Light-bending gravity helps astronomers 'see' dark matter

Astronomers are overcoming a basic frustration - how to "see" the so-called dark matter that constitutes most of the mass of our universe. Dark matter emits no light or other radiation. It makes itself known by the way its gravity helps stars, dust, and gas form galaxies and helps galaxies group together in clusters.

Now gravity is making the invisible visible.

It works like this: Material mass warps space, and light from background objects passing through such warped space is focused to form two or more images of the background objects.

Astronomers have used this so-called gravitational lens effect to study distant galaxies and other dim objects. Lensing can focus light from the more distant objects so the resulting images are bright enough for earth-based telescopes to see. Now astronomers are beginning to use this effect to study the lensing mass itself.

The fact that a gravitational lens is at work reveals the presence of dark matter. There's not enough visible matter in a lensing galaxy or galaxy cluster to do the job. It's like a TV drama featuring invisible aliens. The invaders give themselves away when their see-through bodies distort the background landscape.

David Wittman with Lucent Technologies Bell Labs in Murray Hill, N.J., has reported that he and his colleagues are using gravitational lensing to begin building a 3-D map of the distribution of dark matter in the universe. The way a lensing mass distorts light from background objects holds clues to the distance to that mass, which may be a cluster of galaxies.

Dr. Wittman explains: "This lets us locate the cluster in its proper place in three-dimensional space, all without resorting to studying its light. This is important because most of the mass in the universe is dark."

Dark matter may be a novel form of material particles. Some of it may be dark planets or dead stars. A US-Australian team has been looking for the latter objects in the halo (outer part) of our Milky Way galaxy. They call them Massive Compact Halo Objects (MACHOS).

When a MACHO passes in front of a background star, gravitational lensing should brighten the star's image and reveal the MACHO's presence.

The team recently reported it now has found as many as 40 possible MACHOs in a survey that covered 73 million background stars.

Mapping the distribution of dark matter won't, in itself, reveal what dark matter really is. That's another line of research. Meanwhile, the data gathered on those 73 million stars is a major resource for all astronomers who work on stellar structure and evolution.

University of Pennsylvania astronomer Charles Alcock, leader of the US MACHO team, explains that "the combination of large databases and computerized tools [is] speeding scientific discovery in all fields, and we wanted to expand this capability for astronomers."