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Two thousand years after Lucretius proclaimed that nothing can be created from nothing, inflationary cosmology asserts that he was wrong.

WHEN AN OBSCURE RUSSIAN METEOROLOGIST named Alexander Friedmann proposed, in 1922, that the Universe might be expanding, Albert Einstein was sure that he was wrong. Five years earlier Einstein had published a static model of the Universe, and he was still convinced that it was correct. In a rare but dramatic blunder, Einstein bolstered his unfounded beliefs with an erroneous calculation, and fired off a note to the Zeitschrift fur Physik claiming that Friedmann's theory violated the conservation of energy. Eight months later, however, after a visit from a colleague of Friedmann's, Einstein admitted his mistake and published a retraction. The equations of general relativity do, he conceded, allow for the possibility of an expanding universe.

Today the Big Bang theory, which began with Friedmann's calculations in 1922, has become the accepted view of cosmology. The expansion of the Universe was first observed in the early 1920s by Vesto Melvin Slipher, and in 1929 was codified by Edwin Hubble into what we now know as "Hubble's Law": on average, each distant galaxy is receding from us with a velocity that is proportional to its distance. In 1965 Arno Penzias and Robert Wilson detected a background of microwave radiation arriving at Earth from all directions - the afterglow of the primordial hot, dense fireball. Today we know, based on data from the Cosmic Background Explorer (COBE) satellite (see Beam Line, Vol. 23, No. 3, Fall/Winter 1993), that the spectrum of this background radiation agrees with exquisite precision - to 1/30 of 1 percent - with the thermal spectrum expected for the glow of hot matter in the early Universe. In addition, calculations of nucleosynthesis in the early universe show that the Big Bang theory can correctly account for the cosmic abundance of the light nuclear isotopes: hydrogen, deuterium, helium-3, helium-4, and lithium-7. (Heavier elements, we believe, were synthesized much later, in the interior of stars, and were then explosively ejected into interstellar space.)

Despite the striking successes of the Big Bang theory, there is good reason to believe that the theory in its traditional form is incomplete. Although it is called the "Big Bang theory," it is not really the theory of a bang at all. It is only the theory of the aftermath of a bang. It elegantly describes how the early Universe expanded and cooled, and how matter clumped to form galaxies and stars. But the theory says nothing about the underlying physics of the primordial explosion. It gives not even a clue about what banged, what caused it to bang, or what happened before it banged. The inflationary universe theory, on the other hand, is a description of the bang itself, and provides plausible answers to these questions and more.

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