Section 4: Dark Matter Bends Light
With three independent reasons to believe that dark matter existed—motion of galaxies, structure simulations, and temperature fluctuations in the cosmic microwave background—increasing numbers of physicists and astronomers turned their attention to trying to understand just what the dark matter is made of, and how it is distributed throughout the universe. Gravitational lensing proved a useful tool with which to probe the dark matter.
Quasars, lensing, and dark matter
Images of quasars gravitationally lensed by galaxies provide insight into the distribution of dark matter inside the lensing galaxies. Quasars are distant objects that emit huge amounts of light and other radiation. Since many quasars are visible behind galaxies, their light must pass through those intervening galaxies on the way to us. We know from general relativity theory that the matter in any galaxy—both normal and dark matter—bends space time. That bending distorts the image of any quasar whose light passes through a galaxy.
Figure 9: Gravitational lensing produces more than one image of distant quasars, as seen in this shot from the Hubble Space Telescope.
Source: © NASA/ESA Hubble Space Telescope, NASA/Goddard Space Flight Center. More info
In many cases, this lensing causes several images of the same quasar to appear in our telescopes. Careful measurements of the brightness of the different images of the quasar give hints about the distribution of the matter in the galaxy. Since the matter in each part of the galaxy determines the amount of bending of space time in that part of the galaxy, the brightness of the images tells us how matter, both normal and dark, is distributed. Optical measurements inform astronomers where the normal matter is. They can then use the brightness of the multiple quasar images to trace out the dark matter.
So far, astronomers have identified about 10 such lenses like this. Careful observations have shown that any clumps of dark matter in the galaxies must be smaller than about 3,000 light-years. More sensitive telescopes will find more lenses and will improve our understanding of how dark matter is distributed in galaxies.
Evidence from colliding clusters
Observing colliding galaxy clusters provides another useful way of understanding the nature of dark matter. When two clusters collide, the dark matter in one passes through the other unaffected; dark matter doesn't interact much with either itself or normal matter. But the normal matter in one cluster does interact with the dark matter and the normal matter in the other cluster, as well as with the dark matter in its own cluster. During the collision, the normal matter is dragged forward by the dark matter in its own cluster and dragged back by both the dark matter and normal matter in the other cluster. The net effect of the collision, therefore, is to cause the normal matter in each cluster to fall behind the dark matter in the same cluster.
Figure 10: X-ray and visible light images of the Bullet Cluster reveal strong evidence for the existence of dark matter.
Source: © X-ray: NASA, CXC, CfA, M. Markevitch et al.; Optical: NASA, STScI; Magellan, U. Arizona, D. Clowe et al.; Lensing Map: NASA, STScI; ESO WFI; Magellan, U. Arizona, D.Clowe et al. More info
Astronomers gained solid evidence of that scenario when they imaged a pair of colliding galaxy clusters named the Bullet Cluster in two ways: through its emission of visible light and x-rays. The collision between the normal matter in each subcluster heats up the normal matter, causing the colliding subclusters to emit x-rays. In 2004, NASA's orbiting Chandra x-ray observatory captured an x-ray image of the Bullet Cluster that gives the locations of the normal matter in the two subclusters. At the same time, the entire Bullet Cluster distorts the images of galaxies behind it through the gravitational lensing effect that we reviewed above in the context of quasars. By carefully measuring the shape of the distorted background galaxies, astronomers could determine the average position and mass of each of the subclusters. Since galaxy clusters contain a few times as much dark matter as normal matter, the lensing measurement gives the location of the dark matter, while the x-rays locate the normal matter. The image that combines both measurements shows that the dark matter has run ahead of the normal matter in both subclusters, confirming expectation.
The measurements of the Bullet Cluster were a blow to the MOND theories that we encountered earlier in this unit. Those theories predict no difference between the x-ray and lensing images. Some theorists have tried to modify the MOND approach in such a way that it accommodates the evidence from the Bullet Cluster and other observations, but the clear consensus of astronomers is that dark matter is a reality.
Dark matter in our galaxy
With gravitational lensing successfully being used to "weigh" entire galaxy clusters, the question arose whether it could be brought to bear more locally, to search for dark matter objects in the outer regions of our own Milky Way galaxy. The answer is a resounding yes. A clever gravitational lensing survey to search for clumps of dark matter in the halo of our galaxy began in 1992. The survey was designed to find MACHOs, or massive compact halo objects, which is a fancy term for "chunks of dark matter." It was initially thought that MACHOs would be failed stars or large, drifting planets—familiar objects that don't emit light—but the MACHO project was designed to be sensitive to any lump of dark matter with a mass between the Earth's mass and 10 times the Sun's mass.
Figure 11: This Australian telescope unsuccessfully sought evidence for the existence of MACHOs based on their putative effect on starlight.
Source: © The Australian National University. More info
The MACHO project used a telescope to monitor the light from stars just outside the Milky Way in a very small satellite galaxy called the "Large Magellanic Cloud." If a MACHO passes in front of one of these stars, the gravitational lensing effect predicted by Einstein's general theory of relativity and confirmed in 1979 will increase the measured flux of the starlight by a tiny amount. The Anglo-American-Australian MACHO Project used an automated telescope at Australia's Mount Stromlo Observatory to observe transits. None showed anywhere near enough change in the starlight to account for dark matter as consisting of faint stars or large planets.
A similar project, named "EROS" and run by the European Organisation for Astronomical Research in the Southern Hemisphere at Chile's La Silla Observatory, has had the same negative result. For example, a study of 7 million stars revealed only one possible MACHO transit; in theory, MACHOs would have produced 42 events. But physicists refused to give up the hunt. The SuperMACHO survey, a successor to the MACHO Project, used the 4-meter Victor M. Blanco telescope in Chile's Cerro Tololo Inter-American Observatory to monitor tens of millions of stars in the Large Magellanic Cloud in search of evidence that MACHOS exist. SuperMACHO also found that MACHOs cannot account for the vast amount of dark matter in the galaxy.
The astronomical evidence we have for dark matter ranges from within our galaxy to the farmost regions of space and time that we are able to probe. We now understand that dark matter dominates at the scale of galaxy clusters, normal matter dominates at the subgalactic scale, and they duke it out on the galactic scale. We know that dark matter gravitationally interacts with itself and normal matter, but we still do not know what the dark matter is.