Posted on 04/23/2002 10:34:23 AM PDT by RightWhale
Looking Backward In Time At How Galaxies Clustered
The universe appears to be permeated with an invisible force -- dark energy -- that is pushing it apart faster and faster.
By conducting redshift surveys of galaxy clusters, astronomers hope to learn more about this mysterious force, and about the structure and geometry of the universe.
"Galaxy clusters consist of thousands of galaxies gravitationally bound into huge structures," said Joseph Mohr, a professor of astronomy at the University of Illinois. "Because of the expansion of the universe, the clusters appear denser at larger redshifts, when the universe was younger and denser."
Galaxy cluster surveys that probe the high-redshift universe can potentially provide a wealth of information about the amount and nature of both dark matter and dark energy, said Mohr, who will present the results of an ongoing study of galaxy clusters at a meeting of the American Physical Society, being held in Albuquerque, N.M., April 20-23.
"Till now, galaxy clusters have only been used to study the dark matter component of the universe," Mohr said. "We would measure the total mass in a galaxy cluster, and then determine the fraction of mass that was ordinary, baryonic matter."
Those measurements have shown there is insufficient baryonic and dark matter to account for the geometry of the universe. Astronomers now believe the universe is expanding at ever-increasing speed, and is dominated by a mysterious dark energy that must be doing the pushing.
"The next step is to try to figure out some of the specifics of the dark energy, such as its equation of state," Mohr said. "By mapping the redshift distribution of galaxy clusters, we should be able to measure the equation of state of dark energy, which would provide some important clues to what it is and how it came to be."
Mohr is using data collected by NASA's Chandra X-ray Observatory to study scaling relations -- such as the relationship between mass and luminosity or size -- of galaxy clusters and how they change with redshift.
"These scaling relations are expected to evolve with redshift, reflecting the increasing density of the universe at earlier times," Mohr said.
In particular, Mohr -- in collaboration with John Carlstrom at the University of Chicago and scientists at the University of California and Harvard Smithsonian Center for Astrophysics -- is studying the effect that hot electrons within galaxy clusters have on the cosmic microwave background, the afterglow of the Big Bang.
Galaxy clusters are filled with dark matter, galaxies and hot gas. Electrons in the gas scatter off the protons and produce X-rays. The emission of X-rays diminishes with higher redshift, because of the larger distances involved.
"There also is a tendency for the electrons to give some of their energy to the photons of the cosmic microwave background, which causes the blackbody spectrum to shift slightly," Mohr said. "The resulting distortion - called the Sunyaev-Zeldovich effect -- appears as a cold spot on the cosmic microwave background at certain frequencies. Because this is a distortion in the spectrum, however, it doesn't dim with distance, like X-rays."
By comparing the X-ray emission and the Sunyaev-Zeldovich effect, Mohr can study even faint, high-redshift galaxy clusters that are currently inaccessible by other means. Such measurements, correlating galaxy cluster redshift distribution, structure and spatial distribution, should determine the equation of state of dark energy and, therefore, help define the essence of dark energy.
"Within the context of our standard structure formation scenario, galaxy surveys provide measurements of the geometry of the universe and the nature of the dark matter and dark energy," Mohr said. "But, to properly interpret these surveys, we must first understand how the structure of galaxy clusters are changing as we look backward in time." - By James E. Kloeppel
Thick-Skinned Gravastars Vie to Replace Black Holes, in Theory
By Robert Roy Britt Senior Science Writer
posted: 09:52 am ET 23 April 2002
As if black holes weren't mind-bending enough, a new hypothesis suggests an entirely new idea for Nature's densest objects. In fact, the idea goes black holes aren't holes at all but black bubbles with very thick skins
The new idea, presented this week at a meeting of the American Physical Society, was conceived to provide an alternative to the exotic description of where stuff goes when a star collapses and becomes, in present theory, a black hole.
For most of us, the matter of where the matter goes is no less mysterious under the new notion.
Emil Mottola of the Los Alamos National Laboratory and Pawel Mazur of the University of South Carolina suggest that instead of a star collapsing into a pinpoint of space with virtually infinite gravity, its matter is transformed into a spherical void surrounded by "an extremely durable form of matter never before experienced on Earth."
