Thursday, August 23, 2007
New observations of a massive intergalactic collision are raising more questions about the mysterious nature of dark matter.
The study looked at three huge galaxy clusters that are merging into an even bigger cluster called Abell 520, located about three billion light-years from Earth.
Astronomers describe the collision as a "cosmic train wreck," given the immensity of the forces involved. Each cluster contains about a thousand galaxies, and each galaxy has billions of stars.
The new findings show an unprecedented mix-up in the merging clusters, suggesting the need for an "uncomfortable" revision—or entire rewrite—of our current theories of dark matter.
"Whatever happened did something really unusual to the galaxies," said study lead author Andisheh Mahdavi, an astronomer at the University of Victoria in British Columbia, Canada.
"It moved them all the way on the outer edge of the central region of the cluster, so that only gas and dark matter is left at the center," he added.
"That's never been observed before, and it's really hard to explain."
The findings are slated to appear in an upcoming issue of the Astrophysical Journal.
Dark matter does not absorb or emit light, but scientists believe it makes up about 90 percent of the matter in the universe.
Prevailing theories hold that dark matter is composed of particles that have very weak interactions and move only under the influence of gravity, just like stars.
So when galaxies collide, scientists expect stars and dark matter to move together. Intergalactic gas, however, also responds to pressure and thus is expected to lag behind the other matter.
The bullet cluster observations show dark matter and stars moving together and ahead of the intergalactic gas.
But when Mahdavi and colleagues studied Abell 520, they found a dark matter core separated from most of the galaxies.
Eye in the Sky
Mahdavi and colleagues mapped the distribution of matter in Abell 520 with two ground-based optical telescopes in Hawaii and the orbiting Chandra X-Ray Observatory
Optical telescopes determine the location galaxies from their starlight and infer the location of dark matter by the way its gravity bends the light of other galaxies in the distant background, a technique called gravitational lensing.
X-ray telescopes detect the radiation given off by scorching hot intergalactic gas.
Mahdavi and colleagues plan to re-observe Abell 520 with the Hubble Space Telescope later this year, which will allow for more precise mapping of the dark matter.
If the Hubble observations confirm the distribution of matter in the cluster, astronomers may be forced to revise the physics used to explain the behavior of dark matter, Mahdavi said.
There are two possible explanations, "and they're both equally uncomfortable," Mahdavi said.
The galaxies may have been flung to the outer edge of the cluster via a gravitational slingshot, in the same way astronomers can send satellites around a nearby planet for an extra push to the outer solar system.
"But that has a number of problems, in that we haven't been able to make [with computer simulations] slingshots that are powerful enough," Mahdavi said.
The other possibility is the dark matter just got left behind as the galaxies passed on through the center of the cluster, suggesting that the dark matter interacted through some force other than gravity.
"The problem with that is this is not the theory of dark matter that's generally accepted," he said.
The bullet cluster observations, for instance, appear to confirm the accepted theories that dark matter contains only weakly interacting particles.
Wait and See
Avi Loeb is an astronomer at Harvard University in Cambridge, Massachusetts, who was not involved in the study.
He said he will wait for the results of the Hubble observations "before making a judgment whether the nature of dark matter might be different."
If the result is confirmed, researchers will probably try to explain the phenomenon via the gravitational slingshot effect, which is more plausible than completely revising the physics, Loeb said.
"I don't think we should revise our ideas just based on this example, because there is also the bullet cluster that is showing a different picture and clearly the circumstances are different in the two clusters," he said.
Additional studies of other galactic collisions will help firm up conclusions about dark matter, Loeb added.
"At the moment, we have just two examples of these natural laboratories for dark matter dynamics," he noted.
The world's oldest known diamonds have been found encased in a crystal in Western Australia.The minuscule gemstones are 4.25 billion years old and could provide a rare glimpse into Earth's distant geologic past.
"No one would have really predicted that diamonds were in there," said Simon Wilde, a geologist at Curtin University of Technology in Perth and a member of the team that made the find.
The discovery suggests that seas of molten lava that covered primordial Earth had cooled down faster than had previously been thought.
The find also suggests that plate tectonics, the process by which large shelves of Earth's crust move to create geologic activity, may have already been underway.
"A diamond would never form in a magma ocean," said Thorsten Geisler, a geologist at Westfalische Wilhelms-Universitaet in Munster, Germany, and another team member.
The discovery is a shocker to geologists, many of whom believed that the molten lava and volcanic activity persisted on Earth's surface for at least 500 million years after our planet formed some 4.5 billion years ago.
Diamonds Are Forever
The tiny diamonds were found trapped in zircon, a rare and exceptionally stable mineral that forms under temperatures between 1,112 and 1,652 Fahrenheit (600 and 900 degrees Celsius).
