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universe, totality of matter and energy in existence. The study of the origin of the universe, or cosmos, is known as cosmogony, and that of its structure and evolution, cosmology. The age of the universe depends on which theory of cosmology one accepts. According to the big bang theory, favored by most scientists, the universe is between 10 and 20 billion years old; most recent calculations place its age at c.13.799 billion years. The steady-state theory holds that the universe has been in existence for all time.

Matter and Energy in the Universe

The matter in the universe is subject to various forces, but the greatest force on the cosmological scale is gravitation. This force pulls matter together to form stars, which either exist alone or are part of binary star or multiple star systems, or brown dwarfs, which are also known as “failed stars.” Gravitation also acts to group billions of stars into galaxies and to group galaxies into clusters and superclusters, and gravitation also causes most galaxies to cluster along dense strandlike structures formed by dark matter, with enormous voids among the strands. The main source of energy in the universe is the conversion of the matter of the stars into energy through thermonuclear reactions (see nuclear energy). These reactions continue throughout the different stages of stellar evolution (see also stellar populations) until the star has consumed all its available nuclear fuel.

The Size of the Universe

The first systematic theory of the size and shape of the universe that attempted to explain observed data was constructed by Ptolemy in the 2d cent. In this theory the solar system was thought to be the entire universe, with the earth at its center and the distant stars located just beyond the farthest planet. This belief was held until the 16th cent., when Copernicus advanced the idea that the sun, rather than the earth, is at the center of the system and that the stars are at very great distances compared to the planets. During the first part of the 20th cent., astronomers discovered that the sun is only one of billions of stars in the Milky Way galaxy and is located far from the galactic center.

Estimates of the size of the universe have been refined as methods of measuring galactic and extragalactic distances have improved. Close stellar distances were at first found by measuring a star's trigonometric parallax. A more powerful contemporary method is to analyze the light reaching the earth from an object by means of a spectroscope; the distance of a very faint object can be estimated by comparing its apparent brightness to those of similar objects at known distances. Another method depends on the fact that the universe as a whole appears to be expanding, as indicated by red shifts (see Doppler effect) in the spectral lines of distant galaxies. Hubble's law makes it possible to estimate their distances from the speed with which they are rushing away from the earth. At present the observable universe is believed to be at least 90 billion light-years in diameter; the entire universe may be 7 trillion light-years in diameter or more. One problem with estimating the size of the universe is that space itself (or more properly, space-time) may be curved, as held by the general theory of relativity. This curvature would affect measurements of distance based on the passage of light through space from objects as far away as 5 billion light-years or more.


See A. S. Eddington, The Expanding Universe (1933); J. H. Jeans, The Universe Around Us (4th ed. 1953); G. Gamow, The Creation of the Universe (1961); F. Hoyle, Man in the Universe (1966); L. B. Young, ed., Exploring the Universe (2d ed. 1971).

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(yoo -nă-verss) The sum total of potentially knowable objects. The study of the Universe on a grand scale is called cosmology. Barriers between potentially knowable and unknowable objects exist in even a simple expanding Euclidian Universe with flat space (i.e. in which the surface area of a sphere is 4πr 2). More and more distant objects are seen to recede at faster and faster velocities. At a certain distance objects are receding at the speed of light and suffer an infinite redshift; they are then potentially unobservable.

Within the framework of general relativity the space of the Universe can be curved, and depending on the nature of the curvature the Universe may be closed or open. In a closed Universe gravitational distortion of light would cause space to be spherically curved: space would curve back on itself to form a finite volume with no boundary. The Universe would then be finite and the gravitational attraction among galaxies would be sufficient eventually to halt and reverse the expansion of the Universe so that it would begin to contract. In an open Universe space would still be curved but would not turn back on itself: the curvature would be hyperbolic rather than spherical. The Universe would then be infinite, with gravity too weak to halt the Universe's expansion. In a flat Universe space is not curved: it can be described by Euclidean geometry. The Universe would then be infinite and would expand forever, but less rapidly than an open Universe.

