Searching For Dark Matter Using Gravitational Lensing

A MATTER OF DARK UNDERSTANING - Part Two

Numerical simulation showing the distribution of dark matter in a large volume of the universe. The box shown spans a distance of about 1 billion light-years. The structures are displayed so that the brighter regions have a higher density (that is, more dark matter) than the darker regions.

The dark matter is concentrated into a web-like distribution of filaments that intersect at dense nodes where great clusters of galaxies are expected to form and become visible. At the rear of the cube (to the left), three blue disks represent three distant galaxies.

The yellow lines that cross the box represent light rays from those galaxies propagating through the universe. In the absence of intervening matter, the light would travel on straight lines but in the presence of matter, the paths of the rays are evidently deflected by the gravitational effects of the clumpy matter (the breaks in the yellow lines illustrate the light passing behind a clump of dark matter).

The light from a distant galaxy rarely encounters a clump of mass to strongly bend the light and cause an easily seen distortion. Instead the individual light rays suffer a series of small deflections such that an observer located at the front of the box (to the right), sees that the images of all the galaxies in some small patch of the sky, near to one of our test galaxies say, are all very slightly elongated in a common direction determined by the distribution of dark matter along that particular line of sight. This gravitational distortion is expected to be very small and requires a careful statistical treatment on many patches over the sky but has now been measured by the French team. Numerical simulation by S. Colombi IAP

Searching For Dark Matter Using Gravitational Lensing
Kamuela - March 7, 2000 - An international team of astronomers has obtained the first-ever glimpse of the distribution of dark matter over a large section of sky.

The team used images from the Canada-France-Hawaii Telescope's high-resolution wide-field imaging camera to analyze the light of 200,000 distant galaxies, looking for distortions caused by intervening dark matter.

The results give cosmologists their first clear window into the possible roles of dark matter in the evolution of the universe.

The 13-member team, based in France, was headed by Dr. Yannick Mellier of the Institut d'Astrophysique de Paris and the Observatoire de Paris. By bringing together researchers from France, Germany, Canada and the United States he was able to assemble the wide range of expertise -- cosmology, astrophysics, statistics, data analysis and instrument technology -- needed for the research.

The nature of dark matter is one of the greatest unsolved mysteries of modern science. While dark matter makes up at least 90% of the mass of the universe, both its composition and its distribution are unknown.

Knowledge of dark matter is, however, critical to understanding the evolution and fate of the universe.

"In cosmology we develop models to try and understand what processes underlie the evolution of the universe," explains Dr. Ludovic Van Waerbeke of the Canadian Institute for Theoretical Astrophysics in Toronto.

"We want to know, for example, how galaxies evolved, why we see great voids in space, what is causing galaxies to cluster in large filaments and sheets," he added.

Cosmologists also want models that predict the fate of the universe. At issue is whether the universe will expand forever, contract and collapse, or oscillate between expansion and contraction. But without a knowledge of dark matter, the major constituent of the universe, accurate models are difficult to build.

"To build cosmological models we need to have an idea of the total matter content of the universe," says Dr. Yannick Mellier, the team's leader.

"Since somewhere around 90% of his matter is invisible, it's hard for us to get a precise reading on this. Also, to test our models, to see if they accurately describe the universe, we need to look at the results of our simulations against what is actually out there, what astronomers really see."

But, says Mellier, up until now astronomers could see the distribution of only 10% of the matter in the universe, making it difficult to judge the accuracy of different models.

To determine the distribution of dark matter, Mellier's team used CFHT's wide-field imaging camera, CFH12K, to obtain high-resolution images of a two-square-degree section of sky (10 times the surface of the full moon).

Click For Part Two Of This Report

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