Dark Matter Ineffable

Just when the universe had barely begun to get us used to a new level of weirdness — what with the pervasiveness of dark energy, the anti-gravitational push that wasn’t even on the map until ten years ago — here comes a new discovery that seems to contradict even the little we thought we knew.

Even dark matter isn’t what it used to be. Credit: NASA/CXC/M.Weiss

The weirdness in this case is that of dark energy’s lugubrious pal, dark matter.
Last year, astrophysicists reported remarkable evidence for the existence of dark matter by observing the results of the collision of two clusters of galaxies, collectively known as the bullet cluster. (See Dark Matter Really Exists, the article I wrote about what I declared my favorite science story of 2006.)

In the bullet cluster, the collision had taken the clusters apart. While the intergalactic gas from the two clusters had bungled up together, the galaxies themselves had sped through each other, apparently unaffected. The galaxies’ halos of dark matter in that case had kept traveling along with the galaxies themselves. Dark matter is invisible, but its presence was revealed by how it curved space around itself, squeezing the images of galaxies in the background.
Dark matter has never been caught in the lab, and its nature is largely unknown. The fact that it has never been detected means that it tends to zip through ordinary matter without interacting. In the bullet cluster, scientists saw evidence that dark matter particles from one cluster also did not interact with those from the other cluster.
While the clouds of intergalactic gas from the two clusters were slowed down by mutual friction, the dark matter particles simply kept going, even though the dark matter clouds were probably denser than the gas ones. Dark matter can zip not only through ordinary matter, but also through dark matter itself.
Or not. The new mapping of dark matter, galaxies, and intergalactic gas around Abell 520, a “train wreck” of a galaxy cluster, announced today by NASA (see the NASA Web site) seems to throw everything into discussion. By imaging Abell with multiple methods, astrophysicists found a chaotic dismemberment that no theory seems to explain too well now.
In the artist’s illustration above,

the bulk of the matter (blue) is found compared to the individual galaxies (yellow) and the hot gas (red) in the aftermath of a massive galaxy cluster collision. The material shown in blue is dominated by dark matter. As with the Bullet Cluster there are large separation between the regions where the galaxies are most common (peaks 2 and 4) and where most of the hot gas lies (peak 3). However, unlike the Bullet Cluster, a concentration of dark matter is found (peak 3) near the bulk of the hot gas, where very few galaxies are located. In addition, there is an area (peak 5) where there are several galaxies but very little dark matter [courtesy of the X-ray observatory Chandra].

What this means for our understanding of dark matter, it is probably too early to say (I haven’t talked to any experts about this yet). But one thing is for sure: the universe is a bizarre place.

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