band of radiation from space, from radio signals with wavelengths meters in length to x rays and gamma rays, whose wavelengths are comparable to the size of an atomic nucleus (10-5 angstrom). And one of the most urgent and compelling questions in modern astrophysics concerns matter in space that apparently emits no radiation at all, and hence is referred to as dark matter.

J. Anthony Tyson from AT&T Bell Laboratories told the Frontiers symposium about his deep-space imaging project and the search for the dark matter that most astrophysicists now believe constitutes over 90 percent of all of the physical matter in the universe. If there really is 10 times as much invisible matter as that contained in luminous sources (and maybe even more), most theories about the evolution of the universe and its present structure and behavior will be greatly affected. Ever since the 1930s—soon after modern theories about the universe's evolution, now embraced as the Big Bang paradigm, were first proposed—scientists have been speculating about this ''missing mass or missing light problem," explained Margaret Geller from the Harvard-Smithsonian Center for Astrophysics.

The Big Bang model has been generally accepted for about three decades as the reigning view of the creation of the universe. The whimsical name Big Bang was coined by British cosmologist Fred Hoyle (ironically, one of the proponents of the competing steady-state theory of the universe) during a series of British Broadcasting Corporation radio programs in the late 1940s. The Big Bang hypothesis suggests that the universe—all of space and matter and time itself—was created in a single instant, some 10 billion to 20 billion years ago. This event remains a subject of central interest in modern cosmology and astrophysics, both because its complexities are not fully understood and also because the world ever since—according to the credo of science and the dictates of natural law—may be understood as its sequel.

The event defies description, even in figurative language, but physicists have written an incredibly precise scenario of what probably occurred. There was not so much an explosion as a sudden expansion, away from an infinitely dense point that contained all of the matter of the universe. Both space and time, as currently conceived, did not exist before this moment but were created as the very fabric of the expansion. As space expanded at unimaginable speed, the matter it contained was carried along with it. Today's scientists can observe the resonant echos of this seminal event in many ways. The most significant consequence of the Big Bang is that nearly all objects in the universe are moving away from one another as the expansion continues, and the farther they are apart, the faster they are receding.



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