How the universe originated and evolved are among the most perplexing questions still facing astronomers—in particular cosmologists, who specialize in studying the universe as a whole. Throughout the ages scientists have tried to solve the riddle of the universe’s origin, but it is only in this century that any significant progress has been made.
At the beginning of this century, most astronomers believed that although individual stars and other celestial bodies were moving, these movements “canceled out” each other so that the universe as a whole was static. It was also generally thought that the universe was infinitely large (and, being static, was neither expanding nor contracting) and infinitely old. The universe has always existed in much the same state; there had been no beginning. Then in 1915, serious doubts were cast on the idea of a static universe as a result of Albert Einstein’s general theory of relativity—which, even today, is still the best theory of gravitation. (It is not confined only to the force of gravity but deals also with the effects of acceleration.)When Einstein applied his theory to the universe, he found that his equations allowed for the existence of only non static universes—that is, several different hypothetical models of the universe were possible within the general relativity theory, but none of these possible universes was static. Several years later it was discovered that the spectra of distant galaxies showed red shifts, (Doppler shifts), which indicated that these galaxies were moving away from the earth and, by extension, that the universe was expanding—as predicted by Einstein’s general relativity theory. (Red shifts are an apparent change in the frequency of light because of relative motion between the source and the observer. A frequency shift toward the red end of the visible spectrum means that the source and observer are moving apart.)
Since then other findings have also supported the general relativity theory, and it now forms the basis of almost all modern cosmological models and studies of the universe.
The big bang theory
Testing the big bang theory
According to the big bang theory, most of the helium that is present today was first formed about three minutes after the big bang itself. From this it has been calculated that the universe should now consist of about 23 per cent helium and 77 per cent other elements (mainly hydrogen)—a prediction that has been supported by measurements of the abundances of the various elements in our galaxy and in other galaxies; thus these measurements are evidence in favor of the big bang theory.
The fate of the universe
In addition to extrapolating backward in time to try to discover how the universe originated and evolved, cosmologists have also extrapolated forward in an attempt to predict the ultimate fate of the universe. According to the almost universally-accepted general relativity theory, there are only two possible futures for the universe: it will either continue to expand (at an ever-decreasing rate) indefinitely, or it will eventually stop expanding and collapse back on itself. This second possibility could again form a highly condensed nucleus that will explode in another big bang—the pulsating universe theory. Which of these two possibilities is more likely depends on the amount of matter in the universe: the more matter, the more probable is the pulsating universe.
Read More Evolution of the universe
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