Dr. Fuminobu Takahashi has been actively conducting research on particle cosmology and early universe theory, achieving numerous significant results. In particular, the following contributions have been highly regarded on a global scale:
1. Research on Gravitino Production in the Early UniverseIn supersymmetric particle models, the gravitino, the super-partner of the graviton, is known to potentially cause various cosmological problems, such as affecting the abundance of the light elements produced during Big Bang nucleosynthesis. In papers [1,2], Dr. Takahashi, together with Dr. Endo, Dr. Hamaguchi, and Dr. Yanagida, pointed out that the decay of scalar fields, such as inflaton and moduli, which dominate in the early universe, could produce far more gravitinos than previously thought. They also clarified that in order to avoid the overproduction of the gravitinos, stringent constraints must be imposed on the fields responsible for inflation and the breaking of supersymmetry. This work provided fundamental limitations on the consideration of cosmic evolution based on supersymmetric models, marking a significant achievement.
The axion, a particle motivated by the strong CP problem, has garnered much interest from researchers and is also an important subject in cosmology. Dr. Takahashi has made notable contributions regarding the production mechanisms and evolutionary processes of axions in the early universe. In particular, in paper [3], Dr. Takahashi, together with Dr. Yin and Dr. Guth, independently of Dr. Graham and Dr. Scherlis, pointed out that the initial amplitude of the QCD axion could be determined by quantum fluctuations during inflation. Moreover, they demonstrated that the correct relic abundance of axion dark matter can be achieved without fine-tuning the initial amplitude. This achievement presents a new mechanism for axion dark matter production and has a significant impact on future axion search experiments.
3. Research on Non-Topological SolitonsIn particle models, including supersymmetric ones, scalar field condensations can form localized, soliton-like field configurations that exist as (quasi-)stable states. Since the stability of this type of soliton is not guaranteed by topology, they are referred to as non-topological solitons. In paper [4], Dr. Takahashi, together with Dr. Kasuya and Dr. Kawasaki, pointed out the existence of a type of non-topological soliton that does not possess conserved charges, such as baryon number or lepton number, which had been the focus of previous research. They demonstrated that the stability of these solitons can be understood using adiabatic invariants and discussed their impact on cosmic evolution. This work identified the existence of a new type of non-topological soliton, and its cosmological significance has garnered widespread recognition internationally.
In addition to the aforementioned achievements, Dr. Takahashi has made numerous other significant contributions, establishing himself as a leading figure in the field of particle cosmology and early universe theory. For these reasons, we believe Dr. Takahashi is a deserving recipient of the Yukawa-Kimura Prize.