Moments after the Big Bang, the universe existed as an incredibly hot, dense plasma – a state of matter so extreme it defied everyday experience. Now, physicists at MIT and CERN have provided the first direct evidence that this primordial substance, known as quark-gluon plasma (QGP), behaved like a swirling, sloshing liquid, essentially a “primordial soup” as one researcher described it. This confirmation offers a crucial window into the conditions that existed fractions of a second after the universe’s birth.
The research, detailed recently in the journal Physics Letters B, builds on decades of theoretical work and experimental attempts to recreate the QGP in laboratory settings. Understanding the properties of this early state of matter is key to unraveling the fundamental forces that shaped the cosmos and ultimately led to the formation of the matter we see today. The team’s findings specifically address a long-standing question: did the QGP flow like a fluid, or did its constituent particles scatter randomly?
To simulate the conditions of the early universe, researchers collided lead ions at nearly the speed of light within CERN’s Large Hadron Collider (LHC). These collisions generate temperatures exceeding one trillion degrees Celsius – far hotter than the surface of the sun – briefly creating droplets of QGP. Analyzing the resulting spray of particles, including quarks, proved challenging, but a novel approach allowed scientists to map the energy of the QGP and trace the motion of quarks through it.
“Now we see the plasma is incredibly dense, such that it is able to slow down a quark, and produces splashes and swirls like a liquid. So quark-gluon plasma really is a primordial soup,” said physicist Yen-Jie Lee of MIT. The team observed that as quarks moved through the QGP, they transferred energy to the plasma, creating a wake similar to that of a boat moving through water. This wake effect is a hallmark of fluid behavior.
The challenge lay in isolating the wake created by a single quark amidst the chaotic environment of the QGP, which contains thousands of interacting particles within an incredibly short timeframe – approximately a quadrillionth of a second. Previous experiments often focused on quark-antiquark pairs, which create opposing wakes that complicate analysis. This team employed a different strategy, searching for events where a quark collided with a Z boson, a neutral particle that doesn’t interact with the QGP and therefore doesn’t create a wake.
Out of 13 billion collisions analyzed, only around 2,000 produced a quark-Z boson pair, making the data collection painstaking. However, this rare occurrence allowed researchers to clearly observe the wake generated by the quark, confirming the predictions of theoretical models developed by physicists like Krishna Rajagopal of MIT. Rajagopal explained the analogy to a boat on a lake: “By analogy, when you have a boat moving through a lake, the wake is water behind the boat that is moving in the direction of the boat. The boat has transferred momentum to some region of water, which is ‘following’ it.”
Rajagopal described the findings as “definitive, unmistakable evidence” of the QGP’s liquid-like properties. He further elaborated via email that this research builds on earlier discoveries showing the QGP behaves as a nearly perfect fluid with low viscosity, a surprising result given the extreme conditions. CERN explains that this behavior was unexpected, as many researchers initially anticipated a gas-like state.
This modern technique offers a powerful framework for exploring similar processes in other high-energy collisions, potentially shedding light on other mysteries of the universe. As Rajagopal noted, “In many other areas of science, the way you learn about the properties of a material is to disturb it in some way, and measure how the disturbance spreads, and dissipates.” The ability to “smash” matter at near-light speed, as he put it, provides a unique avenue for understanding the fundamental building blocks of reality.
The research team plans to continue refining their methods and analyzing data from the LHC, seeking to further characterize the properties of the QGP and its evolution. Future studies may focus on exploring the QGP’s response to different types of disturbances, providing a more comprehensive understanding of this exotic state of matter and its role in the early universe. This work represents a significant step forward in our quest to understand the origins of the cosmos and the forces that govern it.
What do you think about the implications of this research for our understanding of the early universe? Share your thoughts in the comments below.
