November 15, 2025
2 minute read
Scientists measured the temperature of the Universe immediately after the Big Bang
Quark-gluon plasma, a bizarre state of matter that mimics early space, is the hottest thing ever created on Earth.
An image of two golden beams colliding at close to the speed of light, June 14, 2000. The collision took place at the Relativistic Heavy Ion Collider (RHIC) operated by Brookhaven National Laboratory in Brookhaven, New York.
Over the past quarter century, scientists using a particle collider on Long Island have smashed the nuclei of gold atoms together at nearly the speed of light, creating the hottest matter ever created on Earth. The soup of particles born from the collision simulates the Universe as it was just after the big bang. Now, researchers have finally measured the temperature of this substance accurately for the first time.
After the gold nuclei collapsed, the protons and neutrons inside them melted into a boiling cloud quark-gluon plasma. This hell recreates the conditions of the beginning of time, when the Universe was too hot and dense to form ordinary atoms or even their constituents, such as protons and neutrons. Instead, the primordial cosmic soup was a flaming mess of fundamental particles called quarks, as well as gluonswho carry strong force which binds atomic nuclei together. “These are the building blocks of particles that make up the visible world, and we're trying to figure out how they work,” says physicist Zhangbu Xu of Brookhaven National Laboratory and Kent State University.
The experiment was conducted at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) as part of the STAR (solenoidal tracker at RHIC) detector. There, gold nuclei speeding through a 2.4-mile loop reach stunning speeds before colliding with each other and breaking apart into quark-gluon plasma. Each primordial cloud lasts only a fraction of a second, releasing many particles as it cools, including photons (particles of light) that decay into pairs of electrons and their antimatter counterparts, positrons.
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Physicists measured the mass ranges of these pairs of particles to measure the energy of the photons that produced them, which in turn revealed the temperature at which the photons were emitted. It revealed temperatures of an incredible 3.3 trillion degrees Celsius (5.94 trillion degrees Fahrenheit)—about 220,000 times hotter than the Sun's core. The scientists reported their results in Natural communications.
Determining this temperature will help physicists figure out when and how the fiery primordial Universe transitioned from quark-gluon plasma to the building blocks of atoms. These two states represent different phases of matter, akin to the more familiar solid, liquid and gaseous phases of everyday life. “We want to map out what might be called the most fundamental 'phase diagram' we know about,” says Frank Geurts of Rice University, a STAR spokesman. “What could be more interesting than a phase diagram of the fundamental building blocks of the Universe?”
The RHIC accelerator and its STAR experiment are in the final stages of their final run after launching 25 years ago. The machines will be stopped over the next few months to make way for a larger facility called Electron-ion colliderwhich is due to open in the early 2030s. Even after STAR's demise, however, scientists will be analyzing the latest batch of data over the next few years to further refine their measurements of this primordial cosmic fire.
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