In the Theoretical Astrophysics Group, we study two main subjects: dynamic phenomena involving black holes and neutron stars, and the origin and evolution of the Universe. Both are closely related to gravitational theory. In objects such as black holes and neutron stars, the relevant scales are quite different from those we are accustomed to on a daily basis. For example, the core of a neutron star is 1015 times more dense than water, rock or metal, whereas the Universe as a whole is 10-30 times less dense than these materials. Therefore, these objects are not as intuitively familiar to us as the Sun and planets. However, studying such extreme aspects of the Universe can lead to a deeper understanding of physics.
The Universe and Gravity
Gravitational interactions play a key role in the large-scale dynamics of astronomical objects, and determine stellar structure and motion. Under conditions of extreme gravity, a black hole can be formed. In the similar way, the evolution of the Universe is also determined by gravitation and the matter content. In Einstein’s General Theory of Relativity, gravity is described as a curvature of space-time. The flat space-time shown in the left panel below represents empty space. In the center panel, it is curved due to the mass of an object. In the case of two objects orbiting each other, time-varying gravitational waves are produced and propagate, as shown in the right panel.
Relativistic Stellar Objects
The gravity associated with a dense collapsed object such as a black hole or neutron star is so strong that general relativity is required to describe it. Astronomical objects are mostly static and rarely exhibit time variations. The number of such compact objects is quite small compared with normal stars. However, they can be detected by sudden brightening, e.g., in the X-ray region. These relativistic stars often emit huge amounts of energy in a short time period, occasionally carried by neutrinos and/or gravitational waves. The image on the right illustrates how the inner structure of a neutron star could be probed by observing flares emitted from it. The variations inside a neutron star shake the exterior magnetosphere, and X-rays and gamma rays can be emitted. A theoretical approach is required in order to obtain meaningful information on strong-gravity environments and high-density states of matter, based on observations of these compact objects.
One of the aims of cosmology is to clarify how the Universe began and evolved. Studies of the cosmic microwave background and the large-scale structure of galaxies have unveiled primordial density fluctuations and their evolution. It has been clarified that inflation and accelerated expansion of the early universe can successfully explain these primordial density fluctuations, and that the current Universe is in a second phase of accelerated expansion. Comparisons between the results of more precise observations and predictions of theoretical models have made it possible to obtain more accurate information about the inflation mechanism, and models involving dark energy can explain the accelerated cosmic expansion. In this regard, there has recently been considerable interest in exploring and testing gravitational theories in the field of cosmology.
Extreme Universe and Astrophysics
Using theoretical approaches, we attempt to shed light on observed phenomena in the Universe, and explore the subjects in depth. For example, the primordial fluctuations in the Universe were generated during the inflationary era in accordance with the principle of quantum mechanics. The energy scale associated with cosmic inflation could be as high as 1016 GeV. Therefore, studying such primordial fluctuations through observational cosmology can lead to a verification of quantum mechanics at such extremely high energies. Astrophysical phenomena are often impossible to reproduce directly in a laboratory, and need to be approached using a combination of theoretical predictions based on physical principles, and astronomical observations.
Yasufumi Kojima (Professor)
In very compact objects such as black holes and neutron stars, gravity is extremely strong, and enormous amounts of energy can be released in very short periods. Such events are rare, but they are very bright, and even distant sources can be seen. There are several unresolved issues in relativistic astrophysics, although they can be roughly estimated. One such issue is related to pulsar magnetospheres, in which electromagnetic energy associated with the central star is converted to the kinetic energy of particles. Another subject of interest is sudden flares associated with black holes or magnetars. The magnetic field produced by magnetars is much stronger than that for normal stars, and dynamic processes may give rise to emission of intense gravitational waves. The ability to detect such waves would open up a new window of observation, different from the electromagnetic processes observed so far. As a member of the theoretical group, I am looking forward to the direct detection of gravitational waves within a few years (maybe in 2017) by the Japanese Gravitational Wave Observatory KAGRA, which is now under construction at Kamioka, Gifu.
Kazuhiro Yamamoto (Associate Professor)
The Universe is believed to have experienced an inflationary era at its beginning, characterized by extremely accelerated expansion. At present, the Universe is again undergoing accelerated expansion, but the energy scale is quite different from that associated with cosmic inflation. This accelerated expansion is quite contrary to the familiar view of gravity as an attractive force, and it’s origin requires investigation. I am interested in theoretical models and cosmological tests associated with these topics. The large-scale structure of the Universe, which originated from quantum fluctuations during the inflationary era, can be used to probe the nature of dark energy, which is believed to cause accelerated expansion, in addition to the physical principles operating at the beginning of the Universe. Modern cosmology is the frontier of astrophysics, having the aspects of the exact science by means of the great observations of the Universe.
Illustration by AND You INC.