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Nature 422, 26-27 (6 March 2003) | doi:10.1038/422026a

Open Innovation Challenges

Cosmology: Filling in the background

Sean Carroll

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Data from NASA's Wilkinson Microwave Anisotropy Probe reveal the cosmic microwave background in more detail than ever before. But will cosmology become a victim of its own success?

Not so long ago, cosmology was, half-jokingly, thought of as "a search for two numbers". The numbers in question were the Hubble constant, which measures the rate at which the Universe is expanding, and the energy density, measured in terms of the critical density for which the Universe is flat (that is, when space has zero curvature).