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Big Bang Theory

Big Bang theory

Big bang theory describes the origin of the knowable universe and the development of the laws of physics and chemistry some 15 billion years ago.

During the 1940s Russian-born American cosmologist and nuclear physicist George Gamow (19041968) developed the modern version of the big bang model based upon earlier concepts advanced by Russian physicist Alexander (Aleksandr Aleksandrovich) Friedmann (also spelled as Fridman, 18881925) and Belgian astrophysicist and cosmologist Abbé Georges Lemaître (18941966). Big bang based models replaced static models of the universe that described a homogeneous universe that was the same in all directions (when averaged over a large span of space ) and at all times. Big bang and static cosmological models competed with each other for scientific and philosophical favor. Although many astrophysicists rejected the steady state model because it would violate the law of mass-energy conservation, the model had many eloquent and capable defenders. Moreover, the steady state model was interpreted by many to be more compatible with many philosophical, social, and religious concepts centered on the concept of an unchanging universe. The discovery of quasars and a permeating cosmic background radiation eventually tilted the cosmological argument in favor of big bang theory models.

Before the twentieth century, astronomers could only assume that the universe had existed forever without change, or that it was created in its present condition by divine action at some arbitrary time. Evidence that the universe was evolving did not begin to accumulate until the 1920s. The theory that all matter in the universe was created from a gigantic explosion called the "big bang" is widely accepted by students of cosmology .

It was German-American physicist Albert Einstein's (18791955) theory of relativity, published in 1915, that set the stage for the conceptual development of an expanding universe. Einstein had designed his theory to fit a static universe of constant dimensions. In 1919, a Dutch astronomer, Willem de Sitter, showed Einstein's theory could also describe an expanding universe. Mathematically, de Sitter's solution for Einstein's equation was sound, but observational evidence of expansion was lacking, and Einstein was skeptical.

In 1929, American astronomer Edwin Powell Hubble made what has been called the most significant astronomical discovery of the century. He observed large red shifts in the spectra of the galaxies he was studying; these red-shifts indicated that the galaxies are continually moving apart at tremendous velocities. Vesto Melvin Slipher, who took photographs of the red-shift of many of the same galaxies, also drew similar conclusions.

Like de Sitter, Lemaître, who worked with Hubble in 1924, developed out a simple solution to Einstein's equations that described a universe in expansion. Hubble's stunning observation provided the evidence Lemaître was seeking for his theory. In 1933, Lemaître clearly described the expansion of the universe. Projecting back in time, he suggested that the universe had originated as a great "cosmic egg," expanding outward from a central point. He did not, however, consider whether an explosion actually took place to initiate this expansion. George Gamow further investigated the origin of the universe in 1948. Because the universe is expanding outward, he reasoned, it should be possible to calculate backward in time to its beginning. If all the mass of the universe was compressed into a small volume 1015 billion years ago, its density and temperature must have been phenomenal. A tremendous explosion would have caused the start of the expansion, left a "halo" of background radiation, and formed the atomic elements that are heavier than the abundant hydrogen and helium. Physicists Ralph A. Alpher and Robert C. Herman established a model to show how such heavier particles could form under these conditions.

Gamow's theory implied there was a specific beginning and end to the universe. However, a number of other scientists, including Fred Hoyle, Thomas Gold, and Hermann Bondi felt that the theory of expansion required no beginning or end. Their model, called the steady state theory, suggested that matter was being continuously created throughout the universe. As galaxies drifted apart, matter would "condense" to form new ones in the void left behind. For nearly two decades, supporters of the competing theories seemed to be on equal footing.

In 1965 Robert H. Dicke made calculations relative to the cooling-off period after the initial big bang explosion. His results indicated that Gamow's residual radiation should be detectable. During the intervening eons it would have cooled to about 5 K (five kelvins above absolute zero). Unknown to him, radio engineers Arno Penzias and Robert W. Wilson already detected such radiation at 3 K in 1964 while looking for sources of satellite communication interference. This was the most convincing evidence yet gathered in support of the big bang theory, and it sent the steady-state theory into decline.

No theory exists today that can account for the extreme conditions that existed at the moment of the big bang. The theory of relativity does not apply to objects as dense and small as the universe must have been prior to the big bang. Cosmologists can project only as far back as 0.01 seconds after the explosion, when the cosmos was a seething mass of protons and neutrons. (It is possible there were many exotic particles that later became important as dark matter.) Based on their theories, cosmologists suggest that during this time neutrinos were produced.

