Dec. 5, 2007 -- The earliest stars in the universe may have been cool expanses of helium and hydrogen, thick with dark matter and spitting with antimatter. There even may be a few still around.
What some astrophysicists are calling "dark stars" would have been dominated by dark matter and could have existed for millennia in the early universe, when dark matter was far more concentrated than today. Some dark stars might have even survived to the present day.
What's more, these theoretical dark matter stars may be the secret behind the giant black holes called quasars, which appear to have come into existence before galaxies had a chance to create them. That timing has that has never made much sense before.
The dark matter in these weird stars would have to be some kind of heavy subatomic particle that only interacts with normal matter by tugging on it with its gravity. Such a substance is currently the most favored hypothesis for the nature of dark matter in the universe.
"The idea that the first stars could be this type is entirely new," said astrophysicist Paolo Gondolo of the University of Utah. He and his colleagues are publishing their dark star theory in next month's issue of the journal Physical Review Letters.
Without much dark matter, stars form when a cloud of interstellar hydrogen and helium gradually cool down, contracting into a smaller and smaller space until it collapses into a hot ball of matter that can ignite nuclear fusion and become a star.
If there's too much dark matter around, as there would have been in the early universe, that scenario might not work, said Gondolo. The thicket of dark matter in the clouds of gas would cause some dark matter particles to annihilate each other, which would possibly emit antimatter electrons (positrons), gamma rays and heat.
"[The dark matter annihilation] puffs up the cloud and keeps it at about the size of our solar system," said Gondolo. Under such conditions a regular star would have a hard time getting started, he explains, but the vast, cool dark star could just continue puffing until it can not longer annihilate dark matter or until it runs out of ordinary matter to suck into it.
Or, Gondolo posits, it could feed on massive amounts of matter, if available, then collapse to form a supermassive black hole.
"This could explain why we see quasars at very high red shift," said Gondolo, referring to the quasar black holes detected at very great distances and therefore very far back in time.
The bottom line is that the potential dark stars and what they might look like to astronomers hunting for them depends entirely on the nature of dark matter -- which is unknown.
"Different dark matter candidates could have different effects on early star formation," explained astrophysicist Alexander Kusenko of the University of California at Los Angeles. By modeling the early effects and then looking for signs of those effects in the distant universe, it might be possible to discover what dark matter really is.
"This is clearly a good place to look for clues," Kusenko told Discovery News.
Related Links:
Larry O'Hanlon's blog: Earth Impacts
Howstuffworks.com: Dark Matter
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