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.