The Baby Universe Really Was a Goopy Soup, Research Suggests

In the moments following the Big Bang, the extreme heat and pressure coerced matter into a goopy mix of tiny particles. But how goopy? The answer had remained rather unclear to physicists—until now.

Quarks and gluons are fundamental particles that make up protons and neutrons. These particles are typically bound together, but the extreme conditions after the Big Bang, physicists believe, led to their separate existence in a soupy form called the quark-gluon plasma (QGP). In a recent Physics Letters B paper, physicists at CERN’s CMS Collaboration and MIT have observed and confirmed for the first time that QGP does indeed behave like a liquid. The quarks in the plasma create waves as they speed through the material, “similar to a duck trailing ripples through water,” the researchers explained to MIT News.

“Now we see the plasma is incredibly dense, such that it is able to slow down a quark, and produces splashes and swirls like a liquid,” explained Yen-Jie Lee, an MIT physicist who led the new research. “So quark-gluon plasma really is a primordial soup.”

The universe’s goopy days

There isn’t much that scientists know for certain about the super early universe. Physicists have proposed a number of theories and models to capture aspects of our universe’s early days. However, the challenge of confirming these ideas through experiments meant scientists were hesitant to draw any firm conclusions.

Related article: New York’s ‘Big Bang Machine’ Passes Critical First Test

That said, the QGP had been one of the few concepts scientists generally agreed on. The primordial stew—boiling at around a few trillion degrees—eventually cooled down to create protons and neutrons that make up matter in the universe. One model, devised by MIT physicist Krishna Rajagopal, argued that a particle flying through the QGP should produce a wake in the plasma, which would ripple and splash like a liquid.

“This is something that many of us have argued must be there for a good many years, and that many experiments have looked for,” said Rajagopal, who wasn’t directly involved in the new work.

Studying the cosmic soup

The new research verifies Rajagopal’s account of the QGP, using a neutral, electrically weak particle called the Z boson as a marker to track the movement of quarks in the plasma. Since the Z boson had virtually no effect on the plasma, any wave-like movement would be from the quark, the researchers hypothesized.

For the experiment, the team used data from CERN’s Large Hadron Collider. But given the instability of the QGP, even the world’s most powerful particle accelerator only held the goop—a “droplet” at that—together for just under a quadrillionth of a second, according to the researchers.

The team looked through 13 billion collisions, of which only 2,000 produced the Z boson they were looking for. They then mapped each of these events according to energy levels in the QGP droplet, finding a consistent, “fluid-like pattern of splashes in swirls”—the wake effect, as predicted by Rajagopal’s model, according to MIT News.

Understanding the universe’s beginnings

What’s more, the researchers anticipate that the methods of the new study will greatly advance our understanding of matter in the early universe. Subsequent experiments will investigate the exact size, speed, and extent of these wakes, which should reveal more about the properties of the plasma.

“[The study] has brought [us] the first clean, clear, unambiguous evidence for this foundational phenomenon,” Daniel Pablos, a physicist at Oviedo University in Spain who was not involved in the study, told MIT News.

“We’ve gained the first direct evidence that the quark indeed drags more plasma with it as it travels,” Lee added. “This will enable us to study the properties and behavior of this exotic fluid in unprecedented detail.”