Gravastars or Blackholes?

Date: Mon Jan 21 2002 - 22:25:12 MST

<<Two U.S. scientists have questioned the existence of black holes
and suggested, in their place, the existence of an exotic bubble of
superdense matter, an object they call a gravastar. The two are pointing out
that physicists have swept some "humiliating" problems with black holes under
the carpet.

By confronting these problems, they claim to have found an alternative fate
for a collapsing star. Emil Mottola of the Los Alamos National Laboratory in
New Mexico and Pawel Mazur of the University of South Carolina in Columbia
think gravastars are cold, dense shells supported by a springy, weird space
inside. They'd look like black holes, lit only by the material raining down
onto them from outside. In fact, they seem to fit all the observational
evidence for the existence of black holes.
So far, however, physicists have mixed feelings about the idea of gravastars.
Their verdicts range from "outstandingly brilliant" to "unlikely." What's
certain is that gravastars will rekindle a great debate of the early 20th
century: are black holes fact or fantasy?

The idea of black holes dates back to the First World War, when German
astronomer Karl Schwarzschild solved the equations of Einstein's newborn
of gravity while serving on the Russian front. He showed that space-time
any massive star would be curved. Squeeze a large enough star into a tiny
space and its density would become infinite and the curvature of space-time
would spiral out of control. The gravity near one of these objects would be
strong that nothing -- not even photons -- could escape its grasp.
Einstein shared the view of most physicists of that time that such objects,
later dubbed black holes, were too outrageous to exist. He argued that it was
all academic anyway, since stars never shrink this small. But scientists
gradually became convinced that they do. If a star is very massive, it will
blast apart in a supernova <>
explosion at the end of its life; and if a core twice as heavy as the Sun
remains, no known force can prevent gravity squeezing it to a point.

The result is a "singularity" with infinite density, where the known laws of
physics break down. The singularity's gravity would be so powerful it would
cloaked in an "event horizon", a boundary beyond which matter or light
escape. The dramatic idea of a black hole, which would rip to shreds anyone
inside it, fired the imaginations of scientists, artists and writers alike.
no one has ever rooted the drama in fact. "So far, there is no direct
observational evidence to show that any of the things astronomers call black
holes have event horizons or central singularities," says Neil Cornish, an
astrophysicist at the University of Montana in Bozeman. htm

We know there are compact objects millions of times as heavy as the Sun
that hog the centers of galaxies. These black hole candidates give themselves
away because hot stars, gas and dust spiraling toward them emit bright
X-rays. But that doesn't mean there's a cataclysmic black hole in the
vicinity; it could simply be a very massive object. The debate petered out
decades ago but there's still no ironclad
proof that black holes exist...>>

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