Scientists have recreated the first matter of the universe

What were the first moments of the Universe like? It's a mystery that scientists have been trying to unravel for decades. The ALICE collaboration at CERN is a specialist in the subject: this detector ( A Large Ion Collider Experiment ) was designed to study quark-gluon plasma, a phase of matter that would have existed just after the Big Bang. And the team recently succeeded in recreating and characterizing this very first hypothetical material, using the Large Hadron Collider (LHC).

What are the properties of matter at the extreme densities and temperatures of the beginnings of the Universe? The scientists of the ALICE collaboration finally have some answers. The resulting material only persisted for a fraction of a second, but long enough that scientists could study its characteristics for the very first time.

It turns out that this plasma is of the liquid type and this discovery could provide a better understanding of how the early Universe evolved during the first microsecond after the Big Bang. To reproduce this primitive material, the team initiated high energy (5 TeV) heavy (lead) ion collisions within the LHC.

Plasma that flows like water

As a reminder, quarks are elementary particles that combine to form protons and neutrons (among others). Quarks are linked together via a strong interaction, mediated by particles called gluons. The collisions that occur in the LHC generate temperatures more than 100,000 times higher than those in the center of the Sun. Under these extreme conditions, the protons and neutrons decompose, releasing the quarks and gluons that constitute them in passing: the famous quarks-gluon plasma is thus obtained.

The objective of the ALICE collaboration is to study this plasma to understand how it was able to give birth to the particles that today make up the matter of our universe. To do this, a gigantic detector was installed 56 meters underground to receive the particle beams from the LHC. Lead particles, launched at a speed close to that of light, thus made it possible to recreate the very first material that appeared after the Big Bang. The experiment had been performed successfully in the past, but this time the scientists had time to probe the characteristics of the plasma in detail.

The quark-gluon plasma exhibited characteristics typical of a perfect liquid, exhibiting almost no resistance to flow. The flow of a fluid is determined by the ratio of its viscosity to its density. While the viscosity and density of quark-gluon plasma are about 16 orders of magnitude greater than that of water, the researchers found that the relationship between the viscosity and density of the two types of fluids was the same. In other words, the very first state of matter would flow in the same way as water!

A surprising similarity

Shortly after the Big Bang, the early Universe consisted of a dense, hot "soup" of quarks and gluons. A few microseconds later, this mixture cooled down to form the first building blocks of matter that makes up the entire universe. Thus, the material which surrounds us today has in theory very different properties from those of this primitive soup. Fluids such as water, for example, rely on assemblies of atoms and molecules much larger than primitive particles, held together by much weaker forces. But these recent experiments show that despite these differences, the kinematic viscosity - or the ability of a fluid to flow out - of primitive plasma is very similar to that of ordinary liquids.

The viscosity of a 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 suggest that the viscosity of the quark-gluon plasma is very close to this universal lower limit of viscosity. "This study constitutes a rather rare and pleasing example of the possibility of establishing quantitative comparisons between extremely disparate systems", underlines in a press release Professor Matteo Baggioli, one of the members of the team.

These results are also a new illustration of the power of physics, which can translate general principles into specific predictions of complex properties, such as liquid flow in exotic types of matter such as quark-gluon plasma, adds the professor.

Quantum chromodynamics is a theory which makes it possible to describe the powerful forces of interactions between quarks and gluons (and therefore the cohesion of the atomic nucleus); but it is not sufficient to fully understand the properties of the original plasma. This astonishing similarity with fluid dynamics is therefore a further step forward in this field of research. Scientists hope to discover even more details about this plasma as CERN's accelerator is upgraded. Further studies will also provide a better understanding of how quarks and gluons organize themselves into protons and neutrons, a step that may have conditioned the extremely rapid expansion (cosmic inflation) of the Universe.

Reference:

S. Acharya et al. Measurements of mixed harmonic cumulants in Pb–Pb collisions at 

$\sqrt{{\mathit{\text{s}}}_{\mathrm{NN}}}=5.02$ TeV. DOI: 10.1016/j.physletb.2021.136354