Inflation and Early Universe

The Universe is expanding; the further away a galaxy is, the faster it is moving, which is known as the Hubble's law. This observational fact implies that, if we go back in time, the Universe was small, dense and extremely hot. The evolution of the early universe is described by the Friedmann-Lemaitre-Robertson-Walker (FLRW) universe, a homogeneous and isotropic solution of the Einstein equations of the general relativity, and the standard big bang theory is based on the FLRW universe. The Hubble's law, the big bang nucleosynthesis (BBN), the comic microwave background (CMB) radiation provide key support for the standard big bang theory. Those three observations still remain important probes of the early Universe.

Despite its great success the big bang theory is plagued with serious theoretical issues such as the horizon problem, the flatness problem, and the monopole problem. Those problems are beautifully solved by the introducing an inflationary expansion at the very early stage of the Universe. What is more important about inflation is that quantum fluctuations of a scalar field driving the inflation (called an inflaton) generate tiny density perturbations, which can account for the seed of the structures such as galaxies and clusters of the galaxies seen in the current Universe. The properties of the density perturbations depend on the inflation models, which can be probed by studying tiny inhomogeneities in the CMB temperature anisotropy.

The recent progress in observational techniques has enabled us to study the evolution of the early universe with unprecedented precision, and our understanding of the Universe has significantly increased. Nevertheless it is not fully known how the inflation occurred, how the universe was reheated after inflation, how the dark matter as well as the baryon asymmetry were created, whether there is large non-Gaussianity in the density perturbations or not, and so on. We would like to tackle those questions in order to reveal how the universe evolved from the inflationary epoch into what it looks like at present.

Group Members

Cosimo Bambi

General Relativity is our current and successful theory of gravity, but it has been tested essentially only in the perturbative and weak field limit. The challenge is to figure out if its predictions are still reliable in other contexts, such as the description of the universe or black hole physics.

Damien Easson

Building concrete models of inflation from string theory. Observ-  able predictions of nonstandard inflationary theories. 

Koichi Hamaguchi

BSM, in particular, SUSY models, their LHC phenomenology and  application to cosmology (baryogenesis, BBN constraints, dark  matter and its signatures). 

Minxin Huang

Non-Gaussianities in the Cosmic Microwave Background from inflation models.

Ken-iti Izawa

Gauge/gravity-mediated supersymmetry breaking, supersymmetric  inflation, united models. 

Takeshi Kobayashi

Cosmology of the early universe through string theory. 

Shinji Mukohyama

Inflation and brane cosmology.

Hitoshi Murayama

Leptogenesis. Models of inflation. 

Seong Chan Park

Two different types of inflation models, the orbifold GUT infla-  tion and the theory with f(φ)R term, so called the nonminimal  coupling term. The (p) reheating of the inflation theory with the  non-minimal coupling term. 

Naoshi Sugiyama

Setting constraints on the inflation models and early universe phenomena such as big bang nucleosynthesis by using observational data.

Fuminobu Takahashi

Mechanism of inflation and subsequent reheating processes. Origin of density perturbations and non-Gaussianity. Baryogenesis. Big Bang nucleosynthesis.

Atsushi Taruya

Probing the early epoch of the Universe through direct and
indirect measurements of the stochastic background of gravitational waves via laser interferometers or observations of CMB anisotropies.

Jun'ichi Yokoyama

Inflation models. Generation of fluctuations. Stochastic inflation.