Tiny Bubbles of Quark-Gluon Plasma Re-create the Early Universe

More accurately than ever, new experiments can recreate the early universe when it was a jumble of elementary particles.

Imagine having a microscope that would enable you to closely examine a single atom. Let's assume that it is an ultra-small hydrogen atom. You can identify the nucleus—in this example, a single proton—if you zoom in past the lone electron circulating at the periphery. High school physics would have you think that this proton is made up of a straightforward quark triad consisting of two up quarks and one down quark. Physicists are still striving to understand the interior structure of a proton and how its constituent parts interact to generate its mass, spin, and other attributes since the reality within a proton is so much more complicated.

Only the "valence quarks"—buoys floating on top of a turbulent sea of quarks and antiquarks (their antimatter counterparts), as well as the gooey "gluon" particles that hold them together—represent the three quarks in the fundamental model of the inside of a proton. A proton has a variable amount of quarks and gluons overall. Quark-antiquark couples appear and disappear often, and gluons frequently divide and proliferate, especially as a proton speeds up. It's essentially total anarchy. This chaos is contained to the interiors of protons and neutrons thanks to the strong force, the most potent of the four basic forces of nature. Unless it doesn't, that is.

The cosmos was too hot and dense in the first minuscule fractions of a second after the big bang for the strong force to bind quarks and gluons together. Instead, they transformed into a quark-gluon plasma, a perfect liquid of particles that flows with practically no resistance. The history of the universe at this point was brief. Quarks and gluons were trapped into protons and neutrons in less than 10-6 seconds. The quark-gluon plasma might potentially be recreated inside particle accelerators 13.7 billion years later. The temperatures and pressures required for droplets of quark-gluon plasma to develop, momentarily, before dissolving are created by the collision of two massive atomic nuclei, such as gold atoms, moving at close to the speed of light.

Towering structures made of stacked detectors and other sensors placed in concentric rings and linked by hundreds of cables are used to record these encounters. Last year, when I went to see two of them at the Brookhaven National Laboratory's Long Island location, I was astounded by the laborious process of vast teams of technicians reaching the devices by mounting numerous levels of scaffolding. These are some of the biggest and most complex machines ever created, all for the study of a drop of primordial slime even smaller than an atom. Standing beneath such a colossus is like witnessing the apex of what humanity can do. Scientists have the opportunity to discover how matter first came into being by examining quark-gluon plasma droplets. Approximately 10 microseconds after the great bang, according to theoretical physicist Bjoern Schenke of Brookhaven, this is what filled the whole cosmos. We can travel back in time as far as we can by studying it.

The study also provides insight into the strong force, which is the least known of all the natural forces. Quantum chromodynamics (QCD), a theory that describes this force, is so challenging that researchers seldom ever use it to perform direct calculations. They can only acquire approximative answers by using supercomputers running simulations. The strong force and quantum chromodynamics are two concepts that are essential to comprehending nature, according to scientist Haiyan Gao, assistant laboratory director for nuclear and particle physics at Brookhaven. To fully comprehend how this theory operates, quark-gluon plasma experiments are required.

Scientists from Brookhaven will start the most recent experiment to examine quark-gluon plasma in April 2023. At the lab's Relativistic Heavy Ion Collider (RHIC), one of the biggest particle accelerators in the world, the gadget, known as sPHENIX, is one of two detectors. The Solenoidal Tracker at RHIC (STAR), the other detector there, is also reopening following significant modifications. The world's largest accelerator, the Large Hadron Collider (LHC), just started a new run at the European CERN physics facility in Geneva. It now has updated detectors and the capacity to smash many more atoms at once. Together, these instruments should provide the most comprehensive image yet of this primordial fluid, advancing our understanding of the mysteries behind the smallest building blocks of matter.


Long before it was ever found, scientists anticipated quark-gluon plasma, albeit they thought it would look very different. Following the identification of quarks in the late 1960s and gluons in 1979, the predictions were realized in the 1970s and 1980s. When liberated from their nuclear bonds, quarks and gluons were predicted by physicists to manifest as a uniformly expanding gas. According to Berndt Mueller, a physicist at Duke University who began developing theoretical models for quark-gluon plasma in the 1980s, fluids often change to gas as they warm up. It seemed sense that quarks and gluons wouldn't leave nuclei until temperatures in the billions of degrees were reached.