The dynamic behavior of the Universe, i.e. whether it is closed, open, or flat, depends on the value of the mean density of matter, and hence on the deceleration parameter q 0 :

if q 0 > ½, Universe is closed
if q 0 < ½, Universe is open
if q 0 = ½, Universe is flat

The Universe exists close to the dividing line between the ever-expanding model and the eventually contracting model, but it is not yet certain which of these radically different prospects will occur. If the Universe is closed it will collapse in upon itself to form a final singularity known as the big crunch. Some theories speculate that there is an infinite cycle of oscillating Universes where the end result of the collapse could be regarded instead as a big bounce. See also age of the Universe; Big Bang theory; cosmological models.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the whole world, unbounded in time and space and infinitely diverse with regard to the forms assumed by matter during its evolution. The universe exists objectively, independent of the human consciousness perceiving it. It contains a vast multitude of celestial objects, many of which are larger than the earth and some even millions of times larger. Any authentic scientific investigation recognizes the objective existence and materiality of the universe.

Materialism regards the various phenomena occurring in the world as correlated and preconditioned. They evolve in space and time. The major task of natural science is to study the laws governing these causations. Unlike philosophic idealism, which maintains that space and time are not objective reality but are forms of human contemplation, materialism acknowledges the objective reality of space and time. Consequently, space and time are also studied from the point of view of natural science.

The distribution of matter in the universe in space and time and the various celestial bodies and systems of bodies that make up the universe are the objects of study of the various branches of astronomy. Astronomy elucidates the structures of all the parts of the universe that are accessible to investigation at a given time. It is the task of cosmology to reach conclusions, based on all scientific knowledge, on the uni-verse as a whole.

Development of views on the structure of the universe. At each stage of society’s development, mankind knew the structure of only some limited part of the universe. The volume of the universe accessible to investigation is continuously extending as methods of scientific research and astronomical instruments become perfected. The actual study becomes more profound, and our knowledge reflects with increasing accuracy the structure and development of the region studied. The history of the knowledge of the universe is one of the most striking examples of Lenin’s theory of knowledge, according to which “Human thought by its nature is capable of giving, and does give, absolute truth, which is compounded of a sum total of truths” (V. I. Lenin, Poln. sobr. soch., 5th ed., vol. 18, p. 137). In the first stages of cultural development man’s conception of the universe was limited to knowl-edge of the rivers, mountains, and forests nearest to his dwelling and to the most prominent celestial bodies. Later this knowledge began to cover considerable regions of the earth’s surface; the next stage was establishing the earth’s roundness and the relative remoteness of the heavenly bodies.

Already in the 16th century the revolution that N. Copernicus effected meant that in the region of the universe whose structure was basically correctly understood, and which was being further studied, the dimensions of the entire solar system were realized. It also became clear that the stars are many times further away from us than the planets. But at that period astronomers did not succeed in measuring the distances even to the nearest stars. Basically, the astronomy of the 17th and 18th centuries was an astronomy of the planetary system, that is, it was limited to the surroundings of one particular star, the sun. The diameter of this system, equal to about 10 billion km, is traversed by light in 10 hours. Accurate determinations of the distances to the nearest stars were first made at the end of the 1830’s by V. la. Struve in Russia, F. Bessel in Germany, and T. Henderson in South Africa. Struve’s statistical investigations, based on star counts, formed a new chapter in the study of the universe. The boundaries of the part of the uni-verse that began to be studied in detail expanded to distances traversed by light in hundreds and thousands of years. The huge task of studying our galaxy was begun, that is, the study of a star system, one member of which was our sun. It was only in the 1930’s that it became possible to determine with certainty the dimensions and main features of the structure of our galaxy, whose diameter was shown to be about 30,000 parsecs (about 100,000 light-years). However, many important features and details of our galaxy’s structure remain un-studied, and intensive investigation of them is proceeding.

