Dark energy, which was introduced to explain the accelerating expansion of the universe, is the biggest mystery in the modern cosmology. It is believed that dark energy is very important not only in the astronomy and the cosmology, but also in the unified theories of the elementary particles such as the superstring theory. Dr. Takeshi Chiba has achieved superior performance in his research of dark energy. In particular, the following three works deserve the Yukawa-Kimura prize this year. These studies are highly praised internationally.

1. Proposal of dark energy   Dark energy is a generalized notion of the Einstein's cosmological constant, including the case where the energy density varies as a function of the time. Dr. Chiba and his collaborators, Dr. Naoshi Sugiyama and Dr. Takashi Nakamura, have generalized the cosmological constant as a "fluid" so that the nature of dark energy may be concisely parametrized by the equation of state. This works have done before the name of "dark energy" has become popular. Furthermore, they have studied the phenomenological constraints on such a hypothesized fluid from the observations of the cosmic microwave background, the large scale structure of the universe and the gravitational lens. They have named this fluid X-matter. It is this fluid that was one of the initial proposals for dark energy widely discussed at present.

2. K-essence   Usually dark energy is modeled by the scalar fields with moderate potentials. Dr. Chaba has, in his work with Dr. Takahiro Okabe and Dr. Masahide Yamaguchi, proposed the model of dark energy which includes only nonlinear kinetic term (without potential terms). He has also shown that the solution of accelerating expansion is an attractor in his model. This is the first application of the inflation model known as K-inflation to the model of dark energy, which is called K-essence nowadays. Recently, the K-essence/inflation has attracted a lot of attention because of their intriguing relations with tachyon effective actions in the string theory and the DBI action for D-branes.

3. Bounds on Modified Gravity   Modified gravity at larger scales is an alternative approach to account for the accelerating expansion of the universe. Many researchers in the world have studied this approach in recent years. One important example is 1/R gravity, in which the term such as 1/R (R is the Ricci scalar) is introduced to the Lagrangian. Dr. Chiba has shown that 1/R gravity is strongly constrained by Solar System test of gravity, and many theories of modified gravity similar to 1/R gravity are in fact excluded by that test. Furthermore, he has derived a consistency relation between distance and density perturbation in general relativity, and proposed a method to test gravity at cosmological scales by using this relation. Recently, he has also derived precise analytical formulas for dark energy evolution, globally leading the research of dark energy.

With the three outstanding works mentioned above, Dr. Chiba has widely made important contributions to the problem of dark energy, which is one of the biggest fundamental problems in modern physics. In the field of the cosmology/gravitational theory involving phenomenology with various observations, he will make more achievements in future. He is surely the appropriate researcher who deserves the Yukawa-Kimura prize this year.