Singularities are the toxic waste of cosmology. Theories, let alone children, are well advised not to touch anything with an infinite density or temperature: the zero time of the big bang, say, or the very center of a black hole. At such places, physics dissolves into metaphysics. These mathematical points admit of no explanation; they just are. To dispose of them, cosmologists usually have opted for burial. For instance, cosmic inflation--the favored mechanism for how our universe expanded from the big bang--does not eliminate the primeval singularity but simply isolates it from today's universe.

Lately, though, a more thorough decontamination is becoming a viable option, especially with the maturing of string theory, physicists' best candidate for a theory of everything. Last fall cosmologists Paul Steinhardt of Princeton University and Neil Turok of the University of Cambridge, building on earlier work with Steinhardt's graduate student Justin Khoury and string theorists Burt A. Ovrut of the University of Pennsylvania and Nathan Seiberg of the Institute for Advanced Study in Princeton, proposed that the big bang is not a one-of-a-kind event but part of a recurring cycle. "What we're motivated by string theory to believe is that the big bang is not what we've always thought--a beginning of space and time, where temperature and energy diverge," Steinhardt says. "Rather it is a transition between the current expanding phase and a preexisting contracting phase."

   
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