Scientists Have Measured the Temperature of the Universe Just After the Big Bang
For the past 25 years, physicists at Brookhaven National Laboratory on Long Island have been recreating one of the most extreme environments imaginable: the conditions of the universe mere microseconds after the Big Bang. By smashing together the nuclei of gold atoms at nearly the speed of light, they have generated the hottest matter ever produced on Earth—a seething, ephemeral state known as quark-gluon plasma. Now, for the first time, researchers have precisely measured the temperature of this primordial cosmic soup, providing new insights into the earliest moments of our universe.
At the heart of this groundbreaking research is the Relativistic Heavy Ion Collider (RHIC), a massive particle accelerator that propels gold nuclei around a 2.4-mile circular track before smashing them head-on. When these nuclei collide, the immense energy causes protons and neutrons inside them to “melt,” freeing their constituent particles—quarks and gluons—into an extraordinary plasma. This quark-gluon plasma is believed to mirror the state of matter that existed just after the Big Bang, before the universe cooled enough for quarks and gluons to combine into the protons, neutrons, and eventually atoms that make up the visible world today.
“This is the closest we can get to the conditions of the early universe here on Earth,” explains physicist Zhangbu Xu of Brookhaven National Laboratory and Kent State University. “By studying quark-gluon plasma, we’re trying to understand how the fundamental building blocks of matter behave under extreme conditions and ultimately how our universe evolved from its fiery origins.”
The experiment relies on the STAR (Solenoidal Tracker at RHIC) detector, which records the shower of particles produced as the quark-gluon plasma rapidly cools and disintegrates. Among these particles are photons—particles of light—that emerge from the plasma and quickly decay into pairs of electrons and positrons (the antimatter counterparts of electrons). By measuring the properties of these electron-positron pairs, the scientists can infer the energy of the photons that created them, which in turn reveals the temperature of the plasma at the moment these photons were emitted.
Using this innovative method, the researchers determined that the quark-gluon plasma reached a staggering temperature of approximately 3.3 trillion degrees Celsius (about 5.94 trillion degrees Fahrenheit). To put this in perspective, this temperature is roughly 220,000 times hotter than the core of our sun, which itself burns at around 15 million degrees Celsius. Such an extreme measurement confirms just how incredibly hot and dense the early universe must have been, and it provides critical data for refining our understanding of the Big Bang and the subsequent evolution of matter.
The results of this research were published in the journal Nature Communications, marking a significant milestone in the field of high-energy nuclear physics. Determining the precise temperature of the quark-gluon plasma is crucial for physicists attempting to map the phase transitions that occurred in the early universe. Much like water can exist as ice, liquid, or steam—different phases of the same substance—matter itself can exist in various phases under different conditions. The transition from quark-gluon plasma to the more familiar protons and neutrons represents one of the most fundamental phase changes in the cosmos.
Frank Geurts, a spokesperson for the STAR experiment and a physicist at Rice University, emphasizes the importance of this research: “We want to map out what you could call the most fundamental ‘phase diagram’ that we know of—the phase transitions of the universe’s fundamental building blocks. What could be more interesting or important than understanding the conditions that led to the formation of everything we see around us?”
The RHIC accelerator and the STAR detector have been instrumental in expanding our knowledge of the early universe for a quarter of a century. However, this marks the final phase of their operations, as the facility prepares to shut down in the coming months. The reason for this transition is the upcoming Electron-Ion Collider (EIC), a next-generation particle accelerator slated to open in the early 2030s. The EIC will offer even more powerful tools to probe the structure of matter and the forces that govern it, promising to deepen our understanding of nuclear physics and the origins of the universe.
Although the STAR experiment will soon conclude its active data collection, scientists will continue analyzing the treasure trove of information gathered during its final runs. This ongoing analysis will help