The boundary between black holes and neutron stars, which has been distinguished under Einstein's general theory of relativity, may disappear under a new gravity theory, according to research findings. The study is being recognized for opening new possibilities for understanding ultra-dense celestial objects and the nature of dark matter.
According to the scientific community on Thursday, a joint research team from the National Institute for Mathematical Sciences (NIMS) and the Korea Astronomy and Space Science Institute recently published research findings showing that the boundary between black holes and neutron stars may disappear under a new gravity theory that goes beyond general relativity.
The current general theory of relativity has limitations in explaining environments with extremely high energy, such as the interior of black holes or the early universe. To address this, the research team applied the "Horava-Lifshitz (HL) gravity theory." HL gravity theory is a candidate quantum gravity theory based on a new concept that time and space can change in different ways.
The research confirmed that under HL gravity conditions, ultra-dense celestial objects such as neutron stars can become much heavier than under existing theories. It was also confirmed that as the density of celestial objects becomes extremely high, the "compactness gap" thought to exist between black holes and neutron stars may disappear.
This could provide a clue to explaining the "objects difficult to distinguish as either black holes or neutron stars" recently discovered by the international gravitational wave observation project "LIGO-Virgo-KAGRA." The research team suggested the possibility that such objects could be a new type of ultra-dense celestial body explained by HL gravity theory rather than existing general relativity.
The research team also suggested the possibility that very small ultra-dense celestial objects could be candidates for dark matter.
The explanation is that certain particles can form celestial bodies that are very small but have strong gravity under the influence of HL gravity, and that such celestial bodies could play the role of "cold dark matter."
"This research is significant in that it presents the mass limit of neutron stars in HL gravity theory for the first time, establishing a standard for related research, and suggests the possibility of the existence of new ultra-dense celestial bodies that are even denser than the neutron stars observed so far and difficult to distinguish from black holes," NIMS researcher Dr. Sohn Jae-joo said.
"This research shows the possibility that new types of celestial bodies may exist between black holes and neutron stars," NIMS researcher Dr. Oh John J. said. "Future gravitational wave observations and pulsar timing observations could serve as important clues for verifying this HL gravity theory." He emphasized, "Modified gravity theories such as HL gravity theory have also shown the potential to contribute to solving the challenges regarding the formation process of dark matter."
However, the research team noted that since these results are still at a theoretical stage, additional observation and verification are needed to determine whether they actually exist in the universe.
This discovery is expected to be utilized in various fields, including research on extreme celestial bodies such as neutron stars and black holes, as well as gravitational wave astronomy, cosmology, and dark matter research. The research findings were published in "Physical Review D," a journal published by the American Physical Society, and in the physics journal "Physics Letters B," respectively.
