HUN-REN CSFK Research Team Probes the Geometry of the Universe with Quasars

Astronomers employ a variety of techniques to study the geometric properties of the Universe. Among the oldest of these is the so-called angular size–redshift test. The basic idea is straightforward: if objects can be identified whose true physical size is approximately known, their apparent size on the sky can be used to infer the expansion history of the Universe.

According to an article published on the CSFK website, an international research team led by Mina Ghodsi Yengejeh of the Konkoly Thege Miklós Astronomical Institute, HUN-REN CSFK, has revisited this classical approach in a novel way by examining the radio-emitting plasma jets of quasars—the luminous cores of distant galaxies, powered by supermassive black holes.

The study, recently published in Astronomy & Astrophysics and incorporating several methodological innovations, focused on relativistic jets associated with active galactic nuclei. These outflows originate in the immediate vicinity of central supermassive black holes and are among the most compact and powerful radio sources known. Their angular sizes can be measured with milliarcsecond precision using Very Long Baseline Interferometry (VLBI). The research team analysed a dataset more than an order of magnitude larger than the samples used in similar studies during the 1990s, making this the first comprehensive reassessment of the method in more than twenty-five years.

The results show that the relationship between the observed angular sizes of quasars and their redshifts follows the trend predicted by cosmological models of the Universe. The researchers also carried out a series of statistical tests, generating one hundred artificial datasets in which redshifts were randomly shuffled among the objects while all other properties were left unchanged. These "scrumbled" samples no longer exhibited the same correlation, providing strong evidence that the relationship seen in the real data has a physical origin rather than arising by chance. In other words, the apparent angular sizes of quasar jets do indeed contain cosmological information.

At the same time, the researchers point out that, in its current form, the method cannot yet provide cosmological parameters with the same precision as other techniques, such as Type Ia supernovae or measurements of the cosmic microwave background. The principal challenge is that the apparent sizes of quasar jets are influenced not only by the geometry of the Universe but also by the intrinsic astrophysical properties of the sources themselves. Because these effects are partially intertwined, the accuracy of the method is significantly reduced. Simulations indicate that the test could become a competitive cosmological probe if the scatter in the observations could be reduced below 20 per cent and if tens of thousands—or even hundreds of thousands—of carefully selected objects were available for analysis.

There is, however, good reason for optimism. Radio astronomy is currently undergoing a period of rapid development. New VLBI networks, highly sensitive radio telescopes, and international observatories now under construction are expected to survey more active galactic nuclei than ever before. The present study has demonstrated that the angular size–redshift relation of quasar jets is a genuine and measurable phenomenon. In the future, it could play an important, independent role in investigating the structure and evolution of the Universe. To facilitate further research, the team has also made the full dataset publicly available.