Suyama Lab

Inflation is a cornerstone of modern cosmology and plays a central role in explaining the origin of cosmic structure. This period of rapid expansion occurred prior to the hot big bang, stretching quantum fluctuations of the inflaton field and spacetime to cosmological scales, thereby generating primordial curvature perturbations. These perturbations eventually grow into the fluctuations observed in the cosmic microwave background (CMB) and the large-scale structure (LSS) of the universe. Our research focuses on the properties and evolution of these perturbations at small scales, where new aspects of inflationary physics may emerge.

To complete our understanding of the early universe, it is essential that, after inflation ends, the universe transitions smoothly into a hot and dense radiation-dominated state. This transition is known as the *reheating phase*, during which the energy stored in the inflaton field is converted into particles of the Standard Model. In many theoretical models, the inflaton oscillates around the minimum of its potential and decays, producing relativistic particles and initiating the radiation-dominated era. This process is crucial for understanding the origin of matter and the possible extensions of high-energy physics. Compared to inflation and Big Bang nucleosynthesis, the reheating phase is much less constrained by direct observations. However, in recent years, high-frequency gravitational waves have emerged as a promising probe of this epoch, offering new insights into early universe physics that are otherwise inaccessible through conventional observational methods.

The early universe is studied through primordial black holes (PBHs), which may form from large density fluctuations re-entering the horizon during the radiation-dominated era. Because PBHs are sensitive to fluctuations at small scales, they offer a unique probe into the final stages of cosmic inflation. Various formation scenarios have been explored, and observational constraints on the abundance of PBHs are used to limit the small-scale curvature power spectrum and the physics of the late inflationary phase.