Universe Today
In its earliest moments, the Universe was hot and dense. A plasma sea of quarks and gluons out of which hydrogen, helium, and humans eventually formed. This early cosmic state is sometimes called the primordial soup, and thanks to new research, we now know just how fitting the term is.
Although we've long known of the early Big Bang, it is difficult to understand its full nature. Some of it we can glean through theoretical calculations and things such as the ratio of hydrogen to helium, but theory can only take you so far. Imagine trying to calculate whether a region of water is solid, liquid, or gas simply from computer simulations of water molecules. There is no bulk material on Earth with the density and temperature of the early Universe.
But the interiors of atomic nuclei come close. So one way to study the early Universe is through particle physics experiments. Recently a team at CERN has been colliding heavy ions. The particles collide with each other at nearly the speed of light, creating a brief mix of quarks and gluons similar to the primordial soup. This state only exists for a tiny fraction of a second, so scientists can't observe the state directly. What they can do is look at the cascade of particles created by the plasma state. This is similar to studying water waves by looking at how the spray wets the shore.
In this recent study, the team focused on the interaction of Z-bosons, which are a type of weak interaction carrier particle, similar to photons for electromagnetism. They compared the observed results with different models of the quark-gluon plasma (QGP). The best model is one in which the plasma is soupy.
What they actually found was evidence of wakes within the plasma field. If you move your hand through a pool of water, it creates a wake of ripples. This is because water is a liquid. If you move your hand through sand, the sand grains move, but they don't create ripples through the sand. By finding evidence of wakes, the team now knows that particles moving through a quark-gluon plasma behave like a stone through water. The plasma has a soupy, fluid-like behavior.
Knowing that matter in the early Universe was a thick soup will help us better understand its earliest moment. Things such as shock waves behave differently through fluids than through gas or solids, and this can affect not only how the first atoms formed but also how the seeds for galaxies and black holes first appeared.
In future studies, the team hopes to pin down more details of these wakes, such as how quickly they move through the plasma and how large they are. This will let them determine bulk properties such as the fluid density and viscosity. In other words, how soupy the primordial soup really was.
Reference: CMS Collaboration. "Evidence of medium response to hard probes using correlations of Z bosons with hadrons in heavy ion collisions." *Physics Letters B* (2025): 140120.
