How Cold Is it Out There?

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Science  07 Dec 2001:
Vol. 294, Issue 5549, pp. 2055-2057
DOI: 10.1126/science.294.5549.2055e

The standard cold dark matter (CDM) cosmology predicts that non-ordinary particles (such as cosmic strings, axions, or supersymmetry particles) would have low-velocity dispersions in the early universe and readily aggregate into galactic-sized clumps. Over time, these clumps would merge to create clusters and superclusters in a bottom-up approach to universe evolution. CDM cosmology thus specifies a large-scale structure for the universe and a small-scale structure for individual galaxies. Unfortunately, recent observations of the dynamics of luminous ordinary matter in galaxies do not lead to the predicted density structure of the unseen dark matter.

Keeton approaches this problem from a different direction by considering the number and sizes of observed gravitational lenses of elliptical galaxies. A gravitational lens can be used to determine the mass and density of the core of a galaxy on the basis of gravitational effects, independently from estimates based on observations of luminosity and galaxy dynamics. The lensing observations indicate that a CDM cosmology requires large-scale structures that are too dense and small-scale structures that are not dense enough, so something exotic, perhaps self-interacting dark matter or warmer dark matter, is needed to smooth out the density discrepancies and resolve the debate between enlightened observers and cold cosmologists.—LR

Astrophys. J.561, 46 (2001).

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