In his thesis, Oleg Komoltsev has shown how Quantum Chromodynamics calculations at very high densities can provide reliable constraints on how matter behaves inside neutron stars.
Oleg Komoltsev defended his doctoral thesis at the Faculty of Science and Technology on May 27, 2025. He defended his thesis Perturbative QCD reveals the softening of matter in the cores of massive neutron stars.
The cores of neutron stars contain the densest matter known in the universe. Recent rapid progress in observing neutron stars now gives researchers a unique opportunity to study cold and extremely dense matter, as described by Quantum Chromodynamics (QCD). By combining these observations with theoretical calculations, we gain new insight into how this matter behaves and how it affects the equation of state and the QCD phase diagram.
In his thesis, Komoltsev shows how QCD calculations at very high densities can provide reliable constraints on how matter behaves inside neutron stars.
What have you researched?
During his PhD, Komoltsev studied the densest matter in the universe: the cores of neutron stars.
«Imagine compressing two suns into a sphere just 12 kilometers across. A single teaspoon of such matter would weigh several billion tons. Despite decades of research, we still don’t have a complete theoretical understanding of how matter behaves under such extreme conditions,» says Komoltsev.
But for the first time in history, astrophysical observations are now offering clues about the behavior of this ultra-dense matter.
«At the same time, particle physicists are conducting experiments here on Earth by smashing particles together to study the fundamental building blocks of matter. The goal of my research is to bridge these two worlds – connecting what we see in the skies with what we learn in the lab – to better understand how matter behaves at the very edge of physical possibility,» says Oleg Komoltsev.
What did you find out?
Oleg Komoltsev describes his findings like this:
«Imagine placing your ear on a train track while a train is approaching in the distance. You’d hear the sound through the metal rails long before you hear the sound of the train arriving through the air. That’s because sound travels much faster in metal than in air. Metal is far stiffer, allowing sound waves to propagate more quickly.
Now apply this idea to a neutron star. Imagine “knocking” on a neutron star and observing how sound propagates through its layers. In my research, I explored how the speed of sound behaves deep inside these stars. Surprisingly, I found that theory developed for high-energy particle experiments, like those at the Large Hadron Collider (LHC), give us much more insight into neutron star interiors than previously thought.»
Komoltsev found that the innermost cores of neutron stars appear to be softer than the outer layers, meaning that the speed of sound actually decreases (starting from outer core) as we move deeper into the star.
What can the research findings be used for?
Basic research like Komoltsev's thesis obviously does not focus on practical applications.
«This is fundamental knowledge that brings us closer to understanding the nature of the densest matter in the universe. At such high densities, normal matter made of protons and neutrons may no longer hold together in its usual form. Instead, it might dissolve into its most basic ingredients: quarks, the elementary particles that make up protons and neutrons. Understanding how and when this transition happens has been a long-standing question in physics, says Komoltsev.
His research shows that data from current particle experiments already offer evidence of the transition from ordinary nuclear matter to exotic quark matter inside the cores of neutron stars.
Oleg Komoltsev describes his four years at University of Stavanger as fun and inspiring.
«For me it was amazing. My collaboration with main supervisor Aleksi Kurkela worked out from day one. I could not have asked for a better supervisor and a better learning environment,» he says.
Oleg Komoltsev has a strong foundation in theoretical physics throughout his academic journey. He completed his bachelor’s degree at the Moscow Institute of Physics and Technology (MIPT), followed by a master’s programme at Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen, Germany.
His PhD thesis includes publications in high-impact journals such as Nature Communications, Physical Review Letters (PRL), The Astrophysical Journal (ApJ), and Physical Review D (PRD). Currently, he is in process of getting a post-doctoral researcher position.