# Outline of Project B03

## Black Holes and Singularities from Quantum Information (21H05186)

The aim of group B03 is to understand fundamental nature of quantum black holes by developing a new approach to quantum gravity that incorporates quantum information into general relativity. Black holes in general relativity are the simplest but one of the most mysterious objects in the universe since their basic constituent is merely the spacetime curvature, but they nevertheless possess thermodynamic properties, such as temperature and entropy. The possibility that a black hole can evaporate by emitting thermal Hawking radiation has raised the fundamental question, called the “black hole information paradox,” which concerns over general relativity and quantum information. The event horizon of a black hole is the causal boundary of domain of outer communications, whose structure and dynamics are governed by the null energy condition via the Einstein equations, and whose sectional area corresponds to the entropy, known as Bekenstein-Hawking entropy. The former aspect suggests that quantum mechanically corrected null energy conditions would play a significant role in understanding the structure and dynamics of quantum black holes. The latter aspect exhibits the close connection between the black hole entropy and quantum entanglement entropy, which characterizes the correlations between subsystems separated by a boundary surface in a larger quantum system. An increasing number of evidence shows that the black hole entropy can be described as an entanglement entropy via the gauge-gravity correspondence. Furthermore, recent development of the holographic principle indicates that geometric description of---not only a black hole event horizon but also---a various type of causal boundaries can admit the holographic description as a quantum entanglement via the Ryu-Takayanagi formula, which connects gravity with quantum information. It is thus anticipated that quantum aspects of gravity would become most tangible around causal boundaries and be described most effectively by using the language of quantum information theory. These observations form our view point of “extreme universe” that gravity/spacetime is a collection of quantum information. Based on these ideas, we will derive basic formulas that govern the dynamics of quantum black holes by unifying geometric and quantum information theoretic aspects of causal boundaries, and thereby attempt to establish a theoretical foundation for extreme universe.

### Members in B03

**[Principal Investigator]**

Akihiro Ishibashi(B03-PI) Department of Physics, Kindai University

**[Co-Investigator]**

Kengo Maeda Faculty of Engineering, Shibaura Institute of Technology

Keiju Murata Department of Physics, Nihon University

**[ExU Postdoctoral Fellow(Research Collaborators)]**

Yoshinori Matsuo Department of Physics, Kindai University

Shunichiro Kinoshita Department of Physics, Nihon University

**[Research Collaborators]**

Takashi Okamura Department of Physics, Kwansei Gakuin University

Gen Kimura Systems Engineering and Science, Shibaura Institute of Technology

Toshifumi Noumi Department of Physics, Kobe University

**[Post-doc Fellows]**

**[Graduate Students]**

Kodai Ueda Department of Physics, Kindai University

Daiki Yamaguchi Department of Physics, Kindai University

Satoshi Matsumoto Department of Physics, Kindai University