Scientists have rebuilt the hot particle mixture that filled the cosmos following the Big Bang, and they encountered a surprise in its behavior. At the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York, researchers smashed gold nuclei together close to the speed of light. The particles that emerged from these collisions displayed a surprising drop in their properties.
Gold Nuclei Collide at Near-Light Speed
Researchers sent gold nuclei hurtling toward each other at great velocity. Upon impact, particles flew outward sideways, and the scientists recorded how forcefully they departed. A steady progression of those fluctuations with changing collision energy was anticipated. That expectation was not met: the fluctuations shrank before swelling once more.
That dip could be a sign of a long-sought “critical point,” a special set of conditions at which nuclear matter changes the way it transforms from one form to another. The signal is strong enough that it is very unlikely to be a statistical accident, but the researchers cautioned that the dip is a hint, not proof.
What the Dip Could Mean
A critical temperature marks the line where liquid and steam lose their separate identities for water. That same kind of transformation, Manikandhan, a postdoctoral physics scholar at The Ohio State University and study co-author, told Live Science, has long been suspected to exist for nuclear matter as well. The “critical point”, which stands for the critical point, serves as a guidepost in the rulebook governing matter pushed to its limits.
When temperatures climb to their highest levels, ordinary matter flows steadily into a state of quark-gluon plasma. However, when density rises instead, the transformation might suddenly occur. That point where the gradual melting gives way to a sudden shift is called the critical point. Certain recent calculations position this very point within the range of RHIC’s lower-energy collisions.
“But all of this is still conjectured and there is nothing concrete yet, either from the experimentalists or theorists,” Manikandhan said.
Why the Fireball Matters
Scientists study how the matter produced in these collisions acts, since its likeness is to the substance that filled the cosmos just a few microseconds after the Big Bang. Tracing its behavior gives researchers insight into how the universe changed from a hot mix of quarks and gluons into the protons and neutrons that form everything around us today.
The researchers ran RHIC at a range of collision energies. The lower the energy, the more tightly the colliding matter was squeezed. For its lowest-energy runs, STAR used a “fixed-target” setup, in which a beam of gold nuclei strikes “a thin gold foil placed inside the detector,” Manikandhan said, instead of a second, oncoming beam. This produces the densest matter RHIC can make.
Electron volts measure collision energies, representing how much energy an electron picks up after being pushed through 1 volt. The field typically uses billions of electron volts, or giga electron volts (GeV), for its calculations. A single GeV corresponds to roughly the energy contained within the mass of a proton, per Einstein’s famous equation E = mc2. The team analyzed roughly 1 billion collisions at energies between 3 and 7.7 GeV per pair of colliding protons or neutrons. That is the bottom of RHIC’s span, which extends to 200 GeV.
Measuring the Dip
For each crash, the researchers tracked how far charged particles were thrown outward from the fireball — a measure known as transverse momentum. They next searched for links between those particles. A link shows whether two things tend to shift together.
The researchers examined pairs of particles from the same collision to see if they were both thrown harder than average, or both more gently. This shows something about the fireball as a whole. A fireball that is slightly hotter, or expands with greater force, gives all of its particles an extra sideways push at the same time.
“Those correlations reflect how much the temperature and the flow of the fireball fluctuate,” Manikandhan said.
Close to a critical point, the matter’s heat capacity — the amount of energy needed to raise its temperature — is expected to shoot up. That makes the fireball’s temperature harder to budge, so the correlations should weaken. “If the matter approaches a critical point or a phase change, we would expect to see those correlations change in an unusual, non-smooth way as we vary the collision energy,” Manikandhan said.
The Signal Is Strong
The team saw the dip in the most head-on collisions. “Instead of changing smoothly with energy, the correlations show a dip,” Manikandhan said. The researchers compared the data with a smooth trend, and the deviation stood out.
Sept. 22, the research was made public in Physical Review Letters.
What Comes Next
The researchers warn that further investigation is required before confirmation can be claimed. However, the signal’s strength suggests that it is extremely improbable to be a mere statistical coincidence.
There is still no proof of the critical point. Neither experimenters nor theorists have produced anything definite. The fresh data makes the case for its existence stronger, but it has not settled the matter.
| Measurement | RHIC Run | Energy Range |
|---|---|---|
| Collision energy | Low-energy fixed-target | 3–7.7 GeV |
| Collision energy | Full RHIC range | Up to 200 GeV |
| Collision count | STAR experiment | Roughly 1 billion |
Cosmology gains from this research as well. Tracing how the early universe cooled and condensed into protons and neutrons sheds light on the structure of the cosmos today. The dip in the data provides a fresh way to study that process.
Source material: “Scientists recreate the universe's first moments and find something they didn't expect,” Live Science.
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