Drops of the Early Universe Spark More Questions

By Rachel Siford and Jessica Opatich

Scientists at Brookhaven National Laboratory published data in late August supporting their creation of tiny droplets of an almost “perfect” liquid using the Relativistic Heavy Ion Collider (RHIC), the second most powerful collider in the world.

This liquid, known as quark-gluon plasma (QGP), could open a tiny research window into the first moments of our universe. Two teams at the Brookhaven National Laboratory (BNL) in Upton, NY are trying to figure out how perfect QGP really is, and how small these perfect droplets can get.

“Right now we’re trying to see how close to a perfect fluid is the quark gluon-plasma that we

created,” physicist at RHIC and STAR collaborator Paul Sorensen said. This plasma is 250,000 times hotter than the center of the sun—the hottest matter humans ever created. “It’s more perfect than any other fluid known in nature.”

RHIC, where Sorensen works, is located in a lab on the BNL campus. The collider is 12 feet tall and made up of thousands of detectors to record ion collisions. These collisions allow scientists to observe what the universe may have been like moments after its creation.

Scientists have been using RHIC for 15 years. It is the first collider in the world capable of colliding heavy ions, which are atoms that have had their outer ring of electrons removed. It collides two beams of ions at almost the speed of light, liberating quarks and gluons, creating the plasma.

“The reason why that’s important is because there were many phase transitions that occurred in the early universe,” Sorensen said. “All of these phase transitions and all of the physics are described by a certain theory. This is the only opportunity that we have to experimentally test that theory. “

Thirteen billion years ago when the universe was created after the Big Bang, there were no protons or neutrons, just free quarks and gluons. But as the universe cooled, they bound together. When the collider separates them, scientists can see how matter was moments after the Big Bang.

“What was predicted actually happened,” Stony Brook University physics professor Edward Shuryak said. “Before scientists weren’t sure what existed at the time, if it was fluid or gas or just a bunch of independent particles.”

The PHENIX team at RHIC is also experimenting with QGP. Their new data was accepted for publication on August 31 in the journal Physical Review Letters. The data revealed that smaller particles can create tiny samples of QGP, but it has also set off a debate in the science world.

“Think of it this way: a tiny drop of water contains about a billion trillion H2O molecules. How many molecules does it take so that you can call it ‘water’? Is one molecule enough? Certainly not, because it would not know the difference between ice, water, or vapor. What about 10 molecules? Or 100? Or 1,000? That’s an interesting and intriguing question,” Berndt Mueller said.

Mueller, the associate laboratory director for nuclear and particle physics at Brookhaven, and his team are now asking the same questions about QGP.

This image shows the tracks from the particles produced after a collision occurs.
This image shows the tracks from the particles produced after a collision occurs.

“What the recent experiments have shown is that the smallest possible drop of QGP that acts as a ‘perfect liquid’ is very small,” Mueller said. “It contains approximately 100 quarks and gluons, not thousands. Many scientists did not think this was possible.”

It’s an experimental approach for the evolution of the Big Bang, RHIC physicist Lijuan Ruan said, but with “small bangs.”

“The entire universe existed in this state during the ​first few microseconds after the Big Bang,” Dimitri Kharzeev senior scientist at Brookhaven National Laboratory said. “The fact that this state in fact represents a ‘perfect fluid’ is fundamentally interesting, and helps us understand better other quantum fluids.​”

Often in science, new discoveries prompt more questions.

“The trendy question of the day became: Is there a QGP created even in these small collisions? Or is there another explanation for what we are seeing? And if there is another explanation for what we are seeing, how do we know that we actually created a QGP in collisions of heavy nuclei?” Sorensen said.

These are the fundamental questions that inspire physicists.

Work at RHIC is about “trying to understand how we got here, what are the laws that govern everything,” Sorensen said. “Those are the kinds of studies that pay off in the time scale of decades or even centuries.”