the model is right must have faith that an enormous amount of mass is hiding from us, escaping detection, perhaps in a uniform and tenuous gas of particles between galaxies.

To summarize, the light-emitting matter we see accounts for sufficient mass to make omega about 0.01; the unseen but gravitationally detected dark matter accounts for another factor of 10 of mass, increasing omega to about 0.1. Advocates of the inflationary universe model must hypothesize that space contains yet 10 times more mass—not only unseen but undetected and composed of some exotic species of matter. As mentioned earlier, some candidate particles will be searched for in the laboratory using “dark matter” detectors currently under development.

Some of the general features of the inflationary universe model are so appealing that many astronomers and physicists believe that some form of the idea is correct. It is sobering to realize that the highly influential inflationary universe model was unknown only a decade ago. Like Supernova 1987A, the idea exploded. We should expect similar observational and theoretical surprises in the future.

Even if the cosmos is infinite in extent, only a limited volume is visible to us at any moment: we can see only as far as light has traveled since the Big Bang. As we look farther into space, we are seeing light that has traveled longer to reach us. Eventually, at some distance, the light just now reaching our telescopes was emitted at the moment of the Big Bang. That distance marks the edge of the currently observable universe, some 10 billion to 20 billion light-years away. We cannot see farther because there has not been time for light to have traveled from there to here. And we have no way of knowing what lies beyond that edge. Some theorists have recently proposed that extremely distant regions might have different properties from the cosmos we know—different forces, different types of particles, even different dimensionalities of space. In such a universe, it would be impossible for us ever to learn about more than a tiny fraction of the possibilities and realities of nature.

Even in the universe we can see, many fundamental surprises surely await us. It is likely that major properties of the universe are yet unknown. The expansion of the universe was unknown in 1920 and the existence of quasars unsuspected in 1960. Who can imagine what astronomers will find by the year 2000?

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