Universe Today
Gamma-ray bursts (GRBs) are the most energetic events ever observed in the Universe. These powerful outbursts can shine a quintillion (1018) times brighter than the Sun. Since they were first detected in 1967 by the Vela 3 and 4 satellites, which were designed to detect nuclear detonations, astronomers have been searching for the origin of these events. At present, it is generally accepted that long-duration GRBs are caused by the collapse of massive stars, while short-duration bursts are caused by the merger of binary objects (neutron stars and/or black holes).
Astronomers have recently considered how these powerful events could be used to examine star formation in early galaxies and measure cosmic distances. In a recent paper, a team of American and Hungarian scientists proposes using GRBs to probe the Universe's large-scale structure. As they explain, this method could allow astronomers and cosmologists to resolve unanswered questions regarding current cosmological models and the structure of the Universe.
The research was led by Istvan Horvath, a Professor of Physics and Natural Sciences at the National University of Public Service (NUPS) in Budapest, Hungary. He was joined by colleagues at the NUPS, Eötvös University, the Research Centre for Astronomy and Earth Sciences at the Konkoly Observatory, the University of Debrecen, the Center for Space Plasma and Aeronomic Research (SCPA) University of Alabama in Huntsville. The paper describing their findings recently appeared in the journal Universe.
According to standard cosmological models, the Universe is homogeneous and isotropic on large scales, meaning that it appears similar in every location and direction and has no specific center. This is known as the Cosmological Principle, which is derived from the Copernican Principle, which argues that no position in the Universe is unique or special. However, several cosmic structures that challenge this principle have been observed using GRBs and other luminous objects. This includes Lopez's "Giant Arc," a galaxy clustering covering an estimated 3.3 billion light-years of space.
As the authors note, GRBs are so bright that they can be observed at great distances. Using current instruments, GRBs have been detected at redshifts of z = 7 or higher, corresponding to cosmic distances of about 13 billion light-years. In recent years, astronomers have used these events to detect other over-densities, including galaxy clusters like the Sloan Great Wall, the South Pole Wall, and the King Ghidorah Supercluster. Then there's the Hercules–Corona Borealis Great Wall (HerCrbGW), the largest cosmic structure ever observed (measuring an estimated 10 billion light-years across).
As the team explains, assuming that large anisotropic regions are common in the Universe, GRBs could be used to explore them. While previous studies of GRBs showed slight anisotropies in the distribution of matter, these efforts were complicated because early instruments could not measure GRB distances. This issue was resolved when astronomers discovered that redshifts could be measured based on observations of GRB afterglows.
In addition, the transient nature of GRBs means that only parts of any large-scale structures can be observed at any given time. The key here is to conduct integrated time-span observations of bursts, which will provide larger samples that can be used to measure structures and compare them to the Universal average. For the team, this consisted of utilizing GRB databases that included measurements of their position, afterglows, and redshifts.
Most of these were detected by NASA's Neil Gehrels Swift Observatory and Fermi Gamma-Ray Space Telescope. At the same time, redshifts were primarily obtained from the Gamma-Ray Burst Online Index (GRBOX), updated data from the Gamma-ray Coordinates Network (GCN), and the publicly available dataset compiled by Jochen Greiner of the Max-Planck-Institute for Extraterrestrial Physics (MPE). From these sources, they identified 542 GRBs with accurately measured redshifts and known angular locations.
Of these, 262 were located in the northern galactic hemisphere, where they concentrated their analysis (and where the HerCrbGW is located). In previous work, Horvath and his colleagues identified three clusters in this structure. In this latest study, they identified a fourth cluster encompassing the third that contained 110 to 120 GRBs and spanned a larger redshift range than the previous two (0.33 ≤ z ≤ 2.43). As they concluded, these findings suggest that HerCrbGW is significantly larger in radial size than previously thought.
Their results further demonstrate the potential of using GRBs to probe distances, structure, and other cosmic parameters. However, they also note that their study was subject to biases and unresolved questions about the spatial distribution of GRBs. "Consequently, large-scale anomalies in the GRB spatial distribution can exist which are not necessarily seen in other cosmic objects," they stated. "Further detailed observations are necessary to obtain a satisfactory solution to this problem."
Further Reading: arXiv
