Tuesday, October 30, 2007

Two Scientists Say Dark Matter Doesn't Exist


Two Canadian astronomers think there is a good reason dark matter, a mysterious substance thought to make up the bulk of matter in the universe, has never been directly detected: It doesn't exist.

Dark matter was invoked to explain how galaxies stick together. The visible matter alone in galaxies—stars, gas and dust—is nowhere near enough to hold them together, so scientists reasoned there must be something invisible that exerts gravity and is central to all galaxies.

Last August, an astronomer at the University of Arizona at Tucson and his colleagues reported that a collision between two huge clusters of galaxies 3 billion light-years away, known as the Bullet Cluster, had caused clouds of dark matter to separate from normal matter. Many scientists said the observations were proof of dark matter's existence and a serious blow for alternative explanations aiming to do away with dark matter with modified theories of gravity.

Now John Moffat, an astronomer at the University of Waterloo in Canada, and Joel Brownstein, his graduate student, say those announcements were premature.

In a study detailed in the Nov. 21 issue of the Monthly Notices of the Royal Astronomical Society, the pair says their Modified Gravity (MOG) theory can explain the Bullet Cluster observation. MOG differs from other modified gravity theories in its details, but is similar in that it predict that the force of gravity changes with distance.

"MOG gravity is stronger if you go out from the center of the galaxy than it is in Newtonian gravity," Moffat explained. "The stronger gravity mimics what dark matter does. With dark matter, you take Einstein and Newtonian gravity and you shovel in more dark matter. If there's more matter, you get more gravity. Whereas for me, I say dark matter doesn't exist. It's the gravity that's changed."

Using images of the Bullet Cluster made by the Hubble, Chandra X-ray and Spitzer space telescopes and the Magellan telescope in Chile, the scientists analyzed the way the cluster's gravity bent light from a background galaxy—an effect known as gravity lensing. The pair concluded that dark matter was not necessary to explain the results.

"Using Modified Gravity theory, the 'normal' matter in the Bullet Cluster is enough to account for the observed gravitational lensing effect," Brownstein said. "Continuing the search for and then analyzing other merging clusters of galaxies will help us decide whether dark matter or MOG theory offers the best explanation for the large scale structure of the universe."

Moffat compares the modern interest with dark matter to the insistence by scientists in the early 20th century on the existence of a "luminiferous ether," a hypothetical substance thought to fill the universe and through which light waves were thought to propagate.

"They saw a glimpse of special relativity, but they weren't willing to give up the ether," Moffat told SPACE.com. "Then Einstein came along and said we don't need the ether. The rest was history."

Douglas Clowe, the lead astronomer of the team that linked the Bullet Cluster observations with dark matter (and now at Ohio University), says he still stands by his original claim. For him and many other astronomers, conjuring up new particles that might account for dark matter is more palatable than turning a fundamental theory of how the univese works on its head.

"As far as we're concerned, [Moffat] hasn't done anything that makes us retract our earlier statement that the Bullet Cluster shows us that we have to have dark matter," Clowe said. "We're still open to modifying gravity to reduce the amount of dark matter, but we're pretty sure that you have to have most of the mass of the universe still in some form of dark matter."

Astronomers Simulate Life And Death In The Universe



Stars always evolve in the universe in large groups, known as clusters. Astronomers distinguish these formations by their age and size. The question of how star clusters are created from interstellar gas clouds and why they then develop in different ways has now been answered by researchers at the Argelander Institute for Astronomy at the University of Bonn with the aid of computer simulations. The scientists have solved -- at least at a theoretical level -- one of the oldest astronomical puzzles, namely the question of whether star clusters differ in their internal structure.

Astronomical observations have shown that all stars are formed in star clusters. Astronomers distinguish between, on the one hand, small and, by astronomical standards, young star clusters ranging in number from several hundred to several thousand stars and, on the other, large high-density globular star clusters consisting of as many as ten million tightly packed stars which are as old as the universe. No one knows how many star clusters there might be of each type, because scientists have not previously managed to fully compute the physical processes behind their genesis.

Stars and star clusters are formed as interstellar gas clouds collapse. Within these increasingly dense clouds, individual "lumps" emerge which, under their own gravitational pull, draw ever closer together and finally become stars. Similar to our "solar wind", the stars send out strong streams of charged particles. These "winds" literally sweep out the remaining gas from the cloud. What remains is a cluster that gradually disintegrates until its component stars can move freely in the interstellar space of the Milky Way.

Scientists believe that our own sun arose within a small star cluster which disintegrated in the course of its development. "Otherwise our planetary system would probably have been destroyed by a star moving close by," says Professor Dr. Pavel Kroupa of the Argelander Institute for Astronomy at Bonn University. In order to achieve a better understanding of the birth and death of stellar aggregations Professor Kroupa and Dr. Holger Baumgardt have developed a computer programme that simulates the influence of the gases remaining in a cluster on the paths taken by stars.

Heavy star clusters live longer

The main focus of this research has been on the question of what the initial conditions must look like if a new-born star cluster is to survive for a long time. The Bonn astronomers discovered that clusters below a certain size are very easily destroyed by the radiation of their component stars. Heavy star clusters, on the other hand, enjoy significantly better "survival chances".

For astronomers, another important insight from this work is that both light and heavy star clusters do have the same origins. As Professor Kroupa explains, "It seems that when the universe was born there were not only globular clusters but also countless mini star clusters. A challenge now for astrophysics is to find their remains." The computations in Bonn have paved the way for this search by providing some valuable theoretical pointers.

The Argelander Institute has recently been equipped with five "GRAPE Computers", which operate at speeds 1,000 times higher than normal PCs. They are being deployed not only in research but also for research-related teaching: "Thanks to the GRAPE facilities, our students and junior academics are learning to exploit the power of supercomputers and the software developed specially for them." The Argelander Institute is regarded world-wide as a Mecca for the computation of stellar processes. Despite their enormous calculating capacity, the machines require several weeks to complete the simulation.

Dazzling Comet Has Hint of Invisible Tail


A comet that has engaged skywatchers worldwide with its sudden outburt has had one disappointing aspect: no tail.

Comet Holmes brightened suddenly and dramatically last week, going from total obscurity to naked-eye brightness that rivaled some of the brighter stars in the sky. But without a tail, the gas and dust ejected by the comet left it looking like no more than a fuzzy tennis ball through backyard telescopes.

Now astronomers think they've found a hint of a tail. Don't expect to see it for yourself, however.

A new image made using near-infrared light, which humans can't see, shows a small, tail-like feature next to the comet's nucleus. The image was obtained by graduate student Sandie Bouchard and assistant Bernard Malenfant on the pre-dawn hours of Oct. 26 using the Ritchey-Chretien telescope at Mont Megantic Observatory in Canada.

A preliminary analysis, performed by astronomers Pierre Bastien and Rene Doyon from University of Montreal and the Centre de Recherche en Astrophysique du Quebec (CRAQ) clearly shows a bright elongated feature.

However, the direction of the feature does not point directly away from the sun, as expected. Comet tails are formed when pressure from sunlight pushes inexorably on the material in the head, which surrounds the solid nucleus. This material—gas, dust, and ice particles—is typically pushed away from the sun to form the tail.

Astronomers don't know why the outburst occurred. Comet Holmes has been known for more than a century but has been quiet for decades, visible only through powerful telescopes.