Researchers create a Little Big Bang: Bowling-pin-shaped nuclei shed new light on the Universe’s first moments

What happened in the first moments of the Universe – before the building blocks of life and the world we know today came into existence?

Physicists at the CERN research facility in Switzerland are trying to answer this question by recreating some of the extreme conditions that prevailed in the Universe during its earliest history. Now, researchers from the Niels Bohr Institute, together with colleagues from the international ALICE collaboration, have come one step closer to understanding them.

At CERN, researchers can make atomic nuclei collide at almost the speed of light, creating tiny droplets of the primordial matter that filled the Universe during its first millionth of a second. This matter is known as quark-gluon plasma and is thought to have been the earliest form of matter in the Universe.

For many years, scientists have assumed that creating this plasma required collisions between very heavy atomic nuclei such as lead. But the physicists from the Niels Bohr Institute have now succeeded in creating the primordial matter by smashing the much smaller nuclei oxygen-16 and neon-20 together.

“We have pushed the boundary for how small the atomic nuclei can be while still recreating this primordial matter – what you could call a Little Big Bang. We now know more about the fundamental conditions required for matter to transition into this extreme state,” says Associate Professor You Zhou, who led the experiment and until recently was employed at the Niels Bohr Institute at the University of Copenhagen.

He adds:

“Hopefully, this will help us better understand how the plasma behaved during the first moments of the Universe – and how it later evolved into the forms of matter that everything around us is made of.”

The research findings, produced as part of the international ALICE experiment, have just been published in the prestigious journal Physical Review Letters.

A microscopic Big Bang shaped like a bowling pin

When the atomic nuclei collide, their constituents are transformed into a tiny droplet of quark-gluon plasma that exists for a tiny fraction of a second. The droplet of extremely hot matter then expands. Researchers cannot observe the plasma directly, but they can measure the particles that the matter turns into shortly afterwards.

Here, it turns out that the movement pattern of the particles reveals the shape of the atomic nucleus. While collisions between two oxygen nuclei produce a more rounded pattern, collisions involving neon produce a bowling-pin-shaped pattern.

“The particles from the primordial matter are directly governed by the geometric shape of the atomic nucleus. If the two nuclei we smash together are spherical, we get one pattern. If they are shaped like bowling pins, we get another. By studying how the particles move after the collision, we can gain insights into atomic nuclei that are otherwise difficult for physicists to obtain,” explains Postdoctoral Researcher Emil Gorm Dahlbæk Nielsen from the Niels Bohr Institute, who is a co-author of the study.

He elaborates:

“It is a bit like shining light on an object and seeing its shadow. You cannot see the object directly, but its shadow reveals its shape. In the same way, the movement of the particles reveals the geometric shape of the atomic nuclei that was present at the beginning of the collision.”

Deep roots at the Niels Bohr Institute

The question of the shape and structure of atomic nuclei has occupied physicists for more than 70 years and has deep roots at the Niels Bohr Institute. Indeed, it was Aage Bohr’s work on the structure of the atomic nucleus that earned him the Nobel Prize in Physics in 1975.

The shape of an atomic nucleus is not merely a matter of geometry. It reveals how protons and neutrons are organised and provides important information about the strong force – one of nature’s four fundamental forces, which scientists are still working to fully understand.

Until now, researchers have primarily investigated nuclear structure at low energies, for example by studying the rotation and vibrations of atomic nuclei.

“A precise understanding of nuclear structure helps us understand the strong force. But instead of carefully investigating nuclei at low energies, we smash them together at the highest energies we can create and can now read their shape from the imprint they leave behind,” says You Zhou.

The researchers describe the potential of the approach as a possible paradigm shift. If the method can be further developed, it could provide a new way of investigating other atomic nuclei whose structures are not yet well understood.

Researchers still do not know the exact boundary for when quark-gluon plasma can be created. The next step is therefore to carry out experiments with even lighter nuclei, such as helium-4.

“What is fascinating is that we can use the same experiment both to learn about the structure of atomic nuclei and to gain a better understanding of what happened during the birth of the Universe. These two things turn out to be much more closely connected than one might initially think,” You Zhou concludes.

WHAT IS QUARK-GLUON PLASMA?

Quark-gluon plasma is an extremely hot and dense state of matter that existed during the first millionth of a second after the Big Bang. At that time, the Universe was so hot that protons and neutrons had not yet formed. Instead, their building blocks – quarks and gluons – moved freely in a kind of hot “soup”.

As the Universe expanded and cooled, quarks and gluons gradually became bound together to form, among other things, protons and neutrons. These later became the building blocks of atomic nuclei and, ultimately, the ordinary matter that makes up stars, planets and ourselves.

 

ABOUT THE STUDY

• The study was carried out as part of the international ALICE collaboration at CERN and has been published in Physical Review Letters as an Editors’ Suggestion.
• You Zhou, Emil Gorm Dahlbæk Nielsen and Zhiyong Lu from the Niels Bohr Institute played central roles in the work.
• The study was supported by the ERC project InitialConditions.