How Slippery Is Dark Matter?

Dozens of galaxy cluster collisions confirm that dark matter particles probably slip right past each other within messy cluster mergers.

The existence of dark matter would solve many problems in our universe: it enables spiral galaxies to rotate quickly at their outer edges, keeps galaxy clusters from flying apart, and forms the universe’s cosmic web structure.

Galaxy clusters from Hubble and Chandra

This collage shows six of the 30 galaxy cluster collisions studied by Harvey and colleagues. They found that dark matter (blue), mapped by gravitational lensing, and galaxies alike slip right past one another. The tenuous X-ray-emitting gas (pink) collides, offsetting it from the dark matter and galaxies.

But dark matter, or at least our understanding of it, creates its own set of problems. Simulations show that dark matter pools in gravitational wells, creating galaxies and clusters of galaxies with far denser cores than observed. That pooling happens when dark matter particles are slippery. Weakly interacting massive particles (WIMPs), physicists’ most popular dark particle, are downright antisocial — they glide right past one another rather than interacting.

One way to overcome the pooling problem is to make dark matter less slippery. In alternative models, so-called “hidden-sector” dark matter particles make contact, says Jonathan Feng (University of California, Irvine). Just like people at a party, he explains, particles that constantly bump into each other tend to spread out, rather than all congregating in the kitchen. Many of these models have the added benefit of explaining a mysterious X-ray spectral line seen in some galaxies and galaxy clusters.

Perfectly slippery dark matter particles should have a self-interaction cross section (a measure of how strongly they interact, measured in square centimeters per gram) of 0. Hidden-sector models propose cross-sections that range between 0.1 and 10.

But now these sticky dark matter models face a challenge. David Harvey (Observatoire de Sauverny, Switzerland, and University of Edingburgh, UK) and colleagues studied 30 galaxy clusters and found that dark matter particles appear to be quite slippery, with a cross section no higher than 0.47 cm2/g.

When Clusters Collide

Bullet Cluster

The Bullet Cluster is the textbook example of merging galaxy clusters. Hot gas (pink) sloshes around the galaxies (red, green, blue) that are anchored in dark matter (blue).
X-ray: NASA / CXC / CfA / M.Markevitch et al.; Lensing Map: NASA / STScI / ESO WFI / Magellan / U.Arizona / D.Clowe et al.; Optical: NASA / STScI / Magellan / U.Arizona / D.Clowe et al.

Astronomers can indirectly measure the stickiness of dark matter by looking at galaxy cluster collisions. When clusters collide, most galaxies sweep right past one another — space is after all very, very empty. But the tenuous gas halos surrounding the galaxies crash into each other and produce beautiful and complex X-ray emission.

If it’s slippery, dark matter will stay closely aligned with the galaxies. (The dark matter isn’t seen directly, but by its gravitational distortion of background light.) But if dark matter particles interact with one another, they’ll do one of two things: the dark mass will slow down, lagging behind the galaxies’ motion, or the dark particles will scatter, displacing the dark mass from the galaxies.

By far the best-studied cluster collision, the Bullet Cluster has already provided one estimate on dark matter slipperiness — observations limit the cross section to less than 1.25 cm2/g, which still leaves plenty of wiggle room for hidden-sector theories. Observations of other clusters haven’t narrowed the possibilities, mostly because the 3D geometry of the systems is difficult to understand.

So Harvey’s team set out to take a statistical approach instead, averaging out the measurements of lots of galaxy clusters to do away with any 3D uncertainty. Drawing from the archives of the Hubble Space Telescope and Chandra X-ray Observatory, Harvey and his colleagues looked at 30 nearby galaxy clusters. In each cluster, the team measured the offset between the hot gas (observed in X-rays) and the galaxies (seen in the visible-light images), which gives the direction of the collision.

Then the team measured the offset between the galaxies and the dark mass, which they pinpointed thanks to its gravitational lensing effect on background light. The astronomers found that across 30 clusters, the offset was essentially zero. Dark matter must be slippery indeed, with a cross section less than 0.47.

The Fine Print

Abell 520, the Train Wreck galaxy cluster

Abell 520, the Train Wreck cluster, at one point appeared to show hints of "sticky" dark matter. Starlight (orange) has been smoothed to show the location of most galaxies. Dark matter (blue) was mapped by weak gravitational lensing.
NASA / ESA / CFHT / CXO / M.J. Jee / A. Mahdavi

There are limitations to this approach, says Douglas Clowe (Ohio University). For one, it’s all archival data, collected for disparate science goals. So Hubble imaged some clusters through multiple filters and some through only a single filter. To level the playing field, the team opted to limit their analysis to a single filter for all the clusters. But that makes it difficult to separate cluster galaxies from foreground and background galaxies. While necessary, the decision added uncertainty to their measurements.

