Dark Matter May Have Powered Universe's First Stars

Anne Minard
for National Geographic News
December 6, 2007
Dark matter may have fueled the formation of the universe's first stars—vast, invisible giants totally unlike the blazing suns of today—scientists say.

According to a new theory, disintegrating fragments of the mysterious substance could have created "dark stars" hundreds of thousands of times wider than the sun around 13 billion years ago, just after the big bang.

Because these stars weren't fueled by fusing hydrogen and helium like known present-day stars, they would have been completely invisible—but scorching hot.

The findings "drastically alter the current theoretical framework for the formation of the first stars," said study co-author Paolo Gondolo, an astrophysicist at the University of Utah.

Scientists still don't know what dark matter is exactly, so the research could shed light on it and other astronomical mysteries, he added.

"We are always searching for ways to determine the nature of dark matter," Gondolo said.

The paper will appear in next month's issue of the journal Physical Review Letters.

Annihilation Engine

According to some theories of the universe, dark matter likely consists of hypothetical particles called neutralinos. (Related: "Dark Matter Mapped in 3-D, Scientists Report" [January 8, 2007].)

The new paper suggests that neutralinos annihilated each other in the early universe, producing subatomic particles called quarks and their antimatter counterparts, antiquarks.

The heat from this process was enough to prevent embryonic hydrogen and helium from cooling and shrinking. Such contraction ignites the self-sustaining fusion process that powers conventional stars.

"The heating can counteract the cooling, and so the star stops contracting for a while, forming a dark star" some 80 million to 100 million years after the big bang, Gondolo said.

Dark stars, made up mostly hydrogen and helium, would be vastly larger than the sun and other stars—up to 15,000 times the size of our solar system. And instead of shining, they would glow in the infrared.

"With your bare eyes, you can't see a dark star," Gondolo said. "But the radiation would fry you."

Wide-Ranging Implications

The theory may have wide-ranging implications about the importance of dark matter in the earliest stages of the universe.

For example, dark matter is widely believed to have helped with early galaxy formation, said Rennan Barkana, an astrophysicist at Tel Aviv University in Israel who was not involved with the new paper.

But until now it was thought that "the dark matter does not play any significant role in the formation of the star itself," he said.

That's important, because the substance is believed to make up most of the universe's matter, partly because galaxies rotate faster than can be explained by the visible matter within them. (Related photos: "The Galaxy Hunters".)

In total, about 23 percent of the universe is thought to be dark matter, as opposed to 4 percent for the ordinary matter that makes up stars, planets, and moons.

The remaining 73 percent is thought to be dark energy, an even more mysterious force helping the universe to expand at increasing rates.

Search Is On

Emanuele Ripamonti, an astronomer at Universita' dell'Insubria in Como, Italy, said that in order for the new research to be plausible, the formation of stars from dark matter must rely on a cascade of events that are not yet well studied.

"Every time they make a choice, the authors pick the 'most likely' option, but in some cases there is no real consensus" about what would happen, he said.

Barkana called the theory intriguing and novel but agreed that more research is necessary.

"It is unclear whether in the end an observational prediction will come out that will allow the dark star possibility to be clearly distinguished from other scenarios," he said.

If dark stars exist, however, they would likely give themselves away by spewing gamma rays, neutrinos, and antimatter, study author Gondolo said. The stars would also be associated with clouds of cold, molecular hydrogen gas that wouldn't normally harbor such energetic particles.

If found, dark stars wouldn't only provide insights into dark matter, he added. They could also help unravel phenomena like the formation of heavy elements—thought to come from exploding conventional stars—and the rapid formation of black holes, which defies theoretical predictions.

"Without detailed simulations, we cannot pinpoint the further evolution of dark stars," he said. "We have to search for them."

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