Research

My research interest lies in expanding and deepening the understanding of fundamental physics, mainly through cosmology. The Standard Model of particle physics (SM) and General Relativity (GR) are well-established physics models. However, there are several evidences that they are not the fundamental theory of physics. In particular, the birth and evolution of the early Universe can hardly be explained by the SM and GR alone. In turn, the early Universe cosmology can be the key to explore the fundamental physics. 

For this purpose, I have constructed models of the early Universe with the particles and fields in the SM as well as well-motivated extensions of the SM (and GR) and studied their verifiability. Since collider experiments have not observed any strong evidence of physics beyond the SM and GR thus far, recently I try to clarify what kind of physics beyond the SM and GR is really needed to explain the birth and evolution of the Universe. The followings are the highlights of my recent studies in this direction. 

Baryogengesis from hypermagnetic fields

Baryon asymmetry or the matter antimatter asymmetry is one of the most important mysteries in cosmology and particle physics. If there were no mechanism to generate the asymmetry, matter and antimatter would have annihilated into photons in the early Universe and there would remain few matter in the present Universe. Since the baryon-number violating interaction in the SM is severely constrained by non detection of proton decay, physics beyond the SM has often been considered to explain this mystery. 

However, the baryon-number is violated within the SM through a quantum effect, namely, the chiral anomaly. If helical hyper magnetic fields existed classically before the electroweak symmetry breaking, it can generate baryon asymmetry through the effect. Although it has been argued that such baryon asymmetry would have been erased by the dynamics around the electroweak symmetry breaking, our detailed study showed that sizable baryon asymmetry can remain until today that can explain the present Universe. Moreover, the hypermagnetic fields that generated the asymmetry also remain until today as intergalactic magnetic fields. 

Future gamma-ray observations might be able to detect such helical intergalactic magnetic fields. The origin of the hypermagnetic fields is then the problem to be solved by the physics beyond the SM and/or GR. 

Generalized Higgs inflation

Primordial inflation is one of the key ingredients in modern cosmology. It can solve the long-standing problems in the Big Bang cosmology such as the monopole problem or the horizon problem at once. Moreover it can explain the origin of the large scale structure of the present Universe and the temperature anisotropies found in the Cosmic Microwave Background Radiation (CMB). Inflation is driven by a scalar field called inflation. The identity of the inflation is its central mystery. 

The SM Higgs is the only (fundamental) scalar field in the SM, but it cannot drive inflation whose observational prediction is consistent with the present Universe as it is. Recently it is found that inflation that is driven by the SM Higgs and is consistent with the present Universe is possible by supposing a nontrivial gravitational or derivative interactions. I constructed a new model of inflation with a nontrivial derivative interaction that had not been considered and found that this model gives a different observational signature in gravitational waves from the existing models. I extended the study and established the framework that can treat all the Higgs inflation models in an equal footing and showed the self-consistency of the model. 

Future observation of CMB might be able to determine the interaction of the SM Higgs that leads to the present Universe. The fundamental theory of physics shall include the origin of the nontrivial gravitational or derivative interaction of the SM Higgs.