The 1920’s saw the elucidation of the extragalactic nature of spiral and elliptic nebulas, which proved to be independent galaxies, that is, systems of the same order as our galaxy. This made it possible to raise the question of the structure of the metagalaxy as a cosmic system of a higher order, a system that includes our galaxy and its neighbors as. separate components. It is not possible to reach the limits of the metagalaxy with modern astronomical instruments; it is not known with certainty whether this system has boundaries. However, with instruments it is possible to observe remote members of the metagalaxy at distances on the order of several billion parsecs. At even greater distances, quasars can be observed; these were discovered in 1963 and are a new kind of cosmic object. The exceptionally high luminosity of many quasars, which, unlike galaxies, are compact bodies, makes it possible to detect them at much greater distances than the largest galaxies.

The boundedness of the part of the universe that has been studied certainly does not contradict the idea of the spatial infinity of the universe. However, the very formulation of the question of the spatial finiteness or infinity of the universe was connected with the classical conceptions of absolute space and absolute time. In accordance with the views of modern physics, the spatial volume occupied by any real or imaginary system is different for observers moving differently relative to the system. Along with the extension of the boundaries of that part of the universe accessible to our investigations, a more detailed and profound study is being made of regions relatively close to us; already many details are known of the structure and properties of the nearest galaxies and galactic clusters. Knowledge is developing both in breadth and depth. The history of science shows that neither the distances in space of various parts of the universe nor the complex nature of the causes of various phenomena excludes the possibility of learning about them. Materialist science, particularly astronomy, convincingly refutes the conclusions of agnosticism, widespread in bourgeois philosophy.

A number of different theories of the structure of the uni-verse as a whole are widespread; these theories are based on the assumption that the observed properties of that part of the metagalaxy containing our galaxy prevail everywhere in the universe and that the metagalaxy encompasses the uni-verse in its entirety. Such simplified schemes can be of value for much specific work aimed at investigating the properties of large volumes of the universe. But the conditional nature of the assumptions made must not be forgotten. Essentially, in solving many simple problems of stellar astronomy, it is also convenient to assume that our galaxy extends infinitely far. Such an assumption makes it possible, for example, to obtain the first theoretical conceptions of the distribution of stars according to apparent stellar magnitudes or to deter-mine the laws of the fluctuation of the brightness of the Milky Way. However, in assuming the infinite nature of our galaxy when solving a specific problem, the scientist understands the arbitrariness of such an assumption. In like manner it is impossible to consider the aforementioned theoretical schemes, which are based on simplified assumptions and serve for particular studies, as theories of the universe as a whole. Sometimes they are only useful working schemes. The extension to the entire universe of properties of that part that has been studied by us in varying degrees contradicts all accumulated data on the study of the universe. It is known that the appearance of new techniques in observation, making it possible to considerably expand the limits of the area of the universe accessible to observations, brings about the discovery of qualitatively new structural features. Thus, large differences between the structures of the solar system and the galaxy were found. The difference between these systems is not limited to different dimensions or the number of bodies they contain. More significant is the qualitative difference in the nature of the subordination and intersubordination of members within each system. While the sun contains almost all of the solar system’s mass so that its gravitational field basically determines the motion of the planets, the major part of our galaxy’s mass is distributed over tens of billions of stars, and the gravitational field is primarily determined by the action of this aggregate of stars. The same qualitative difference in structure is revealed in passing from our galaxy to the metagalaxy.

