On a fine scale, the Universe seems lumpier than it should be.
Ars Technica said:While the idea of dark matter was originally proposed to explain the structure of galaxies, one of its great successes was explaining the nature of the Universe itself. Features of the Cosmic Microwave Background can be explained by the presence of dark matter. And models of the early Universe produce galaxies and galaxy clusters by building on structures formed by dark matter. The fact that these models get the big picture so right has been a strong argument in their favor.
But a new study suggests that the same models get the details wrong—by an entire order of magnitude. The people behind the study suggest that either there's something wrong with the models, or our understanding of dark matter may need an adjustment.
Under a lens
The new study, performed by an international team of researchers, took advantage of a phenomenon called gravitational lensing. Gravity warps space itself, and it can do so in a way that bends light, analogous to a lens. If a massive object—say, a galaxy—sits between us and a distant object, it can create a gravitational lens that magnifies or distorts the distant object. Depending on the precise details of how the objects are arranged, the results can be anything from a simple magnification to circular rings or having the object appear multiple times.
Because dark matter's effects are detectable via gravity, we can "see" the presence of dark matter via its gravitational-lensing effects. In a few cases, we've even detected lensing where little matter is present. That's one of the many pieces of evidence in favor of dark matter.
The researchers used gravitational lensing to set up a test that, at least conceptually, was very simple. We've built models of the early Universe that indicate how dark matter helped structure the first galaxies and drew them into clusters of galaxies. These models, when run forward, provide a description of what that dark matter distribution should look like at different points in the Universe's history up to the present. So the researchers decided to use gravitational lensing to determine whether the dark matter distribution seen in the models matched where we see it via gravitational lensing.
According to these models, the Universe was built hierarchically. Via gravitational interactions with itself, dark matter formed filaments that intersected in a complex, three-dimensional meshwork. The additional gravitational pull at the points where filaments intersected would draw in regular matter, leading to the first galaxies. Over time, the continued draw of gravity pulled galaxies together, forming large clusters. By examining the output of these models, we can get a look at the expected distribution of dark matter around clusters. And by zooming in, we can see how dark matter should be distributed in the area of individual galaxies.
That distribution of dark matter can be viewed as a prediction of the models.
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