Gravitational lensing is tricky enough to measure, but how can we use it to track down the elusive ‘dark matter’ that constitutes the great bulk of the matter in the universe? Remarkably, researchers at Johns Hopkins, working with the Space Telescope Science Institute, think they have found a way. Using the Hubble telescope, they’ve measured how gravity from unseen dark matter creates small distortions in the shapes of galaxies as seen from Earth. Their work has focused on two galactic clusters in the southern sky roughly 7 billion light years away; each contains more than 400 galaxies.
That dark matter is a mystery needs no elaboration here, as it’s always been a reminder that our knowledge of the universe is limited to a small subset of the things we can see and understand. Indeed, dark matter is only part of the story. Some 70 percent of the entire universe is now thought to be ‘dark energy,’ an even more mysterious ingredient that plays a role in the expansion of the cosmos. With another 25 percent of all things being tied up in dark matter, that leaves the normal, visible matter we take for granted in things like water, trees, stars and galaxies as merely five percent of everything that is out there.
Dark matter is thus a confounding challenge to our everyday perceptions, but one slowly yielding to analysis. The current dark matter work proceeds on a key assumption: that visible matter and dark matter should coalesce at the same places because gravity pulls them together. In other words, concentrated dark matter should attract visible matter, and thus have a role in the formation of stars and galaxies themselves. The team’s work has resulted in computer-simulated images showing the location of dark matter in relation to the galactic clusters under study.
One finding is that dark matter clumps around the cluster galaxies, which implies that dark matter particles do not collide and scatter, but actually pass through each other. From a Johns Hopkins news release, quoting Myungkook James Jee, an assistant research scientist at Hopkins’ Krieger School of Arts and Sciences:
“Collision-less particles do not bombard one another, the way two hydrogen atoms do. If dark matter particles were collisional, we would observe a much smoother distribution of dark matter, without any small-scale clumpy structures,” Jee said.
Jee goes on to say “The images we took show clearly that the cluster galaxies are located at the densest regions of the dark matter haloes, which are rendered in purple in our images,” an observation that supports the association of dark and visible matter. In such ways do we learn how to study a form of matter that emits no light, and whose composition and effects may have much to say about how the visible universe around us formed.
The team’s findings appear as Jee, White, Ford et al., “Hubble Space Telescope Advanced Camera for Surveys Weak-Lensing and Chandra X-Ray Studies of the High-Redshift Cluster MS 1054-0321,” Astrophysical Journal 634 (December 1, 2005). An abstract is available here. 12/14 update: Cosmic Variance has been carrying on an interesting discussion on these findings.