Because the metagalaxy occupies first place in the hierarchy of studied cosmic systems, we really mean the properties and phenomena of the metagalaxy when speaking about the most general or large-scale properties of the universe. By the 1970’s the joint efforts of astronomers of different countries have established the following important properties of the metagalaxy: (1) Its galaxies are not uniformly distributed; almost all of them are concentrated in clusters and groups of galaxies. Our galaxy is part of the Local group of galaxies, which is relatively small in number. (2) The law of the relative recession of galaxies with speeds approximately proportional to their mutual distances (Hubble’s law) holds. Thus, galaxies 10 million parsecs apart recede from one another at speeds of about 600 km/sec. This expansion, by the Doppler principle, produces the observed red shift of spectral lines in the galaxies’ spectra. This entire immense phenomenon is often called an expansion of the universe. (3) In the millimeter wavelength our part of the universe is uniformly filled with radio-frequency radiation whose density corresponds to radiation from a blackbody at 3° K. The radiation is called relict radiation, because it is assumed that it is a residue of radiation processes that occurred at a very remote era of the beginning of the metagalaxy’s existence.

These three facts form the basis of numerous modern schemes of cosmology. However, in the future cosmology will undoubtedly have to take into account not only these basic facts but also many other more detailed phenomena and circumstances.

Structural features of the universe. Up to the middle of the 20th century it was customary to assume that an overwhelmingly large part of the matter accessible to our observations of part of the universe was concentrated in the stars and that interstellar matter, planets, and comets formed only a small fraction of it. However, after the role of galactic nuclei as active centers of galaxies had been established and after the discovery of quasars it became clear that the universe contains bodies with masses exceeding the masses of stars by at least a million times. It is difficult to estimate the total mass per unit volume of these starlike bodies and to compare it with the mass of stars. Nonetheless there is no doubt that these masses play an important role in the process of the evolution of the universe. Quasars of the highest luminosity are known to be a hundred times more powerful generators of radiant energy than the aggregate of stars of the most massive individual galaxies. It is significant, however, that stars together with interstellar matter and various small bodies form star systems that we observe as galaxies. The assertion according to which the overwhelming part of the matter in the universe is concentrated in galaxies seems to be a rather precise description of the actual picture, particularly if one takes into account that quasars can be regarded as the limiting case of galaxies with very bright nuclei and relatively low star populations and that we already know many objects with properties showing them to be intermediate between quasars and galaxies of the classical type.

However, galaxies are far from being the largest structural units in the observed universe. They are concentrated in clusters and groups of galaxies; isolated galaxies are rarely encountered. The tendency of galaxies to crowd together is one of the most important structural properties of the uni-verse. A number of investigations makes it possible to assume that there are systems of higher order than clusters and groups of galaxies: clusters of clusters or superclusters of galaxies. The Local system of galaxies (including our galaxy) along with the abundant galactic cluster in the constellation Virgo and some nearer groups are part of one such supercluster. The study of superclusters of galaxies is greatly impeded by individual superclusters being superimposed on one another in the sky, and it is often impossible to separate them with sufficient clearness. It is all the more difficult to settle the question of the existence of a system of even higher order than the supercluster. There is no basis for asserting that superclusters are uniformly distributed in the universe, the more so as observational data have always shown the existence of inhomogeneities on ever increasing scales. Inhomogeneity and a tendency to crowd are characteristic traits of that part of the universe which is accessible to study.

Stars and interstellar matter consist of ionized gases. Hence it can be concluded that the basic physical form of matter in the universe is not the solid, liquid, or neutral gas but plasma consisting of ions and electrons. However, the discovery of pulsars afforded the first observational evidence in favor of superdense bodies consisting basically of degenerate baryon gas.

Red shift. Hubble’s law, which states that the red shift of spectral lines (and hence also the velocities of recession of extragalactic objects) is proportional to the distances to them, is valid only for velocities that are small compared with the velocity of light. Extragalactic objects over 2 billion par-sees away also show continually increasing rates of recession with increasing distance, but the simple proportionality law is no longer valid. The rates of recession along the line of sight for the most remote galaxies, whose radial velocities are found using the Doppler principle, are close to half the velocity of light. Because of their high luminosities, quasars can be comparatively easily observed at distances exceeding 2 bil-lion parsecs. Already there have been observed quasistellar objects whose red shifts are so large that their radiation wavelengths are three and even almost four times greater than the laboratory values. All attempts to explain the red shift in the galactic spectra in terms of non-Doppler effects have proved futile. At present (1970’s) similar attempts are being made regarding the red shift in the spectra of quasars. However, analysis of the results shows that these attempts are also futile. Furthermore, the fact that the red shift is equally observed in galaxies, quasars, and intermediate-type objects (for example, N-galaxies) confirms the fact that the red shift represents a manifestation of the large-scale geometric and kinematic properties of space-time, which depend little on the physical properties of the actual radiating objects and are, to a known degree, independent of the evolution of these objects.

