cosmology, area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe 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.
..... Click the link for more information. .

Modern Cosmological Theories

Present models of the universe hold two fundamental premises: the cosmological principle and the dominant role of gravitation gravitation, the attractive force existing between any two particles of matter .

The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it
..... Click the link for more information. . Derived by Hubble, the cosmological principle holds that if a large enough sample of galaxies is considered, the universe looks the same from all positions and in all directions in space. The second point of agreement is that gravitation (or an antigravitation force, called dark energy dark energy, repulsive force that opposes the self-attraction of matter (see gravitation ) and causes the expansion of the universe to accelerate. The search for dark energy was triggered by the discovery (1998) in images from the Hubble Space Telescope of a distant
..... Click the link for more information. ) is the most important force in shaping the universe. According to Einstein's general theory of relativity relativity, physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.
..... Click the link for more information. , which is a geometric interpretation of gravitation, matter produces gravitational effects by actually distorting the space about it; the curvature of space is described by a form of non-Euclidean geometry non-Euclidean geometry, branch of geometry in which the fifth postulate of Euclidean geometry, which allows one and only one line parallel to a given line through a given external point, is replaced by one of two alternative postulates.
..... Click the link for more information. . A number of cosmological theories satisfy both the cosmological principle and general relativity. The two main theories are the big-bang hypothesis and the steady-state hypothesis, with many variations on each basic approach.

The Steady-State Theory

According to the steady-state theory, now of historical interest only, the universe expands, but new matter is continuously created at all points in space left by the receding galaxies. The theory implies that the universe has always expanded, with no beginning or end, at a uniform rate and that it always will expand and maintain a constant density.

The Big-Bang Theory

According to big-bang theories, at the beginning of time, all of the matter and energy in the universe was concentrated in a very dense state, from which it "exploded," with the resulting expansion continuing until the present. This "big bang" is dated between 10 and 20 billion years ago. In this initial state, the universe was very hot and contained a thermal soup of quarks, electrons, photons, and other elementary particles elementary particles, the most basic physical constituents of the universe.

Basic Constituents of Matter

Molecules are built up from the atom , which is the basic unit of any chemical element .
..... Click the link for more information. . The temperature rapidly decreased, falling from 10¹³ degrees Kelvin after the first microsecond to about one billion degrees after three minutes. As the universe cooled, the quarks condensed into protons and neutrons, the building blocks of atomic nuclei. Some of these were converted into helium nuclei by fusion; the relative abundance of hydrogen and helium is used as a test of the theory. After many millions of years the expanding universe, at first a very hot gas, thinned and cooled enough to condense into individual galaxies and then stars.

Several spectacular discoveries since 1950 have shed new light on the problem. Optical and radio astronomy complemented each other in the discovery of the quasars quasar (kwā`sär), one of a class of blue celestial objects having the appearance of stars when viewed through a telescope and
..... Click the link for more information. and the radio galaxies. It is believed that the energy reaching us now from some of these objects was emitted not long after the creation of the universe. Further evidence for the big-bang theory was the discovery in 1965 that a cosmic background noise is received from every part of the sky. This background radiation has the same intensity and distribution of frequencies in all directions and is not associated with any individual celestial object. It has a black body black body, in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, approximates an ideal black body.
..... Click the link for more information. temperature of 2.7°K; (−270°C;) and is interpreted as the electromagnetic remnant of the primordial fireball, stretched to long wavelengths by the expansion of the universe. More recently, the analysis of radiation from distant celestial objects detected by artificial satellites satellite, artificial, object constructed by humans and placed in orbit around the earth or other celestial body (see also space probe ). The satellite is lifted from the earth's surface by a rocket and, once placed in orbit, maintains its motion without further
..... Click the link for more information. has given additional evidence for the big-bang theory.

Development of Modern Cosmology

The earliest pre-Ptolemaic theories assumed that the earth was the center of the universe (see Ptolemaic system Ptolemaic system (tŏl'əmā`ĭk), historically the most influential of the geocentric cosmological theories, i.e.
..... Click the link for more information. ). With the acceptance of the heliocentric, or sun-centered, theory (see Copernican system Copernican system, first modern European theory of planetary motion that was heliocentric, i.e., that placed the sun motionless at the center of the solar system with all the planets, including the earth, revolving around it.
..... Click the link for more information. ), the nature and extent of the solar system began to be realized. The Milky Way Milky Way, the galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon; if not blocked by the horizon, it would be seen as a circle around the entire sky.
..... Click the link for more information. , a vast collection of stars star, hot incandescent sphere of gas, held together by its own gravitation , and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy .
..... Click the link for more information. separated by enormous distances, came to be called a galaxy and was thought to constitute the entire universe with the sun at or near its center. By studying the distribution of globular star clusters star cluster, a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction.
..... Click the link for more information. the American astronomer Harlow Shapley was able to give the first reliable indication of the size of the galaxy and the position of the sun within it. Modern estimates show it to have a diameter of about 100,000 light-years light-year, in astronomy, unit of length equal to the distance light travels in one sidereal year . It is 9.461 × 10¹² km (about 6 million million mi). Alpha Centauri and Proxima Centauri, the stars nearest our solar system, are about 4.
..... Click the link for more information. with the sun toward the edge of the disk, about 28,000 light-years from the center.

