For years scientists have struggled with the fact that based on current knowledge, there isn't enough matter in the universe to hold it together. Specifically, the rotation speed of many observed galaxies is too high for the amount of matter that they contain, and should, as a result, fly apart. But instead, they remain cohesive. As a result of this observation, two theories arose: one says that our understanding of gravity is flawed and the other speculates Dark Matter.
One group of scientists speculate that you can account for current state of the universe by speculating that gravity becomes stronger the futher away from a body of mass you get and also is stronger in weak fields. The model for this theory is called Modified Newtonian Dynamics. But this has been hard to reconcile with general relativity.
The other group speculates that the majority of matter in the universe does not interact with electromatgnetic radiation in the same way as ordinary matter does and therefore is undetectable by direct observation. This type of matter has been dubbed Dark Matter.
But if you can't detect the matter, how is it science? Science, as you may recall from my criticism of Intelligent Design, can only consist of theories that can be tested. So how does one go about finding Dark Matter?
Well Dark Matter may not be able to be seen but, according to the theory, it must have a gravitational effect or there would be no need to speculate its existence in the first place.
Recently astronomers cleverly observed the collision of two galaxies to observe the effects of Dark Matter and as a result claim to have the proof that the pervasive substance exists
NASA's Chandra X-ray Observatory, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes were used to observe the violent collision between two large galaxy clusters 3 billion light years away. The force of the collision separated the dark and luminous matter, allowing a clear identification. Although scientists are yet to determine what form this mysterious dark matter may take, the observations are strong evidence that most of the matter in the universe is dark (Astrophysical J. and Astrophysical J. Letters to be published).
It is clear that these new observations make it more difficult to justify continued work on modified gravitation theories.
“Regardless of how one modifies gravity, it should still generally point to where most of the mass is,” says Maxim Markevitch at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, who was involved in the research. “If the only matter in this cluster was visible matter, the mass map would approximately follow the interstellar gas map. Instead, we found most of the mass elsewhere, exactly where if it were dominated by collisionless dark matter.”
Behind these observations lies a remarkable bullet-shaped cloud of hot gas produced by the collision of two clusters. As they cross at 10 million miles per hour, the luminous matter in each interacts with the other and slows down. But the dark matter does not interact at all, passing right through without disruption. This causes the dark matter to sail ahead, separating each cluster into two components: dark matter in the lead and luminous matter lagging behind.
To detect this separation, researchers compared x-ray images of the luminous matter with measurements of the cluster's total mass through gravitational lensing. This involves the observation of the distortion of light from background galaxies by the cluster's gravity -- the greater the distortion, the more massive the cluster. The team discovered four separate clumps of matter: two large clumps of dark matter speeding away from the collision, and two smaller clumps of luminous matter trailing behind, proving two types of matter exist.
The results have captured the imagination of the cosmology community. “This is an exciting discovery -- dark matter is not merely a trick of the light,” says Robert Caldwell, who is a cosmologist at Dartmouth College, in New Hampshire. “This result helps confirm we're on the right track in trying to solve the mystery of dark matter.”
This observation is winning scientists over to the Dark (Matter) side
"I don't think there is any other way to work around it," said Marusa Bradac, one of the study's authors and a researcher at Stanford's Kavli Institute for Particle Astrophysics and Cosmology. "This is the clear-cut example where we do see there is something else in the system."
The bullet cluster formed when two different groupings of galaxies passed through each other. As the two galaxy clusters collided, the visible gas—which constitutes regular matter—slowed down as a result of frictional forces. But dark matter, which gives off no light or heat, is not subject to these same forces and would not have slowed down during the collision. The researchers hypothesized that if the colliding galaxy clusters contained dark matter, the visible and invisible matter would have separated as the dark matter traveled more rapidly past the collision site.
When the scientists measured the mass after the collision, they found that much of the bullet cluster's mass lay beyond the observable gas, where they had predicted the dark matter would be.
"Because normal and dark matter usually are closely intertwined in galaxies and clusters, it has always been difficult to distinguish unambiguously between the dark matter and the modified gravity paradigms," Maxim Markevitch, a Harvard University astronomer and another of the study's coauthors, wrote in an email. "This is the first time when dark matter and normal matter have been seen separated in space, and the reason is the energetic cluster collision."
Still skeptics remain
Mordehai Milgrom, the man who originally proposed Modified Newtonian dynamics (MOND) as an alternative to dark matter, said that just because the researchers didn't see all the matter they measured doesn't mean dark matter had to be present.
"Everyone knows there is still a lot of normal matter out there that we have yet to detect," he said via email. "While this is also 'dark,' it is not what we call the 'dark matter,' which people say has to be there. So it is definitely not a proof of the dark matter paradigm."
The new study's authors say their paper is not an attempt to disprove MOND, but insist that their results point directly to dark matter.
Science is best when debate is vigorous.
