When the Sun goes down, the night sky darkens; lit up only here and there by points of lights we now know to be distant suns. But have you ever wondered why the sky is dark at night? You might be inclined to argue that the sky is bright during the day because the Sun is so close, and therefore its light fills the sky when it is visible; at night, conversely, the distant stars cannot brighten the sky. But this argument is inadequate, a fact which Kepler was one of the first to recognize.
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If the universe is infinite and contains an infinite number of stars that live forever, then every line of sight must end on a star. It is true that a star’s light diminishes as the distance squared, but as distances become greater, the volume of space sampled increases by exactly the same factor. Thus the night sky should be everywhere as bright as the average surface of a star; both night and day would be ablaze. Yet we do not observe this, and this has important cosmological implications.
At the beginning of the twentieth century, many of the era’s leading scientists had refused even to accept that atoms existed; less than four decades later, the structure of the nucleus was nearly established. Even the stars themselves had yielded their deepest secret, the source of their energy. The nucleus itself soon seemed to represent only one aspect of the interactions among particles; after the Second World War, particle physics began to emerge as one of the most active subfields of physics.
Although the proton, neutron, electron, and possibly the neutrino are the most important of the elementary particles in the present universe, there are many others. As physicists studied cosmic rays, which are high-energy rays impinging upon the Earth from space; they found new kinds of particles. With the construction of accelerators, even more, particles were discovered. Many of these less-familiar particles are unstable; after some very short half-life, they decay into other particle species. As more and more “elementary” particles were found, physicists realized that there had to be a classification scheme to make sense of them. (Malcolm, 181)
Quantum mechanics is the system of physical laws that governs the behavior of elementary particles and of nuclei and atoms. It is a formal, mathematical system that developed from the work of many of the greatest scientists of the twentieth century, such as Niels Bohr, Max Planck, Albert Einstein, Werner Heisenberg, and Erwin Schrödinger. Quantum mechanics made it possible to sort out the confusing extravagance of particles and to understand their behaviors.
The salient feature of any quantum property is that it is quantized; it cannot occur in arbitrary amounts but only in multiples of a certain inherent value. Electric charge is an example of a quantum property; any particular particle, such as an electron, always has the same, specific quantum of electric charge. Particles possess many quantum properties and may be classified in various ways, depending upon the problem at hand; for now, we shall be concerned with only one important property. According to modern particle physics, there are two fundamental classes of particles, with the division based upon the spin of the particle.
The extreme conditions of the early universe require that our understanding of its history be inextricably linked with particle physics. Throughout this century, there have been occasional interactions between cosmology and other branches of physics; some of the most distinguished physicists of the first half of the twentieth century, such as Enrico Fermi, George Gamow, Robert Oppenheimer, and many others, made important contributions to cosmology. For the most part of cosmology, nuclear and particle physics advanced independently. Particle physicists have sought to build ever-larger accelerators, in order to study physics at higher energies. But such accelerators take an increasing toll on effort and resources.
Our study of special relativity showed how difficult it is to accelerate even elementary particles to relativistic speeds; if we wish to create even more exotic states, such as significant quantities of antimatter, the engineering problems become considerable, even overwhelming. The Superconducting Supercollider was to have consisted of an evacuated ring, 54 miles in circumference, about which nearly infinitesimal particles would have been driven to ever-higher energies by the magnetic field from superconducting magnets.
The cost of this great machine proved prohibitive, however. And even the Supercollider could not have reached the energies for which the particle physicists ultimately yearn. In order to test the leading edge of particle physics to the utmost, much greater energies are necessary. With any realistically foreseeable technology, only the early universe itself could be an appropriate laboratory.
The German astronomer Heinrich Olbers repeated this argument. For whatever reason, the name of Olbers stuck to this awkward difficulty, and it has become generally known as Olbers’ paradox. But the explanation was incorrect, as John Herschel showed in the middle of the nineteenth century. Any fluid that filled the universe and absorbed starlight would, according to the laws of thermodynamics, heat up until its temperature was equal to the average temperature of the stars; it would then radiate just as much light as if it were itself a source of starlight.
Herschel himself favored a hierarchical view of the universe, in which stars clump into clusters, the clusters bunch into larger clusters, and so forth ad infinitum. In a hierarchical universe, or for that matter in any non-uniform distribution of stars, there do exist lines of sight that are empty; this is the salient feature that distinguishes uniform from non-uniform. But if the universe is to be isotropic and homogeneous on large scales, as the modern view requires, then a strictly hierarchical model is ruled out.
Olbers’ paradox hung over cosmology well into the twentieth century. With the discovery of the expanding universe, many cosmologists immediately accepted it as the answer. The light from the most distant stars is so red-shifted that it contributes no appreciable energy to light our skies. Some authors have gone so far as to assert that the darkness of night is sufficient proof that the universe is expanding.