Because of the broad range of theoretical possibilities and the impending arrival of experimental evidence, Mueller was drawn to the field. I was approximately 30 years old at the time, and you search about for new projects to work on when there is so much fascinating information to learn. During this time, scientists were inventing methods to collide heavy ions, which are nuclei that contain several protons and neutrons. They predicted that these collisions would produce temperatures and densities that would shatter subatomic particles. The Super Proton Synchrotron (SPS) accelerator at CERN started its own heavy ion collisions in 1986, and those produced the first proof for the new state of matter. The earliest heavy ion collisions, which took place in the 1970s at Lawrence Berkeley National Laboratory, were not strong enough to create quark-gluon plasma.

It took some time. The CERN team ultimately made its findings public in 2000, but even at that time there was disagreement among scientists on whether the data was reliable enough to declare a discovery. The RHIC at Brookhaven debuted the same year and began smashing heavy ions at energy greater than those at the SPS. This accelerator gathered enough evidence in just five years for scientists to proclaim quark-gluon plasma to be definitively discovered.

They had no idea what it would be like. The quark-gluon plasma appeared more like a nearly flawless liquid with practically no viscosity than an expanding gas. Particles move independently in a gas while cohesively in a liquid. The "better" the liquid is at being a liquid, the greater the interactions between the particles are—the more they can drag one another along. The RHIC measurements revealed that quark-gluon plasma has the lowest flow resistance of any material yet seen. This "was very much unexpected," according to Mueller.

The temperature of the quark-gluon plasma was for the first time measured in 2010, according to RHIC researchers. It was a blistering four trillion degrees Celsius, almost 250,000 times hotter than the center of the sun and much hotter than any other material ever made by humanity. According to Mueller, "things often become less of a perfect fluid the hotter they get." However, when you get to the critical temperature, it does the reverse and changes into a liquid. Scientists surmise that this peculiar behavior is caused by a powerful force. The strong force works across the whole plasma when the particles are hot enough to break free from protons and neutrons. This causes the particle mass to interact strongly with one another.


The precise moment at which the quarks and gluons escape their confinement is one of the most important unanswered mysteries about quark-gluon plasma. Where does quark-gluon plasma and ordinary matter meet? Gao queries. Where is the alleged crucial region where the quark-gluon plasma and nuclear matter coexist? One of the primary objectives of the new and improved tests will be to determine when and where that transition occurs as well as the minimum number of particles required to start the collective behavior.

Whether quark-gluon plasma is a fractal, or if its structure has a complicated, repeating pattern that seems the same at any size, whether you zoom in or out, is another matter of debate. These two characteristics of quark-gluon plasma, according to some researchers, might be explained by fractal theory, which may also provide light on the plasma's behavior. Quark-gluon plasma appears to have fractal structure, claims Airton Deppman of the University of So Paulo's Institute of Physics. We are also looking at whether the fractal structure endures the plasma to proton phase change.

Understanding the strong force, the most perplexing of nature's fundamental forces, may be improved with the aid of the answers to these questions. The interactions between quarks and gluons are described by quantum chromodynamics by giving them a characteristic termed color charge. It also explains why quantum chromodynamics spirals out of control so fast. In the theory of electromagnetism, color charge is analogous to electrical charge. Unlike electromagnetism, which only has positive or negative charges, QCD has three different charges: red, green, or blue. Furthermore, antimatter particles can be charged in antired, antigreen, or antiblue.

The electromagnetic force is carried by a particle called a photon in electromagnetism, which makes things very straightforward. But in QCD, the force carrier, the gluon, also has a color charge and may interact with both itself and quarks through the strong force. QCD has become unreasonably difficult due to these self-interactions and additional fees. The idea can be expressed in simply two lines, although Schenke claims that the problem still hasn't been fully solved. It has not yet been figured out how quarks and gluons, for instance, get confined in the proton during the confinement process.

Researchers believe that investigating quark-gluon plasma, the only environment in which they have ever been able to investigate unbound quarks, would help them understand confinement better. Freeing them and seeing how they recombine into protons, neutrons, and other particles that we can see from the detector is one method to discover that, says Schenke. As a result, the mechanisms in QCD that cause confinement may be better understood using experimental data from heavy-ion collisions.


