Observations by two powerful space telescopes have revealed that the mysterious stuff that makes up nearly 85 percent of the universe's total matter is weirder than we ever thought.
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By observing massive colliding galaxy clusters, astronomers have been able to deduce how dark matter behaves during these vast encounters. Until now, we've studied a handful of cluster smashups, only allowing us a snapshot of dark matter interactions.
But a new survey by the NASA/ESA Hubble Space Telescope and NASA's Chandra X-ray Observatory has focused on 72 galactic cluster collisions from all angles and at different times during their collisions. This has given us the unprecedented opportunity to see how dark matter interacts with itself over time.
And in results to be published on March 27 (Friday) in the journal Science, researchers have pieced together a chronology of sorts; using the series of cluster collisions to see how the interactions between dark matter clouds these clusters are known to contain.
"We know how gas and stars react to these cosmic crashes and where they emerge from the wreckage. Comparing how dark matter behaves can help us to narrow down what it actually is," said lead author David Harvey of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland.
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Dark matter's presence is known only by its interactions with normal matter through gravity. It does not, however, interact via the electromagnetic force, which is why we cannot directly see it - it does not emit, scatter or reflect light - it is more "invisible" than "dark."
In this new research, Harvey and his team realized just how invisible this stuff is, even to itself.
As two galactic clusters collide, the stars, gas and dark matter interact in different ways. The clouds of gas suffer drag, slow down and often stop, whereas the stars zip past one another, unless they collide - which is rare. On studying what happens to dark matter during these collisions, the researchers realized that, like stars, the colliding clouds of dark matter have little effect on one another.
Thought to be spread evenly throughout each cluster, it seems logical to assume that the clouds of dark matter would have a strong interaction - much like the colliding clouds of gas as the colliding dark matter particles should come into very close proximity. But rather than creating drag, the dark matter clouds slide through one another seamlessly.
"A previous study had seen similar behavior in the Bullet Cluster," said co-investigator Richard Massey of Durham University in the UK. "But it's difficult to interpret what you're seeing if you have just one example. Each collision takes hundreds of millions of years, so in a human lifetime we only get to see one freeze-frame from a single camera angle. Now that we have studied so many more collisions, we can start to piece together the full movie and better understand what is going on."
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Now we know that dark matter particles do not experience strong frictional forces when in close proximity to other dark matter particles, their properties can be further explored. Next, the researchers want to see whether there is evidence of dark matter particles bouncing off oneanother like billiar balls. Such a kinetic interaction could display scattering properties in these vast galactic cluster collisions. They also hope to see how dark matter acts when single galaxies collide - an event that occurs more frequently than galaxy cluster collisions.
"There are still several viable candidates for dark matter, so the game is not over, but we are getting nearer to an answer," said Harvey. "These ‘Astronomically Large' particle colliders are finally letting us glimpse the dark world all around us but just out of reach."