In the moments following the Big Bang, intense heat and pressure forced matter into a mixture of tiny particles. But how goopy? The answer has been elusive to physicists—until now.
Quarks and gluons are the fundamental particles that make up protons and neutrons. These particles are usually bound together, but extreme conditions after the Big Bang, physicists believe, led to their separate existence in the form of a soup called quark-gluon plasma (QGP). In a recent Physics Letters B paper, physicists from CERN’s CMS Collaboration and MIT have observed and confirmed for the first time that QGP behaves like a liquid. The quarks in the plasma create waves as they run through the material, “like a duck walking on water,” the researchers explained to MIT News.
“Now we see the plasma is incredibly dense, so much so that it can submerge quarks, and produce splashes and oscillates like a liquid,” explained Yen-Jie Lee, the MIT scientist who led the new study. So the quark-gluon plasma is a really important soup.
The goopy days of the world
There is not much that scientists know for sure about the great early universe. Physicists have proposed a number of theories and models to capture the characteristics of the early days of the universe. However, the challenge of validating these ideas experimentally meant that scientists were reluctant to draw any firm conclusions.
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That said, the QGP was one of the few theories that scientists generally agreed upon. The primordial stew—boiling at about a few billion degrees—eventually cooled to create the protons and neutrons that make up the matter in the universe. One model, devised by MIT physicist Krishna Rajagopal, argued that a particle flying through the QGP should produce a wake in the plasma, which would flow and spread like a liquid.
“This is something that many of us have been arguing about for many years, and with a lot of research,” said Rajagopal, who was not directly involved in the new project.
Reading the cosmic soup
The new study confirms Rajagopal’s account of QGP, using a neutral, weak electron particle called the Z boson as a marker to track the movement of quarks in the plasma. Since the Z boson has no effect on the plasma, any wave-like motion would have come from the quark, the researchers reasoned.
For the study, the team used data from CERN’s Large Hadron Collider. But given the instability of QGP, even the world’s most powerful particle accelerator held only a goop—a “drop” at that—together for just under a millionth of a second, according to the researchers.
The team looked for 13 billion collisions, of which only 2,000 produced the Z boson they were looking for. They then mapped each of these events according to the energy levels in the QGP droplet, finding a consistent pattern, “similar to liquid splashes in swirls” – a wake effect, as predicted by Rajagopal’s model, according to MIT News.
Understanding the origins of the universe
In addition, the researchers expect that new research methods will greatly improve our understanding of matter in the early universe. Subsequent experiments will investigate the exact size, speed, and extent of this wake, which should reveal more about the properties of the plasma.
“[The study] bring it [us] the first clean, clear, unambiguous evidence of this fundamental state,” Daniel Pablos, a physicist at Oviedo University in Spain who was not involved in the research, told MIT News.
“We found the first direct evidence that a quark drags more plasma with it as it moves,” added Lee. “This will enable us to study the properties and behavior of this rare liquid in unprecedented detail.”