The improved STAR detector and the brand-new RHIC experiment sPHENIX should allow researchers to monitor the plasma with the highest level of accuracy ever. For instance, the superconducting magnet on sPHENIX is nearly three times more powerful than the one on STAR. According to Brookhaven physicist David Morrison, "that's critical for many of the things we want to measure." "When particles collide, their courses are bent by magnetic fields because they shoot out in all directions. We can take a look at that to begin determining the type of particle it was and how much energy and momentum it had. For instance, the team is looking for composite particles called upsilons. Upsilon particles, which are made up of a bottom quark and an anti-bottom quark, can arise in collisions and then fly through the quark-gluon plasma as test probes to see how the plasma affects them. He continues, "We can actually untangle the physics underpinning many of the peculiar features of the quark-gluon plasma.

The experiment will also gain from the ability to capture far more data than was previously feasible, which entails recording many more collisions and the particles they produce. Around 10 petabytes of data are collected yearly by STAR; 150 petabytes will be collected annually by sPHENIX. With such growth, hitherto unanswerable questions will become possible.

STAR also possesses cutting-edge capabilities, including brand-new calorimeters for calculating particle energies and tracking detectors for recognizing particles with various electrical charges. The "forward" detectors, which can capture particles flying out of collisions at greater angles than before, including particles going in the same direction as the beams that flowed into the crash, are among the most important innovations, according to Lijuan Ruan of Brookhaven, one of STAR's spokesmen. Ruan, who has been working on STAR for many years and helped to create some of its first components about 20 years ago as a doctoral student, says, "Now that's basically it—we're not going to update anymore." When you really construct a detector that the entire cooperation can use, it's a different experience from when you only use one, she claims. "I feel honored," Before it shuts down, STAR, one of the initial RHIC experiments that contributed to the identification of quark-gluon plasma, will continue to run for a further three years.

The LHC's third phase, which started in July 2022 and will last until 2025, just got going in Europe. Approximately 100 times as many lead-lead collisions may now be studied at the LHC thanks to the most recent enhancements. The additional collisions will also improve measurement accuracy. Understanding the start of the quark-gluon plasma and how it progressively develops is one of the key objectives for run three, according to Luciano Musa, an ALICE experiment participant at the LHC.

The LHC collisions take place at greater energy than the RHIC experiments and result in a hotter, denser, and more durable quark-gluon plasma. In addition to producing a wider range of particles, these violent collisions also produce information on the plasma's characteristics. According to Musa, "the research at RHIC and LHC actually complement one other." Every time a discovery is made at the LHC, the RHIC team investigates if the same phenomena have been seen at lower energies.

Different facets of the plasma are shown by the various energy ranges. Vanderbilt University physicist Raghav Kunnawalkam Elayavalli completed their doctoral work at the LHC, but more recently joined the STAR and sPHENIX teams to concentrate on the particles emerging from lower-energy collisions. According to Kunnawalkam Elayavalli, "They are closer to the size of the plasma; they talk to it a lot more." "Imagine it as a party where there are plenty of people and you're running for the door. However, if you take your time and don't want to go right away, you have the opportunity to chat with individuals as you depart. They can get more information from the quark-gluon plasma at RHIC because particles have to go through it more slowly. The average distance a particle may travel without interacting with another particle is what scientists call the "transport characteristics," they continue. It provides information about the plasma's basic scale.


Quark-gluon plasma research is entering a new age that ought to take it beyond the fundamentals and toward real solutions to open problems. At RHIC, there came a time when physics was essentially, "Wow, this is happening—this is new physics," according to Kunnawalkam Elayavalli. "And right now, we're in the era of accuracy. We might inquire, "Why is this taking place?

The attempt to comprehend this unique form of matter is being led by RHIC and the LHC, but planned experiments elsewhere will also provide insights. In addition to the LHC, the SPS accelerator is still operational at CERN. The critical point at which protons and neutrons transform into quark-gluon plasma will be determined by colliding moving ions onto a stationary target in the NA61/SHINE experiment, which is scheduled to take place there. The Facility for Antiproton and Ion Research (FAIR) at GSI Darmstadt in Germany is slated to launch a second fixed-target experiment in 2028. Additionally, a collider known as the Nuclotron-based Ion Collider Facility (NICA) at the Joint Institute for Nuclear Research in Dubna, close to Moscow, will also test the critical point.

It's a thrilling period, adds Mueller. We are aware that the quark-gluon plasma was there in the early cosmos, but we lack the means to investigate it. This is how we can investigate a physical state that would be impossible to attain otherwise.