The Beginning and the End of the Universe
The Beginning and the End of the Universe
A complete understanding of time would have to include the whole history of the universe. Most people who lived prior to the 20th century believed their universe had no beginning or ending, and if this were true, the same could be said about time. Time would
be infinite, lasting forever. But today most physicists believe the universe began about 14 billion years ago. The big bang theory suggests that the universe began as a singularity—an infinitely dense point—and expanded to its present size. There are several reasons why this theory is widely accepted. Careful measurements by astronomers show that the galaxies are moving apart, with a speed that increases with dis- tance—distant galaxies are moving at high speeds, in some cases a considerable fraction of the speed of light. This movement occurs despite gravitational attraction, which is always at work. (The force of gravitation is strong enough to cause the closest galaxies to move together, but in general the galaxies of the universe are mov- ing away from each other.) The universe is expanding, and physi- cists account for this by saying that space itself is expanding. Other astronomical measurements also support the big bang theory—for instance, energy called cosmic background radiation is believed to
be leftover energy from the creation of the universe.
Time 123
If the universe had a beginning, how did it get its start? This is one of the basic questions of a branch of science called cosmol- ogy (the term cosmos comes from a Greek word referring to the universe). The physics is complicated, and while scientists have proposed theories to explain many of the early events of the uni- verse—such as how the forces of nature arose and why matter formed the stars and galaxies—the laws of physics, at least as they are currently understood, do not permit a mathematical descrip- tion of the earliest moments of the universe. At a time of about
10 -35 seconds and earlier, the universe was so hot and dense that its description is beyond the present reach of physics. The 10 -35 seconds is an unimaginably tiny period of time, but, nevertheless, it means a complete understanding of the beginning of the uni- verse—and of time—is not yet possible.
Some people are uncomfortable with the notion of a beginning for time. If time began at the big bang, what came before? Perhaps this question has no meaning, or at least no meaning in terms of physics. Although people can ask, and wonder, about this question, it may have no answer. Everything that people experience or can imagine experiencing has a beginning, so the question is a natural one, but the beginning of the universe is an event well beyond the range of human experience—and perhaps understanding as well. On the other hand, science proceeds by asking questions, and no one knows for sure whether this question has an answer or not.
Much of the universe is still a mystery, but the second law of thermodynamics has something to say about its evolution over time. The entropy of an isolated system increases, and the universe is certainly an isolated system—the universe is all that exists, so nothing from the outside should be able to influence it. According to the second law, the entropy of the universe is increasing. The universe is becoming more and more disordered as time goes by.
If time and the universe had a beginning, it is possible they have an ending as well. What kind of ending will eventually occur depends on whether the universe continues to expand or not. The big bang sent the universe flying out in every direction, but mat- ter also experiences an attractive force due to gravitation. If the density of the universe is large enough, gravitation will win, and
124 Time and Thermodynamics
the expansion will come to a halt. If not, the expansion will prob- ably continue forever. Measurements of the amount of matter in the universe and measurements of the present expansion rate are difficult to make and sometimes present conflicting evidence, but at the present time, the best guess is that the universe will continue to expand. Recent measurements indicate that the expansion is even accelerating, perhaps by the presence of some kind of “dark energy” that propels matter further apart.
As the expansion goes on, entropy will continue to increase. This gives rise to one possible fate of the universe—“heat death.” Entropy rises to a maximum, and at this point, the universe becomes totally disordered. As discussed in the last chapter, the second law of thermodynamics requires heat engines to discard some heat in the engine’s exhaust so that entropy increases, oth- erwise the engine cannot do any work. When entropy reaches a maximum, this will not be possible and, according to the second law of thermodynamics, there can be no more work from heat engines.
If the universe continues expanding, then the notion of time, if not time itself, would probably have an end. Time always involves change—some sort of regular, periodic change, whether it is the rotation of the Earth, the swing of a pendulum, or the oscillation of an atom’s radiation. When the universe gets old enough for entropy to rise to a maximum, there will be few orderly changes, so time would be difficult or impossible to measure. But there is no cause for concern at present—time has billions of years more to go before all is said and done.