Red Shifts and the Expanding Universe
In this chapter we enter into the domain of extra-galactic cosmology and discuss the big bang theory and the evidence for an expanding universe. We illustrate how scientists construct theoretical models that are mistaken for reality by millions of people who encounter them in authoritative textbooks and in popular presentations. We show, however, that even within the narrowly defined domain of astrophysics, new observations may arise that fundamentally challenge these models and reveal them to be mere systems of speculative ideas. In the example that we consider, this fundamental challenge shows the need for drastic revisions in the modern understanding of the universe as a whole-revisions that thus far have not been seriously considered in the scientific community.
Let us begin with the theory of the big bang. Basically, this is the idea that in the beginning (or before the beginning, if you will), all matter in the universe was concentrated into an infinitely small volume at an infinitely high temperature and pressure. Then, according to the story, it exploded with tremendous force. From this explosion rushed a superheated, ionized gas, or plasma. This plasma expanded uniformly until it cooled sufficiently to form ordinary gas. Within this cooling cloud of expanding gas formed galaxies, and within the galaxies took birth generations of stars. In turn planets such as our own earth formed around the stars.
But here's a fact that few people realize: Even with the most powerful telescopes, it is not possible to actually see galaxies moving away from us. The images we see are static, and scientists would not expect them to show visible motion, even if observations could go on for centuries.
So how do we really know the universe is expanding? All we have to go on is the light and other kinds of radiation that travel to us from across the reaches of interstellar space. Images formed from this radiation do not directly show universal expansion, but subtle features of the radiation have convinced scientists that this expansion is taking place. What scientists do is first assume that the earthly laws of physics apply without change throughout the universe. They then try to figure out how processes obeying these laws could produce the observed light.
To understand how scientists have used this way of analyzing light to conclude that the universe is expanding, let us go back into the history of astronomy and astrophysics. Examining the heavens, astronomers long ago observed that in addition to individual stars and planets, there were many faintly glowing bodies in the sky. They called them nebulae, a Latin word meaning "clouds," and later on, as their conceptions evolved, they called them galaxies.
Larger than the full moon in the night sky, yet so dim that it is hardly visible to the unaided eye, is the nearby galaxy Andromeda. In the early part of this century, astronomers turned powerful new telescopes on this and other galaxies and found that they appeared to be vast islands of billions of stars. At further distances are found entire clusters of galaxies.
Until the discovery of stars in Andromeda, it was generally thought that all celestial bodies were located within the boundaries of our local Milky Way galaxy. But with this development and the discovery of other, more distant galaxies, all that was changed. The dimensions of the universe expanded beyond comprehension.
Up until the early part of this century, scientists believed that the basic objects in the universe were static in relation to one another. Then in 1913 the American astronomer Vesto Melvin Slipher came to study the spectra of light coming from a dozen prominent nebulae and concluded that they were moving away from the earth at speeds of up two million miles per hour.
How did Slipher reach this astonishing conclusion? For some time, astronomers had been using spectrographic analysis to determine the elements present in the stars. It was known that the spectrum of light associated with a particular element will show a characteristic pattern of lines that serves as a kind of signature for the element.
Slipher noticed that in the spectra of galaxies he studied, the lines for certain elements were shifted toward the red part of the spectrum. This curious phenomenon is called a "red shift." Slipher interpreted the red shift as a Doppler effect, indicating that the galaxies were moving away. This was the first major step toward the idea that the entire universe is expanding. (If the lines in the spectrum had been shifted toward the blue end of the spectrum, that would have indicated that the galaxies were moving toward the observer.)
The Doppler effect is often explained by using the example of a train whistle, which seems to change pitch as the train goes by. This phenomenon was first scientifically studied in 1842 by Christian Johann Doppler, an Austrian physicist. He proposed that the intervals between the sound waves emitted from an object moving toward a listener are compressed, causing the sound to rise in pitch. Similarly, the intervals between sound waves reaching a listener from a source moving away are elongated, and thus the sound's pitch is lower. It is reported that Doppler tested this idea by placing trumpet players on a flatcar drawn by a locomotive. Musicians with perfect pitch listened carefully as the trumpet players moved by them, and they confirmed Doppler's analysis.
Doppler predicted a similar effect for light waves. For light, an increase in wavelength corresponds to a shift toward the red end of the spectrum. Therefore the spectrum of an object moving away from an observer would tend to be shifted toward the red. Slipher chose to interpret his observations of galaxies in this way, as a Doppler effect. He noted a red shift and decided the galaxies must be moving away.
Another step toward belief in an expanding universe took place in 1917, when Einstein published his theory of general relativity. Before Einstein, scientists had always assumed that space extended to infinity in all directions and that the geometry of space was Euclidean and three-dimensional. But Einstein proposed that space could have a different kind of geometry-four-dimensional curved space-time, in which space could curve back on itself.
There are many forms that space could take, according to Einstein's theory. One is a closed space without a boundary, like the surface of a sphere; another is a negatively curved space that extends to infinity in all directions.
Einstein himself thought the universe should be static, and he adjusted his equations to insure this outcome. But almost immediately, Willem de Sitter, a Dutch astronomer, found solutions to Einstein's equations that predicted a rapidly expanding universe. The geometry of space would change with time.
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