmopolitan king in front of the enthroned sun god.
Earlier cosmologies were based on the principle of recording astronomical data and then interpreting the data . The mythologies developed from the interpretations and prophecies . In addition, the astronomical records provided useful information for the historical calendar, e.g. B. Ur-3 calendars, with the help of which the processes in agriculture were arranged. The process of rationalization began with the Greek scholars Thales of Miletus , especially Anaximander (6th century BC). For the first time Anaximander designed a worldview that was based on lawful causal relationships and assigned a physical nature to celestial objects. According to Anaximander, the infinite universe is the source of an infinite number of worlds, of which the experienced world is only one that has split off and collected its parts by turning. The cosmological designs of the atomists Democritus and Anaxagoras went in the same direction .
Anaximenes worked out the ideas of Anaximander further and saw the air as primordial matter. Pythagoras - for whom all things were really numbers or ratios - held the view that heaven breathed in infinity to form groups of numbers.
Another important development was the first historically transmitted system in which the earth was not in the center, which was developed by Philolaos , a Pythagorean , in the 5th century BC. Was designed. Another Pythagorean, Archytas of Taranto , gave an argument for the infinity of the cosmos ( "Rod of Archytas" ).
In the cosmology of Plato (5th / 4th century BC), which he describes in the Timaeus , he described the celestial objects as divine beings endowed with intellect and personal . In Plato's mind the earth was a sphere that rested in the center of the cosmos.
In his cosmology, Plato's student Aristotle partially contradicted the view of his teacher regarding the divine nature of celestial objects. He calls the heavenly bodies divine and endowed with intellect; they consist of the " fifth element " and are explored by the " first philosophy ". The movements of the celestial bodies and spheres are ultimately caused by a first immobile mover (in the sense of a changer ). Aristotle represented a model of the universe which assumed a central fire (he explicitly did not mean the sun), around which the celestial bodies ran in circles.
Eudoxus of Knidos designed at the beginning of the 4th century BC. A model of the spheres, which was further developed by Kallippos and was able to describe the retrograde loop movements of the planets for the first time. This influenced the Aristotelian and the Ptolemaic worldview. Measurements by Eratosthenes , who lived in the 3rd century BC. BC determined the circumference of the earth with good accuracy, and Aristyllus and Timocharis showed deviations of the planetary motions from the positions calculated by Eudoxus' method. Apollonios von Perge developed in the 3rd century BC A method of calculating planetary orbits with the help of epicycles , he allowed circular movements of the planets, the center of which was again on a circular path.
A heliocentric world model was represented by Aristarchus of Samos (3rd / 2nd century BC). He was therefore accused of wickedness; his world model could not prevail.
Ptolemy described a geocentric cosmology in his Almagest in the 2nd century , which was to be reconciled with most of the observations of his time and was generally accepted until the Copernican worldview was established.
The Copernican Turn
In his book De revolutionibus orbium coelestium , published in 1543, Nicolaus Copernicus created the first view of the world which, in its completeness and accuracy, matched the Ptolemaic system, but was much more simply structured. What is important about the Copernican system is the assumption that the earth is also just a planet of the sun, i.e. no longer enjoys a special position. In the heliocentric universe of Copernicus, the planets move in combinations of uniform circular motions around a point that is close to the sun and is also orbited by it.
The universe described by Copernicus, like the Ptolemaic universe, was limited by a material sphere of fixed stars, which, however, had to be assumed to be much larger than previously thought in order to explain the lack of an observable fixed star parallax . Nikolaus von Kues (1401–1464) had already anticipated the important idea of an unlimited universe without a specific center as a place for the earth. Thomas Digges (1576, A Perfit Description of the Caelestiall Orbes) represented a modified Copernican view of the world without material fixed stars with infinite Euclidean space. By Giordano Bruno (1548-1600) an infinite universe was postulated an infinite number of suns and planets, where the observed fixed stars are distant suns. Because of this and other statements that contradicted the Catholic principles of faith, Bruno was convicted of a heretic and executed at the stake.
Other important reasons for turning away from the Ptolemaic view of the world were Tycho Brahe's observations that the supernova of 1572 and the comet of 1577 had to be outside the lunar orbit , thus refuting the unchangeability of the sky, as taught by Aristotle. Tycho Brahe increased the precision of planetary observation considerably. On the basis of his more precise observation data, his assistant Johannes Kepler developed a worldview in which every planet, including the earth, moves at variable speed on an ellipse around the resting sun , instead of, as Copernicus assumed, on a combination of several uniformly traversed circles around a point near the sun. Kepler formulated the laws for planetary motion, which are now known as Kepler's laws , and viewed the sun as a source of magnetic force that moves the planets on their orbits, giving them their variable speed. He thus turned to a mechanistic picture of planetary motion in which the planets were no longer animated as in Ptolemy's . Kepler's heliocentric system made calculations of the planetary positions approx. 10 times more accurate than before with Copernicus and Ptolemy. However, Kepler again assumed a finite universe and proved this with arguments that later became known as the Olbersian paradox . The Copernican system was further supported by Galileo Galilei , who with his new type of telescope discovered the moons of Jupiter and mountains and their shadows on the lunar surface, although the fixed stars continued to appear point-like.
