Strong Gravity

Gas accretion onto black holes is thought to power some of the most energetic astrophysical phenomena observed. Black hole accretion disks are efficient engines for converting binding energy into light, and for launching relativistic unbound flows (jets) such as in gamma ray bursts, microquasars and radio-loud active galactic nuclei (AGN). Some systems individually exhibit a wide variety of spectral and bolometric states while others remain remarkably predictable. As their brightest emission usually emanates near the black hole's event horizon, they serve as excellent environments for exploring different theories of gravity or for constraining the black hole's geometry. In this talk I will explain how investigators use modern general relativistic magnetohydrodynamic computer simulations to understand accretion observations and probe the strong-field regime of gravity. In particular, I will focus on three topics. First, I will describe how dynamical models of the accretion flow around Sagittarius A*, the supermassive black hole at the center of our galaxy, can help us predict what we will see when observations at the sub-horizon scale are made soon for the first time. Second, I will explain recent developments in simulating cooled thin disks and how their results may affect estimates of black hole spin from the disks' thermal spectra. Last, I will describe how temporal variability analysis of our dynamical simulations can offer insight into the common behavior seen in high-energy emission from black holes with masses of 10 solar masses to a billion solar masses.

Galaxy mergers, which are a natural consequence of hierarchical assembly of galaxies, are expected to produce binary black holes, which subsequently merge. The detection and analysis of gravitational waves from these sources is the major aim of the next generation gravitational wave detector: LISA, the Laser Interferometric Space Antenna. These gravitational waves encode a tremendous amount of information, but to make the connection with astrophysics and cosmology, it is necessary to identify the galaxies hosting these mergers via the associated electromagnetic counterpart to these mergers. I will describe these mergers events and discuss the various regimes where potential electromagnetic counterparts can be found. I will also describe some recent work, which holds much promise for the prompt identification of these mergers -- an electromagnetic precursor from tidal forcing.

Binary neutron stars are among the most important sources of gravitational waves which are expected to be detected by the current or next generation of gravitational wave detectors, such as LIGO and Virgo, and they are also thought to be at the origin of very important astrophysical phenomena, such as short gamma-ray bursts. In order to describe the dynamics of these events one needs to solve the full set of general relativistic magnetohydrodynamics equations through the use of parallel numerical codes. I will report on some recent results obtained with the use of the fully general relativistic magnetohydrodynamic code Whisky in simulating binary neutron stars which inspiral and merge forming an hypermassive neutron star which eventually collapses to form a black hole surrounded by a torus. I will in particular describe how mass, equation of state and magnetic fields can affect the dynamics and consequently the gravitational waves emitted by these systems and discuss about their possible connection with the formation of short gamma ray bursts.

Einstein’s general theory of relativity is the standard theory of gravity, especially where the modern needs of astronomy, astrophysics, cosmology and fundamental physics are concerned. As such, this theory is used for many practical purposes involving spacecraft navigation, geodesy, time transfer and etc. Series of recent experiments have successfully tested general relativity to a remarkable precision. Various experimental techniques were used to test relativistic gravity in the solar system namely spacecraft Doppler tracking, planetary ranging, lunar laser ranging, dedicated gravity experiments in space and many ground-based efforts. We will discuss the recent progress in the tests of relativistic gravity and motivation for the new generation of high-accuracy gravitational experiments in space. We also discuss the advances in our understanding of fundamental physics that are anticipated in the near future and evaluate the discovery potential of the recently proposed solar system gravitational experiments.

