April 19, 2005
New results from a particle collider suggest that the universe behaved like a liquid in its earliest moments, not the fiery gas that was thought to have pervaded the first microseconds of existence.
By revising physicists' conception of the early universe, the new discovery offers opportunities to better learn how subatomic particles interact at the most fundamental level. It may also reveal intriguing parallels between gravity and the force that holds atomic nuclei together, physicists said Monday at a Tampa, Fla., meeting of the American Physical Society.
"There are a lot of exciting questions," said Samuel Aronson, associate lab director for high energy and nuclear physics at Brookhaven National Laboratory on New York's Long Island.
Between 2000 and 2003, the lab's Relativistic Heavy Ion Collider, known as RHIC, repeatedly smashed the nuclei of gold atoms together with such force that their energy briefly generated trillion-degree temperatures. Physicists think of the collider as a time machine, because those extreme temperature conditions last prevailed in the universe less than 100 millionths of a second after the big bang.
Everything was so hot then that quarks and gluons, which are now almost inextricably bound into the protons and neutrons inside atomic nuclei, were thought to have flown around like BBs in a blender.
By reproducing the conditions of the early universe, the collider has shown that unconstrained quarks and gluons do not fly away in all directions so much as squirt out in streams.
"The matter that we've formed behaves like a very nearly perfect liquid," Aronson said.
The word "perfect" refers to the liquid's viscosity, a frictionlike property that affects a fluid's ability to flow and the resistance to objects trying to swim through it. Honey has a high viscosity; water's viscosity is low. A perfect liquid has no viscosity at all, which is impossible in reality but useful for theoretical discussions.
"You always have to have a little bit of viscosity," said Brookhaven physicist Peter Steinberg. "The excitement is that we might be achieving the lowest viscosity that's possible."
Theoretical physicists recently have proposed that material swallowed by black holes might also have extremely low viscosity. That notion, based on a branch of mathematical physics known as string theory, has led some physicists to hypothesize that there might be a deeper connection between what happens in a black hole and what goes on when two gold nuclei collide at RHIC. For physicists, any chance to draw parallels between two vastly different phenomena is an opportunity to advance toward the field's holy grail, a unified theory of nature's forces.