By Isaac Newton ( Philosophiae Naturalis Principia Mathematica cosmology was first with an elaborate, 1687) mechanics linked. With his concepts of force and inertia as well as the postulate of a general gravity , Newton brought a physics into cosmology in which the same laws applied to heavenly (planetary motion) and earthly areas (gravity). An important step in this development was the preceding development of mechanics, especially the preparation of the concept of inertia ( Galileo , Descartes ). The Newtonian celestial mechanics allowed the consideration of the mutual orbital disturbances of the planets due to their mutual gravitation and led in the 18th century with increasing mathematical knowledge to a further increase of the accuracy by about 50 times. According to this, the planetary movement is no longer given solely by the action of the sun, rather all bodies, including the sun, move under the influence of mutual forces around the common center of gravity of the solar system ( barycenter ), which is at most a few solar radii away from the sun's center.
In the 18th century, Thomas Wright did not regard the sun as the center of the universe, but as one of the fixed stars. He rejected the assumption of a homogeneous star distribution and identified the Milky Way as a disk consisting of single stars in whose plane the sun is located. He viewed the "nebulae" observed by astronomers as other galaxies. In 1755, Immanuel Kant developed in the General Natural History and Theory of Heaven not only a cosmology similar to that of Thomas Wright, but a cosmogony in which an initially chaotically distributed matter agglomerates under the action of gravity to form the celestial bodies observed. A similar development scheme was developed by Laplace . The astronomer Wilhelm Herschel tried to derive a chronological development scheme by classifying the stars and galaxies.
From the world of gods and myth to natural science
The transitions from philosophy to natural science took place depending on the different attitudes towards metaphysics . However, the cosmos has been used in philosophy as an archetype and model to represent the appropriate behavior for a person. The harmonious order of the universe was exemplary for Greek philosophy and served both as an ideal of a contemplative life and, in particular, the ideal of the scientific attitude, the bios theoreticos . It was possible to adapt to this cosmic order through mimesis , as in the case of Artes liberales, which originated in ancient Greece . The forces that were understood in the Greek cosmological tradition as gods and superhuman powers, philosophy later regarded as forces of the soul. Hence the connection and, in some cases, equating astronomy and astrology until the late Middle Ages and beyond until the 18th century. Hannah Arendt sees a similar view of the world in the parable of the clock , which tends to split between subject and object due to an imperfect knowledge of nature . Corresponding mystifications explain the attempt to overcome this split and give rise to the “objectivist appearance” criticized by Jürgen Habermas . Cosmos as a Greek word, which means something like jewelry, ornament, order, division, furnishing, design, world order and universe, has on the one hand aesthetic and on the other hand technical- practical aspects. This term conveys the framework ideal for Greek values of preoccupation with the beautiful, which was the basis of the old ontology and included a kind of sacred attitude. The following Kantian "decision" may tie in with this:
"Two things fill the mind with ever new and increasing admiration and awe, the more often and more persistently the reflection occupies itself with it: The starry sky above me and the moral law in me."
Thoughts on the distant future
The current cosmology is essentially dependent on information obtained from the universe itself (existence of other galaxies, redshift, background radiation, element abundances, etc.). Such sources of information will be lost over time as the universe expands. In the distant future (> 100 billion years) scientists will deduce from their observations a picture of the universe that resembles ours from the beginning of the last century: A static universe consisting of a galaxy without a big bang . The reason given is:
- The event horizon continues to expand, but is becoming smaller and smaller compared to the expansion of the universe. Objects located beyond, e.g. B. other galaxies, are then withdrawn from observation.
- The cosmic background radiation is getting longer and longer waves. At a wavelength of 300 kilometers, it is no longer able to penetrate the Milky Way, it is reflected by its dust.
- Due to the ongoing nucleosynthesis in the stars, the traces of the primordial nucleosynthesis are becoming more and more blurred. The proportion of helium in the universe will increase from 24% (primordial) over 28% (today) to 60% (in a trillion years).
- The Milky Way, Andromeda Nebula, and a few smaller, nearby galaxies will unite into a single giant galaxy. On longer timescales this applies to all galaxies in a super galaxy cluster (in our case: the Laniakea supercluster).
All of this means that 100 billion years from now it will appear to an observer in this supercluster as if he is depicting the entire universe. No more conclusions can be drawn about the Big Bang. Astronomers who might be alive would get a completely different picture of the structure and development of the universe than those who are currently living. This has led to the question of the extent to which such a loss of information may have already occurred, and thus to the question of the reliability of today's cosmological theories. After all, with the inflationary phase , these already contain such a loss of information as, shortly after the Big Bang, large areas of the universe were shifted beyond the observable.
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