WORDPLAY
- the scientific study of the form, content, organization, and evolution of the universe
- the branch of metaphysics dealing with the origin and structure of the universe
FAST FACTS
Edwin Powell Hubble, (1889-1953), U.S. astronomer, born in Marshfield, Mo.; at Mount Wilson Observatory 1919-53; at Mount Wilson and Palomar observatories 1948-53 (‘Realm of the Nebulae’)
Big bang theory, a general theory held by many astronomers that the universe may have originated about 12 to 15 billion years ago as the result of a violent explosion of some primordial mass; since then the universe has been expanding and evolving; a refinement of the theory states that the universe pulsates, expanding and contracting every 80 billion years
The joint effort of astronomers and physicists to understand the origin, evolution, and fate of the universe is the field of cosmology. The era of modern cosmology began in 1905 when the physicist Albert Einstein published his special theory of relativity. Before then space and time had been regarded as separate and unrelated. Special relativity showed that space and time must be regarded as aspects of a single entity: space-time. Einstein’s general theory of relativity, published in 1916, incorporates gravity by describing itin terms of space-time and by relating space-time to the distribution of matter in the universe. It is in the light of general relativity that modern notions of the structure, evolution, and fate of the universe are described.
Today there is strong evidence that the universe is homogeneous such basic properties as the average density of matter (the mass in a given very large volume of space) do not vary greatly from place to place. Similarly, the universe is isotropic, which means that the nature of what an observer sees does not depend upon the direction in space he or she looks.
When the concepts of homogeneity and isotropy were applied to Einstein’s equations relating space-time structure to matter distribution, it was discovered that the universe must be dynamic: It must be expanding or contracting. Einstein’s equations require that at later or earlier times the distribution of matter in the universe will remain homogeneous but the distance scale between objects will change. In effect, space itself expands or contracts. If the universe were expanding, the distance between any two points would be increasing in proportion to the distance between them. In other words, the farther apart the points are, the faster they would be moving away from each other.
Just as the pitch, or frequency, of a train whistle lowers as the train moves away from a listener, the frequency of light from a receding galaxy should appear lower to an observer than if the galaxy and observer were motionless relative to each other. This kind of change is called a “red shift” because visible light is shifted downward in frequency toward the long-wavelength, red end of the electromagnetic spectrum. The theory of an expanding universe was confirmed in 1929 by the astronomer Edwin Hubble of the United States, who showed from an analysis of the red shifts of distant galaxies that they are indeed receding from our galaxy and with velocity of recession that is proportional to distance. Hence, the universe is expanding.
The Big Bang
When a mathematical excursion into the past is made, a very striking thing happens. The farther back one goes in time, the more contracted was the universe and the more rapid was the rate of expansion. In fact, general relativity indicates that about 10 billion to 20 billion years ago the universe was infinitely contracted: The distance between any two points was zero, the density of matter was infinite, and the volume of the entire universe was zero. According to this picture the universe came into being in a highly singular state, the moment of its origin being referred to as the “big bang.”
A common misconception is that matter suddenly poured into an empty universe from an explosion at a point. This is not the case. Not only did matter come into existence at the big bang, but so also did the structure of the space-time. Space itself was shrunk to zero volume at the time of the big bang, and before the big bang, there was no “before.”
For the first second or so after the “big bang” the matter in the universe was very hot and dense. Extremely energetic elementary particles, both stable and unstable, were present in large numbers. Following that second, however, expansion and cooling of the universe proceeded so rapidly that most of the unstable particles decayed away. During the next 15 minutes nuclear reactions took place. Theory indicates that about one quarter of the original mass of protons and neutrons in the universe was converted to helium at this time. This fraction of helium is in good agreement with its observed abundance in the present universe. After 15 minutes the density and temperature of matter had dropped sufficiently that no further nuclear reactions could occur until much later in the evolution of the universe when the stars were formed.
