Jonathan Nichols
San Francisco

According to Krauss and Scherrer, a direct implication of the accelerating expansion of the universe they describe is that three quarters of the energy in the universe is "dark"—in other words, a mystery. I have to question whether the authors have extrapolated the consequences of accelerating expansion beyond the point where it is reasonable to be so authoritative.

Steve Didcott
Great Missenden, England

KRAUSS REPLIES: Regarding Nichols's letter, in general relativity superluminal travel is possible, but one has to be careful about what one means. Special relativity implies that nothing can travel through space faster than light speed. General relativity, however, allows for the possibility that objects that are locally at rest in their background inertial frame can be separating from distant objects at greater than light speed because the space between these objects is expanding faster than light. There is no limit on the expansion rate of space. Even in a universe of non-accelerating expansion (that is big enough), there will always be objects receding from one another at greater than light speed. A Scientific American article by Charles H. Lineweaver and Tamara M. Davis addresses the subject at www.SciAm.com/jul2008.

As to Didcott's letter, it is true that one must be wary of extrapolations into new domains in physics. I should point out, however, that it is not simply the acceleration of the universe that implies dark energy. All the other observations in cosmology, particularly the estimates of big bang nucleosynthesis, the measurements of galaxy clustering, and the inference that the universe is flat (confirmed by observations of the cosmic microwave background), appear to be consistent only if we assume dark energy's existence. Because we do not know the nature of dark energy, we cannot say with any real certainty that the rest of the universe will disappear in the far future. But the possibility is fascinating, not least because it points out how dependent we are on our observations in framing conceptions of the cosmic past and future.

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