Whirlpools might have stirred up baby universe’s soup

Complex swirls and vortices can appear in the souplike phase of matter that existed just moments after the Big Bang. Computer simulations show that this substance, called the quark-gluon plasma, can contain “the hottest smoke ring in nature,” says Xin-Nian Wang of Lawrence Berkeley National Laboratory, coauthor of a paper published in the Nov. 4 Physical Review Letters.

Wang and colleagues simulated collisions like those at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, N.Y., where gold ions are slammed together at nearly the speed of light. Such smashups produce an extremely hot, dense fluid, in which particles called quarks and gluons — the constituents of protons and neutrons — roam free. This quark-gluon plasma hits temperatures of trillions of degrees Celsius, hundreds of thousands of times hotter than the core of the sun (SN: 03/13/10, p.8).
Studying the whorls that appear in this quark-gluon plasma “is a truly new direction,” says physicist Michael Lisa of Ohio State University. When scientists initially began investigating the plasma, they thought it would behave like a simple fireball, expanding rapidly outward. But the reality has been more complicated.

In the aftermath of the simulated collisions, the quark-gluon plasma churned like a smoke ring, with spinning, doughnut-shaped regions. Pairs of whirlpoollike eddies appeared in the fluid as well. A similar effect can be seen if you drag your hand through a swimming pool, says Wang. Vortices are produced on either side, which rotate in opposite directions. “This is exactly what we find is happening here in our simulation,” says Wang.
The appearance of these structures is “both surprising and not surprising,” says cosmologist Kevork Abazajian of the University of California, Irvine. “We don’t know what to actually expect.” The result may have implications for the quark-gluon plasma in the early universe, he says, but there are notable differences between the quark-gluon plasma in the infant cosmos and that produced in ion collisions. The early universe is thought to have been very uniform, whereas the quark-gluon plasma produced in collisions can be irregular. But, says Abazajian, if swirls like these did form in the early universe, “something novel or something very different could actually happen,” like the formation of black holes early in the universe’s history.
To observe eddies in a real quark-gluon plasma, scientists could study the particles produced in the ion collisions. These particles have a quantum property called spin, an intrinsic type of angular momentum, which tends to trace the vorticity of the fluid. Scientists can detect the influence of this spin by observing decays of certain particles, and measuring the angles at which their decay products are emitted. By studying the correlation between pairs of such particles, scientists could determine the vorticity of the fluid.

Though swirls have not yet been seen in experiments, searches for them are now under way. “The experiments are working hard on it, so we hope to have news soon,” says Lisa.