It is argued that the laws of physics and chemistrymanifested in the properties of the fundamental forces of gravity , the strong force, electromagnetism, and the weak force (electromagnetism and the weak force are now known to be different manifestations of a more fundamental electroweak force)formed in the first few fractions of a second of the big bang. Protons and neutrons began to form atomic nuclei about three minutes and 46 seconds after the explosion, when the temperature was a mere 900,000,000 K. After 700,000 years hydrogen and helium formed. About one billion years after the big bang, stars and galaxies began to appear from the expanding mass. Countless stars would condense from swirling nebulae, evolve and die, before our Sun and its planets could form in the Milky Way galaxy.

Although the big bang theory accounts for most of the important characteristics of the universe, it still has weaknesses. One of the biggest of these involves the "homogeneity" of the universe. Until 1992, measurements of the background radiation produced by the big bang have shown that matter in the early universe was very evenly distributed. This seems to indicate that the universe evolved at a constant rate following the big bang. But if this is the case, the clumps of matter that we see (such as stars, galaxies, and clusters of galaxies) should not exist.

To remedy this inconsistency, Alan Guth proposed the inflationary theory, which suggests that the expansion of the universe initially occurred much faster. This concept of accelerated expansion allows for the formation of the structures we see in the universe today.

In April 1992, NASA made an electrifying announcement: its Cosmic Background Explorer (COBE), looking 15 billion light-years into space (hence, 15 billion years into the past), detected minute temperature fluctuations in the cosmic background radiation. It is believed these ripples are evidence of gravitational disturbances in the early universe that could have resulted in matter to clumping together to form larger entities. This finding lends support to Guth's theory of inflation.

See also Astronomy; Atom; Atomic theory; Catastrophism; Cosmic microwave background radiation; Cosmology; Earth (planet)

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Big Bang Theory

Big bang theory

The big bang is the foremost model that scientists use to describe the creation of the universe. This theory proposes that the universe was created in a violent event approximately 12 to 15 billion years ago. In that event, the lightest elements were formed, which provided the building blocks for all of the matter that exists in the universe today. A consequence of the big bang is that we live in an expanding universe, the ultimate fate of which cannot be predicted from the information we have at this time.

The evolution of the universe

Cosmologists (scientists who study the origin of the universe) believe the universe began as an infinitely dense, hot fireball. They call this single point that contained all the matter in the universe a singularity. Time began at the moment this fireball exploded, stretching space as it expanded rapidly. (Space into which the fireball exploded did not exist separately, but was a part of the fireball at the beginning.) The universe, at first no bigger than the size of a proton, expanded within a microsecond to the size of a basketball. Gravity came into being, and subatomic particles flooded the universe, slamming into one another, forming protons and neutrons (elementary particles that form atoms).

Three minutes after the big bang, the temperature of the universe had cooled to 500,000,000°F (277,777,760°C). Protons and neutrons began to combine to form the nuclei of the simple chemical elements hydrogen, helium, and lithium. Five hundred thousand years later, atoms formed. Some 300 million more years passed before the universe expanded

and cooled enough for stars and galaxies to form. Our solar system, formed from a cloud of dust and gas, came into being a mere four-and-a-half billion years ago.

The search for the beginning

A key assumption on which the big bang theory rests is that the universe is expanding. Prior to the twentieth century, astronomers assumed that the universe had always existed as it was, without any changes. In the 1920s, however, American astronomer Edwin Hubble (18891953) discovered observable proof that other galaxies existed in the universe besides our Milky Way galaxy. In 1929, he made his most important discovery: all matter in the universe was moving away from all other matter. This proved the universe was expanding.

Hubble reached this conclusion by looking at the light coming toward Earth from distant galaxies. If these galaxies were indeed moving away from Earth and each other, the light they emitted would be stretched or would have a longer wavelength. Since light with a longer wavelength has a reddish tone, this stretching is called redshift. Hubble measured the redshift for numerous galaxies and found not only that galaxies were moving away from Earth in all directions, but that farther galaxies seemed to be moving away at a faster rate.

Inflationary theory and the cosmic microwave background

By the mid-1960s, the big bang theory had received wide acceptance from scientists. However, some problems with the theory still remained. When the big bang occurred, hot radiation (energy in the form of waves or particles) given off by the explosion expanded and cooled with the universe. This radiation, known as the cosmic microwave background, appears as a weak hiss of radio noise coming from all directions in space. It is, in a sense, the oldest light in the universe. When astronomers measured this cosmic microwave background, they found its temperature to be just under 450°F (270°C). This was the correct temperature if the universe had expanded and cooled since the big bang.

