What were the first moments of the universe? This is a mystery that scientists have been trying to solve for several decades. The ALICE collaboration at CERN is an expert on this subject: this detector (A Large Ion Collider Experiment) was designed to study quark-gluon plasma, the phase of matter that should have existed immediately after the Big Bang. And recently, the team was able to recreate and characterize this very first hypothetical matter thanks to the Large Hadron Collider (LHC).
What are the properties of matter at the extreme densities and temperatures of the early universe? Scientists from the ALICE collaboration finally got answers to some questions. The resulting matter lasted only a fraction of a second, but long enough for scientists to study its characteristics for the first time.
It turned out that the plasma is liquid, and this discovery may provide a better understanding of how the early Universe evolved in the first microseconds after the Big Bang. To reproduce this primordial matter, the team initiated collisions of heavy (lead) ions at high energy (5 TeV) within the LHC.
Plasma flowing like water
Recall that quarks are elementary particles that combine to form protons and neutrons (among others). Quarks are bound together by a strong interaction mediated by particles called gluons. In LHC collisions, the temperature is more than 100,000 times the temperature at the center of the Sun. In these extreme conditions, protons and neutrons decay, releasing their constituent quarks and gluons: the famous quark-gluon plasma is obtained.
The goal of the ALICE collaboration is to study this plasma to understand how it could have produced the particles that make up the matter of our universe today. To do this, a giant detector was installed at a depth of 56 meters underground, receiving particle beams from the LHC. Lead particles launched at close to the speed of light were used to recreate the very first matter to appear after the Big Bang. The experiment has already been successfully conducted in the past, but this time the scientists managed to test the characteristics of the plasma in detail.
The Quark-gluon plasma showed characteristics typical of an ideal liquid, with virtually no resistance to the flow. The flow of a liquid is determined by the ratio between its viscosity and density. Although the viscosity and density of quark-gluon plasma are about 16 orders of magnitude higher than that of water, the researchers found that the ratio of viscosity to density of the two types of liquid was the same. In other words, the very first state of matter would flow just like water!
A surprising similarity
Shortly after the Big Bang, the early universe consisted of a dense, hot “soup” of quarks and gluons. After a few microseconds, this mixture cooled and formed the first building blocks of the matter that makes up the entire universe. Thus, the matter that surrounds us today, theoretically, has very different properties than that primitive soup. Liquids like water, for example, are based on a collection of atoms and molecules that are much larger than primitive particles and are held together by much weaker forces. But recent experiments show that, despite these differences, the kinematic viscosity – the ability of a liquid to flow – of primordial plasma is very similar to that of ordinary liquids.
The viscosity of the liquid can vary significantly depending on the temperature. However, there is a lower limit to this almost universal viscosity, which depends on fundamental physical constants (such as Planck’s constant). However, the results of the study show that the viscosity of the quark-gluon plasma is very close to this universal lower limit of viscosity. “This study is a fairly rare and encouraging example of the possibility of making quantitative comparisons between extremely disparate systems,” Professor Matteo Baggioli, one of the team members, said in a statement.
“These results are also a new illustration of the power of physics to translate general principles into concrete predictions of complex properties, such as fluid flow in exotic types of matter, such as quark-gluon plasma,” he added. Quantum chromodynamics is a theory that can describe the powerful forces of interaction between quarks and gluons (and thus the cohesion of the atomic nucleus), but it is not enough to fully understand the properties of primordial plasma. Therefore, this striking similarity to hydrodynamics is a new step forward in this field of research. Scientists hope to discover even more details about this plasma as the CERN accelerator is upgraded. Further research will also provide a better understanding of how quarks and gluons organize into protons and neutrons – a step that could lead to extremely rapid expansion (cosmic inflation) of the universe.