Formulation Of The Big Bang Theory

Both Friedmann and Lemaitre had devised cosmological models on the basis of general relativity that hypothesized that the universe had expanded outward from some initial time when the scale factor R was zero. In such a universe the density of matter and energy will increase as one goes backward in time, reaching infinite values at time zero. The general relativistic models were geometric structures satisfying the gravitational field equations, and the analysis of the physical conditions that must hold in them was not a matter of close concern. Friedmann died in 1925, only dimly award of the new developments in nebular astronomy. Lemaitre, by contrast, was present in January 1925 at the meeting of the American Astronomical Society, where Hubble's demonstration of the extragalactic character of the spiral nebulae was announced. With the discovery of the red shift law four years later the relativistic models became something more than mathematical curiosities; the very real possibility existed that they described the physical universe in which one lived.

Lemaitre believed that if the expansion of the universe was extrapolated backward in time, one was led, other things being equal, to an initial moment of creation involving conditions of extremely high density. He rejected the relativistic model proposed by Eddington, in which the universe began in a static state and then, at some moment, began to expand outward. He hypothesized that the universe began in the radioactive disintegration of a "primeval atom," a fantastic explosion that propelled the subsequent expansion of the universe. Like many cosmological theorists in the 1930s, Lemaitre also believed that the expansion might be propelled by a kind of repulsive cosmic force corresponding to the cosmological constant in the field equations of general relativity. With the discovery of universal expansion Einstein himself rejected the cos-mological constant, stating that its earlier introduction by him—done to preserve a static cosmos—was a mistake.

Lemaitre is regarded as the father of modern physical cosmology. His idea that the universe began with an explosive event in conditions of high density attracted the attention of many theorists in the two decades following 1931. This idea formed the basis for what became known as the big bang theory of the universe. The name itself was coined by British scientist Fred Hoyle (1915-2001) in 1949 in a BBC radio lecture. Ironically, Hoyle was a proponent of an alternative cosmology (the steady state theory discussed below) and used the phrase big bang in a rather disparaging way to criticize his scientific opponents.

A key idea of the big bang theory is that the universe is evolutionary. It originated at a finite time in the past—believed by current estimates to be around 12 to 14 billion years ago—and has undergone a steady expansion and decrease in density since then. As one looks out in space, one looks back in time; it follows, according to the big bang theory, that the universe should look younger and therefore less evolved the farther one looks out. The theory is a historical one since its account of the large-scale structure of the universe is also an account of the temporal origins of the universe. In this respect, the big bang theory stands in striking contrast to both ancient Greek cosmology and to Copernican heliocentric cosmology, both of which involved no assumptions about the origins of the planetary system.

Lemaitre's notion of a disintegrating primeval atom was an interesting idea, but it proved difficult to develop into a consistent quantitative model describing conditions in the very early universe. The modern hot big bang theory had its origins in the writings, during the 1940s and early 1950s, of three American specialists in nuclear physics, George Gamow (1904-1968), Ralph Alpher (1921-), and Robert Herman (1914-1997). Of the three, Gamow was most vigorous in promoting big bang cosmology, which he did in research papers as well as in popular writings aimed at a broad scientific audience.

Gamow was initially concerned with the problem of stellar nucleosynthesis, that is, with how heavier elements are synthesized from lighter elements in the interiors of stars. This problem was closely connected to the question of how stars evolve. By the 1930s it was recognized that a star's source of energy involved thermonuclear fusion in its hot and dense core. A major breakthrough occurred in 1938, when Hans Bethe (1906-2005) in the United States explicitly identified the chain of reactions by which hydrogen is converted to helium, the so-called carbon-nitrogen cycle. Carl von Weizsäcker (1912-) in Germany obtained a similar result at roughly the same time. In the proposed sequence of nuclear reactions, hydrogen is converted to helium in the cores of stars, carbon playing the role of a catalyst in the reactions. The carbon-nitrogen cycle is the main source of stellar energy. Serious problems arose when physicists tried to derive corresponding reaction cycles for the heavier elements. Gamow and others were attracted to cosmology and the big bang idea because it allowed in principle for the possibility of prestellar synthesis of the heavier elements.

The essential idea as it developed in the work of Gamow, Alpher, and Herman in the late 1940s was that the very early universe was dominated by radiation, matter being present at this time in the form of a soup consisting of protons, neutrons, and electrons. As the universe expanded, thermonuclear processes produced helium nuclei from the protons and neutrons. Further element formation followed, although the precise mechanisms for this were not spelled out. At a certain time the universe had expanded and cooled to such a degree that the matter density exceeded the radiation density; at this moment, later referred to as the decoupling time, the universe as we know it was born. In a paper in 1948 Alpher and Herman carried out some computations and concluded that "the temperature in the universe at the present time is found to be about 5° K" (Kragh 1996, 119). No one at the time viewed this as a serious empirical prediction subject to test, and the work of Gamow, Alpher, and Herman failed to attract much interest.

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