Nevertheless, Clowe adds, “Within the limitations of their data, they have produced an excellent analysis.” In addition to improving on the Bullet Cluster measurements, he says, these types of statistical studies, which pull in huge amounts of information on dozens of galaxy clusters, show what we can look forward to in five to ten years, when next-gen telescopes such as WFIRST, LSST, and Euclid come online.

But don’t count out hidden-sector models just yet, says Feng. Even with the tighter limit from Harvey’s study, these models have enough wiggle room to remain viable. “The really interesting message here,” Feng adds, “is that these observations are getting tighter and tighter in a very interesting region of parameter space.” In the near future, studies like this one might rule out self-interacting dark matter . . . or better yet, find evidence for its existence.

7 thoughts on “How Slippery Is Dark Matter?

  1. Mike-Cavedon

    There is evidence of dark matter every time a double slit experiment is performed; it’s what waves.

    Dark matter has mass. Dark matter physically occupies three dimensional space. Dark matter is physically displaced by the particles of matter which exist in it and move through it.

    The Milky Way’s halo is not a clump of dark matter anchored to the Milky Way. The Milky Way is moving through and displacing the dark matter.

    The Milky Way’s halo is the state of displacement of the dark matter.

    The Milky Way’s halo is the deformation of spacetime.

    What is referred to geometrically as the deformation of spacetime physically exists in nature as the state of displacement of the dark matter.

    A moving particle has an associated dark matter displacement wave. In a double slit experiment the particle travels through a single slit and the associated wave in the dark matter passes through both.

    Q. Why is the particle always detected traveling through a single slit in a double slit experiment?
    A. The particle always travels through a single slit. It is the associated wave in the dark matter which passes through both.

    What ripples when galaxy clusters collide is what waves in a double slit experiment; the dark matter.

    Einstein’s gravitational wave is de Broglie’s wave of wave-particle duality; both are waves in the dark matter.

    Dark matter displaced by matter relates general relativity and quantum mechanics.

  2. ground-potential

    It’s always a worry when we have to invent a new substance to explain a theory, that’s why I turned my attention to the rotation curves of Galaxies, and sure enough I soon discovered that the observed rotation curves are perfectly normal, and there was no need to postulate additional matter. Here is a brief explanation on my forum explaining why rotation curves look the way they do.

    Steven Sesselmann

  3. David CorlissDavid Corliss

    In the article, we read “Weakly interacting massive particles (WIMPs), physicists’ most popular dark particle” – correctly stated. However, the article goes on to use “dark matter” as if WIMPs were the only type. For example, the authors concludes with “In the near future, studies like this one might rule out self-interacting dark matter . . . or better yet, find evidence for its existence.” Evidence for the discovery of WIMPs, you mean , not any dark matter. Another type of dark matter – non-luminous baryonic matter, such as planets and dust – is very well known to exist. Here the question is an entirely different one: how can there possibly be enough to explain what we see in observations of galaxies? It’s a good question.

    The point here is that the author uses WIMPs as if they were the only possible object or hypothesis in the category of dark matter. They are not.

    David Corliss

  4. Jim-BaughmanJim-Baughman

    Sorry, but the hypothesis that dark matter particles, which supposedly provide the bulk of the gravitational force in galaxies, can “glide right past one another rather than interacting” is beyond absurd. If dark matter is the source of powerful gravity, by this very definition they cannot just flit by one another heedless of gravity’s effects. If they could we would have a weird particle indeed, one that can switch its gravitational powers on and off at will, apparently. As I said—absurd.

    What is needed is an entirely new theory that approaches the gravitational anomalies seen in galaxies and galaxy clusters from an entirely new direction, much as Einstein approached the anomalies in Mercury’s orbit from an entirely new path and came up with relativity. Time to send “dark matter” to the showers.

    1. Mike-Cavedon

      The notion that dark matter is a clump of stuff traveling along with the matter is incorrect.

      ‘Empty’ space has mass which physically occupies three dimensional space and is physically displaced by the particles of matter which exist in it and move through it.

      The Milky Way is moving through and displacing the mass which fills ’empty’ space.

      The Milky Way’s halo is the state of displacement of the mass which fills ’empty’ space.

      The state of displacement of the mass that fills ’empty’ space is the deformation of spacetime.

      The Milky Way’s halo is the deformation of spacetime.

  5. PhxMarkER

    “Dark matter is a hypothetical kind of matter that cannot be seen with telescopes but accounts for most of the matter in the universe. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.”
    Dark matter – Wikipedia, the free encyclopedia

    Old School Above/\ As Above/\ Answers Here/\
    New School Below\/ So Below\/ More complete Answers Here\/

    The Surfer, OM-IV.

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