During the first two decades of the 20th cent. astronomers came to realize that some of the faint hazy patches in the sky, called nebulae, are not within our own galaxy, but are separate galaxies at great distances from the Milky Way. Willem de Sitter of Leyden suggested that the universe began as a single point and expands without end. After studying the red shift red shift or redshift, in astronomy, the systematic displacement of individual lines in the spectrum of a celestial object toward the red, or longer wavelength, end of the visible spectrum. The effect was discovered by V. M.
..... Click the link for more information. (see Doppler effect Doppler effect, change in the wavelength (or frequency) of energy in the form of waves, e.g., sound or light, as a result of motion of either the source or the receiver of the waves; the effect is named for the Austrian scientist Christian Doppler, who demonstrated
..... Click the link for more information. ) in the spectral lines of the distant galaxies, the American astronomers Edwin Hubble and M. L. Humason concluded that the universe is expanding, with the galaxies appearing to fly away from each other at great speeds. According to Hubble's law Hubble's law, in astronomy, statement that the distances between galaxies (see galaxy ) or clusters of galaxies are continuously increasing and that therefore the universe is expanding.
..... Click the link for more information. , the expansion of the universe is approximately uniform. The greater the distance between any two galaxies, the greater their relative speed of separation.

At the end of the 20th cent. the study of very distant supernovas led to the belief that the cosmic expansion was accelerating. To explain this cosmologists postulated a repulsive force, dark energy, that counteracts gravity and pushes galaxies apart. It also appears that the universe has been expanding at different rates over its cosmic history. This led to a variation of the big-bang theory in which, under the influence of gravity, the expansion slowed initially and then, under the influence of dark energy, suddenly accelerated. It is estimated that this "cosmic jerk" occurred five billion years ago, about the time the solar system was formed. This theory postulates a flat, expanding universe with a composition of c.70% dark energy, c.30% dark matter, and c.0.5% bright stars.

A number of questions must be answered, however, before cosmologists can establish a single, comprehensive theory. The expansion rate and age of the universe must be established. The nature and density of the missing mass, the dark matter dark matter, material that is believed to make up (along with dark energy ) more than 90% of the mass of the universe but is not readily visible because it neither emits nor reflects electromagnetic radiation , such as light or radio signals.
..... Click the link for more information. and dark energy that is far more abundant than ordinary, visible matter, must be identified. The total mass of the universe must be determined to establish whether it is sufficient to support an equilibrium condition—a state in which the universe will neither collapse of its own weight nor expand into diminishing infinity. Such an equilibrium is called "omega equals one," where omega is the ratio between the actual density of the universe and the critical density required to support equilibrium. If omega is greater than one, the universe would have too much mass and its gravity would cause a cosmic collapse. If omega is less than one, the low-density universe would expand forever. Today the most widely accepted picture of the universe is an omega-equals-one system of hundreds of billions of galaxies, many of them clustered in groups of hundreds or thousands, spread over a volume with a diameter of at least 10 billion light-years and all receding from each other, with the speeds of the most widely separated galaxies approaching the speed of light. On a more detailed level there is great diversity of opinion, and cosmology remains a highly speculative and controversial science.

Bibliography

See D. W. Sciama, Modern Cosmology and the Dark Matter Problem (1993); J. D. Barrow, The Origin of the Universe (1994); P. Coles and F. Lucchin, Cosmology: The Origin and Evolution of Cosmic Structure (1995); M. S. Longair, Our Evolving Universe (1996); B. Green, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (2000); S. Hawking, The Universe in a Nutshell (2001); R. P. Kirshner, The Extravagant Universe: Exploding Stars, Dark Energy, and Accelerating Cosmos (2002); S. Singh, Big Bang: The Origin of the Universe (2005).

cosmology

Field of study that brings together the natural sciences, especially astronomy and physics, in an effort to understand the physical universe as a unified whole. The first great age of scientific cosmology began in Greece in the 6th century BC, when the Pythagoreans introduced the concept of a spherical Earth and, unlike the Babylonians and Egyptians, hypothesized that the heavenly bodies moved according to the harmonious relations of natural laws. Their thought culminated in the Ptolemaic model (see Ptolemy) of the universe (2nd century AD). The Copernican revolution (see Copernican system) of the 16th century ushered in the second great age. The third began in the early 20th century, with the formulation of special relativity and its development into general relativity by Albert Einstein. The basic assumptions of modern cosmology are that the universe is homogeneous in space (on the average, all places are alike at any time) and that the laws of physics are the same everywhere.

cosmology

1. the philosophical study of the origin and nature of the universe

2. the branch of astronomy concerned with the evolution and structure of the universe
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Modern Cosmological Theories

The Law of Universal Gravitation

The Steady-State Theory

The Big-Bang Theory

Basic Constituents of Matter

Development of Modern Cosmology

Bibliography

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