In quantum field theory it is possible to create negative local energy densities. This would violate the Generalized Second Law (GSL) unless there is some sort of energy condition requiring the negative energy to be counterbalanced by positive energy. TO explore what this energy condition is, I will assume that the GSL holds in semiclassical gravity for all future causal horizons. From CPT symmetry it follows that the time-reverse of the GSL, properly understood, holds for all past causal horizons. These two conditions together then imply that the Averaged Null Energy Condition (ANEC) holds on any null line, i.e. a complete achronal lightlike null geodesic. In curved spacetimes, the ANEC can be violated on general geodesics. But even if the ANEC only holds on null lines, theorems by Sorkin, Penrose and Woolgar, and by Graham and Olum imply that semiclassical gravity should satisfy positivity of energy, topological censorship, and should not admit closed timelike curves. These results can thus be seen as consequences of the GSL. However, these theorems don't apply when gravitational fluctuations are taken into account. In that case, the GSL argument suggests a modification to the ANEC which may make these theorems applicable to perturbative quantum gravity.

One of the main science objectives for the Laser Interferometer Space Antenna (LISA) is to quantitatively map the strong field regions around compact objects using Extreme-Mass-Ratio Inspirals (EMRIs). This idea has been shown to be possible in principle, however in practice only inspirals in a Kerr spacetime have been studied in detail. A spacetime mapping algorithm for an EMRI inspiral into a generic compact object is formulated using ideas from integrable systems. I discuss several aspects of the theoretical development required to make the problem tractable. Some recent results about particle orbits around "Bumpy Black" holes are highlighted.

I present an overview of how inspiral-merger-ringdown (IMR) waveforms are currently being used within LIGO and Virgo search efforts. I'll discuss search strategies from the two major astrophysics working groups within t he LIGO/Virgo collaboration searching for transient gravitational-wave signals - the Compact Binary Coalescence group and the Burst Group. For masses where the inspiral, merger and ring-down phases are prominent in the LIGO/Virgo band both working groups have developed pipelines that are sensitive to these systems and are now trying to work together to make a joint statement about LIGO and Virgo's sensitivity to IMR systems.

With the imminent detection of gravitational waves by ground-based interferometers, such as LIGO, VIRGO and TAMA, pulsar timing observations, and proposed space-borne detectors, such as LISA, we must ask ourselves: how much do we trust general relativity? The confirmation of general relativity through Solar System experiments and binary pulsar observations has proved its validity in the weak-field, where velocities are small and gravity is weak, but no such tests exist in the strong, dynamical regime, precisely the regime of most interest to gravitational wave observations. Unfortunately, because of their inherent feebleness, the extraction of gravitational waves from detector noise relies heavily on the technique of matched filtering, where one constructs waveform filters or templates to clean the data. Currently, all such waveforms are constructed with the implicit assumption that general relativity is correct both in the weak and strong, dynamical regimes. Such an assumption constitutes a fundamental bias that will introduce a systematic error in the detection and parameter estimation of signals, and in turn can lead to a mischaracterization of the universe through incorrect inferences about source event rates and populations. In this talk, I will define this bias, explain its possible consequences and propose a remedy through a new scheme: the parameterized post-Einsteinian framework. In this framework one enhances waveforms via the inclusion of post-Einsteinian parameters that both interpolate between general relativity and well-motivated alternative theories, but also extrapolate to unknown theories, following sound theoretical principles, such as consistency with conservation laws and symmetries. The parametrized post-Einsteinian framework should allow matched filtered data to select a specific set of post-Einsteinian parameters without {\emph{a priori}} assuming the validity of the former, thus allowing the data to either verify general relativity or point to possible dynamical strong-field deviations.

The uncertainty in the equation of state of cold matter above nuclear density is notorious. Despite four decades of neutron-star observations, recent observational estimates of neutron-star radii still range from 8 to 16 km; the pressure above nuclear density is not known to better than a factor of 5; and one cannot yet rule out the possibility that the ground state of cold matter at zero pressure might be strange quark matter -- that the term "neutron star" is a misnomer for strange quark stars. The last few orbits of binary inspiral are sensitive to the stars' distortion, and a major goal of the next generation of gravitational wave detectors is to extract parameters characterizing the high-density equation of state from inspiral waveforms. This talk reports a first study that uses numerical simulations to estimate the accuracy with which the equation of state can be measured.