The ‘concordance’ model of the universe suggests that it is mostly composed of dark matter. In fact, a cluster of hundreds of galaxies would, in the concordance scenario, house fully 90 percent of its mass in dark matter. Never mind that we know almost nothing about dark matter — theorists have still been able to simulate how it would clump together, forming an intricate substructure that should be capable of observation.
Now a Yale astronomer and her colleagues have used gravitational lensing to study galactic clusters, finding a tight fit between the concordance model’s predictions and their observations. A gravitational lens might be a galaxy that intervenes between us and a more distant object, focusing the light from that object so that we see things we would not otherwise be able to observe.
Says assistant professor of astronomy and physics Priyamvada Natarajan:
“We used an innovative technique to pick up the effect of precisely the clumps which might otherwise be obscured by the presence of more massive structures. When we compared our results with theoretical expectations of the concordance model, we found extremely good agreement, suggesting that the model passes the substructure test for the mass range we are sensitive to with this technique.”
Do these clumps provide the insights we’ll need to understand what dark matter is? If so, expect to hear a great deal more about them, and soon. Natarajan’s work appeared as “Substructure in Clusters of Galaxies,” Astrophysical Journal Letters 617: L13-L16 (December 10, 2004). Also see “Quantifying substructure using galaxy-galaxy lensing in distant clusters,” available here (PDF warning). Other articles on substructure in galactic clusters and gravitation lensing can be found at her Yale Web page.
Centauri Dreams‘ take: gravitational lensing is rapidly becoming a key tool for distant observations, and we’ve seen a variant of it called ‘microlensing’ suggested as a possible way to detect planets around stars in the galactic core. A mission to the gravitational lensing point some 550 AU from the Sun will one day allow us to use our own star as a gravitational lens. Gregory Matloff reviews gravity lens mission concepts — including a 60-year mission of his own design — in his book Deep Space Probes (Chichester, UK: Springer-Praxis, 2000), pp. 27-29.
