by Playfuls Staff |
22nd August 2006
Astronomers have now discovered the long expected proof that the mysterious form of matter which is responsible for the expansion of our Universe exists.[more]
Up until now, scientists could only speculate based on other observations that the dark matter fills the “empty space” and pushes the “borders” of our Universe furthermore.
Its presence is inferred indirectly from the motions of astronomical objects, specifically stellar, galactic, and galaxy cluster/supercluster observations. It is also required in order to enable gravity to amplify the small fluctuations in the Cosmic Microwave Background enough to form the large-scale structures that we see in the universe today.
Dark matter - which does not emit or reflect enough light to be "seen" - is thought to make up 25% of the Universe. By comparison, the “usual” matter we can see is believed to make up no more than about 5% of our Universe.
The term was invented to account for the fact that despite not having enough mass to hold themselves together under their own gravity, galaxies still spectacularly failed to tear themselves apart. Astronomers reasoned that something invisible, but massive, must be holding things together. Hence, dark matter.
A team of US researchers are now boasting with the discovery of what is effectively known as the gravitational signature of dark matter.
This type of astronomical signature was created by dark matter and ordinary matter being wrenched apart by the immense collision of two large galaxy clusters, more than 3 billion light years away from us.
"The kinetic energy of this collision is...enough to completely evaporate and pulverise planet Earth ten trillion trillion times over," said team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, US.
Study leader Doug Clowe, from the University of Arizona, said: "This provides the first direct proof that dark matter must exist and that it must make up the majority of the matter in the Universe."
The finding will have potentially great impact on an active debate among physicists and cosmologists about not only dark matter, but the workings of gravity. Indeed, the theory of dark matter evolved largely to explain the finding several decades ago that there was not enough visible matter in the universe to produce and account for the gravity needed to keep galaxies from flying apart.
“A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking,” said Doug Clowe of the University of Arizona in Tucson, leader of the NASA-Harvard University study. “These results are direct proof that dark matter exists.”
Another explanation offered for the unusual behavior of massive, but fast galaxies which travel at dazzling speeds through space without breaking apart, was the fact the gravity has different effects inside massive star-clouds than it has on Earth.
But scientists said a collision between the enormous “bullet” cluster of galaxies more than 3 billion light years away and another smaller galaxy cluster proved the existence of dark matter by, in effect, stripping the dark matter away from visible matter.
Once stripped, dark matter was clearly identified by the strong gravitational pull that it exerted.
“We now have direct evidence” of dark matter, said Sean Carroll, a cosmologist in the Physics Department of the University of Chicago, who did not participate in the study. “There is no way to explain the observations without dark matter.”
Astronomers believe dark matter consists of some sort of invisible particles, which account for 80 or 90 percent of the mass of the universe but do not interact with ordinary, visible matter.
"It could be that there are these particles streaming through us and we just don't know it," Clowe said.
The breakthrough came using data from NASA's orbiting Chandra X-ray Observatory, and involved information from what researchers called the most massive release of detected energy in the universe since the Big Bang.
The astronomers also used the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes to determine the location of the mass in the clusters by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity.
But although very powerful, these observations are still questioned by the group of researchers who imply that a small “tweak” in Newton’s laws about gravity, should explain the phenomenon better than the dark matter.
Stacy McGaugh, an astrophysicist at the University of Maryland, has been one of the dark matter skeptics, and he said yesterday that he remained unconvinced.
“I've been aware of this result some time, and I agree that it is interesting, and may make more sense in terms of dark matter than alternative gravity,” he said. “However, it is premature to say so."
He said a definitive detection of dark matter particles would mean “grabbing them in the laboratory, not just inferring that their effects can be the only possible explanation for an observation before the alternatives have actually been checked.”
But the recent observations of the US team of astronomers gives scientists more confidence that the Newtonian gravity familiar on Earth and in the solar system also works on the huge scales of galaxy clusters.
"We've closed this loophole about gravity, and we've come closer than ever to seeing this invisible matter," Clowe said.
