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First objects in the universe
were invisible blobs, physicists calculate
Posted Jan. 26, 2005
Courtesy Arizona State University
and World Science staff
Ghostly haloes of dark matter as heavy as the
Earth and as large as our solar system were the first structures to form in the universe, according to new calculations from scientists at the University of Zurich, published in this week's issue of
the research journal Nature.
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| Top: a supercomputer simulation of the distribution of dark matter in the universe. Galaxies form within the complex network of massive filamentary dark matter structures. The region shown below is a billion light years
across.
Bottom: a zoom into the first object to form in the universe. The two inset regions are each expanded by a scale of a hundred so you can see the single earth mass dark matter halo which is about the size of the solar system. The large blue region is 10,000 light years across.
(Credit: University of Zurich Institute for Theoretical Physics.)
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Our own galaxy still contains quadrillions of these halos with one expected to pass by Earth every few thousand years, leaving a bright, detectable trail of
radiation behind it, the scientists say. Day to day, countless random dark matter particles rain down upon the Earth and through our bodies undetected.
"These dark matter haloes were the gravitational 'glue' that attracted ordinary matter, eventually enabling stars and galaxies to form," said
Ben Moore of the Institute for Theoretical Physics at the University of Zurich, a co-author
of the report. "These structures, the building blocks of all we see today, started forming early, only about 20 million years after the big bang."
Dark matter comprises over 80 percent of the mass of the universe, yet its nature is unknown. It seems to be intrinsically different from the atoms that make up matter all around us. Dark matter has never been detected directly; its presence is inferred through its gravitational influence on ordinary matter.
The Zurich scientists based their calculation on the leading candidate for dark matter, a theoretical particle called a neutralino, thought to have been created in the big bang. Their results entailed several months of number crunching on the zBox, a new supercomputer designed and built at the University of Zurich by Moore and
colleagues.
"Until 20 million years after the big bang, the universe was nearly smooth and
homogenous," Moore said. But slight imbalances in the matter distribution allowed gravity to create the familiar structure that we see today. Regions of higher mass density attracted more matter, and regions of lower density lost matter. Dark matter creates
centers of gravitational attraction in space, which suck in ordinary matter. Galaxies and stars started to form as a result about 500 million years after the
"big bang," explosion believed to have begun the Universe. The universe is 13.7 billion years old.
Using the supercomputer, the team calculated how neutralinos created in the big bang would evolve. The neutralino
is a particle proposed by many physicists as the chief component of dark matter.
In this model, the dark matter it is known as "cold dark matter." This means it does not move fast and can clump together to create a
center of gravitational attraction. The neutralino has not yet been detected. This is a proposed "supersymmetric" particle, part of a theory that attempts to rectify inconsistencies in
the standard model of elementary particles.
For the past two decades scientists have believed that neutralinos could form massive dark matter haloes and
envelop entire galaxies today. What has emerged from the Zurich team's zBox supercomputer calculation are three new and salient
facts, the researchers said: "haloes" or blobs, weighing about as much
as the Earth, formed first; these structures have extremely dense cores enabling quadrillions to have survived the ages in our galaxy; also these "miniature" dark matter haloes move through their host galaxies and interact with ordinary matter as they pass by. It is even possible that these haloes could perturb the Oort cometary cloud far beyond Pluto and send debris through our solar system.
"Detection of these neutralino haloes is difficult but possible", the team said. The halos are constantly emitting gamma rays, the highest-energy form of light, which are produced when neutralinos collide and self-annihilate.
"A passing halo in our lifetime (should we be so lucky), would be close enough for us to easily see a bright trail of gamma rays," said Diemand, now at the University of California at Santa Cruz.
The best chance to detect neutralinos, however, is in galactic centres, where the density of dark matter is the highest, or in the centres of these migrating Earth-mass neutralino
haloes, the scientists said. Denser regions will provide a greater chance of neutralino collisions and thus more gamma rays. "This would still be difficult to detect, like trying to see the light of a single candle placed on Pluto," said Diemand.
NASA's GLAST mission, planned for launch in 2007, will be capable of detecting these signals if they exist. Ground-based gamma-ray observatories might also be able to detect gamma rays from neutralino interactions.
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