Once zircon has crystallized it may be moved around by geological processes, but its chemical makeup and structure don't change. This makes its age easy to pinpoint.
Zircon crystals represent the only record of the first 400 million to 500 million years of Earth's history, Wilde explained.
By analyzing a crystal's trace minerals and structure, geologists can deduce the conditions under which it formed.
John Valley, a geologist at University of Wisconsin in Madison who was not involved in this study, notes that there are four known "recipes" that create diamonds.
But the 4.25-billion-year-old diamonds "suggest the additional possibility that the diamonds have formed by some process that is not yet understood."
Study co-author Wilde said, "The bottom line is that we really honestly don't know why they're there."
The study, led by Martina Menneken, a master's student at the Westfaelische Wilhelms-Universitaet, appears in this week's issue of the journal Nature. Alexander Nemchin from Curtin University of Technology also contributed.
Clues to Earth's Earliest Life?
One exciting prospect is that if Earth cooled down earlier in its existence, then it's possible that life on Earth cropped up earlier too, Geisler said.
(Related: "Weird Australia Rocks Are Earliest Signs of Life, Study Says" [June 7, 2006].)
Geisler hopes that analyzing the various types of carbon in these diamonds could reveal whether this was the case.
"We don't know yet, but this is potential information contained in the carbon," he said.
Valley, the Wisconsin geologist, added, "Even though these diamonds are too small to be of commercial value as gems, scientists will find them even more valuable for the information they carry about the Earth."
Wednesday, August 22, 2007
Vampire bats in Latin America are turning their fangs on cattle as rain forest is being cleared to make way for livestock, new research shows.
Scientists made the find by studying changes in the breath of vampire bats in Costa Rica.
The researchers discovered that the bats are finding meatier victims to sink their fangs into as the habitat of wild forest mammals disappears and is turned into livestock pasture.
A study led by Christian Voigt from the Leibniz Institute for Zoo and Wildlife Research in Berlin, Germany, found the blood-seeking bats are switching to cattle from rain forest prey such as tapirs and piglike peccaries.
The study team investigated which animals the bats were targeting by analyzing the chemical signatures, called isotopes, in the carbon dioxide they exhaled soon after eating.
Cattle and rain forest mammals feed on different plants that can be distinguished by their carbon isotopes. Since these chemical clues are present in prey's blood, the signature in the bats' breath varies with their meals.
The study clearly indicated that the vampires' most recent victims were almost always cattle, the team said.
The findings don't mean that vampire bats prefer bovine blood, the team said. Instead, they suggest that livestock are simply easier for the bats to find.
Voigt compared the vampire's dining options to that of a hungry human looking for a hot dog.
"One supplier is moving with a small van through the streets of the town, and it is not predictable for you where the van shows up," he said.
"The other supplier is in a snack bar with a permanent address. You most likely wouldn't go searching [for] the hot dog van but [would instead] go to the snack bar. Vampires are basically doing the same."
The research, which is published online in the Journal of Comparative Physiology B, came in response to reports from Costa Rican cattle ranchers of increased vampire attacks in the region.
Only three bat species are vampires, all of which are confined to Latin America. Just one of these, the common vampire bat, feeds on mammal blood.
Attacking at night, the bat doesn't suck its victims' blood but laps it up from tiny puncture wounds made with two sharp fangs. An anticoagulant in the bat's saliva keeps its nutritious meal from clotting.
Anecdotal evidence suggests that vampire bat numbers have increased significantly over the past 50 years, Voigt said.
This rise is widely attributed to deforestation, a theory bolstered by recent surveys of bat populations conducted by Voigt's team, he added.
A survey in virgin rain forest in the remote Amazon detected only a few vampires, Voigt said, but a survey of disturbed rain forest in the Andean foothills showed the winged blood-seekers to be the most abundant bat species.
No livestock were found near the Amazonian site, Voigt said, "whereas at the other site numerous pastures with cattle were present."
Livestock farming is seen as a leading cause of rain forest destruction in Central and South America.
(Related: "Amazon Deforestation Drops 25 Percent, Brazil Says" [August 14, 2007].)
In the Amazon, around 60 percent of deforestation between 2000 and 2005 can be attributed to cattle ranching, according to estimates based on figures from the Brazilian National Institute of Space Research and the United Nations Food and Agriculture Organization.
The growing number of vampire bats in cattle-ranching areas has also been blamed for the spread of disease among both humans and domesticated animals.
In just two months in 2005, 1,300 people in northern Brazil were treated for rabies after suffering bat bites. Twenty-three of the patients died, according to reports.
Voigt said that habitat conservation, along with efforts to reduce wildlife poaching, is crucial to minimizing the overall impact of vampire bats and their late-night raids.
"If rain forest mammals such as tapirs and peccaries are … killed, there is no alternative left for vampires to get blood from," he said.