GRAVITY THEORY SEMINARS 2009
"Quantum fluctuations of an evaporating black hole and de Sitter spacetime"
While general relativity provides an accurate description of macroscopic gravitational phenomena, a
fully satisfactory microscopic theory of quantum gravity is still lacking. Nevertheless, an effective
field theory (EFT) of the gravitational interaction has been developed with the goal of describing the
quantum mechanical effects of the gravitational field at length-scales much larger than the
fundamental scale of the theory (usually believed to correspond to the Planck length). Here we will go
beyond the usual applications of such an EFT to weak field situations and consider the quantum
fluctuations of an evaporating black hole and those of de Sitter spacetime. Our approach is based on
introducing a mean field approximation for the background spacetime and quantizing the metric
perturbations around it.
Employing those tools I will describe our finding that the horizon fluctuations of an evaporating black hole can build up in time and become important at late times, but well before reaching the Planckian regime. Next, I will present the calculation of the quantum correlation function of the stress tensor in de Sitter spacetime, which is a key element in the study of the metric fluctuations induced by quantum matter fields. The existence of an interesting discontinuity in the masslesss limit of minimally-couped fields will be discussed as well as the long-range correlations exhibited by fields with very small (but non-vanishing) mass.
"Spin effects in gravitational waveforms from inspiralling black hole binaries"
In this talk I will discuss the importance of spin effects in gravitational waveforms from inspiralling black holes. Spin corrections appear in the equations of motion for the binary, and the phase and amplitude of the gravitational waveforms. The talk will begin with a review of the derivation of gravitational waveforms and their spin corrections in post-Newtonian theory. Waveforms applicable for the general case of a spinning, precessing black hole binary will be presented, as well as frequency-domain waveforms for the special case when spins are colinear with the orbital angular momentum. By studying these waveforms, we find that spin effects can significantly alter the features of the gravitational radiation. The importance and implications of spin effects for observations by ground- and space-based gravitational wave detectors will also be discussed.
"Rheology and the quark-gluon plasma"
In recent years, experiments have discovered an exotic new state of matter known as the strongly coupled quark-gluon plasma (sQGP), which seems to behave like a nearly perfect fluid. In parallel developments, string theory has provided a theoretical laboratory for studying the hydrodynamic properties of plasmas in certain strongly interacting gauge theories. I will describe efforts to measure the physical properties of the sQGP and the possible connections to the string theory calculations.
"Was Einstein right-handed"
What if the gravitational interaction possessed a preferred handedness and violated parity? We know that Maxwell's electromagnetism respects this symmetry but the weak-sector of the standard model violates it maximally. In this talk, we shall explore a string-theory motivated effective theory, Chern-Simons modified gravity, that models such parity-violation by introducing a novel curvature-squared term to the Einstein-Hilbert action. We shall analyze observables directly related to gravitational parity violation, including orbital precession and gravitational radiation. We shall conclude with a discussion of binary pulsar and Solar System observations that have already constrained such signatures, as well as planned gravitational wave detectors that hold the promise to bring these constraints to a new level.
"Binary black hole simulations and implicit time-stepping"
Numerical simulations of black hole binaries have made tremendous progress over the last years. The usefulness of such simulations is limited by their tremendous computational cost, which ultimately results from a separation of time-scales: Emission of gravitational radiation drives the evolution of the binary toward smaller separation and eventual merger. The time-scale for inspiral is far longer than the dynamical time-scale of each black hole. Therefore, the currently deployed explicit time-steppers are severely limited by Courant instabilities. Implicit time-stepping algorithms provide an obvious route around the Courant limit, thus offering a tremendous potential to speed up the simulations. However, the complexity of Einstein's equations make this a highly non-trivial endeavour. This talk will first present a general overview of the status of black hole simulations, followed by a status report on the ongoing work aimed at implementing modern implicit/explicit (IMEX) evolution schemes for Einstein's equations.
"Primordial non-Gaussianities from inflation: Current bounds and future prospects"
The hint of a possible detection of primordial non-Gaussianities in the recent WMAP5 Cosmic Microwave Background data has recently brought the studies of primordial non-Gaussianities into the forefront of precision cosmology. In this talk, I will give an overview of both the theoretical and observational bounds on primordial non-Gaussianties, focusing primarily on the CMB. On the theory side, I will review the current computational techniques and discuss future improvements and applications. On the observational side, I will talk about the latest constraints from the WMAP5 data and prospects of obtaining even better bounds from future experiments and data analysis techniques.
"Second-order spectral evolutions"
Spectral evolutions of Einstein's equations normally require a fully first-order reduction of the system. While this is the only well-known way to obtain a stable system, it has the disadvantage of introducing additional constraints and equations. A new method for evolving second-order (in space) equations spectrally is derived for the simplest analogous system, the scalar wave equation. Surprisingly, even this simple case turns out to be non-trivial. The derivation of the method and its application to binary black hole spectral evolutions will be presented and discussed.
