Researchers at the University of Copenhagen have recreated the primordial state of matter believed to have filled the Universe shortly after the Big Bang, using collisions between atomic nuclei far smaller than scientists once thought possible. These microscopic versions of the early Universe could help researchers understand both the first moments of cosmic history and some of the deepest questions in nuclear physics.
What was the Universe like before stars, planets, atoms, and the other familiar forms of matter existed?
At CERN in Switzerland, physicists are trying to answer that question by reproducing some of the extreme conditions that existed in the Universe shortly after its birth. Researchers from the Niels Bohr Institute, working with scientists in the international ALICE collaboration, have now taken an important step toward that goal.
Recreating the Universe’s Primordial Matter
At CERN, atomic nuclei can be accelerated to nearly the speed of light and smashed together. These collisions can create tiny droplets of quark-gluon plasma, the extraordinary state of matter believed to have filled the Universe during its first millionth of a second.
Scientists had long thought that producing this plasma required collisions between very heavy nuclei, such as lead. The new experiments show that much smaller nuclei can also generate the primordial material. Researchers successfully created it by colliding oxygen-16 and neon-20 nuclei.
“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 Tiny Big Bang With a Bowling Pin Signature
When atomic nuclei collide at enormous speeds, their constituents can transform into an extremely small droplet of quark-gluon plasma. The droplet survives for only a tiny fraction of a second before expanding and converting into other particles.
Scientists cannot observe the plasma itself directly. Instead, they measure the particles that emerge immediately afterward and study how those particles move.
The new results show that these movement patterns preserve information about the original shape of the colliding nuclei. Collisions between two oxygen nuclei generate a relatively rounded pattern, while collisions involving neon create a distinctive 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.”
Using Extreme Collisions to Probe Nuclear Structure
Physicists have been trying to understand the shapes and internal structures of atomic nuclei for more than 70 years. The question has particularly deep connections to the Niels Bohr Institute. Aage Bohr received the Nobel Prize in Physics in 1975 for his work on the structure of the atomic nucleus.
The shape of a nucleus is important for far more than geometry. It reflects how protons and neutrons are arranged and can provide valuable information about the strong force – one of nature’s four fundamental forces, which scientists are still working to fully understand.
Traditionally, physicists have investigated nuclear structure using relatively low-energy experiments, including measurements of how atomic nuclei rotate and vibrate.
The new approach turns that strategy around. Instead of gently probing nuclei, researchers collide them at the highest energies available and reconstruct their shapes from the patterns left behind.
“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 say the technique has the potential to represent a paradigm shift. If it can be developed further, it could offer scientists a new way to investigate atomic nuclei whose internal structures remain poorly understood.
How Small Can a Little Big Bang Get?
Scientists still do not know exactly how small a collision system can become while still producing quark-gluon plasma. Determining that boundary is one of the next major goals.
The team therefore plans to conduct additional experiments using even lighter atomic nuclei, including 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. During that period, temperatures were so extreme that protons and neutrons had not yet formed. Instead, the particles that make them up – quarks and gluons – moved freely in a kind of hot “soup.”
As the Universe expanded, its temperature fell. Quarks and gluons eventually became bound together, forming, among other things, protons and neutrons. Those particles later became the components of atomic nuclei and, ultimately, the ordinary matter found in stars, planets and ourselves.
