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
Using the general relativity theory, astronomers have extrapolated back in time from the universe as it now is to try to discover how it originated. Measurements of its expansion rate indicate that the universe originated in an unimaginably violent explosion that occurred from 10 to 20 billion years ago—the big bang theory of the universe. According to this theory, all the matter that now exists was originally conglomerated in a state of enormous density; this condensed “nucleus” then exploded, throwing out the matter from which the stars, galaxies, planets—and ultimately ourselves—later developed.
But the origin of the matter in the nucleus and the details of the conditions at the precise moment of creation are still unsolved problems. The general relativity theory proposes that at the moment of creation all matter was condensed in a unique state called singularity, a state of infinite density and infinitesmal volume. The laws of physics do not apply to this state; without such a frame of reference, it is impossible to describe or explain what happened at the moment of creation. Nevertheless, some scientists believe that the big bang could not have occurred had matter reached a state of singularity. It has, therefore, been postulated that little understood “quantum gravitational processes” intervened to prevent singularity from occurring, thereby enabling the big bang to take place.
Immediately after the big bang, the rapidly-expanding matter was inconceivably hot. At such high temperatures, elementary particles (protons, neutrons, and electrons, for example) can be created, seemingly from nothing. These particles, which are produced in matter-antimatter pairs (electrons and positrons—positive electrons—for example), came into existence about 1 millionth of 1 second after the explosion. By about 100 seconds later, the universe had cooled to a temperature low enough for electrons and positrons to interact with protons and neutrons to form deuterium nuclei.
About three minutes after the big bang, helium nuclei were formed from deuterium. Other nuclei—for example, beryllium nuclei— were also formed, but only in extremely small amounts. (The relative abundance of the heavier elements in the present-day universe is a result of their more recent formation in stars.)
The temperature of the universe continued to decrease, and by about a million years after the big bang, it was cool enough to enable the formation of hydrogen atoms. This point marked the end of the radiation-dominated era, during which the effects of energy (in the form of heat radiation) predominated. At the beginning of the matter-dominated era (which is still continuing), interactions between matter, rather than energy, were (and are) the principal changes.
The big bang was extremely violent and created a great deal of turbulence, as a result of which the rapidly expanding matter was not distributed uniformly throughout the universe. Thus, there were dense regions containing large amounts of matter and less dense areas with relatively little matter. It is generally believed that the high-density regions then began to slowly attract increasing amounts of matter until, about 1 billion years after the big bang, they developed into galaxies. Subsequently, the individual galaxies gradually grouped together to form clusters and super-clusters. About 3 billion years later, the first stars began to develop in the galaxies. After a further 6 billion years, our solar system (and therefore the earth) started to form. Life evolved on earth relatively quickly; it is thought that the first life-forms (probably simple single-celled organisms) developed about 3 billion years ago—within about 1 billion years of the earth’s formation. Modern man (Homo sapiens) is a newcomer, the earliest fossil remains being only some 300,000 years old.
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.
Another test of the validity of the big bang theory derives from a discovery, made by the American physicists Arno Penzias and Robert Wilson, concerning the intensity of background radiation. If the universe began with an enormous explosion, there should still exist a much-cooled relic of the incredibly high temperature that originally existed. Because the big bang involved the entire universe, this relic should be in the form of a fairly uniform background radiation (called isotropic radiation), with minor local variations in intensity reflecting inhomogeneities in the universe when the radiation-dominated era came to an end.
In 1965, Penzias and Wilson detected isotropic background radiation in the microwave region of the electromagnetic spectrum—the type of radiation expected if the big bang theory is correct. Subsequent measurements of the intensity and frequency of the isotropic radiation and of its average temperature, about 3 K (—454° F.; —270° C), also tended to support the big bang theory.
In 1989, NASA launched the Cosmic Background Explorer (COBE) satellite to study the microwave background radiation. COBE confirmed that there is very uniform background radiation at about 2.7 K (—454.8° F.; —270.5° C). Big bang theory also predicts that the background radiation should have irregularities left over from the explosive turbulence of the big bang itself. In 1992, NASA scientists announced that analysis of COBE data had revealed the first such small irregularities, strengthening the case for the big bang. Most scientists believe that the big bang theory provides the most plausible explanation of the origin of the universe.
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.
Astronomers have calculated that an average density (throughout the entire universe) of 100 atoms per 10 cubic yards (7.6 cubic meters) is necessary for the universe eventually to stop expanding and collapse. But the most recent measurements indicate that the average density is, at most, one atom per cubic yard. Thus, it is generally thought that the universe will continue to expand indefinitely, gradually becoming less active until, more than a 100 billion years from now, it will be completely “dead,” except for the occasional explosion of a black hole.
But some astronomers doubt the validity of this hypothesis. They believe that there is a large amount of material (such as dust, dead stars, and black holes) that has not yet been detected and, therefore, that there might be sufficient matter for the universe to eventually collapse back on itself. If this is the case, the universe will not merely “fade away” to a cold, dead expanse, empty of everything, even gas and dust. Instead, it will eventually stop expanding, become static for a short time, and will then begin to contract.
As this contraction begins, the galaxies will start to converge, slowly at first, then with increasing speed as they are mutually attracted by gravity. About 100 million years before the “big crunch” (the time at which all matter will be conglomerated in an extremely dense nucleus), the galaxies will begin to merge and their constituent stars will collide, exploding as they do so. As the universe continues to contract, it will become hotter. According to the pulsating universe theory, eventually all matter will become condensed into a single, incredibly hot mass, which will then explode— as in the original big bang—thereby causing the universe to expand again. Thus, the universe would undergo a perpetual cycle of big bang, expansion, momentary stasis, contraction, and big bang again. But this theory depends for its plausibility on there being more matter in the universe than has been detected, and the available evidence therefore supports the continuously-expanding universe theory.