Our universe started with a bang that blasted everything into existence. But what happened next is a mystery. Scientists think that before atoms formed—or even the protons and neutrons they’re made of—there was probably a hot, soupy mix of two elementary particles called quarks and gluons, churning through space as a plasma. And because no one was around to observe the first moments of the cosmos, a coalition of researchers is trying to re-run history.
Using the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, they have essentially created a “Little Bang” and are using it to probe the properties of that quark-gluon plasma. The findings will help cosmologists refine their still-fuzzy picture of the early universe, and how the oozy, blistering state of infant matter cooled and coalesced into the planets, stars, and galaxies of today.
“We think about a microsecond after the Big Bang, the universe was in this stage,” says physicist Rongrong Ma, who works with the Solenoidal Tracker at the Relativistic Heavy Ion Collider, or STAR, a detector devoted to investigating the quark-gluon plasma. “So if we can understand from experiments the properties of such matter, this will feed into our understanding of how the universe evolved.”
Scientists aren’t sure how long this plasma stage lasted—it could have been anywhere from a few seconds to thousands of years. It might even still exist today in the dense cores of neutron stars, or get made when super-high-energy particles crash into Earth’s atmosphere, so learning about its properties could help characterize the physics of the most extreme cosmic environments.
These early days of the universe are impossible to study with telescopes, which can only reach as far back as the cosmic microwave background—the first light that emerged from the dense early universe, a hundred thousand years after the Big Bang. Everything before that is both literally and figuratively a dark era of cosmology. Theoretical simulations can help fill in that gap, says Jaki Noronha-Hostler, a nuclear physicist at the University of Illinois Urbana-Champaign, but detectors like STAR “allow you to experimentally understand a system that’s very similar to the Big Bang.”
In addition, quarks and gluons are never found solo in nature, making it difficult to study them in isolation. “We can’t just pluck one out and examine it,” says Helen Caines, a physicist at Yale University and spokesperson for the STAR experiment. Instead, they’re stuck in composite states: protons, neutrons, and more exotic matter like upsilons, pions, and kaons. But at high enough temperatures, the boundaries between these composite particles begin to blur. “And that is the quark-gluon plasma,” Caines says. They’re still confined to some volume, but the quarks and gluons within this space are no longer fused together. In fact, she says, “plasma” might be a bit of a misnomer, because it actually behaves more like a fluid, in that it flows.
In March, scientists at Brookhaven reported in Physical Review Letters that they were able to generate the quark-gluon plasma for a brief blip in time by accelerating two beams of gold nuclei close to the speed of light, then smashing them into each other. Then came the clever bit: They used this collision to calculate how hot the post-Big Bang plasma would have been.
To do this, they needed to look for upsilons, which weren’t actually present at the beginning of the universe but are a byproduct of the Brookhaven beam collisions. Upsilons are comprised of a quark and its antimatter twin bound together in one of three configurations: a tightly tethered “ground state” and two excited states, one looser than the other. Slamming the gold nuclei together produces a slew of them in each of these three states.
“The idea is to use these particles as a thermometer,” Caines says. A plasma like the one that theoretically existed microseconds after the Big Bang can rip these upsilons apart; interactions with the free quarks and gluons melt them down to their most basic elements. And each state has its own “melting point.” Ground-state upsilons would need the most energy—the hottest temperatures—to fall apart, and the more loosely bound quark-antiquark pairs would need less. So recreating post-Bang plasma conditions, then counting how many upsilons of each state survived, would reveal what the temperature was in those first moments of the universe.
That, in turn, would tell physicists about other properties of the quark-gluon plasma, because its temperature is intrinsically linked to its density, pressure, and viscosity. Ultimately, scientists want to be able to solve what they call an equation of state: a mathematical expression describing all of the plasma’s properties, how they influence each other, and how they evolve with time.
The quark-gluon plasma is a unique system: It’s extremely hot but also tiny—on the order of the diameter of a proton, Noronha-Hostler says. So it doesn’t obey the usual laws of how fluids act. “We can write down equations, but we can’t solve them,” she says. Once this behavior is understood, cosmologists can extrapolate how long the universe must have been in this soupy state, and what physical processes drove a transition into the more familiar protons, neutrons, and other particles that matter is made up of today.
This was actually the second time scientists had done such a test; the first was in 2012 using the Large Hadron Collider at CERN, which accelerates particles to energies a factor of 25 higher than what can be achieved at Brookhaven. Studying the plasma at lower energies helps scientists understand the temperature dependence of its properties, giving them another data point that can be used to tune theoretical models of the early cosmos. “In the field that we’re in, you really want to do things at a range of energies,” says Brookhaven physicist David Morrison, who was not involved in the work. Hotter plasma is a better probe for earlier in the universe, but the lower temperature state made at Brookhaven is closer to what the system may have looked like when the quarks and gluons began to merge.
This time, after smashing gold nuclei in the STAR detector, the researchers counted how many upsilons they saw in each state and compared it to how many should have been created by the collision—before the plasma melted them. They found that about 60 percent of the upsilons in the ground state, and 70 percent of those in the intermediate state, were missing, presumed melted. Upsilons with the most loosely bound quark and antiquark pair seemed to be completely gone.
Their newly collected data confirmed past measurements from the STAR team, which determined a lower limit on the temperature needed to make the plasma: at least a trillion degrees. (That’s almost a million times more sizzling than the center of the sun.) Their atom smashing had managed to achieve this temperature for an incredibly brief 10-23 of a second.
The STAR team is gearing up to redo their upsilon measurement at Brookhaven with about 20 times more data, which will help nail down whether the particles with the most loosely bound quark-antiquark pair truly vanished or just survived at rates too low to be detected. A different detector, called sPHENIX, will also turn on at the lab within the next month. The thousand-ton instrument, built around an ultracold, superconducting magnetic core, will be able to investigate this melting effect with even higher precision. “This STAR paper had hundreds of upsilons,” says Morrison, who is a spokesperson for the sPHENIX collaboration. “We’ll be measuring tens of thousands.”
Ultimately, upsilons are only one part of the puzzle when trying to understand the properties of the quark-gluon plasma, Ma says. Physicists can also look for individual quark collisions, study photons emanating from the plasma, or try to figure out the types and production rates of other particles resulting from the gold nuclei blasts. These different types of measurements will help physicists connect phenomena they understand into explanations for what they don’t. “We try to put all these together, using a multi-messenger approach to build a full picture of the quark-gluon plasma,” Ma says—“for a theory that can explain everything.”
Update 4-5-2023 7:15 pm ET: This story was updated to correct some details about how the STAR team calculated their results.
