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RESEARCH

How did the universe begin? How did it evolve, and how will it end? These fundamental questions have not yet been answered. Based on observational and experimental data, we are trying to tackle with these fundamental problems by making use of cosmology, gravitational theories, string theory, astrophysics, computational science and quantum information science.

Projects

Cosmology

Recent cosmological observations suggest that our universe experienced a period of extremely rapid expansion of space called inflation during its first few moments. The most surprising prediction of the inflationary universe is that the origin of cosmic structure stems from quantum fluctuations. However, any compelling observational evidence of the quantum nature of the origin has not yet been found. Our universe has also been observed to enter another phase of accelerating expansion, driven by an unexplained mechanism called dark energy. The figure below shows the evolution of the universe. To reveal truths about the universe, we challenge to make prediction to the level of precision by the observations.

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Quantum Information Science, the Universe, and Gravity

Various experiments have been conducted successfully in the field of quantum information science, including an Earth-scale experiment that used satellites to achieve quantum teleportation and involved the transmission of quantum states via quantum entanglement. This field has rapidly developed and aims to apply next-generation quantum communication and quantum computing. The rapid development of this field has had an extremely significant effect on fundamental problems in theoretical physics, such as the black hole information paradox and open quantum system dynamics. In fact, although no one has been able to verify whether gravity itself obeys quantum mechanics, verifying this point using quantum entanglement may be possible. Richard Feynman, who received the Nobel Prize in Physics, proposed a thought experiment in 1957 to verify the quantum nature of gravity. Although it has been only a thought experiment to date, recent developments in quantum control technologies make it possible to perform this experiment on quantum gravity as a tabletop experiment. This problem is also important to cosmology. A gravitational field is a spacetime curvature according to general relativity; thus, if gravity follows quantum mechanics, it would follow that spacetime curvatures could exist in a state of quantum mechanical superposition. Quantum entanglement is only produced through non-local quantum interactions; thus, if the occurrence of quantum entanglement through gravitational interactions were detected, verifying whether gravity obeys quantum mechanics would be possible. We are theoretically studying these possibilities.

Theory of Quantum Fields in Curved Space–Time

If quantum fluctuations were enlarged during the inflationary epoch, they may have given rise to the source of the universe’s structure as well as primordial gravitational waves. However, quantum fluctuations in a vacuum also predict various physical phenomena. For example, there is a theoretical prediction that, from the perspective of an accelerating observer, a vacuum will appear to be in a thermal state at a temperature proportional to the degree of acceleration; this is called the Unruh effect. This prediction has a corresponding relation with Hawking radiation, which predicts the evaporation of black holes. These predictions are deeply connected to the non-local correlation of vacuums, wherein a vacuum is described as the state of quantum entanglement of the quantum states that compose a partial region of space–time. We are studying these structures and their potential for verifiability.

​The Cause of the Universe’s Accelerating Expansion

The cosmological theory introduced by Einstein became the Standard Model, which explains the current accelerating expansion of the universe. However, the final superstring theory suggests that scalar fields, which are dynamical energy components with negative pressure, may be the cause of this expansion. We are exploring the cause of the accelerating expansion of the universe and investigating the observational consequences of a new, spatially non-uniform theoretical model wherein the energy components that cause accelerating expansion exist on a scale larger than the observable scale of the universe.

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