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An excess of small-scale gravitational lenses observed in galaxy clusters

Gravitational lenses in galaxy clusters

The large mass of a cluster of galaxies deflects light from objects in the background, a phenomenon known as a gravitational lens. The large-scale gravitational lensing caused by the entire cluster can be changed by small-scale mass concentrations within the cluster, such as individual galaxies. Meneghetti et al. examined these small-scale gravitational lenses in observations of 11 galaxy clusters. They found tiny lenses an order of magnitude smaller than would be expected from cosmological simulations. The authors conclude that there is an unidentified problem with the prevailing simulation methods or with standard cosmology.

Science, this number p. 1



Cold dark matter (CDM) makes up the majority of matter in the universe. The interaction between dark and luminous matter in dense cosmic environments, such as galaxy clusters, is theoretically studied using cosmological simulations. Observations of gravitational lenses are used to characterize the properties of substructures – the small-scale distribution of dark matter – in clusters. We derive a metric, the probability of strong lens events produced by the dark matter substructure, and calculate it for 11 clusters of galaxies. The observed cluster substructures are slower more efficient than CDM simulations predicted, by over an order of magnitude. We suggest that systematic problems with simulations or wrong assumptions about the properties of dark matter could explain our results.

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