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16.13 Dark Matter on the Largest Scales

Visible matter only makes up less than 5% of the universe. The rest of the cosmos is made up of dark matter and dark energy. In resent years, it has become possible for the first time to trace out the distribution of dark matter by observing how it bends light and changes object's centers of mass. The process of bending light is termed gravitational Lansing. Gravitational Lansing was a prediction of Einstein's theory of general relativity, a consequence of his premise that mass-energy alters the geometry of space-time. As a ray of light passes near a mass, its trajectory is bent toward the mass, just as the trajectory of a space craft is bent by the gravitation of a planet or moon that it passes near. As light from distant galaxies travels toward the Earth, where we observe it, the light passes near various concentrations of dark matter. The twists and turns gravity causes the light allows us to map the dark matter.

Different teams have taken high resolution images looking for this light bending in the light from distant galaxies. When averaged together, the shapes of galaxies on the sky should be circles or ellipses with no preferred direction. While one individual galaxy may be a cigar stretching from 2 to 8 on an imaginary clock, and another galaxy may be S-shaped, and another Z-shaped, with another being an edge on disk reaching from 11 to 5 on the same clock, put together all the galaxies fill in a circle. By looking at a collection of galaxies and measuring deviations from circular in these collections, the amount of intervening dark matter can be measured. This is done by determining what amount and distribution of dark matter is needed to make the observed deviations from circular. More importantly, by looking at deviations from circular in collections of galaxies at a variety of distances along a column of space, the 3-dimensional distribution of dark matter can be measured. The typical distortion to a galaxy shape is only a few tenths of a percent, so thousands of galaxies have to be studied to detect dark matter this way.

In general, a map of the three dimensional structure of dark matter shows that luminous matter traces out the densest regions of the dark matter, but it isn't a direct mapping — the density of luminous matter doesn't exactly scale with the density of dark matter, and their are places that are primarily dark matter. These are important results that allow theorists to better understand how structures may have grown in the early universe. It is now believed that dark matter may have begun to collapse into structures first, with the luminous matter gravitationally falling in afterwards.

Gravitational Lansing isn't our only evidence of the distribution of dark matter. In colliding galaxy clusters like the Bullet cluster, careful measurements of the distribution of luminous matter and of the gravitational potential have revealed that the center of mass of the colliding systems is not aligned with the center of mass of the visible mass. This indicates that the dark matter is able to keep moving at high velocities during the collision, undeterred by particle-on-particle collisions. This indicates that whatever particles makeup dark matter have very small cross-sections.

While these measurements don't allow us to determine the detailed distribution of dark matter within individual galaxies or non-interacting galaxy clusters, they have allows dozens of clusters to be "weighed$quoted; and the result confirm the dominance of dark matter over visible matter. Gravitational Lansing shows that dark matter exists on all scales in the universe, inside galaxies and beyond galaxies. In the past, there were competing theories that involved modifying Newtonian dynamics as a way of doing away with the need for dark matter. These theories added an additional term to gravity that at extremely large distances increased the strength of gravity. However, these alterations wouldn't cause the observed gravitational Lansing and are now largely ruled out.