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The human understanding of the universe has dramatically changed over time, with advances made in extrapolating evidence from carefully observed phenomena, and with discoveries in disciplines such as particle physics. Each change in the cosmological model has diminished human significance in a vast and ancient universe.
Cosmology is the study of the universe as a whole, its structure, composition, history, and probable future. Because human records only extend over a tiny fraction of the lifetime of the universe and human exploration has been restricted to a very small region, cosmologists are forced to rely on extrapolations and speculation beyond what would be acceptable in the other sciences. While much of cosmological theory has no immediate relevance to understanding the environment of living organisms on earth, cosmology sheds light on the time scale of biological evolution and the possibility of new resources being discovered. Cosmology touches at several points on issues also of concern to religion and has for many thinkers required a revision of their estimate of the place of humans in nature.
The Geocentric Universe and Astrology
As ancient humans watched the daily transit of the sky by the sun, moon, and stars, it was only natural to assume that the Earth was the center about which these celestial bodies moved. On closer observation it became apparent that a few star-like objects moved a bit differently, seeming to wander against the background of stars that traveled together across the sky as if attached to a transparent sphere. The movements of the wanderers, called planets, was thought to influence human affairs and a group of professional astrologers began to predict the future for individual clients by casting horoscopes, based on the position of the planets at the time of their births. Tables of planetary positions were worked out, assuming the planets moved in circular orbits about the earth. The most famous of these, the Almagest of Ptolemy (90–168 CE) is the basis for astrological tables used today.
A Heliocentric Universe
The Ptolemaic model held sway until the Polish clergyman Nicolas Copernicus (1473–1543) published On the Revolutions of the Celestial Orbs in 1512. Realizing that his views would meet with disapproval by church authorities, he postponed publication of his book until his dying year. The Italian mathematician Galileo Galilei (1564–1642) popularized the Copernican model and incurred censure for his views. In actuality, though, both the Ptolemaic and Copernican systems were defective in assuming, on philosophical grounds, that planetary motion must be circular. Based on the careful observations of the Danish nobleman Tycho Brahe (1546–1601), the German scholar Johannes Kepler (1571–1630) demonstrated that the planets moved about the sun in elliptical orbits. It remained for the great English mathematician Sir Isaac Newton (1642–1727) to discover the laws of motion and of universal gravitation that explained the elliptical orbits and confirmed the heliocentric model.
A Very Large and Old Universe
By the beginning of the twentieth century, astronomers were in general agreement that the sun was just one of many stars and that the visible stars in the sky were clusters together in a large structure, the Milky Way galaxy. A group of indistinct fuzzy objects called nebulae had yet to be explained. In 1918, the American astronomer Harlow Shapley (1875–1972) was able to demonstrate that the sun was quite far removed from the center of the Milky Way. In 1923 the American astronomer Edwin Hubble measured the distance to the Andromeda nebula, showing that it and the other nebulae were actually galaxies in their own right. The universe was understood then to consist of many galaxies like our own and thus to be very much larger than anyone had previously expected.
A theoretical result known as Olber’s paradox implies that the universe, or at least the stars in it, could not be of infinite age, since, if it were, the sky would be as bright at night as during the day. In 1929 Edwin Hubble, working with the world’s then largest telescope, on Mount Palomar in California, was able to demonstrate that galaxies far from the Earth were moving away from it with a velocity that increased with distance. This fact along, with other evidence, lead to the conclusion that the universe we know, was at its beginning, some ten to twenty billion years ago, confined to a very small region of space and began to separate as the result of a giant explosion, the “big bang.”
Life Cycles of Stars and the Origin of Chemical Elements
Until the twentieth century the energy source of the sun was a mystery. In the 1930s it was discovered that the fusion of small nuclei to form a large nuclei would result in the transformation of a small fraction of the smaller nuclei’s mass into heat energy according to Einstein’s famous formula, E=mc2. It is now known that the power source of the sun is the fusion of hydrogen nuclei to form helium nuclei. Computer modeling now shows that after the sun, which is now about five billion years old, has transformed most of its hydrogen into helium, its inner regions will contract and become hotter until the combining of helium nuclei to form carbon and oxygen nuclei becomes feasible. After that further contractions will occur until atoms as heavy as iron will be formed. The formation of nuclei heavier than iron occurs, either by a slow process in which a heavy nucleus absorbs neutrons that are then converted to protons through beta decay, or when massive stars explode as supernovas.
Our current understanding of the distribution of chemical elements is as follows: hydrogen and a certain amount of helium were formed during the big bang, about 13 billion years ago. The basic elements of life and those found in large quantities on earth had to be formed in stars that had passed the hydrogen burning stage and ultimately returned their substance to the interstellar medium where it could condense as a new star, the sun, and planets formed. Elements with atomic numbers higher than iron are formed in very old stars or in supernova explosions, and therefore are destined to remain rare regardless of how thoroughly the mineral resources of Earth are explored.
Modern cosmologists apply the principles of relativity theory and the discoveries arising from experiments in particle physics to try to understand the origin and the ultimate fate of the universe as we know it. They also must assimilate observations from the Hubble telescope and a number of artificial satellites that show the universe to be relatively uniform when averaged over large distances. In contrast to the assumption that the Earth held a privileged position in the universe, most cosmologists today make assumptions of isotropy and homogeneity, that is, that the universe looks pretty much the same from any point in it and in any direction in space. Perhaps the biggest questions in cosmology today are the reason for this uniformity, which has been taken as evidence for a period of especially rapid expansion or inflation shortly after the big bang, and the nature of the so-called dark matter that is needed to explain the rotational speeds of galaxies as observed. Once these questions are resolved, cosmologists will have a better understanding of the early history of the universe and may speak with greater confidence about its distant future.
- Chaisson, E. J. (2001). Cosmic evolution. Cambridge, MA: Harvard University Press.
- Hartquist, T. W., & Williams, D. A. (1995). The chemically controlled cosmos. New York: Cambridge University Press.
- Kaufmann, W. J., III & Comins, N. F. (2005). Discovering the universe (7th ed.). New York: W. H. Freeman.
- Kippenhahn, R. 100 billion suns. (1983). New York: Basic Books.
- Primack, J., & Abrams, N. (2007). The view from the center of the universe: Discovering our extraordinary place in the cosmos. New York: Penguin/Riverhead.
- Seeds, M. A. (2007). Foundations of astronomy (10th Ed.). Belmont, CA: Wadsworth.
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