Search Talks

Understanding NP-complete problems is a central topic in computer science. This is why adiabatic quantum optimization has attracted so much attention, as it provided a new approach to tackle NP-complete problems using a quantum computer. The efficiency of this approach is limited by small spectral gaps between the ground and excited states of the quantum computer's Hamiltonian. We show that the statistics of the gaps can be analyzed in a novel way, borrowed from the study of quantum disordered systems in statistical mechanics. It turns out that due to a phenomenon similar to Anderson localization, exponentially small gaps appear close to the end of the adiabatic algorithm for large random instances of NP-complete problems. We show that this effect makes adiabatic quantum optimization fail, as the system gets trapped in one of the numerous local minima. We will also discuss recent developments including the effect of the exponential number of solutions and Hamiltonian path change. Joint work with Boris Altshuler and Hari Krovi Based on arXiv:0908.2782 and arXiv:0912.0746

We introduce a class of probability spaces whose objects are infinite graphs and whose probability distributions are obtained as limits of distributions for finite graphs. The notions of Hausdorff and spectral dimension for such ensembles are defined and some results on their value in koncrete examples, such as random trees, will be described.

Using a formulation of the post-Newtonian expansion in terms of Feynman graphs, we discuss how various tests of General Relativity (GR) can be translated into measurement of the three- and four-graviton vertices. The timing of the Hulse-Taylor binary pulsar provides a bound on the deviation of the three-graviton vertex from the GR prediction at the 0.1% level. For coalescing binaries at interferometers, because of degeneracies with other parameters in the template such as mass and spin, the effects of modified three- and four-graviton vertices at the level of the restricted PN approximation, is to induce an error in the determination of these parameters and it is not possible to use coalescing binaries for constraining deviations of the vertices from the GR prediction.

While gravitational waves offer a new, and in many ways clean, view of compact objects, most of what we presently know about these has been obtained by careful study of their messy interactions with surrounding material. I will summarize what we know about a variety of potential gravitational wave sources, how this astrophysical hair has helped to illuminate some of the same questions gravitational wave observations promise to address, and how future observations may begin to relate the gravitational and electromagnetic properties of compact objects.

In addition to the dominant oscillatory modes gravitational waves contain non-oscillatory components which arise as drifts or offsets in the signals. Nonlinear gravitational memory arises from a change in mass multipole moments of a boundsystem due to contributions from the emitted gravitationalwaves. In practice it appears as a slowly monotonically growingsignal during the inspiral which sees a rapid rise at thetime of merger. The low amplitude and non-oscillatory natureof these signals present unique challenges for modeling.I discuss recent efforts to evaluate these signals in numericalsimulations using characteristic extraction as well as theirpotential relevance to detection.

We present a short overview on the current state of core-collapse supernova modeling and the set of processes expected to emit gravitational waves in a core collapse event. We go on to show new results from 3D GR simulations focusing on failing black-hole forming supernovae and present the gravitational wave signature of such events.

In this work we investigate the electromagnetic radiation induced by a binary black hole merger when they are surrounded by a force-free environment (i.e. plasma with inertia terms negligible compared to the electromagnetic stresses). We discuss the relevance of this system for possible multimessenger astronomy with binary black holes.

I will report on some recent results obtained using the fully general relativistic magnetohydrodynamic code Whisky in simulating equal and unequal-mass binary neutron star (BNS) systems during the last phases of inspiral, merger and collapse to black hole surrounded by a torus. BNSs 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. I will in particular describe both the gravitational wave signals generated by these sources and the properties of the tori that can be formed. I will also describe how the Effective One Body (EOB) model can be used to accurately compute the gravitational wave signal generated during the inspiral of BNSs by comparing it with the longest general relativistic numerical simulations of BNSs performed up to now.

We describe a Markov-Chain Monte-Carlo technique to study the source parameters of gravitational-wave signals from the inspirals of stellar-mass compact binaries detected with ground-based detectors such as LIGO and Virgo. We can apply this technique to both spinning and non-spinning waveforms and we use a variety of tools like parallel tempering to improve the sampling efficiency of the algorithm in a multi-dimensional parameter space. We describe new developments in model-selection techniques for distinguishing between alternative signal models. We present preliminary results from the application of these techniques to data sets containing injections of numerical-relativity waveforms into simulated Gaussian detector noise. We study the source parameters of signals from the inspirals of stellar-mass compact binaries detected with ground-based gravitational-wave detectors such as LIGO and Virgo. We use automatic adaptation of the step size and take into account the correlations between parameters to efficiently probe the parameter space while keeping the algorithm suitable for a wide range of signals. We shall discuss the performance of the MCMC algorithm and the typical measurement accuracy of the source parameters as a function of the binary parameters and the number of detectors in the network. We will show that despite the lower positional accuracy compared to other astronomical observations an association of a gravitational-wave event with e.g. an electromagnetic detection is possible with three or even two 4-km-size interferometers.

Direct detection of gravitational wave stands at a cross roads; the first generation of interferometric detectors will soon be decommissioned and the second generation projects are underway. In this talk, I will describe the Initial LIGO and VIRGO generation of instruments, the techniques required to achieve a strain sensitivity of 3 x 10^{-23} and an NS / NS inspiral range of 15 Mpc. I'll follow with a description of the Advanced detectors and the differences that should improve the sensitivity by a factor of ten. Finally, I will describe projects from radio and microwave astronomy to measure gravitational waves using pulsar timing and the CMB B-mode polarization.