A few hundred million years after the big bang, the universe was dark. Oceans of hot hydrogen atoms and negative hydrogen ions pervaded space. The cosmos as we know it started to take shape when atoms and ions paired up to form molecular hydrogen, which expelled heat out of the gas clouds, allowing them to cool down enough to form the first stars.
But how long did it take molecular hydrogen to form? That chapter of cosmic history has been unclear. Now, by recreating the chemistry of those early gas clouds in the lab, researchers have determined the rate at which hydrogen atoms and negative hydrogen ions combined in the primordial soup. The result gives astrophysicists a firmer handle on the mass of the first stars, reducing the uncertainty in the estimated mass down from a factor of 20 to two, the scientists report in tomorrow's issue of Science.
The experiment has "eliminated a major uncertainty in theoretical simulations of the chemistry and cooling rate of the first gas clouds," says Avi Loeb, a theoretical physicist at Harvard University. Now that theorists have a better handle on the chemistry, Loeb says, they can plug the information into computer models to explore the properties of the first stars.
Even though the combination of H and H– is an "amazingly simple reaction, it's been poorly understood" because it's hard to bring the ingredients together in the lab, says Daniel Savin, one of the paper's authors and a researcher at Columbia University's Astrophysics Laboratory. To make it happen, Savin and his colleagues, including Holger Kreckel, who is now at the University of Illinois, Urbana-Champaign, first generated a beam of negatively charged hydrogen ions and sent it barreling down a tube. The beam passed through a chamber where a laser knocked the extra electrons off of about 7% of the ions, leaving a mix of hydrogen and negatively charged hydrogen ions to react with each other farther down the tube. In the final leg of the apparatus, the researchers counted how many hydrogen molecules the reaction produced.
"It turns out that molecular hydrogen forms faster than previously thought," Savin says. "That means the first stars likely formed faster than previously expected." Knowing the rate at which the reaction proceeds is an improvement, but it's not enough to nail down the mass of the first stars. "Because we don't fully know the initial conditions from which the first stars formed," he says, "we don't yet reliably know the distribution of masses."
Knowing how rapidly molecular hydrogen formed would help scientists to model both the first stars and the evolution of cosmic structure over time, writes Volker Bromm, an astrophysicist at the University of Texas, Austin, in a related Perspectives article. That's because the properties, behavior, and fate of the first stars affected cosmic events that followed, such as the formation and distribution of primordial galaxies. "Indeed, it is a fascinating aspect of this study that microphysical processes can have such large-scale, cosmological implications," Bromm writes.