Colloquiums 

The exterior of the Schwarzschild black hole is known to be stable against linear perturbations. The complete proof is established by exploiting the energy integral method and spectral theory. I will first review the standard arguments for the stability analysis. Then, I will discuss possible challenges in applying these arguments to the recently discovered holographic semiclassical instability of de Sitter and AdS spacetimes induced by the backreaction of CFT stress-energy.

This colloquium will begin with a review of the fundamental knowledge necessary for simulating the dynamics of primordial black hole formation. Following this, we will compare previously proposed analytical PBH formation conditions with numerical simulation results, focusing on the effects of inhomogeneity and ellipticity in the matter-dominated phase of primordial black hole formation. Finally, we will present the results of numerical simulations using the relativistic particle method and offer a discussion based on these findings.

  Ringdown gravitational wave emitted after merger of black hole (BH) or neutron star binaries are called quasi-normal modes (QNMs) because their (complex) frequencies are characterized by mass and angular momentum of the final state, stationary vacuum BHs. Therefore, testing general relativity using this is often discussed. However, to perform precise tests, it is necessary to take into account the effects of factors such as matter distribution that may exist around the actual BHs.

  In this colloquium, I will first set up the background spacetime of spherically symmetric BH solutions growing via the accretion of dilute perfect fluid, then derive the master equation satisfied by the metric perturbation. The ringdown gravitational waveform is derived numerically in the time domain. Finally, I will define two observables derived from the waveform, then discuss the qualitative behavior of deviations from the QNM frequency and what information can be extracted about the BH’s surrounding environment.

Black hole ringdown waves may provide a way to test whether astrophysical black holes are exactly described by vacuum solutions or carry additional “hair.” In this seminar, I present a perturbative framework for studying how effective black-hole hair, modelled as an anisotropic fluid, modifies quasinormal-mode frequencies. Using the eikonal correspondence between quasinormal modes and unstable circular null geodesics, the ringdown frequency is related to the photon-orbit angular frequency and Lyapunov exponent. This allows the shifts in oscillation and damping to be derived from geodesic quantities rather than from the full wave equation. I discuss the static case, applications to Bardeen, Hayward, and Kiselev black holes, and the extension to rotating hairy black holes.

We formulate a local geometric criterion for weak cosmic censorship in black hole overcharging and overspinning thought experiments. Under the null convergence and generic conditions, matter injection turns a horizon cross section into a closed trapped surface. Any final spacetime unable to accommodate this surface is ruled out. This trapped surface criterion excludes superextremal Reissner-Nordström, Reissner-Nordström-de Sitter, and Kerr-Newman final states, as well as Weyl-class naked singularities. Our criterion does not rely on asymptotic charges or on an extremal condition characterizing naked singularities. 

In this talk, we propose the sector of typical state in Large N symmetric orbifold CFT is corresponding closed universe sector of boundary CFT. This sector of typical state has the same nature to closed universe: (1) Pure state cannot be distinguished to mixed state from the simple correlation function. (2) The dimension of the sector of typical state is one-dimensional in Large N. This talk is based on https://arxiv.org/abs/2606.04575

Assuming an isometry on a spacetime, one can reduce field equations on it into ones on the reduced spacetime (orbit space). It is well known that the Einstein field equation reduces to a two-dimensional problem by assuming two different symmetries; for instance, assuming time translation symmetry (stationarity) and rotation symmetry (axisymmetry) in the Einstein-Maxwell system, the field equations reduce to a couple of complex potential equations called the Ernst equations. It is important fact that the solution space of the Ernst equations possesses SU(2,1) symmetry, so one can generate new non-trivial solutions from a given seed solution (e.g. Harrison transformation, which produces electromagnetized solutions from a given vacuum seed).

In this talk, I briefly introduce the Ernst formalism first, then study the solution generating techniques under the formalism with some examples.

We construct stationary rotating wormhole solutions in five-dimensional spacetime with equal angular momenta that asymptotically approach the Kaluza-Klein spacetime with a compactified extra dimension.
For wormholes supported by a phantom field, it has been shown that rotation can reduce the violation of the null energy condition. We investigate whether the same behavior occurs in our solutions and how it depends on the size of the compactified extra dimension.
Furthermore, since analytical solutions for Kaluza-Klein black holes exist, we aim to clarify the relationship between wormholes and black holes.

We have been observing the star S0-2 orbiting the massive BH Sgr A*, and performing the so-called "Parametrized-Post-Newtonian (PPN)" test of General Relativity (GR).
I report a preliminary result of Bayesian fitting between S0-2 observational data and General Relativistic modelling of S0-2 motion around Sgr A*.
The posterior probability distribution of the PPN parameters implies Dark Matter Accumulation (or Violation of GR) within a few thousand AU region around Sgr A*, with more than 99.9% credibility. 

This study investigates quasinormal modes (QNMs) in stationary and axisymmetric black hole spacetimes. QNMs can be defined as the eigenmodes of a wave equation subject to specific boundary conditions: purely ingoing waves at the event horizon and purely outgoing waves at spatial infinity. Because energy can escape through the horizon and to infinity, the corresponding QNM frequencies are generally complex. The real part describes the oscillation frequency, while the imaginary part determines the damping rate.
  One useful approach to calculating QNM frequencies is the QNM–geodesic correspondence. : the real part of the QNM frequency is related to the orbital angular frequency of an unstable circular null geodesic, while the imaginary part is related to its Lyapunov exponent, which characterizes the instability of the orbit.
  In this study, we extend this correspondence to more general stationary and axisymmetric black hole spacetimes. This framework allows us to obtain QNM frequencies for a wide class of black holes in a relatively simple way by using the properties of unstable null geodesics. In this talk, I focus on the analytical results of the formalism and examine consistencys with previous papers.

Primordial black holes (PBHs) can form from the gravitational collapse of large curvature perturbations re-entering the horizon in the early universe. Peaks theory offers a natural statistical description of this process, but as usually applied it relies only on the typical (mean) peak profile, which is not sufficient on its own. We address this in two ways. First, we show that the angle-averaged, rather than pointwise, field is the correct initial data for spherically symmetric collapse simulations, correcting how the typical profile should be constructed. Second, we incorporate the profile's variance, sampling profiles s standard deviations from the mean, into the collapse threshold and abundance calculation. This raises the estimated PBH abundance relative to mean-profile-only estimates, and shows that both the threshold and the critical-scaling PBH mass depend systematically on s and the peak wavenumber.

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