Enrico Barausse (IAP/CNRS) slides
Environmental Effects for Gravitational-wave Astrophysics
The detection of gravitational waves by terrestrial interferometers has ushered in the era of gravitational-wave astronomy. This will be even more true when space-based detectors will come of age and measure the mass and spin of massive black holes with exquisite precision and up to very high redshifts, thus allowing for better understanding of the symbiotic evolution of black holes with galaxies, and for high-precision tests of General Relativity in strong-field, highly dynamical regimes. Such ambitious goals require that astrophysical environmental pollution of gravitational-wave signals be constrained to negligible levels, so that neither detection nor estimation of the source parameters are significantly affected. Here, we consider the main sources for space-based detectors -- the inspiral, merger and ringdown of massive black-hole binaries and extreme mass-ratio inspirals -- and account for various effects on their gravitational waveforms, including electromagnetic fields, cosmological evolution, accretion disks, dark matter, "firewalls" and possible deviations from General Relativity. We discover that the black-hole quasinormal modes are sharply different in the presence of matter, but the ringdown signal observed by interferometers is typically unaffected. The effect of accretion disks and dark matter depends critically on their geometry and density profile, but is negligible for most sources, except for few special extreme mass-ratio inspirals. Electromagnetic fields and cosmological effects are always negligible. We finally explore the implications of our findings for proposed tests of General Relativity with gravitational waves, and conclude that environmental effects will not prevent the development of precision gravitational-wave astronomy.
Abraham Harte (Dublin City University) slides
Flatlanders never forget: Self-interaction in lower dimensions
While the Capra community has traditionally been motivated by astrophysics, many of the tools we have developed are much more general: They can and should be applied to also understand other areas of physics. One interesting class of problems are those associated with motion in two spatial dimensions, as relevant for various condensed-matter and fluid-mechanical systems. Tail effects are much stronger here than in any "ordinary" 3+1D system, implying the existence of extremely persistent memory effects. These provide a far richer and less intuitive phenomenology than we're used to. For example, there is a sense in which mechanics takes on an almost-Aristotelian (instead of Newtonian) character.
Yi-Zen Chu (National Central University, Taiwan)
Causal Structure of Gravitational Waves in Cosmology
Despite being associated with massless particles, electromagnetic and gravitational waves do not propagate strictly on the null cone in curved spacetimes. They also develop tails, traveling inside the light cone. This tail effect provides a contribution to the self-force of compact bodies orbiting super-massive black holes, which in turn are believed to be important sources of gravitational waves for future space based detectors like LISA, TianQin and Taiji. I will summarize my efforts -- which were inspired by the EMRI self-force problem -- to explore novel methods to understand the tail effect in curved geometries, primarily in cosmological spacetimes. Some of the spin-offs include the (small) discovery of new type of gravitational wave memory effect induced by tails.
Peter Diener (Louisiana State University) slides
Progress in self-consistent evolution with a time domain scalar charge self-force code
I will describe the progress made in performing self-consistent evolution of a scalar charge in orbit around a Schwarzschild black hole. I will briefly describe the main technical obstacle that was overcome, the obstacles that remain and show the first preliminary comparison between self-consistent and geodesic evolution. I will finally outline the plans for future development.
Sumanta Chakraborty (Department of Theoretical Physics, Kolkata, India) slides
Signatures of extra dimensions in gravitational waves from black hole quasi-normal modes
In this work we have derived the evolution equation for gravitational perturbation in four dimensional spacetime in presence of a spatial extra dimension. The evolution equation is derived by perturbing the effective gravitational field equations on the four dimensional spacetime, which inherits non-trivial higher dimensional effects. The gravitational perturbation has further been decomposed into a purely four dimensional part and another piece depending on extra dimensions. The four dimensional gravitational perturbation now admits massive propagating degrees of freedom, courtesy to the existence of higher dimensions. We have also studied the influence of these massive propagating modes on the quasi-normal mode frequencies, signalling the higher dimensional nature of the spacetime and have contrasted these massive modes with the massless modes in general relativity. Surprisingly, it turns out that the massive modes experience much smaller damping compared to the massless modes in general relativity and may even dominate over and above the general relativity contribution if one observes the ring down phase of a black hole merger event at sufficiently late times. Furthermore, the whole analytical framework has been supplemented by the fully numerical Cauchy evolution problem as well. In this context we have shown that except for minute details the overall features of the gravitational perturbations are captured in both the Cauchy evolution as well as in the analysis of quasi-normal modes. The implications are also discussed.
Chris Kavanagh (IHES) slides
Overlap of self-force, post-Newtonian and effective-one-body approaches
The effective-one-body (EOB) model continually proves to be a flexible platform for combining information from numerical relativity and post-Newtonian theory into a comparatively simple form. This has allowed it to become one of the most widely used tools in waveform generation for LIGO. In this talk I will overview the EOB method, and how the developments in conservative self-force calculations have allowed considerable overlap between EOB, PN and self-force.
Andrea Antonelli (AEI Potsdam) slides
Progress at the interface between Effective One Body and the Small Mass Ratio approximation
In this brief talk, several aspects at the interface between EOB theory and the Small Mass Ratio (SMR) approximation will be reviewed. The light ring problem will be discussed and an outline of its resolution presented. The state-of-the-art EOBSMR model will then be introduced, followed by a sketch of the roadmap for its further improvements.
Justin Vines (AEI Potsdam) slides
A test black hole in curved spacetime
The Mathisson-Papapetrou-Dixon equations, governing the motion of an extended test body in a curved background, depend on the body's infinite series of multipole moments. From the quadrupolar level and up, the multipole tensors are completely unconstrained in the MPD analysis; they would be determined by the body's internal structure and dynamics. Is there a particular form for the MPD multipoles appropriate for a "test black hole"---an extended test body with spin-induced (and tidally induced?) multipoles matching those of black hole? We will explore this question in the context of an effective action approach to the MPD dynamics, and will mention some intriguing relationships to questions about scattering amplitudes for higher-spin quantum particles.
Anna Heffernan (University of Florida) slides
Mode sum regularization in generic Kerr spacetime
The self-force mode-sum technique has proven itself the current leading technique in self-force calculations. The removal of a singular field via regularization parameters is a crucial part of this method. We show a covariant form of the Detweiler-Whiting singular field in the scalar case. We give a rotated coordinate system which allows for the production of high order regularization parameters in generic Kerr spacetime. We show the success of our parameters and outline the steps towards the gravity case.
- Morning: Barry Wardell
- Afternoon: Niels Warburton