Record hot temperature: Trillions of degrees measured shortly after the Big Bang.

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Technological Innovation Website Editorial Team - October 30, 2025

Partial view of the giant STAR detector, designed to study the early moments of the Universe. [Image: BNL]

Record temperature

If you like records, you'll like this one: It's impossible to imagine a higher temperature than that of the primordial soup, called quark-gluon plasma , which came into existence shortly after the Big Bang.

Scientists have just captured the temperature profile of this ultra-hot state of matter, generated by a detector known as STAR, located at the Relativistic Heavy Ion Collider (RHIC) in the USA. STAR is also the name of the international collaboration that built and operates the equipment, bringing together hundreds of scientists from 55 institutions in 12 countries.

By analyzing rare electron and positron emissions from atomic collisions, the team determined the precise temperatures at different stages of the primordial plasma's evolution. The results not only confirm theoretical predictions but also refine the phase diagram of quantum chromodynamics (QCD), which maps the behavior of matter under extreme conditions.

"Our measurements reveal the thermal signature of the quark-gluon plasma," said Frank Geurts, a member of the STAR Collaboration. "Tracking dilepton emissions allowed us to determine the plasma's temperature and when it began to cool, providing a direct insight into conditions just microseconds after the Universe formed."

The diagram illustrates the properties of matter with baryonic chemical potential (equivalent to net baryonic number density) and temperature, with reference points of normal nuclei, neutron stars, and the phase transition to quark-gluon plasma. [Image: STAR Collaboration - 10.1038/s41467-025-63216-5]

Trillions of degrees

The results showed two distinct temperature ranges, depending on the mass of the emitted electron pairs.

In the low-mass range, the average temperature reached approximately 2.01 trillion Kelvin, consistent with theoretical predictions and with the temperatures observed when plasma transitions to ordinary matter. In the higher-mass range, the average temperature was approximately 3.25 trillion Kelvin, representing the initial, hotter phase of the plasma.

This contrast suggests that low-mass electrons were produced later in the evolution of the plasma, while high-mass electrons were born in its earlier, more energetic stage.

By precisely measuring the temperature of quark-gluon plasma at different points in its evolution, scientists obtained crucial experimental data to complete the QCD phase diagram. This diagram is essential for mapping how fundamental matter behaves under immense heat and density, similar to the conditions that existed moments after the Big Bang, but which are also present today in astrophysical phenomena, such as neutron stars.

"With this thermal map, researchers can now refine their understanding of the lifespan of quark-gluon plasma and its transport properties, thus enhancing our understanding of the early Universe," said Geurts. "This advance means more than a simple measurement; it heralds a new era in the exploration of the most extreme frontier of matter."

Schematic representation of an Au+Au collision reconstructed with the STAR detector. [Image: STAR Collaboration - 10.1038/s41467-025-63216-5]

What type of thermometer was used?

Measuring temperatures in environments where no instrument can physically survive required a great deal of creativity and hard work.

The team overcame this challenge by studying thermal pairs of electrons and positrons released during high-speed collisions of atomic nuclei in the collider. These emissions allowed them to reconstruct the plasma temperature during its formation and cooling.

"Thermal lepton pairs, or electron-positron emissions produced over the lifetime of quark-gluon plasma, have emerged as ideal candidates," said Geurts. "Unlike quarks, which can interact with the plasma, these leptons pass through it virtually unscathed, carrying undistorted information about their environment."

This required unprecedented technological improvements to the STAR detector to enable it to isolate low-energy lepton pairs and reduce background noise. The central idea is that the energy distribution of these pairs directly reveals the plasma temperature, an approach known as "penetrating thermometer"—the emissions are integrated to produce an average thermal profile.

Previous estimates of the post-Big Bang temperature had a great deal of uncertainty, in addition to being distorted by movement within the plasma, which created deviations similar to the Doppler effect, or by confusion about whether the readings reflected the plasma itself or later stages of its decay.

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