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.
Various self-similar spherically symmetric spacetimes admit naked singularities, providing a challenge to the cosmic censorship hypothesis. However, it is not clear if the naked singularities are artefacts of the high degree of symmetry of the spacetimes, or if they are potentially generically present. To address this question, we consider perturbations of (various cases of) these spacetimes, focusing particularly on the behaviour of the perturbations as they impinge on the Cauchy horizon.
The effective field theory framework yields a systematic treatment of gravitational bound states such as binary systems. Gravitational waves emitted from compact binaries are one of the prime event candidates at direct detection experiments. Due to the multiple scales involved in the binary problem, an effective field theory treatment yields many advantages in perturbative calculations. My talk will review the setup of the effective field theory framework and report on recent progress in gravitational wave phenomenology.
The local and global properties of the retarded and Feynman Green functions to the wave equation in curved spacetime are crucial for radiation reaction in the classical theory and for renormalisation in the quantum quantum theory. Building on an insight due to Avramidi, we provide a system of transport equations for determining key fundamental geometrical bitensors determining the local Hadamard singularity structure of these GreenÃ¢ÂÂs functions.
The upcoming launch of the space-based gravitational wave interferometer detector LISA will yield an unprecedented amount of astrophysical and cosmological science from a variety of gravitational wave sources. Among these, the extreme mass ratio inspirals (EMRIs) of stellar-mass compact objects into supermassive black holes will provide a unique opportunity to test the predictions of General Relativity for strongly gravitating systems since the masses and spins of these sources are expected to be measured with precisions better than about 1 part in 10^4.
Assuming exotic matter, several models representing static, spherically symmetric wormhole solutions of Einstein's field equations have been considered in the literature. We examine the dynamical stability of such wormholes in one of the simplest model, in which the matter is described by a massless ghost scalar field, and prove that all solutions are unstable with respect to linear fluctuations and possess precisely one unstable, exponentially in time growing mode.
The long awaited discovery of the double radio pulsar system, PSR J0737-3039A/B, surpassed most expectations, both theoretical and observational, as a tool to probe general relativity, stellar evolution and pulsar theories. The Double Pulsar provides a unique and the most complete and clean test of theories of gravity in a regime sensitive to possible strong-gravitational self-field effects. All six post-Keplerian parameters have been measured (including the measurement of the relativistic spin precession), some parameters to a precision of 10^{-4}.
Inflation is one of the foundational paradigms of our picture of the Universe. Yet distinguishing between the multitude of different inflationary models presents major observational challenges. In this talk, I will discuss a number of inflationary observables, specifically the tilt and running of the primordial power spectrum, compensated isocurvature modes, and non-Gaussianity, and the extent to which they might be constrained by future galaxy surveys and 21 cm experiments.
NANOGrav is a consortium of radio astronomers and gravitational wave physicists whose goal is to detect gravitational waves using an array of millisecond pulsars as clocks. Whereas interferometric gravitational wave experiments use lasers to create the long arms of the detector, NANOGrav uses earth-pulsar pairs. The limits that pulsar timing places on the energy density of gravitational waves in the universe are on the brink of limiting models of galaxy formation and have already placed limits on the tension of cosmic strings.
There has been a growing interest in electromagnetic counterparts to gravitational wave signals. Of particular interest here, are counterparts to gravitational wave signals from super-massive black hole mergers. We consider a circumbinary disk, hollowed out by torques from the binary, and provide an analytic solution to its response following merger. There are two changes to the potential which occur during the merger process: an axisymmetric mass-energy loss and asymmetric recoil kick given to the resulting super-massive black hole.
Numerical simulations of binary black holes with spin have revealed some surprising behavior: for antialigned spins in the orbital plane, 1) one sees an up-and-down "bobbing" of the entire orbital plane at the orbital frequency and 2) the merged black hole receives an enormous kick that depends on the phase at merger. Natural questions are: What causes the bobbing? Can the kick be viewed as a post-merger continuation of the bobbing?