A method to measure celestial distances using blazars—active galactic nuclei previously considered too bright and prone to distortion—has been verified for the first time.
The Korea Astronomy and Space Science Institute (KASI) announced on the 12th that a research team led by Dr. Lee Sang-sung has verified a technique to accurately measure cosmic distances through brightness variations and intrinsic luminosity of active galactic nuclei (AGN), among the brightest objects in the universe.
The standard methods for measuring distances from our galaxy to other galaxies are "standard candles," objects with known intrinsic brightness, and "standard rulers," objects with known intrinsic size. For objects with known intrinsic brightness, distances can be calculated from apparent brightness as seen from Earth; for objects with known intrinsic size, distances can be determined from apparent angular size.
However, active galactic nuclei—celestial bodies that appear bright and emit massive amounts of energy even from great distances—present a different challenge. Blazars, a type of AGN, could not be used as standard rulers due to extreme relativistic brightness amplification.
The research team succeeded in measuring the distance to a high-redshift blazar by using standard candles and standard rulers simultaneously, thereby canceling out relativistic effects. The study utilized 15 years of observational data from the U.S. Very Long Baseline Array (VLBA) and other long-term radio observations.
Through this analysis, the team precisely measured the distance to blazar TXS 0506+056, located approximately 3 billion light-years from Earth. TXS 0506+056 appears as a very bright radio source even at great distances due to strong relativistic brightness amplification and is the first AGN from which high-energy neutrinos were detected. These characteristics have made it an important observational target for studying cosmic structure and expansion.
The research team focused particularly on radio flare phenomena, where radio brightness increased dramatically following neutrino detection, calculating celestial distances based on brightness variation rates and emission region angular sizes.
Results revealed that radio flares originate from the blazar's central region, and the most accurate distance measurements are possible using observations taken when flares reach peak brightness. The team explained, "The calculated distance to TXS 0506+056 was approximately 3.07 billion light-years, which matched very well within the margin of error with distances predicted by the current standard cosmological model (ΛCDM)."
Song Chan-woo, a KASI researcher and first author of the study, said, "We confirmed that blazars, which appear bright even at very great distances due to relativistic effects but can have distorted observational values, can be used as distance measurement tools. We will attempt additional verification targeting multiple bright high-redshift blazars."
Dr. Lee Sang-sung added, "In future research, we will use the Korean VLBI Network (KVN) to challenge distance measurements to galaxies in more distant parts of the universe. This will become a new key to verifying cosmological models and could illuminate the edge of the universe."
The research team plans to measure distances to more distant AGN and verify their potential use as standard candles and standard rulers. The study was published in the February 2026 issue of Astronomy & Astrophysics.