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Gravity is one of the oldest known interactions in nature, yet research into it continues at the forefront of theoretical physics. General relativity has been a great success as a classical theory of gravity, and no contradictions have yet been found with observed results. However, verification is still ongoing in various situations, such as in cosmology, strong gravity regions around black holes, and gravitational interactions at short distances of less than 1 mm. In addition, from a quantum theoretical perspective, general relativity is completely different from the interactions that represent the other three fundamental forces (electromagnetism, weak force, and strong force), and the corresponding quantum field theory is not yet known due to difficulties such as the non-renormalizability. On the other hand, with the development of the holographic principle that connects various quantum physics other than gravity with general relativity across dimensions, research into the relationship between gravity and quantum physics is being conducted in various forms at the forefront of theoretical physics. Research into general relativity continues to expand in scope as understanding deepens. In our laboratory, we conduct theoretical research centered on general relativity to understand phenomena under strong gravity such as black holes, and various problems in cosmology from the early universe to the present. We also conduct fundamental research aimed at understanding the relationship between gravity and quantum mechanics by applying quantum information theory to non-trivial space-time structures.
Below, we will introduce recent research content by theme.
We are conducting research into the mathematics of general relativity and the relationship between gravity and quantum mechanics, focusing on the global structure of space-time, such as the early universe, black holes, and singularities. Recently, our understanding of the correspondence between gravity theory and quantum field theory defined on the boundary of space-time (AdS/CFT correspondence) has progressed, and our approach to research on gravity and quantum mechanics has also changed. As part of our research in this direction, we are attempting to incorporate quantum information into the geometry of space-time by deepening our understanding of the thermodynamic properties of black holes using the holographic principle, generalized entropy, and quantum energy conditions. We are also conducting research to elucidate the nature of quantum correlations in quantum fluctuations in the inflationary universe.
Some high-energy phenomena in our universe, such as AGN and gamma-ray bursts, are thought to involve the extraction of energy from black holes. The Blandford-Znajek (BZ) mechanism is considered the most likely method for extracting the rotational energy of a black hole using a magnetic field, and research is still in progress to fully understand it. We are attempting to understand the BZ mechanism as the limit of wave phenomena in the black hole magnetosphere. We are also conducting research on more general problems involving the interaction of black holes with waves (formulation of wave optics in black hole spacetime, analogue models of black holes).
Primordial black holes are a general term for black holes formed in the early universe, and their total amount and distribution give us a trace of the nonlinear inhomogeneity of the primordial universe. A typical formation process is gravitational collapse, which proceeds when the amplitude of density fluctuations generated during inflation happens to be large. For black holes formed by the gravitational collapse of normal stars, the existence of a limiting mass is known, and black holes lighter than the solar mass are not formed. On the other hand, primordial black holes are formed in a different process, so any mass is possible in principle. Primordial black holes are actively discussed as candidates for dark matter, the origin of black hole binaries observed with gravitational waves, and as a clue to exploring the early universe. However, many points remain unclear, such as how the mass and angular momentum of primordial black holes change depending on the environment at the time of their formation. We are actively conducting research on primordial black holes, using numerical simulations and other methods, making use of our knowledge of relativistic cosmology and nonlinear gravity, which are the characteristics of our laboratory.
We are also conducting basic research to verify the distribution of matter around black holes and the theory of gravity from observations near black holes. For example, the gravitational field can be probed by observing the motion of stars and pulsars around the black hole at the center of the galaxy. By theoretically estimating the effects of gravity theories modified from general relativity and hypothetical matter distributions on observables, we aim to organize information obtained from future observations and propose more efficient observational tests. We also expect similar effects of modified gravity and matter distributions for black holes with masses of tens to hundreds of solar masses formed by neutron star mergers. This information is thought to affect the damping oscillation of gravitational waves soon after the formation, known as ring-down gravitational waves, and we are conducting basic research to verify this using future observations.