|
We have just crossed over the threshold into the twenty-first century, making this a propitious moment to recall what we knew about our universe at the beginning of the twentieth century. The discoveries made over the past century have been dramatic, and they have revolutionized our perception of the universe in which we live.
At the outset of the twentieth century, we inherited the then 350-year-old, initially blasphemous Copernican idea of a Sun-centered (rather than Earth-centered) universe. Eight planets were then known to orbit our Sun. A controversy arose as to whether the collection of stars in our own Milky Way constituted the entire universe or was merely one of many galaxies of stars. With the construction of giant reflecting-mirror telescopes in California in the early part of the century, major discoveries followed rapidly:
First, our Sun was displaced from the center of the universe. We know today that the Sun is located about two-thirds of the way from the center of our own Milky Way Galaxy and is one of about a hundred billion stars in total.
Next, Carnegie Institution astronomer Edwin Hubble (of the eponymous space telescope) discovered that the Milky Way is not unique. There turn out to be about as many galaxies in the visible universe as there are stars in the Milky Way.
By 1929 Hubble had demonstrated not only that there are myriads of other galaxies but also, most incredibly, that these galaxies are in motion, expanding outward with tremendous velocities that increase with the distance from us. In 1915 Albert Einstein had formulated his general theory of relativity, describing the nature of gravity, and had recognized that a stationary universe would not survive for very long—that is, it would tend to either contract or expand. In an important example of the interplay between theory and experiment in science, general relativity provided a framework for understanding the unexpected motions of galaxies observed by Hubble. The universe itself is expanding, and galaxies are being carried along with the expansion of space.
The implications of this result are enormous. If space is expanding, then galaxies and the matter that gave rise to them must have been closer together at some time in the past. Early in the universe, then, the density and temperature of matter must have been extraordinarily high. The theory and observations led to the striking conclusion that the universe began with a colossal explosion, the "big bang." Experimental evidence for the remnants of the big bang came in 1965 with the discovery, by Arno Penzias and Robert Wilson, that the universe is bathed in a sea of cool (3 degrees above absolute zero) radiation—a theoretically predicted remnant of the big bang. Now, as the twenty-first century begins, we face a number of fascinating unsolved mysteries, and we are still exploring the implications of recent discoveries:
The first discovery of planets outside of our own solar system. Until 1995 no planets outside of the 9 in our solar system were known to orbit sunlike stars. For the first time in history, we now know that planets exist elsewhere. As of mid-2000 the count of extrasolar planets stands at 50. To date, though, measuring techniques are sensitive mainly to planets of Jupiter-like mass (about 300 times more massive than Earth) and greater. Over the next decade, more sensitive techniques will become available; then planets with characteristics similar to Earth's could be discovered, if they exist. Experiments are already being designed that will make it possible in the future to study the atmospheres of extrasolar planets and discover if they contain ozone, carbon dioxide, and water—that is, evidence of life.
Closer to home, in harsh and unexpected environments on Earth, biologists have recently found life ranging from primitive, ubiquitous bacteria in subterranean locations to exotic species of worms and crustaceans living near thermal vents on the ocean floor. We have learned in the past decade that one of Jupiter's moons, Europa, is covered with ice, under which there may be a liquid ocean. Imagine how our worldview will change with the scientific discovery of life elsewhere in the universe—a discovery that will rank among the most profound of all time.
The matter that we see (the stars and galaxies that shine) is not all of the matter that exists. If there were only the matter that we see, then the motions of stars within galaxies, or of galaxies within clusters of galaxies, would be significantly smaller than what we observe. Similarly, other recent determinations of the masses of clusters would be smaller than measured. The additional matter that does not omit light has come to be referred to as "dark matter." An extraordinary consequence of the existence of dark matter is that we (along with the luminous matter in stars and galaxies) are made of only about 5 percent of the overall matter plus energy density of the universe. At this time, however—although the field of particle physics offers plausible ideas about the possible composition of most of the mass of the universe, and although numerous ongoing experiments are designed to search for dark-matter particles—we do not know how dark matter is distributed in the universe, what it is made of, or how to detect it.
There may be an additional force in the universe that acts counter to gravity and is causing the universe to speed up its expansion. Recent measurements of exploding stars or supernova events early in the universe suggest that in addition to dark matter, there may be a source of dark energy—a repulsive force that results in an acceleration of the universe. This result is relatively new, and it has not been tested for very long, but a number of other, indirect lines of evidence appear to support it. If it is confirmed, it poses an enormous challenge for fundamental physics. There is currently no physical explanation for this force. In fact, straightforward predictions lead to a startling contradiction at the level of 60 orders of magnitude (that is, a number given by multiplying 10 x 10 all the way up to 60 factors of 10-a very large discrepancy).
As we reflect back on what we have learned about the universe over the past century and consider how dramatically our horizon has expanded from that of a small, sun-centered universe, we realize that we can look forward to answering in the twenty-first century many of our current questions about the nature of the universe, its origin, and its fate, as well as about life elsewhere in the universe. But perhaps even more exciting, we can also look forward to confronting completely new questions that we cannot now even contemplate.