Talk about a contradiction in terms. The first stars in the universe might have been so deeply mired in clouds of dark matter that they could not ignite, a team of physicists has calculated. If confirmed, the idea could cause a radical rethinking of theories about stellar evolution and black hole formation in the young cosmos.
Most astronomers think the universe started forming its recognizable structure around clumps of dark matter, the mysterious substance that collectively weighs six times more than all the visible matter and so far has eluded all attempts to detect it directly (ScienceNOW, 13 September). Researchers surmise that those dark matter clumps acted as gravitational anchors in the early universe, about 13 billion years ago. Clouds of hydrogen and helium gas condensed around them and grew into the first stars and galaxies.
Now, researchers led by physicist Paolo Gondolo of the University of Utah in Salt Lake City have presented a different scenario. According to their calculations--which haven't yet been tested in standard computer simulations--stellar and galactic evolution took a step sideways for a while. The results show that dark matter particles, or neutralinos, apparently smashed into each other and heated the star-forming clouds, keeping them from condensing enough to ignite their nuclear furnaces. And thus, as the team will report in an upcoming issue of Physical Review Letters, the stars remained dark for an as-yet-undetermined period of time and were far larger than any stars seen today--perhaps as much as 15,000 times the size of our solar system. It's also possible that these giant dark stars still exist, the team says. If so, astronomers could hunt for them by detecting gamma rays, neutrinos, and even antimatter radiating from interstellar and intergalactic clouds of hydrogen gas.
Astrophysicist Charles Bennett of Johns Hopkins University in Baltimore, Maryland, calls the study "very exciting." He says it will open up new research into the evolution of stars and black holes. "They have shed new light on dark stars," he quips. However, others are skeptical. Astronomer Avi Loeb of Harvard University says the idea of dark matter stars "relies on idealized assumptions that may not be realized in nature." The problem, he explains, is that dark matter could not concentrate in sufficient amounts on the scale of stars--even giant ones--to produce the effects suggested.
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