All this time, matter was everywhere absorbing and emitting electromagnetic radiation and the early universe was filled with a homogeneous “soup” of matter and radiation. Just as the colour, or frequency, of thermal radiation emitted by a hot body is associated with its temperature, so also was the frequency of this cosmic radiation associated with the temperature of the early universe. As the universe expanded and cooled, the frequency of this primordial radiation lowered until by the present era it should correspond to a temperature only a few degrees above absolute zero. Thus, if the big-bang theory predicted by general relativity is correct, the universe should be filled with a uniform sea of very low temperature electromagnetic radiation.
Exactly such a “cosmic background radiation” was discovered in 1965 by radio astronomers Arno A. Penzias and Robert W. Wilson of the United States. It was later established that this radiation reaches the Earth equally from all directions. The existence of this radiation is very strong confirmation of the big-bang theory, which predicts it and accounts for it in the simplest way. Furthermore, the high degree of isotropy of the radiation provides strong evidence that the universe is homogeneous and isotropic, as was assumed. In 1992, after support for the big-bang theory had been declining due to the increasing support for rival theories such as the steady-state theory, further evidence of the correctness of the big-bang theory was found. The Cosmic Background Explorer satellite detected tiny temperature differences in microwave radiation coming from wispy clouds of gas surrounded by slightly less dense bands of matter arranged in a sort of rippling effect. The rippling arrangement is thought of as an after-effect of the big bang.
According to Einstein’s equations, if the average density of matter in the universe is equal to a certain critical value, then the universe will expand at an ever slower rate and eventually stop expanding. If the average density is below the critical value, the universe is open and will continue to expand forever. If, on the other hand, the average density is above the critical value, the universe is closed that is, it will eventually stop expanding, begin to contract, and finally come to an end, no earlier than 20 billion years from now, in a “big crunch” reversal of its origin in the big bang. Hence, one of the most important questions in modern cosmology is the determination of whether the universe is open or closed.
Using the most generous estimate of the mass of all the galaxies, the mass density of the universe is still about 20 times smaller than the critical density. This strongly suggests that the universe is open, but the possibility remains that significant amounts of matter may exist between galaxies.
A second measurement that could determine whether the universe is open or closed is that of the decrease in the expansion rate. The velocity of expansion is expressed in the equation
v = Hr
in which v is velocity, r is distance between two points, and H, called Hubble’s constant, is a proportionality value that relates velocity and distance. Although H decreases with time whether the universe is open or closed, it decreases more rapidly for a closed universe. To measure H it is necessary to measure the intrinsic brightness of distant galaxies. Unfortunately, it is quite likely that the intrinsic brightness of a galaxy varies significantly during its evolution and what astronomers see now may be misleading. If so, such methods of distance measurement break down, and the deceleration cannot be determined at least until astronomers develop a much better understanding of the evolution of galaxies.
A third method that astronomers use for determining whether the universe is open or closed is to measure the age of the universe that is, the time elapsed since the big bang. The amount of time from the big bang to the present assuming that the present rate of expansion holds throughout this period is called Hubble time.
Hubble time can be considered a measure of the time required for a galaxy to achieve its present distance from our galaxy. In other words it is the maximum time since the galaxies separated, or the maximum possible age of the universe. For a universe either open or closed whose expansion rate was higher in the past, the actual expansion time must be shorter than the Hubble time. (Hubble time is expressed as 1/H.) Einstein’s equations demonstrate that a closed universe must expand faster than an open universe. Specifically, if the actual age of the universe is less than two thirds of the Hubble time, the universe must be closed; otherwise it is open. Two thirds of the Hubble time is called the critical age of the universe.
By estimating the age of the oldest known star clusters and the age of long-lived radioactive elements, scientists have arrived at an estimate of 15 billion years as the present age of the universe. Because there is a large margin of error in this estimate, this number is rather uncertain; the correct age quite possibly could be as little as 10 billion or as much as 20 billion years.
To compare this with the critical age one needs to know the numerical value of Hubble’s constant, H. The problem of measuring H was discussed above. The best current estimate of the critical age of the universe is about 13 billion years. Although this value is in good agreement with the observed age, the uncertainties in both are too great to conclude whether the universe is open or closed.
Thus, the present observational evidence points to an open universe. It may well require another half-century of observational data, however, before a firm conclusion can be drawn on this question.
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