Thus observations confirm the general basis of the interpretation of the red shift given by relativistic cosmology. However, the question of specific relativistic models of the metagalaxy is still the subject of discussion.

Age characteristics of the universe. The discovery of numerous evolutionary processes in various systems and bodies forming the universe has made it possible to study the regularities of cosmic evolution on the basis of observed data and theoretical calculations. The determination of age of cosmic objects and their systems is one of the most important problems. Since it is difficult in most cases to decide what is to be understood by the ’’moment of birth” of a body or system, one generally has in mind two, generally speaking, different quantitative assessments in establishing the age characteristics: (1) the time during which the system has al-ready been in the observed state (or in states close to that observed in the present era) and (2) the total life-span of the given system from the moment of its appearance to its destruction. Obviously this second characteristic, as a rule, can be obtained only on the basis of theoretical calculations. Usually, the first of the indicated quantities is called the age and the second, the life-span.

The fact of the mutual recession of the galaxies forming the metagalaxy indicates that in some previous time it was in a qualitatively different state and was denser. The most probable value of the Hubble constant (the proportionality coefficient in the relationship connecting the recession velocity of extragalactic objects and the distances to them) is 60 km/(sec • megaparsec), and this gives the time for the expansion of the metagalaxy to its present state of approximately 17 billion years. Such is the estimate of the age of the largest system. It seems very natural that the age of the galaxies and of the individual stars is less.

The presence of inhomogeneities in the composition of many galaxies indicates that despite a differential rotation, galaxies have not attained a complete mixing of the stars. This signifies that each galaxy has made not more than a few tens of rotations around its axis. The time for a single rotation of our galaxy around its axis is about 200 million years, and other galaxies rotate at about the same rate. Thus the mean age of the galaxies is estimated at 10 billion years. However, this does not mean that individual galaxies or even groups of galaxies cannot be much younger. But probably there are no galaxies with ages substantially exceeding 20 billion years.

Extragalactic astronomical data show that some galactic clusters and groups have such a large dispersion of velocities for their members, that the forces of mutual attraction between constituent galaxies cannot keep them in clusters, so that such clusters must disintegrate. In most cases the disintegration period is estimated at 1-2 billion years, although there are groups of galaxies disintegrating in shorter times, 200-500 million years. Since modern stellar dynamics rejects the possibility of clusters and groups forming from galaxies that were previously independent, it has to be admitted that in these cases these numbers also determine the age of the members of these groups. This means that galaxies some-times contain very young (compared with the mean age) objects, that is, the process of the creation of new galaxies is also going on at the present stage of development of the metagalaxy.

Existing methods of determining age characteristics of galaxies and galactic clusters enable us to evaluate the life-span. But because age is always less than the life-span, an upper age limit is thus also obtained. Since the upper limits always turn out to be less than the indicated age of the metagalaxy, it can be asserted that the age characteristics of the individual members of the metagalaxy do not contradict the existing estimate of its age.

The age of the metagalaxy is sometimes taken as the age of the universe; this is characteristic of advocates who identify the metagalaxy with the universe as a whole. Actually, the hypothesis that the universe contains numerous metagalaxies simply placed at various distances has not been confirmed. However, it is necessary to take into account the possibility of more complex relationships between the metagalaxy and the universe as a whole and even between individual metagalaxies; in such large volumes of space the principles of Euclidean geometry are already inapplicable. These relation-ships can also be complex topologically. It is also impossible to exclude the possibility that each charged elementary particle can be the equivalent of the entire system of galaxies, that is, consist of such a system. The possibilities of such, more complex, relationships must also be taken into account by cosmology. Hence, it is still premature to say that there are any data on the age of the universe as a whole.

