Physicists capture trillion degree heat from the Big Bang’s primordial plasma

Tracking Heat in the Early Universe

Measuring temperatures in environments where no instrument can physically survive has long challenged scientists. The team overcame this by studying thermal electron-positron pairs released during high-speed collisions of atomic nuclei at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York. These emissions provided a way to reconstruct how hot the plasma became as it formed and cooled.

Earlier temperature estimates had been uncertain, often distorted by motion within the plasma that created Doppler-like shifts or by confusion about whether the readings reflected the plasma itself or later stages of its decay.

"Our measurements unlock QGP's thermal fingerprint," said 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."

Opening a New Thermal Window

The quark-gluon plasma is a unique state of matter where the basic building blocks of protons and neutrons, quarks and gluons, exist freely rather than being confined inside particles. Its behavior depends almost entirely on temperature. Until now, scientists lacked the tools to peer into this hot, fast-expanding system without distorting the results. With QGP reaching temperatures of several trillion Kelvins, the challenge was to find a "thermometer" capable of observing it without interference.

"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."

Detecting these fleeting pairs among countless other particles required extremely sensitive equipment and meticulous calibration.

Experimental Breakthrough at RHIC

To achieve this, the team refined RHIC's detectors to isolate low-momentum lepton pairs and reduce background noise. They tested the idea that the energy distribution of these pairs could directly reveal the plasma's temperature. The approach, known as a penetrating thermometer, integrates emissions across the QGP's entire lifetime to produce an average thermal profile.

Despite challenges in distinguishing genuine thermal signals from unrelated processes, the researchers obtained highly precise measurements.

Distinct Temperature Stages Revealed

The results showed two clear temperature ranges, depending on the mass of the emitted dielectron pairs. In the low-mass range, the average temperature reached about 2.01 trillion Kelvin, consistent with theoretical predictions and with temperatures observed when the plasma transitions into ordinary matter. In the higher mass range, the average temperature was around 3.25 trillion Kelvin, representing the plasma's earlier, hotter phase.

This contrast suggests that low-mass dielectrons are produced later in the plasma's evolution, while high-mass ones come from its initial, more energetic stage.

"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.

Mapping Matter Under Extreme Conditions

By precisely measuring the temperature of the QGP at different points in its evolution, scientists gain crucial experimental data needed to complete the "QCD phase diagram," which is essential for mapping out how fundamental matter behaves under immense heat and density, akin to conditions that existed moments after the big bang and are present in cosmic phenomena like 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."

Contributors to the study include former Rice postdoctoral associate Zaochen Ye (now at South China Normal University), Rice alumnus Yiding Han (now at Baylor College of Medicine), and current Rice graduate student Chenliang Jin. The work was supported by the U.S. Department of Energy Office of Science.