Title: Black-Hole Perturbation Theory: What Can We Ask and Learn

Abstract: Black-hole perturbation theory provides a powerful and flexible framework for studying strong gravitational environments. By studying test fields on fixed black-hole backgrounds, one can extract observables such as superradiance, quasinormal modes, and others. We will use two examples to show what questions this approach allows us to ask.

The first example concerns superradiance, in which incident waves are amplified by extracting energy from a black hole. Although superradiance appears in contexts ranging from quantum many-body systems to black holes, a unified framework remains incomplete. We will discuss an information-theoretic approach in which relative entropy is used to characterize information in the black-hole system, study the outgoing radiation, and identify the conditions under which superradiance occurs.

The second example concerns quasinormal modes of black holes surrounded by matter. Since astrophysical black holes are not isolated, future observations will require increasingly accurate modeling of environmental effects. We will present a simple model with anisotropic matter around the black hole and study scalar-field perturbations in this background. To compute the spectra, we develop an extension of Leaver’s continued-fraction method and a new automatic-differentiation-based method to explore challenging parameter regimes.

Together, these examples illustrate the flexibility of black-hole perturbation theory as a framework for computing observables and formulating new questions. They also highlight the need for tailored analytical and numerical tools when extending perturbation theory to more complex physical settings.

Title: Sensitivity of Neutron-Star Observables to Microscopic Nuclear Parameters in the Chiral Mean Field Mode.

Abstract: The equation of state of matter at supranuclear densities governs the astrophysical observables of neutron stars, including their masses, radii, compactnesses, and tidal deformabilities. Realistic dense-matter models such as the Chiral Mean Field model provide a physically motivated description of this regime, but they depend on many microscopic nuclear-physics parameters whose individual impact on neutron-star structure is not always transparent. In this seminar, I will present a sensitivity analysis of neutron-star observables to the parameters of the Chiral Mean Field model in beta equilibrium, using SLy as the crust. We compute neutron-star sequences, extract the corresponding observables, and construct a dimensionless sensitivity matrix from logarithmic derivatives with respect to the model parameters. A principal-component analysis then identifies the dominant combinations of microscopic parameters that control the macroscopic stellar response. We find that the most important parameters are primarily associated with the scalar and chiral sectors of the model, especially the vacuum value of the dilaton field \chi_0, the scalar singlet strength g_1^X, and the quadratic scalar-potential term k_0. This framework provides a reproducible, data-driven way to quantify parameter sensitivities in dense-matter models and to guide future inference of nuclear information from multi-messenger neutron-star observations.