The earliest stars in the universe might have been beasts of a different nature than modern stars, a new model suggests. While nuclear reactions between ordinary chemical elements fuel the fire of stars like Earth’s own sun, mysterious dark matter might have powered the first stars.
In the standard account of star formation, clouds of hydrogen and helium become unstable and start to cool and condense into small protostars. Shrinking under their own gravity, protostars eventually become dense and hot, and their atoms begin to fuse. This ongoing fusion reaction is the power behind starlight.
Katherine Freese and her collaborators, though, point out that this model doesn’t necessarily take all the parts of the early universe into account. About 85 percent of the matter in the universe is thought to be of an invisible or dark form, which scientists can detect only indirectly.
“You’ve got this whole reservoir of dark matter sitting out there too. It must play some role in stars,” says Freese, of the University of Michigan in Ann Arbor.
So she, along with Paolo Gondolo of the University of Utah in Salt Lake City and Douglas Spolyar of the University of California, Santa Cruz, created a model of what would happen to star formation if lots of dark matter got caught up in dense clouds of hydrogen and helium. Although the nature of dark matter is not known, the researchers made standard assumptions about the kind of particles of which it probably consists. The model predicts that the dark matter particles would interact among themselves, undergoing annihilation reactions that would create immense amounts of heat. This outpouring of energy would keep the helium and hydrogen from cooling and condensing.
“You get this heat source that prevents you from getting fusion, and you get stuck in this new place,” explains Freese, who says this phase could go on for as long as 600 million years. Eventually, though, the dark matter would run out, since every time two dark matter particles interacted, they would turn into ordinary matter. Then the helium and hydrogen could spawn protostars that would enter the standard fusion-powered stage.
Objects in this newly hypothesized phase of star evolution have been dubbed dark stars but the researchers say that the name, which comes from a Grateful Dead song, is misleading. “They’d actually give off lots of light,” says Freese, “although we’re not sure exactly what that light would look like.”
For ultimate validation of their idea, the researchers hope the next generation of powerful telescopes will actually detect the dark stars, which could be 10 times as large as the solar system.
Until then, some scientists remain unconvinced. The new calculations shed light on one way dark matter may have interacted in the early universe, says Dan Hooper of Fermi National Accelerator Laboratory in Batavia, Ill. But he says the calculations, which is slated to appear in Physical Review Letters, rely on many assumptions.
“A lot of it is speculative,” says Hooper. “We don’t know that stars in the early universe were forming under these particular conditions.”
If these environments did exist, though, Hooper says dark stars could have existed. “It’s not implausible by any means,” he says.