However, cosmologist E. R. Harrison has emphasized that the resolution of the paradox does not require expanding space. The crucial flaw in the traditional argument was the assumption that the stars could shine forever. Of course, with our modern understanding of energy conservation, this could not possibly be the case. Light carries energy, and thus stars must liberate energy in order to shine. Stellar lifetimes are very finite. When we look sufficiently far out into space, and therefore back in time, eventually we look to a time before any stars existed.
Moreover, in any universe that is not infinitely old, or that expands, the size of the observable universe is finite, because of the finite speed of light. The finite volume of the observable universe contains a finite number of stars, so most lines of sight never intersect the surface of a star at all. Even if multiple generations of stars live and die, the sky will still be dark. The number of stars is too small, and stellar lifetimes are simply too short, to fill the vastness of space with light. (Rees, 55) The darkness of the night sky quite elegantly rules out the simplest naive model of an infinite universe filled with infinitely old stars.
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The two greatest cosmological observations of the twentieth century were the discovery of the expansion of space, and the discovery of the cosmic background. In both cases, the discoveries were astonishing and revolutionary; but we can also see, in retrospect, that cosmological theory was prepared for them. Einstein’s theory of general relativity, and the difficulties in obtaining a static model, provided an immediate theoretical interpretation for Hubble’s finding. Similarly, the existence of the CBR was anticipated by cosmologists investigating the early history of expanding models.
The explanation of the background radiation as a relic from a hot, dense phase in the history of the universe was sufficiently persuasive to create coalescence in cosmological theory; indeed, it is a primary piece of the evidence that makes the “standard” models standard. (Halliwell, 130)
Prior to 1965, however, little was known with certainty, and the case that could be made for any of the standard models was no more compelling than arguments for other models. During the era between the two World Wars, many astronomers developed models, based more upon philosophy than upon data, since hardly any data were available at the time beyond the bare knowledge that distant galaxies were receding. (Hetherington, 122) Some of these models were developed in Great Britain, whose scientists had long taken a more philosophical approach to their cosmology than had those in the United States. The British tendency was to avoid an explicit beginning for the universe.
Arthur Eddington, a distinguished British astronomer who was one of the first to realize that nuclear processes must power the stars, devised his own model in the 1930s, in which the universe, although it had a beginning, emerged calmly and gradually from a nearly static initial state. (Bernstein, 79) The Eddington model was essentially an Einstein static model with a positive cosmological constant that, after an unknown length of time, began expanding. Through this contrivance, Eddington avoided the question of an initial state in the finite past.
Another British astronomer and stellar theorist, E. A. Milne, rejected entirely any cosmological explanation in terms of general relativity; with a few exceptions such as Eddington, British scientists, and mathematicians of the 1920s and 1930s were never very receptive to the general theory. Milne’s model, which he derived in the 1930s and continued to defend until his death in 1950, was based on special relativity.
There was no gravity at all on the cosmological scale. He adopted the point of view that the apparent expansion of the universe was simply the result of the infinity of galaxies expanding outward at ever-increasing velocities approaching the speed of light. The outer edge of this ensemble was identified with a sphere, expanding at the speed of light into flat Minkowski space. Within the sphere, increasingly distant galaxies were Lorentz-contracted by just the right amount to fit an infinite number of galaxies into the finite volume of the sphere. Although it may not be immediately obvious, such a universe obeys the cosmological principle.
Because the speed of light can never be reached, the view from every galaxy is the same; that is, each observes surrounding galaxies moving according to the Hubble law. As it turns out, Milne’s model is mathematically equivalent to the empty hyperbolic standard model, to which Milne’s name is sometimes now attached. Milne himself recognized the equivalence but disliked the idea of curved space, preferring his own interpretation.
Eddington’s and Milne’s models represent interesting but futile attempts to explain the existing data of their time in a manner consistent with their philosophical prejudices. We should not immediately dismiss such efforts as foolish or old-fashioned; aesthetical considerations have continued to guide many cosmologists throughout the twentieth century. When few data are at hand, not much else is available to help with the construction of models.
Better data from space-based and improved ground-based, telescopes should continue to make it possible for cosmology to rely in the future more upon observations and less upon speculation, but all cosmological observations are very difficult and not always good enough to be of much help. (Barbara, 115) Significant progress has often occurred because some scientists held stubbornly to a particular viewpoint in the face of apparently contradictory data which later proved to be wrong. Yet cosmologists must always be prepared to give up their preferred models if the weight of data refutes them. It is a fine line to walk, but there is no other option.
Barbara Ryden: Introduction to Cosmology: Benjamin Cummings; 1st edition (2002).
Bernstein Jeremy. An Introduction to Cosmology: Englewood Cliffs, New Jersey: Prentice Hall, 1995.
Halliwell Jonathan J. “Quantum Cosmology and the Creation of the Universe.” Scientific American, 265, No. 6, 76: 1991.
Hetheringon Noriss S., editor. Cosmology: Historical, Literary, Philosophical, Religious, and Scientific Perspectives. New York: Garland Publishing, Inc., 1993.
Malcolm S. Longair: Our Evolving Universe: Cambridge University Press (1997).
Rees M. Perspectives in Physical Cosmology: Cambridge: Cambridge University Press, 1995.