But the radiation seemed smooth, with no temperature fluctuations. If the radiation had cooled at a steady rate, then the universe would have had to expand and cool at a steady rate. If this were true, planets and galaxies would not have been able to form because gravity, which would help them clump together, would have caused fluctuations in the temperature readings.

In 1980, American astronomer Alan Guth proposed a supplemental idea to the big bang theory. Called the inflationary theory, it suggests that at first the universe expanded at a much faster rate than it does now. This concept of accelerated expansion allows for the formation of the stars and planets we see in the universe today.

COBE and MAP

Guth's inflationary theory was supported in April 1992, when NASA (National Aeronautics and Space Administration) announced that its Cosmic Background Explorer (COBE) satellite had discovered those fluctuations. COBE looked about 13 billion light-years into space (hence, 13 billion years into the past) and detected tiny temperature fluctuations in the cosmic microwave background. Scientists regard these fluctuations as proof that gravitational disturbances existed in the early universe, which allowed matter to clump together to form large stellar bodies such as galaxies and planets.

In late 2000, scientists added further supporting evidence to the validity of the big bang theory when they announced that they had analyzed light from a quasar that was absorbed by a distant cloud of gas dust billions of years ago. At that time, the universe would have been about one-sixth of its present age. Based on their findings, the scientists estimated that the background temperature at that point was about 443°F (264°C), a temperature mark that agrees with the prediction of the big bang theory.

Present-day astronomers liken the study of the cosmic microwave background in cosmology to that of DNA (deoxyribonucleic acid; the complex molecule that stores and transmits genetic information) in biology. They consider it the seed from which stars and galaxies grew. To widen the scope and precision of that study, NASA launched a satellite called the Microwave Anisotropy Probe (MAP) in 2001. Orbiting farther away from Earth than COBE, the goal of MAP is to measure temperature differences in the cosmic microwave background on a much finer scale. Astronomers hope the information gather by MAP will reveal a great deal about the universe, including its large-scale geometry.

[See also Cosmology; Redshift ]

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Big Bang

Big Bang In cosmology, theory advanced to explain the origin of the Universe, developed from the ideas of Georges Lemaître and advanced in the 1940s by George Gamow. According to ‘Big Bang’ theory, a giant explosion 10 to 20 thousand million years ago began the expansion of the Universe, which still continues. Everything in the Universe once constituted an exceedingly hot and compressed gas with a temperature exceeding 10,000 million degrees. When the Universe was only a few minutes old, its temperature would have been 1000 million degrees. As it cooled, nuclear reactions took place that led to material emerging from the fireball consisting of about 75% hydrogen and 25% helium by mass, the composition of the Universe as we observe it today. There were local fluctuations in the density or expansion rate. Slightly denser regions of gas, the expansion rates of which lagged behind the mean value, collapsed to form galaxies when the Universe was perhaps a tenth of its present age. The cosmic microwave background radiation detected in 1965 is considered to be the residual radiation of the ‘Big Bang’ explosion. See also oscillating Universe theory; steady-state theory

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Big Bang Theory

Big Bang Theory


The Big Bang Theory is based on the observation that all the stars and galaxies of the universe are in motion and not stationary. The American astronomer Edwin Hubble (18891953) discovered in 1929 that the light of all visible stars was redshifted. Hence the movement of the myriad of galaxies is not random but everything is moving further away. If all galaxies are now racing away from one another then at one point all matter must have been clustered together in an infinitely dense space and its present motion might best be explained by an original explosion of matter. Hence the term Big Bang. The 1965 discovery by Arno Penzias (b. 1933) and Robert Wilson (b. 1936) of the background radiation produced by the intense heat of this "explosion" served to further confirm the theory. The Big Bang Theory brought to an end the idea of a static universe and made respectable again discussions of the beginning and possible creation of the universe.

See also Big Crunch Theory; Cosmology, Physical Aspects; Creation; Inflationary Universe Theory

mark worthing

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big bang theory

big bang theory The current explanation for the origin of the universe, in which it expands and evolves from an initial very high-temperature condition about 15–20 billion years ago. The expansion time is given from the reciprocal of the Hubble constant (the rate at which galaxies are receding). All all-pervasive background radiation of 3K is considered to be residual from the big bang and is the strongest supporting evidence for the theory.

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big bang

big bang (also Big Bang) • n. Astron. the explosion of dense matter that, according to current cosmological theories, marked the origin of the universe.

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