Pushing scientists closer than ever to understanding the Big Bang, Rice University physicists have finally measured the temperature of quark-gluon plasma (QGP) over its evolution.
The work illuminates a state of matter that scientists believe was common in the short microseconds immediately following the Big Bang. As revealed in a paper in Nature Communications, the study is a monumental step toward finally constraining the universe’s first moments.
Following the Big Bang
Understanding the early conditions that occurred immediately after the Big Bang is challenging. Not only were the typical temperatures so extreme that few present-day events recreate them, but direct access is also impossible. The closest researchers can come is to artificially create QGP, a state of matter which theoretically filled the universe before cooling over hundreds of thousands of years to produce the first atoms. Researchers have only been observing QGP directly for a quarter century, with CERN making the first discovery in 2000, based on predictions dating back to the 1970s.
Decoding QGP’s thermal profile required researchers at New York’s Brookhaven National Laboratory to observe thermal electron-positron pairs emitted by ultra-relativistic heavy-ion collisions in the Relativistic Heavy Ion Collider (RHIC). Uncertainty had plagued previous attempts to get an accurate temperature measurement, including whether a Doppler-like effect was interfering with the readings.
“Our measurements unlock QGP’s thermal fingerprint,” said co-author Frank Geurts, a professor of physics and astronomy and co-spokesperson of the RHIC STAR collaboration. “Tracking dilepton emissions has allowed us to determine how hot the plasma was and when it started to cool, providing a direct view of conditions just microseconds after the universe’s inception.”
Measuring Quark Gluon Plasma
Properly observing QGP is exceptionally challenging, yet essential to understanding the state of matter. Temperatures impact the quarks and gluons’ properties, but these temperatures are so extreme, exceeding trillions of Kelvins, that they tend to affect the measurement devices themselves. The challenge was to develop an unobtrusive measurement concept with the necessary resolution and penetration power to measure QGP’s thermal conditions accurately.
“Thermal lepton pairs, or electron-positron emissions produced throughout the QGP’s lifetime, emerged as ideal candidates,” Geurts said. “Unlike quarks, which can interact with the plasma, these leptons pass through it largely unscathed, carrying undistorted information about their environment.”
They hypothesized that the pair’s energy distribution would record the QDP’s temperature, observable by tracking emission data over the plasma’s lifetime and producing a temperature average with a technique involving use of a penetrating thermometer. While these pairs carry the information that scientists needed, measuring them was no simple task either. They are quite rare among a large sea of particle debris, which requires that any device put to the task feature unprecedented sensitivity.
The team had to utilize a special detection system at RHIC, which they carefully calibrated to seek the rare lepton pairs. Despite their success in utilizing this novel technique, there were more hurdles to overcome. Background processes mimicking thermal signals added hard-to-remove noise to the observations and statistical data, which brings with it technological limitations.
Filling in the Early Universe
The final results identified two separate average temperatures, determined by whether the pair was high or low mass. Lower mass pairs displayed an average temperature of 2.01 trillion kelvin, as predicted by theoretical models. However, the higher mass pair greatly exceeded that figure, reaching an average of 3.25 trillion Kelvin. From these results, the team determined that lower temperature thermal radiation correlates with later emissions near the phase transition, while high mass emission reflects the hotter stage at the beginning of QGP evolution.
“This work reports average QGP temperatures at two distinct stages of evolution and multiple baryonic chemical potentials, marking a significant advance in mapping the QGP’s thermodynamic properties,” Geurts said.
This study lays the groundwork for further interrogation of the universe’s earliest stages. Next up is filling in the QCD phase diagram, which the team hopes their experimental data will help understand the QGP’s temperature across its evolutionary phases. The QCD diagram will map out how matter behaves under the severe heat and density conditions found immediately following the Big Bang, but now only exists in some of the universe’s most extreme environments, such as neutron stars.
“Armed with this thermal map, researchers can now refine their understanding of QGP lifetimes and its transport properties, thus improving our understanding of the early universe,” Geurts said. “This advancement signifies more than a measurement; it heralds a new era in exploring matter’s most extreme frontier.”
The paper, “Temperature Measurement of Quark-Gluon Plasma at Different Stages,” appeared in Nature Communications on October 14, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.