Aleksi Vuorinen looks into the heart of neutron stars
Neutron stars may contain a completely new phase of matter, believes Academy Research Fellow Aleksi Vuorinen from the University of Helsinki.

Aleksi Vuorinen received a grant from the European Research Council (ERC) worth over €1 million to study the densest matter in the universe. Vuorinen studies quark matter, which could be found in the cores of extremely dense neutron stars.

The matter in neutron stars is so dense that a piece the size of a sugar cube would be able to contain all humans presently alive.

“Massive neutron stars are on the brink of collapsing into black holes, so no denser system can exist even in principle outside the event horizons of black holes.”


In addition to phases of matter, Vuorinen specializes in novel computational methods. Even though the theory of quantum chromodynamics (QCD) was developed more than 40 years ago to describe the properties of nuclear and quark matter, it continues to provide mathematical challenges. There is room for new approaches.

Theoretical predictions of the properties of neutron stars continue to be imprecise compared with the predictions regarding the properties of other stars, explains Vuorinen.

 “There isn’t anything inherently mysterious about the matter inside neutron stars. Stars are made of extremely dense nuclear matter, in which neutrons, protons and some heavier atomic nuclei are compressed together.”

At the moment, the most important unanswered question is whether nuclear matter in the cores of the most massive neutron stars becomes quark matter.

According to Vuorinen, theorists have long ago established the existence of quark matter, a new phase of matter in which the nuclear particles such as protons and neutrons are under such extreme pressure that they compress into one another. In these circumstances, the quarks and gluons that make up these composite particles are liberated, i.e. they behave as the new degrees of freedom in the system.


Unlike the interiors of black holes, which cannot be observed past the event horizon caused by gravitational forces, it is possible to empirically study the matter inside neutron stars. In particular, the electromagnetic radiation and gravitational waves emitting from stars contain immense amounts of information on the properties of the cores of the stars.

 “Additionally, precise observation of the macroscopic properties of the neutron stars, such as their mass or radius, can reveal some very significant things about the behaviour of quark matter.”

Vuorinen emphasises that this is the reason why neutron stars may serve as laboratories for particle physics.

 “However, it is difficult to find practical applications for neutron star research outside of the discipline itself. This is primarily research for the sake of curiosity, like most of particle physics.”

Vuorinen does not want to assume all the credit for discovering the potential new phase of matter.

 “There is a whole crowd of researchers chasing quark matter, and even my own research is far from a one-scientist show.”


Vuorinen’s most important research partner is Aleksi Kurkela, who works at CERN. The physicists have worked together for more than a decade, and their cooperation continues. The two Aleksis shared a dissertation supervisor, Keijo Kajantie. In fact, most Finns studying quark matter are Kajantie’s academic protégés.

 “It’s extremely easy and pleasant to work with Aleksi. We’ve also become good friends: he was the best man at my wedding last summer, and I am the godfather of his child,” Aleksi Vuorinen explains.

Scientists must be social. Vuorinen’s most important partners include colleagues in the same field everywhere in the world as well as the postdoctoral researchers working in his research group at the University of Helsinki.

Vuorinen also lavishes praise on the doctoral and undergraduate students of physics. The future of the field looks bright.

 “Our students are extremely motivated and intelligent, so I really can’t complain about my working conditions.”


According to Aleksi Vuorinen, theoretical physicists tend to represent one of two main types. One is driven by interest in the natural or engineering sciences, while the other is fascinated by pure mathematics.

 “You can see this already in the first-year students. Some are driven by the desire to qualitatively understand physical phenomena, while others are happiest when performing complicated calculations,” says Vuorinen.

"I personally firmly belong to the latter group."

Vuorinen has found his niche in theoretical particle physics – or in more general terms, high energy physics, which also covers some areas of cosmology.

 “In this field, I get to use the machinery of mathematics to solve interesting real-world problems which tell us something about how nature works at its most fundamental level.”

Vuorinen says he turned to neutron stars because the research subject seemed both topical and challenging:

 “I realised that there is a big unanswered question in neutron star physics, and that I can be a part of answering it.”

This article was published in Finnish in the Y/03/17 issue of Yliopisto magazine.

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