Slovenian astronomer from the UL FMF Gregor Rihtaršič awarded prestigious ESA fellowship
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Gregor Rihtaršič, a doctoral student at the Faculty of Mathematics and Physics of the University of Ljubljana, has been awarded a research fellowship from the European Space Agency (ESA), one of the most competitive research programs in astrophysics in Europe. His achievement represents an important recognition of both his research work and Slovenian astrophysics.
Research on dark matter and gravitational lensing
In his project, Gregor Rihtaršič will investigate the nature of dark matter. Dark matter makes up approximately 27% of the total mass-energy content of the universe and is about five times more abundant than ordinary matter. However, dark matter still has not been directly detected on Earth and its nature remains a mystery. Gregor will attempt to address this by observing collisions of galaxy clusters. Such collisions can provide insight into the nature of dark matter if its distribution is measured precisely using gravitational lensing. The most striking example is the Bullet Cluster (1E 0657–56), considered one of the strongest pieces of evidence for the existence of dark matter. For this cluster, Gregor recently obtained observing time with the JWST (James Webb Space Teles) telescope. A study of it, with him as lead author, was accepted for publication in April this year in the journal Astronomy & Astrophysics.
The ESA fellowship will enable Gregor Rihtaršič to conduct independent research at the ESTEC center in the Netherlands—the largest ESA research facility—which will contribute to a better understanding of the physical properties of dark matter and its role in the formation of galaxies such as our own.
“The European Space Agency research fellowship will give me an exceptional opportunity to further develop my research. Alongside NASA, ESA is one of the most productive research organizations in astronomy in the world in terms of the number and impact of space missions. Joining their science department, which works with mission data and ensures their operation, will provide invaluable experience essential for my future research career. Of course, my research at ESA will be carried out in close collaboration with the FMF astronomy group and my mentor, Prof. Dr. Maruša Bradač,” emphasized Gregor Rihtaršič.
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ESA Fellowship: Recognition of excellence in space science
The ESA fellowship program in space science is highly selective and intended for outstanding young scientists, enabling them to spend up to three years at ESA centers across Europe and the United States conducting research directly related to the agency’s scientific missions. The selection of fellows is based on the excellence of their past research and the quality of proposed projects. This year’s cohort of six fellows covers a wide range of topics, from Jupiter’s Great Red Spot and the properties of exoplanets to dark matter. ESA, of which Slovenia has been a member since 2025, is Europe’s leading organization in space exploration. In addition to participating in human spaceflight missions, such as the recent Artemis II mission, ESA runs an extensive scientific program with numerous space missions, including the James Webb Space Telescope (JWST) and Euclid.
The awarded fellowship is a recognition of Gregor Rihtaršič’s research achievements to date, which include lead and co-authorship in 28 scientific publications in the most prestigious astrophysics journals, the design and leadership of two approved observing projects with the James Webb Space Telescope, and participation in the ESA PRODEX research program.
At the same time, this achievement confirms the visible role of Slovenian astrophysics in the international arena and strengthens the collaboration of the University of Ljubljana’s Faculty of Mathematics and Physics with leading research institutions.