The researchers call the objects gravastars. And, as with a black hole, you wouldn't want to get to close.
"Since this new form of matter is very durable, but somewhat flexible, like a bubble, anything that became trapped by its intense gravity and smashed into it would be obliterated and then assimilated into the shell of the gravastar," Mottola said.
There is no proof that this new form of matter exists, and thus gravastars remain for the moment no more than a potentially convenient proposal. But other astronomers are intrigued, both because black holes themselves remain mere theory, not fact, and because gravastars might explain strange physical observations that black holes don't.
Losing their grip
Black holes were conceived during World War 1 by the German astronomer Karl Schwarzschild, who while serving in the war was scratching solutions to Einstein's theories.
Black holes are theorized to be so dense that nothing, not even light, escapes the gravitational grip. Einstein first thought the idea was nuts. In a way, though, the concept is not as odd as one might think. Astronomers have clearly seen how any large object, such as the Sun or another star bends light and sends it on a new course. Black holes just bend light a whole lot more, folding its photons right into the object, Schwarzschild proposed.
Such excruciating bends cause the warping of both space and time, or space-time, as the theorists put it. Gravastars would be no less forgiving of what we traditionally call reality.
Inside a gravastar, space-time would be "totally warped," the researchers say. Further, the inner space would exert an outward force, which would enhance the durability of the bubble.
Mottola and Mazur have not worked out all the details of how gravastars might form. Yet they say the objects solve a flaw in black hole theory.
Physicists have long struggled to account for the tremendous entropy, or information, that a black hole would harbor. Theory holds that a black hole should have a billion, billion times more entropy sometimes referred to as states, than the star it formed from.
"Where are all these zillions of states hiding in a black hole?" Mottola said in a recent article in New Scientist magazine. "It is quite literally incomprehensible."
Gravastars don't have the same problem, as their entropy is said to be very low.
Remarkable properties
From the outside, a gravastar would appear much like a black hole; visible only by the high-energy emissions it spits from its jowls while consuming matter. Astronomers use X-ray observations, created by such cosmically carnivorous activity, to detect black holes. By noting the small region of space that can't be seen within a sphere inside those emissions, and by looking at the gravitational effects that the space has on surrounding matter, the black hole is deduced.
But inside, the material in a gravastar would have undergone a phase change, something like when water freezes to the solid state. The newly conceived, wildly dense phase of matter is theoretically rooted in a recent discovery.
In 1995, researchers cooled matter to near absolute zero and created a new form of matter called a Bose-Einstein condensate , in which the motion of electrons, protons, and everything else comes to a complete halt. Everything reaches a single state, called a quantum state, creating what's been called a "super atom."
The matter inside a gravastar would be akin to the Bose-Einstein condensate. It would exist in a vacuum, surrounded by an ultra-thin, ultra-cold, ultra-dark bubble, hence the name gra (vitational) va (cuum) star, or gravastar.
The "unique and remarkable properties" of a gravastar "could explain several high-energy astrophysical phenomena that now are puzzling," says Marek Abramowicz, a black hole expert at Gothenburg University.
Abramowicz thinks the violent creation of a gravastar might explain gamma ray bursts , distant explosions of incredible energy that puzzle researchers.
But Neil Cornish, an astrophysicist at the University of Montana, wonders whether an exploding star could shed enough entropy to become a gravastar. "I don't think that is a likely scenario," he told New Scientist.
Other theorists have criticized the gravastar hypothesis. Mottola and Mazur defend it but admit they have work to do before they can explain how the objects actually develop when a star collapses.
Yet even before they've figured this out, Mottola and Mazur have taken their extreme idea to a mentally dizzying new level: The say our entire universe may be the interior of a giant gravastar.
True. It's great to see fresh ideas come out. The brane theory is still there, waiting for confirmation of gravitational blueshift. Who is to say that the Big Bang itself is the best model just because it is the most popular? They are all just theories and we may never know.
gravitas - gravitas vacuas [somebody check this, my Latin sucks]
Seems so. However, it might open up another direction for travel, which would be to go to entirely different universes. There might be a way to pass from this universe to the inside of a gravastar, which could be an entire universe in itself once you are inside. And the other way, too, if there is a way, you might enter the universe that contains this one.
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