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New evidence that dark matter could be self-interacting

Colliding galaxies suggest dark matter interacts with forces other than gravity.

Abell 3827, imaged by the Hubble Space Telescope. The four main objects at the center are the galaxies discussed in the article. The strange pale blue objects surrounding them are lensed images of a much more distant background galaxy. The blue contour lines show the distribution of dark matter. The top-left galaxy of the four has dark matter clearly offset from the visible galaxy, perhaps implying lag due to friction, caused by self-interactions.

A new study examined the galaxy cluster Abell 3827 and found indications that dark matter could be self-interacting. If confirmed, this would mark a significant step forward in the ongoing quest to understand the substance that helps structure the Universe.
The team used the MUSE instrument on the Very Large Telescope (VLT) along with images from the Hubble Space Telescope to map out the cluster. Because large masses such as galaxies and galaxy clusters bend the paths of light, they act as lenses, a process called (surprise!) gravitational lensing. The team made use of the complex web of lensing effects throughout the cluster to map out the dark matter there. The presence of strong gravitational lensing is fortunate for the study, as the dark matter would be invisible without it.

Dark matter and tidal stripping

Every galaxy sits within a roughly spherical blob, called a halo, of dark matter. That halo makes up most of the galaxy’s mass. In normal situations this configuration is stable, but when multiple galaxies interact with each other, a process called tidal stripping can take place, in which gravity from one galaxy pulls in material from another. This can separate the dark matter from the stars in the galaxy.
What happens next depends on whether dark matter particles are self-interacting or not—that is, whether the individual dark matter particles are colliding with each other. It’s generally assumed, in the currently accepted model, that dark matter particles aren’t interacting with one another at significant levels, for the same reason that they’re not generally interacting with normal matter such as the Earth: the only known force that can affect dark matter is gravity. If the dark matter particles are colliding with one another, it means they’re susceptible to some force other than gravity.
“We used to think that dark matter just sits around, minding its own business, except for its gravitational pull,” said Richard Massey of Durham University, the paper’s lead author. “But if dark matter were being slowed down during this collision, it could be the first evidence for rich physics in the dark sector—the hidden Universe all around us.”
There are some minor observational problems with interaction-free dark matter, problems which could be solved if dark matter were self-interacting. A number of studies, including a recent one, saw unusual gamma ray signals that might be caused by dark matter collisions, though these results have been inconclusive.
Other observations have looked at lag effect in galaxy clusters and galaxy cluster collisions. If dark matter doesn't self-interact, then galaxy halos will pass through each other without incident. If there is some interaction, then the dark matter should slow down a bit—lagging behind the regular matter. So far, these searches have found nothing. The absence of obvious lag in the Bullet Cluster, one of the best-known dark matter observations, put a constraint on the degree of interaction—the “cross section” of dark matter particles—that occurs.

Constraints and a pot of gold

The cross-section is essentially a measure of how often the dark matter particles are colliding. The more they collide (or the higher the cross-section), the more a blob of dark matter should lag, due to the friction created by the colliding particles. In the Bullet Cluster observations, no lag was found, but the study was not precise enough to rule it out entirely. But the study did constrain it; if dark matter self-interactions were taking place, they weren't doing so that often (the cross-section had to be 1.25 square centimeters per gram or less).
Other studies have failed to achieve the precision necessary to rule out self-interactions with smaller cross sections. A recent study, performed by members of the same team as the present one, examined a number of colliding galaxy clusters and used a statistical analysis to look for the lag effect. They were able to further constrain the cross-section; the new maximum possible cross-section for dark matter was 0.47 square centimeters per gram, less than half of the original constraint.
The further down it’s constrained, the less likely it is that dark matter is self-interacting. It’s sort of like looking for the pot of gold at the end of the rainbow. If you could find the end of a rainbow and see no pot of gold there, you might say “well, maybe the leprechauns only store them at some rainbows.” As you search more and more rainbows, however, and find no gold, you could repeatedly refine your expectation—“Well, maybe less than 30 percent of rainbows have gold.” In this way, the possibility of pots of gold will never be completely ruled out, but with each failure to discover any, it becomes less and less likely there’s ever any gold at all.
Like a pot of gold, whatever you're looking for often remains enticing even as its likelihood diminishes. If the actual cross-section were just slightly smaller than the recent constraint of 0.47 square centimeters per gram, it would be just right to solve the observational issues with the leading dark matter model, which calls for a cross-section between 0.1 and one square centimeter per gram.

Abell 3827

 A wider view of Abell 3827, as taken by Hubble.

The galaxy cluster Abell 3827 is an ideal candidate to look for dark matter self-interactions. For one thing, its core (a region of radius about ten kiloparsecs) holds four almost equally bright galaxies (previously reported as five, but one of them turned out to be a star in the Milky Way that just happened to be in our line of sight to Abell). For another, there's a rich system of gravitational lenses woven throughout the cluster, enabling the team to map out the cluster’s dark matter. Even better, it’s located at a distance where we can effectively measure even small separations between stripped dark matter halos and their bright galaxies.
Once the team had mapped out the region, they discovered that one of the galaxies’ dark matter halo was offset from the visible galaxy’s position by a fair margin: about 5,000 light-years. Even a small cross-section (meaning very infrequent dark matter particle collisions) could produce a noticeable lag, like this one.
The other three galaxies are also separated from their dark matter, but it’s harder to make certain determinations of the distance relative to the bright portion, as those galaxies aren’t lined up as well with the background object being gravitationally lensed.
Further work is still needed to rule out other factors besides dark matter self-interactions as the cause of the observed lag. But under the assumption that self-interacting dark matter is the culprit, the team was able to determine the approximate cross-section: it’s about 0.00017 square centimeters per gram. A far cry, unfortunately, from the minimum 0.1 square centimeters per gram needed to resolve the issues with the current dark matter model, but present nonetheless.
That value, however, has some error, mainly because there’s a large uncertainty as to the time it’s taken these galaxies to reach the current stage in their collision. The friction that slowed the dark matter could have been a weak force acting over a very long time or a strong force acting over a shorter time.
It’s worth noting that this difference is also the reason that this study was able to get a better measurement of the cross-section than the statistical study mentioned earlier in this article. While that study looked at collisions of galaxy clusters, this one looks at collisions of individual galaxies within one cluster.


As noted, more work needs to be done to confirm these results and to firmly nail down the cross-section once and for all. “We know that dark matter exists because of the way that it interacts gravitationally, helping to shape the Universe, but we still know embarrassingly little about what dark matter actually is,” wrote Liliya Williams of the University of Minnesota, another of the paper’s authors. “Our observation suggests that dark matter might interact with forces other than gravity, meaning we could rule out some key theories about what dark matter might be.”
If nothing else, finding out that dark matter interacts by forces other than gravity would finally allay the fears of physicists—there's at least a chance that we could identify such a particle in Earth-based detectors.
Crucially, as noted by Williams, the result would also rule out certain dark matter candidates, continuing to narrow down the field of possibilities and slaying some of the monstrous creatures at the edges of the dark matter map.

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