After the discovery of quasars and other quasistellar sources, new possibilities emerged for studying the past of the universe. The most remote quasars are observed in the far regions of the metagalaxy, and their light reaches us after about 10 billion years. Thus by observing these cosmic objects, it is possible to judge the state of these regions of the metagalaxy in the remote past. Analysis of observation data shows that in the remote past the state of these metagalactic regions differed greatly from the state observed in the present era near our galaxy. It is true that the available statistical data are still insufficient to uniquely interpret the results of these observations. It can be considered that formerly quasars had a mean radioluminosity greater than that at present. It is not excluded that in the remote past the density of the spatial concentration of quasars was higher. However, it may be considered proven that the metagalaxy is actually evolving and that the so-called theory of the stationary universe al-ready has almost no supporters.

Life in the universe. Since each gigantic galaxy contains over 100 billion stars and since the number of such galaxies in the metagalaxy is not less than 100 million, then obviously the total number of stars in the universe exceeds 1019. Hence there naturally arises the question of the frequency of occurrence of organic life on planets whose existence around these stars is thought very likely.

It is obvious that the forms of life existing on the earth cannot exist under all the possible physical conditions on other planets. Such forms do not exist on the moon and prob-ably do not exist on Saturn or Uranus. Unfortunately, biology has not succeeded in determining the extreme values of the parameters of the physical conditions on planets that permit the existence of terrestrial forms of life. These extremes impose limits on the temperature, the value of the atmospheric density, the acceleration due to gravity, the length of the day, and atmospheric composition. The presence of liquid reservoirs must also play some part. Nonetheless it is hard to imagine that these extremes should prove so narrow that only an insignificant number of planets has satisfied them. Hence, it is highly probable that the metagalaxy contains billions of planets where conditions are more or less favorable to the origin of organic life. Naturally, the possibility of intelligent beings as the highest stage in the process of biological evolution is connected with the stricter restrictions that are imposed on the range, constancy, and length of time of the preservation of definite physical conditions. Hence it is quite likely that extraterrestrial civilizations would rarely be encountered. However, the only progress that it has been possible to make so far in this problem amounts to a few guesses.

Finally, a wider formulation of the question is possible. It is possible, since one is not restricted to terrestrial forms of life, to make a general study of the possibility of the existence of systems receiving, preserving, and processing information from the most elementary to the most complex. There is no doubt that a wider class of planets can prove favorable to such systems developing on them, if the forms of life known to us are not actually the only ones. However, only further progress in modern biology will make it possible to study this question intensively.

Man and the universe. The use of working tools has made it possible for man to become master of the earth. The appearance of the tools of intellectual labor and the development of technology made it generally possible for man to pass beyond the limits of the terrestrial globe and master near-cosmic space. In the future, man will visit all the planets and even penetrate beyond the solar system. Technology has made it possible for man to alter the character of the earth. Canals are built, new seas are made, large tunnels are driven in mountains, and gardens arise in deserts. Unfortunately, it is true that negative phenomena are seen: individual forms of animal life become extinct and the atmosphere becomes polluted. But on the whole, the transformation proceeds in accordance with the needs of human society. There is no doubt that in the course of time man will begin to re-model the universe. The creation of new celestial bodies—artificial satellites—is only the first step in that direction. The expansion of our civilization outside the earth’s limits was the most important development in the 1950’s and 1%0’s. There is no doubt that the continuation of human activity in this direction will greatly affect the future progress of human society.


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The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


The totality of astronomical things, events, relations, and energies capable of being described objectively.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


Astronomy the aggregate of all existing matter, energy, and space
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


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