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article imageDark matter is a fluid — Its particles aren't particles at all

By Stephen Morgan     Mar 27, 2015 in Science
The mystery of dark matter just got more mysterious. The latest research into the effects on dark matter of galaxy collisions, suggests that it may not be made up of particles, but is, instead, a fluid-like substance.
Dark matter is invisible, because it neither absorbs or emits light like other matter in the universe does. We can only see its effects, because it bends space-time in a similar way to gravity.
Quoted in the Mail Online, Dr Richard Massey from Durham University's Institute for Computational Cosmology, who co-led the new study, explained this in a very simple and clear way,
"We can’t observe wind, but you know it’s there because leaves are floating around," he said
"Dark matter is similar, we can’t see it, but it pulls things around a bit. "
(Dark matter shouldn't be confused with dark energy, which has the opposite effect to the gravitational-like pull of dark matter on cosmic bodies. Instead, dark energy pushes matter apart and is responsible for accelerating the expansion of the universe.)
In order for dark matter to have an attractive power towards other objects – like visible matter does – scientists have assumed that it was also made of some form of particles, even if we couldn't see them.
However, the latest research into galaxy collisions seems to debunk previous theories about dark matter. The research suggests that it is not made of particles at all, but a fluid-like substance, capable of passing straight through other matter and even of passing through itself.
Normally speaking, subatomic particles exchange momentum when they interact. Consequently, galactic collisions should also lead to interaction by dark matter particles and by studying it, we would have a better idea of what dark matter really is.
To verify whether dark matter is made up of particles, the researchers explored two possible outcomes. The first, was that particles of dark matter often interact, but don't exchange much momentum. The second was that they seldom interact, but exchange a lot of momentum.
If the first case was true, dark matter would slow down after the collision, since the greater frequency of the interactions would have drag-like effect. If the second case was proven to be true, then dark matter would likely be dispersed into space.
Surprisingly, the study discovered that the dark matter particles from two colliding galaxies, simply pass through each other, suggesting that dark matter particles do not interact with themselves, but have a more fluid-like character instead.
Science Daily explained,
"The team found that, like the stars, the dark matter continued straight through the violent collisions without slowing down. However, unlike in the case of the stars, this is not because the dark matter is far away from other dark matter during the collisions. The leading theory is that dark matter is spread evenly throughout the galaxy clusters so dark matter particles frequently get very close to each other. The reason the dark matter doesn't slow down is because not only does it not interact with visible particles, it also interacts even less with other dark matter than previously thought."
Phys.org quotes David Harvey, co-author of the present study at EPFL's Laboratory of Astrophysics.
"The study challenges the view that dark matter consists of proton-like particles - or perhaps any particles whatsoever," he said.
"We have now pushed the probability of two 'dark matter particles' interacting below the probability of two actual protons interacting, which means that dark matter is unlikely to consist of just 'dark-protons'. If it did, we would expect to see them 'bounce' off each other."
However, Harvey added that,
"There are still several viable candidates for dark matter, so the game is not over, but we are getting nearer to an answer."
The team studied observations from the NASA/ESA Hubble Space Telescope and NASA's Chandra X-ray Observatory involving 72 collisions. They plan to continue their research with the help of the Large Hadron Collider (LHC) in Cern.
Their results have been published in the journal Science on 27 March 2015
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