In this division the elementary particle physicists and cosmologists are collaborating and pushing forward the interdisciplinary study of the theory of the universe and elementary particle theory, to tackle the problems such as cosmological dark matter, dark energy, origin of baryon abundance in the universe, big-bang nucleosynthesis, and inflation models.
Recent progress of the technology in observation of the universe made the precise comparison of the theory and observation possible. As a result, the study of cosmology is becoming a quantitative science based on the rich observational data. Our division conducts such a study utilizing the latest observational data and at same time tries to clarify the theory of particles behind them.
The goal of theoretical particle physics is to understand elementary particles and those interactions in a unified manner. As a candidate of such a unified theory, theories with the space-time dimension larger than four, like super string theories, have been considered. Fundamental interactions in nature are the four of the electromagnetic, weak, strong and gravitational interactions. The first three interactions among these can be described by quantum field theories. However, only gravity is of a very different nature and the corresponding quantum field theory is not understood. As a candidate of the quantum theory, almost unique formulation known is that of super string theories. Provided that the gravity is indeed described by a higher dimensional theory such as super string theories, a significant difference from general relativity would show up where the gravity is strong, i.e., for black holes or in early universe. Moreover, the accelerated expansion of the present universe which has recently been found is a pretty unusual phenomena from the current understandings of theoretical particle physics. By investigating this phenomena too, we expect to obtain the clue of the gravitational theory beyond general relativity.
The structure of this universe where we live is very rich and has hierarchy with accurate composition. Every structure of the universe was given its form taking as long as 14 billion years since the creation of the universe to the present time. By solving theoretically how the structure of the universe has been built, we make approach to the true nature of the universe and try to find the theory of gravity and the theory of particles.
The research field to try to quantitatively show the figure of the whole universe based on the observational data is called observational cosmology. Also here, theoretical studies to explain the real observational data play an important role, predicting the observed quantities and making it possible to evaluate what the most suitable observations for cosmology are. From the observational data, neutrino masses and the nature of dark energy are found by using statistical method through large scale Monte Carlo simulations with parallel computers, and we proceed with studies of giving constraints on models in the early universe etc., whereas we participate in observational projects form theoretical point of view.
Moreover, numerical simulations take an important part as ``numerical experiments'', in particular, they have played a big role in the studies of the structure formation of the universe. We expect to obtain the deeper understanding of the evolution of the large scale structure of the universe as well as the clue of gravitational theories beyond general relativity.
