Seminars archive 2023

The standard cosmological model is recently suffering severe shortcomings and tensions due to the fact that, very likely, it has to be improved at IR and UV scales. In view to reconstruct a self-consistent cosmic history, cosmography revealed a model-independent approach capable of fixing reliable models starting from observations. Without claim to completeness, we are going to sketch an overview of the method and the possible realizations towards the solutions of various cosmological tensions.

In this seminar I will try to investigate the interplay between General Relativity (GR) and f(R) gravitational theories, exploring their implications for our understanding of the fundamental nature of gravity. I will delve into the theoretical frameworks of GR and f(R), examining the mathematical formulations and physical consequences of these two approaches on the construction of black holes.

We investigate the charged Vaidya spacetime with conformal symmetry by classifying the horizons and finding its connection to Hawking temperature. We find a conformal Killing vector whose existence requires the mass and electric charge functions to be proportional, as well as linear in time. Solving the Killing equations for the conformally transformed metric from the linear charged Vaidya metric yields the required form of the conformal factor. From the vanishing of the norm of the conformal Killing vector, we find three conformal Killing horizons which, under the transformation, are mapped to the Killing horizons of the associated static spacetime, if the spherical symmetry is maintained. We find that the conformal factor is not uniquely determined, but can take any function of the ratio of the radial coordinate to the dynamical mass. As an example, we illustrate a static spacetime with our choice of the conformal factor and explicitly show that the surface gravity of the conformal Killing horizons, which is conformally invariant, yield the expected Hawking temperature in the static spacetime. This static black hole spacetime contains a cosmological horizon, but it is not asymptotically de Sitter. We also investigate the case when the mass parameter is equal to the constant electric charge. While in this case the standard pair of horizons, the loci of the time component of the metric, degenerate, the conformal Killing horizons do not degenerate. This therefore leads to a non-zero Hawking temperature in the associated static spacetime. [Slides]

This lecture is intended to provide basic concepts of the global structure of spacetime in general relativity. For this purpose, we will take up some particular topics concerning theoretical and mathematical aspects of black hole spacetimes, including singularity theorems, black hole thermodynamics, and asymptotic symmetries, each of which points to some recent research directions.

Talk session: "Quantum black hole in the asymptotic safe gravity (漸近安全な量子重力における量子ブラックホール)" [Slides]

Talk session: "Crossing of observational cosmology and quantum gravity (観測的宇宙論と量子重力の交差点)"

It is explained the concept of generalised holographic dark energy. It is shown that entire evolution of the universe from the inflation till dark energy maybe provided by holographic dark energy with specific cut-offs. All known specific holographic dark energies, like Tsallis, Renyi, etc are shown to be particular examples of such generalised holographic dark energy introdced in Nojiri-Odintsov, Gen.Rel.Grav. 38 (2006) 1285-1304. The relation of holographic dark energy with modified gravity is also explained.

At the second part of talk novel 4 or 5 parameters dependent entropy which generalises the Bekenstein-Hawking, Tsallis, Renyi, Barrow, Sharma-Mittal, Kaniadakis and Loop Quantum Gravity entropies is introduced following Nojiri-Odintsov-Faraoni, Phys.Rev.D 105 (2022) 4, 044042. We address the implications of the generalized entropies on black hole thermodynamics as well as on cosmology, and discuss various constraints of the entropic parameters from different perspectives. The relation to holographic dark energy is briefly discussed. Our findings indicate that entropy is not fundamental quantity but may serve the basis for qualitatively new approaches to modern cosmology.