Observing the pre-bang universe may sound like a hopeless task, but one form of radiation could survive from that epoch: gravitational radiation. These periodic variations in the gravitational field might be detected indirectly, by their effect on the polarization of the cosmic microwave background (simulated view, right), or directly, at ground-based observatories. The pre¿big bang and ekpyrotic scenarios predict more high-frequency gravitational waves and fewer low-frequency ones than do conventional models of inflation (chart). Existing measurements of various astronomical phenomena cannot distinguish among these models, but upcoming observations by the Planck satellite as well as the LIGO and VIRGO observatories should be able to.

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