Research

Abstract

Although gravity is the oldest known interaction in nature, its research continues to be at the forefront of theoretical physics. General relativity has achieved great success as a classical theory of gravity, and no contradictions with observational results have yet been discovered. However, verification is still ongoing in various situations, such as the strong gravity region around black holes and gravitational interactions over short distances of less than 1 mm. Furthermore, although general relativity is a successful classical theory, if we look at the quantum aspect, it is completely different from the other three fundamental interactions  (electromagnetic, weak, and strong interactions). This is because the corresponding quantum field theory is not understood due to difficulties such as not being able to perform renormalization. Research exploring the quantum properties of gravity is being advanced from various aspects at the forefront of theoretical physics. In our laboratory, we use general relativity and gravity theory to conduct research to understand phenomena under strong gravity such as black holes, and various issues in cosmology from the early universe to the present. We are also conducting theoretical research that will form the basis for observational tests of modified gravity theory that go beyond general relativity. On the other hand, we are also conducting basic research aimed at understanding the relationship between gravity and quantum theory by applying quantum information theory to non-trivial space-time structures. 

Below, we will introduce recent research contents by theme. 

Quantum information and gravity

Recently, importance of the relation between classical gravitational theory and quantum theory defined on its boundary is recognized (AdS/CFT correspondence), and attempts to understand quantum phenomena with gravity from the viewpoint of quantum information theory are progressing. We are investigating quantum correlations of the primordial fluctuations originated from the cosmic inflation based on the quantum information. We are also interested in the Hawking radiation from black holes and aim to understand its nature from qunatum informational viewpoint.

Primordial Black Hole

Primordial black hole (PBH) is the collective term for the black holes formed in the very early universe differently from gravitational collapse of a star. The amount of PBHs and those distribution trace the non-linear inhomogeneity in the early universe and provide the information about the statistical property of cosmological perturbations. The commonest scernario of PBH formation is due to the gravitational collapse in a rarely high density region originates the quantum fluctuation during inflation.  For the standard black hole formation due to the gravitational collapse of a star, there is the lower bound for the black hole mass. Therefore, a black hole with the mass lower than the solar mass cannot be formed. On the other hand, in principle, any value of the mass is possible for PBH. There are observational constraints on the fraction of PBHs to the dark matter components in the universe at each mass scale. Comparing these observational constraints with theoretical predictions, one can narrow down the possible range of the theoretical models (e.g., constraints on parameters of an inflationary model). However, details of the condition of the black hole formation and characters of the resultant PBH are not well-understood. Environmental dependence of the mass and angular momentum of a PBH has not been deeply investigated. In QG-Lab., taking the advantage of our experience about the relativistic cosmology and non-linear gravitational phenomena, we actively study the PBH formation and its statistical distribution. 

Black hole physics (classical and astrophyisical aspect)

In our universe, there are many astrophysical objects with high activities such as AGN and gamma-ray burst, and extraction of erergy from their central black holes plays an essential role to explain their activities. The BZ mechanism explains this extraction process  from the Kerr black hole via magnetic field, but its complete theoretical understanding is still missing. We are investigating the BZ mechanics as a limit of wave amplification process (superradiance). We are also treating problems of interactiong between black holes and waves (wave optics in black hole spacetimes, analog models of black holes).

Observational test of gravity around black holes

We also conduct fundamental research to test the matter distribution and gravitational theories from observations in the vicinity of a black hole. For example, the gravitational field can be tested by observing the motion of stars and pulsars around the black hole in the galactic center. We are conducting research to organize the information could be obtained from future observations and to propose more efficient methods of observational test by making theoretical estimates of the effects of a modified gravity theory and hypothetical matter distribution on the observed quantities there. Similarly, the effects of modified gravity and matter distribution are also expected for black holes of tens to hundreds of solar masses formed by neutron mergers and other processes. This information is expected to have an effect on the decaying oscillation of gravitational waves, known as ring-down gravitational waves, soon after the formation of black holes, and we are conducting basic research to extract this information using future observations.