Dark matter may actually interact with itself

Chuck Bednar for redOrbit.com – @BednarChuck

By observing colliding galaxies with the ESO’s Very Large Telescope (VLT) and the Hubble Space Telescope, astronomers have found evidence suggesting for the first time that dark matter may interact with other dark matter in a way other than through gravity.

The discovery, which was reported Wednesday in the Monthly Notices of the Royal Astronomical Society, appears to contract what we already known about the mysterious substance that accounts for 85 percent of the mass of the universe and suggests that it may not be invisible after all.

Possible non-gravitational dark matter interactions found

Dr. Richard Massey, a research fellow at the Durham University Department of Physics as well as a member of the Centre for Extragalactic Astronomy, and his colleagues used Hubble and the VLT’s MUSE instrument to get an up-close look at four colliding galaxies in the cluster Abell 3827, which Discovery News said is a common “hunting ground” for gravitational lenses.

While dark matter cannot be directly seen, its location can be deduced using the technique of gravitational lensing, which occurs when its gravitational pull bends light from distant galaxies. As it happens, the collision they observed took place directly in front of a distant and unrelated source, and it allowed them to both pinpoint the location of the mass in the system and compare the distribution of the dark matter with the position of the luminous galaxies.

“Although dark matter is invisible, we can map it using a effect first predicted by Albert Einstein and known as gravitational lensing, Looking past something heavy (like dark matter) is similar to looking through a bathroom window,” Dr. Massey told redOrbit via email. “Although the glass is transparent, you can tell it is there because street lights over the road appear distorted.”

“By calculating the extent of the distortion, it’s possible to work out the thickness of the glass,” he added. “We do the same with very distant galaxies (6 or 7 times further away than the ones we’re interested in). The distant images appear distorted, and we can work out where the dark mass was along our line of sight, to cause that distortion.”

Even if you remove the gravitational influence of the visible galaxies, there is still a tremendous gravitational component remaining, which allowed Dr. Massey’s team to accurately measure the quantities and locations of dark matter clouds within the cluster. Using this method to map out the four colliding galaxies, they determined that the dark matter associated with each galaxy has a lag of approximately 5,000 light-years behind the normal matter there.

So exactly what’s causing that lag to happen?

In their study, the researchers report that it is likely caused by some type of interaction between the galactic halos of dark matter contained within those colliding galaxies. As they collide, the visible matter within the galaxies interact as expected, but there appears to be some sort of drag effect impacting the dark matter halos and causing the drag, according to Discovery News.

Dr. Massey explained that astronomers had previously believed that dark matter remains in one place and has no influence on its surroundings other than its gravitational pull. However, if dark matter is being slowed down during the collisions, it would provide the first-ever evidence for rich physics in the hidden parts of the universe known as the dark sector. Additional research is required to rule out other possible causes of this lag, however, he added.

This potential dark matter interaction appears to contract a recent survey of 72 galaxy cluster collisions which found little to no self-interaction amongst dark matter. However, the new study looks at individual colliding galaxies, not entire clusters, and these collisions have likely lasted far longer than those observed in the previous study, increasing the observed lag effects.

Dr. Massey explained to redOrbit that the two papers tell two parts of the same story. While the previous results found that dark matter interacts “very little – in particular units, by a value lower than 0.5,” the professor said that the new result “is that it interacts very very little, but not zero. In the same units, it is a value bigger than 0.001.”

“There have been lots of negative results over the past decade, saying that dark matter interacts less than this,” he said via email. “Our previous result dropped the bar a lot further, but all these results were fundamentally saying that we see dark matter being boring.”

“This is the first time we’ve caught dark matter in the act of doing something interesting,” Dr. Massey added. “I’m excited to finally to give a positive result because there is a HUGE difference between zero interactions (as the old ‘Cold Dark Matter’ theory would suggest), and nonzero (albeit tiny) interactions. Once the dark Universe around us is allowed to interact at all, the possibilities for what it could get up to are rich and varied.”

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Source: http://www.redorbit.com/news/space/1113372691/dark-matter-may-actually-interact-with-itself-041615/