## Research

In our laboratory, we are investigating gravitational physics including higher dimensional theories like superstring. We try to solve the problems of the inflation epoch in the early universe, and accelerating expansion of the present universe, and the phenomena under the strong gravity as in the black hole. We aim to find any constraint and/or any clue for the understanding of the nature of the gravity theory from the cosmological observations etc. In the following, we will explain the themes of recent researches.

### Dark energy

One of the main themes is “dark energy”. By the observation of the cosmic microwave background radiation, we know that the universe is flat but in order that the universe could be flat, the average of the density in the universe should be equal to the critical density (∼10^{−29}g/cm^{3}). But the total density of usual matters in stars and interstellar matters could be estimated to be only 4% of the critical density. The density of the dark matter could be also estimated to be 23% of the critical density. Then there should be something unknown, whose density should be about 70% of the critical density. We call this substance as “dark energy” (Figure 1). On the other hand, by the observation of the supernovae, we believe that the expansion of the universe is accelerating from about five billion years ago, which tells that the dark energy should have large negative pressure. We are now investigating, various models of the dark energy and we aim to obtain any clue of the theory coming from higher dimensions, like superstring theory, and we are considering how we could verify this kind of theory by the experiments and/or observations.

Figure 1: The ratio of density of ordinary matter, dark matter and dark energy in the universe.

As a viewpoint opposite from the elementary particle physics, we are also trying to explain the accelerating expansion of the present universe based on the reaction and the influence to the distance due to the inhomogeneity of the universe.

### Space-time and quantum theory

Various phenomena in physics occurs in the container called “space-time”, whose shape is deformed by the gravity. It is believed that the quantum theory of the gravity could be obtained by summing up the shapes of the space-time. A formulation of the summation is obtained by discretizing the space-time into a set of simplices and to sum up all the possible configuration of the simplices (Figure 2). By using this formulation, we try to clarify the nature of the gravity as a quantum theory.

Figure 2: Discretization of the space-time into simplices.

A phenomena where the quantum theory plays the important role in the early universe is the inflational expansion of the universe, which generates primordial fluctuations via quantum effects to provide the structure formation of the present universe. We are studying the process where the quantum fluctuations develop to the classical structure of the present universe by considering the quantum entanglement.

In the black hole predicted by the general relativity as a classical theory, anything which fell into the region inside the horizon cannot escape from the black hole. If we include quantum effects, however, there could occur a phenomena called the Hawking radiation, which is very similar to the thermal radiation, and the black hole would lose its energy. We are investigating this quantum process by considering the fluid system which has an acoustic horizon, which has properties similar to the event horizon of the black hole.

### Astronomical phenomena of black hole

In the nucleus of active galaxies, which show very active high energy phenomena, it is expected to exist an extremely large black holes, whose mass could be expected to be about 10^{8} times of the solar mass. There could occur the accretion of large quantities of plasma around the nucleus, which produces X-ray radiation and jet ejection under the strong gravity and magnetic field. We try to clarify the roles of the general relativity in such astronomical objects and verify the existence and the affects of the black hole by the comparison with the observations.

### Recent papers

For our recent papers, please refer to the following link.