This series consists of talks in areas where gravity is the main driver behind interesting or peculiar phenomena, from astrophysics to gravity in higher dimensions.
I
will review recent work in two very different topics. First, I will discuss the
quasinormal mode spectrum of nearly extremal Kerr black holes, where a
bifurcation of the frequency spectrum is observed. In addition, collective
oscillations of many modes is possible, resulting in a power-law rather than
exponentially decaying ringdown. Next, I will discuss a recent proposal for how
tidally induced, multimode coupling of normal modes in neutron stars can
destabilize the stars. Such an instability could hamper gravitational wave
Pulsars have enormous magnetic fields whose energy density
dwarfs the rest mass density of their plasma magnetosphere. In this
regime of a plasma, the particles drop out of the description, leaving a set of
equations for the electromagnetic field alone. This non-linear,
deterministic system is known as force-free electrodynamics, and turns out to
have some beautiful and bizarre features. I will give a pedagogical
introduction to these equations and their role in astrophysics and then discuss
Implications of
recently well-measured neutron star masses, particularly near and above 2 solar masses, for the equation of state (EOS) of neutron star matter will be highlighted. Model independent upper
I describe recent work with with Stefan Hollands that establishes a new criterion for the dynamical stability of black holes in $D \geq 4$ spacetime dimensions in general relativity with respect to axisymmetric perturbations: Dynamic stability is equivalent to the positivity of the canonical energy, $\mathcal E$, on a subspace of linearized solutions that have vanishing linearized ADM mass, momentum, and angular momentum at infinity and satisfy certain gauge conditions at the horizon. We further show that $\mathcal E$ is related to the second order variations of mass, angular momentum, and
Spins play a major role in the strong-field dynamics of
black-hole binaries and their gravitational-wave emission. By detecting spin
effects in the waveforms, existing and future gravitational-wave detectors
therefore provide a natural way to test gravity in strong-field, highly
dynamical regimes.
In the first part of my talk, I will show that the
inclusion of the spins in the gravitational templates for future space-based
detectors will permit testing scenarios for the formation and cosmological
We discuss well-posed initial-boundary value formulations
in general relativity. These formulations allow us to construct solutions of
Einstein's field equations inside a cylindrical region, given suitable initial
and boundary data. We analyze the restrictions on the boundary data that result
from the requirement of constraint propagation and the minimization of spurious
reflections, and choosing harmonic coordinates we show how to cast the problem
into well-posed form. Then, we consider the particular case where the boundary
In the last few years several interesting phenomena associated to the interaction between massive black holes and fundamental bosonic fields have been discovered. I present a selection of them, including superradiance instabilities of spin-0, spin-1 and spin-2 fields, floating orbits in extreme-mass ratio inspirals and black-hole spontaneous scalarization. The theoretical potential of these effects
as almost-model-independent smoking guns for exotic particles and modified gravity, as well as their limitations in realistic astrophysical scenarios, are discussed.
The last few years have seen new opportunities for
constraining the physics of neutron star interiors. I will first discuss the
current state of neutron star radius measurements and then go on to discuss
thermal tomography as a probe of the nuclear, magnetic, and transport
properties of neutron star crusts. In each case, I will emphasize the
astrophysics that must be understood to make reliable inferences about the
properties of dense matter from observations of neutron stars.