On May 29, the team led by Professor Zhang Xin from the School of Science, NEU published their latest research findings in the top astronomy journal Astrophysical Journal Letters. For the first time, they used the measured data of the gravitational-wave standard siren to calibrate the distance of the strong gravitational lensing time delay system. Furthermore, the Hubble constant was measured in a cosmological model-independent manner. Song Jiyu, a doctoral student from the School of Science, NEU, is the first author of the paper. Associate Professor Qi Jingzhao and Professor Zhang Xin are the co-corresponding authors.
The Hubble constant describes the current expansion rate of the universe, and its precise measurement is of great significance for the study of the evolution of the universe and the fundamental physical laws. In recent years, there has been a significant difference of more than 5σ between the Hubble constant value measured by the cosmic microwave background radiation (CMB) observations and that measured by the nearest neighbor cosmic distance ladder method, which has triggered a new cosmological crisis, known as the "Hubble Crisis". The inconsistency in measurement may imply new physics beyond the standard cosmological model, or may stem from unexplored systematic errors in the measurement. To explore the root cause of the "Hubble Crisis", Zhang Xin's team innovatively used 47 gravitational-wave standard siren events observed by the LIGO-VirVirgo KAGRA detection network to calibrate the distance of the strong gravitational lensing time delay system RXJ131 -1231. Thus, a new measurement value of the Hubble constant that is independent of the cosmological model is given through the distance summation relationship.
Under the assumption that the universe is flat, the Hubble constant measured by the research team is approximately 73.22 km/s/Mpc, which leans towards the result of the distance ladder. Furthermore, the team also analyzed the dependence of the measurement results on the method for inferring the mass distribution of lensed galaxies. The results show that when inferring based on the dynamic information of lensed galaxies, the measured value of the Hubble constant decreases, but the deviation of the two results is still within the range of 1σ.
Another major innovation of this research lies in the first use of a gravitational-wave "dark siren" (a gravitational-wave event without an electromagnetic counterpart) to calibrate the distance in a strong lensing system. Compared with the "bright siren" event that relies on electromagnetic counterparts for observation, the "dark siren" is usually a merger event of two black holes with greater mass, which can cover a higher redshift range and is therefore more suitable for calibrating high redshift lensing systems. The research found that the Hubble constant value obtained by combining 42 binary black hole events with the strong lensing system RXJ131 -1231 has an accuracy improvement of approximately 40% compared to using the "bright siren" event GW170817 alone. This means that in the future, with the accumulation of more high redshift "dark siren" events and the addition of more strong lensing time delay systems, this method is expected to achieve high-precision measurements of the Hubble constant that do not rely on the cosmological model, opening up a brand-new way to solve the "Hubble Crisis".
This research was jointly funded by the SKA Special Project of the National Key Research and Development Program, the National Natural Science Foundation of China, the CSST Project of the Space Station Survey Telescope of the Manned Space Program, and the 111 Project of the National Program for Introducing Foreign Experts. The Key Laboratory of Cosmology and Astrophysics of Liaoning Province, Institute of Theoretical Physics, Chinese Academy of Sciences - NEU Peng Huanwu Science and Education Cooperation Center, the National Frontier Science Center for Industrial Intelligence and System Optimization, and the Key Laboratory of Intelligent Industrial Data Analysis and Optimization of the Ministry of Education have also provided strong support for the research of Professor Zhang Xin's team.
