Image: Courtesy of John M. Pierre

STRINGS live in a 10-dimensional spacetime, which requires that every point in our four-dimensional universe is associated with a tiny ball of six-dimensional space. These tiny dimensions can come in the form of spheres or donuts, as illustrated above

"Science progresses by having a debate between alternatives," says cosmologist Abraham Loeb of Harvard University. "The problem in cosmology recently was all alternatives to inflation were out." String theory may be helping to change that, if the ekpyrotic model is any indicator. In fact, well before Steinhardt and Turok's work, it had already spawned one attempt to tame the big bang singularity.

In the early 1990s, Gabriele Veneziano of CERN in Geneva and Maurizio Gasperini, then of the University of Turin, began exploring a cosmology based on the idea that the tiny but finite size of strings set a limit on how tight the universe could have squeezed during the big bang singularity. This limit would have kept temperature and density from skyrocketing to infinity.

Running the tape of the cosmos back past such a stringy singularity revealed a universe growing increasingly flat and cold--reminiscent of what preceded the ekpyrotic model's bouncing branes. The pre-big bang state would have been inherently inflationary, not to mention neatly symmetric with the thinned out universe we have to look forward to thanks to dark energy.

Some physicists--including Steinhardt--were critical of the need for special initial conditions, but Veneziano says his enthusiasm was only cooled after more detailed microwave background data came in a few years ago, as the nature of the variations conflicted with what the model predicted. Recent work has revived his hope that this tension can be resolved, however. He adds, "I think that the theoretical objections to it were mostly stemming from people's difficulty in accepting such a radically different starting point." Steinhardt and Turok's work may be helping to change all that, he remarks.

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