Dr. Kazuya Koyama has conducted consistent and comprehensive research, ranging from fundamental studies of extended gravity theory beyond Einstein's general theory of relativity to its verification through cosmological observations, and has made many accomplishments. In particular, he has received worldwide recognition in the following three areas.

Brane cosmology, which holds that the four-dimensional space-time in which we live is actually a membrane embedded in a higher-dimensional space-time, has taken the world by storm because of its novel picture of the universe and its potential for solving the hierarchy problem in particle physics. Dr. Koyama, in collaboration with Dr. Maartens, correctly calculated the evolution of cosmological perturbations in brane cosmology and gave an observational prediction for the cosmic microwave background radiation. In particular, he has shown that the propagation of gravity in higher dimensional space causes effective anisotropic stress on the brane and its effect on the evolution of density fluctuations. This is a widely accepted feature of this theory today.

The accelerated expansion of the universe, discovered at the end of the 20th century, suggested the possibility of a breakdown of general relativity, and a trend was building to explain this by extensions of gravitational theory without a cosmological constant. Dr. Koyama et al. showed that the typical model along that line, the DGP Brane model, has ghost instability and cannot be used to explain accelerated expansion. He proposed a method to distinguish between extended gravity theories and dark energy models based on general relativity by observation, and made exhaustive comparisons with various observational data, including galaxy distributions, galaxy cluster distributions, and spectrum of the cosmic microwave background radiation, obtaining useful limits for many theories.

Due to the nonlinearity of quantum fluctuations generated during inflation in the early universe, their statistical distribution is expected to deviate slightly from a Gaussian distribution. The three-point correlation has been actively studied as a non-Gaussianity parameter. Dr. Koyama made a major breakthrough in this area of research when he discovered that in order to incorporate the nonlinearity of gravity and calculate the correlation correctly, the correct curvature fluctuation variables must be quantized. Based on this consideration, he performed systematic calculations of three-point and four-point correlations in the inflationary cosmology based on the Dirac-Born-Infeld action derived from brane theory, and created a definitive version of this theory.

His research style, which begins with a detailed examination of fundamental theories and ends with the test of gravity theories through comparison of observational data of the large-scale structure of the universe and the cosmic microwave background radiation, is unrivaled worldwide. He is expected to continue to play a leading role as the world's leading researcher in cosmology and gravity theory, and is truly deserving of the Yukawa-Kimura Prize.