"Atom interferometers for gravitational-wave detection and other gravitational measurements. Report on the Florence conference"
The detection of gravitational waves (GWs) will open a completely new window for observing the universe with deep implications for astrophysics and cosmology. However, it will require detectors with unprecedented sensitivity. This quest is currently led by several kilometer-size laser interferometers around the world. On the other hand, atom interferometers are being used to measure gravitational properties with increasing precision, and several schemes based on atom interferometery have been proposed to achieve sensitivities competitive with that of gravitational-wave detectors employing laser interferometers. I will present a careful theoretical analysis of the response to GWs for these kind of atom interferometers and provide a critical comparison of their performance with that of laser interferometers. In addition, I will summarize the main results presented at the international conference in Florence on "Gravitational Wave Detection with Atom Interferometry", and conclude by discussing possible future directions in the field.
"Fiberbundle-based visualization of a stir tank fluid"
We describe a novel approach to treat data from a complex numerical simulation in a unified environment using a generic data model for scientific visualization. The model is constructed out of building blocks in a hierarchical scheme of seven levels, out of which only three are exposed to the end-user. This generic scheme allows for a wide variety of input formats and results in powerful capabilities to connect data. We review the theory of this data model, implementation aspects in our visualization environment, and its application to computational fluid dynamic simulation covering a fluid in a stir tank. The computational data are given as a vector field and a scalar field describing pressure on 2088 blocks in curvilinear coordinates.
"NLO calculations and parton showers"
Many inclusive processes relevant for collider physics have by now been calculated to NLO accuracy. Unfortunately, fully exclusive event samples are required by the experimental collaborations in order to implement experimental cuts and correct the theoretical predictions for detector effects. In this talk, I will describe how to combine NLO calculations with parton shower algorithms and will explain why resummed NLO calculations are necessary. I will present results that allow this combination to be performed without the need for computationally costly numerical integrations.
"Eccentric binary black hole systems in numerical relativity and post-Newtonian theory"
An introductory discussion of the inflationary and cyclic models of the universe, including connections to observations past and future.
"Strongly coupled quark-gluon plasma"
Experiments made at the Relativistic Heavy Ion Collider suggest a new phase of QCD, known as the quark-gluon plasma, to be a near-perfect liquid with small viscosity. The first 1/3 of the talk is about the main phenomena which lead to this conclusion. The next 2/3 is about attempts to explain it. One is based on electric-magnetic duality and the role of (color) magnetically charged quasiparticles -- monopoles and dyons. Another duality is the so called AdS/CFT correspondence, based on string theory gravity methods, which allows to address strongly coupled gauge theories. Two transport observables -- the diffusion constant and viscosity -- obtained in both ways will be compared at the end.
"Using black hole perturbation theory to understand extreme mass ratio inspirals"
Extreme mass ratio inspirals (EMRIs), where stellar mass compact objects (1-100 M_sun) spiral into massive black holes (10^6-10^8 M_sun) at the centers of galaxies, are particularly interesting sources of gravitational waves. I will present some recent developments in modeling EMRIs using black hole perturbation theory. We now have a toolkit that can model the inspiral trajectory and gravitational waveforms from EMRIs. I will also present some recent estimates of recoil velocities from supermassive black hole mergers using perturbation theory.
"Graphics hardware and GPU computing: Past, present and future"
Modern GPUs have emerged as the world's most successful parallel architecture. GPUs provide a level of massively
parallel computation that was once the preserve of supercomputers like the MasPar and Connection Machine. For example,
NVIDIA's GeForce GTX 280 is a fully programmable, massively multithreaded chip with up to 240 cores, 30,270 threads
capable of performing up to a trillion operations per second. The raw computational horsepower of these chips has
expanded their reach will beyond graphics. Today's GPUs not only render video game frames, they also accelerate
physics computations, video transcoding, image processing, astrophysics, protein folding, seismic exploration,
computational finance, radioastronomy -- the list goes on and on. Enabled by platforms like the CUDA architecture,
which provides a scalable programming model, researchers across science and engineering are accelerating applications
in their discipline by up to two orders of magnitude. These success stories, and the tremendous scientific and market
opportunities they open up, imply a new and diverse set of workloads that in turn carry implications for the evolution
of future GPU architectures.
In this talk I will discuss the evolution of GPUs from fixed-function graphics accelerators to general-purpose
massively parallel processors. I will briefly motivate GPU computing and explore the transition it represents in
massively parallel computing: from the domain of supercomputers to that of commodity "manycore" hardware available to
all. I will discuss the goals, implications, and key abstractions of the CUDA architecture. Finally I will close with
a discussion of future workloads in games, high-performance computing, and consumer applications, and their
implications for future GPU architectures.
In this talk I will discuss the evolution of GPUs from fixed-function graphics accelerators to general-purpose massively parallel processors. I will briefly motivate GPU computing and explore the transition it represents in massively parallel computing: from the domain of supercomputers to that of commodity "manycore" hardware available to all. I will discuss the goals, implications, and key abstractions of the CUDA architecture. Finally I will close with a discussion of future workloads in games, high-performance computing, and consumer applications, and their implications for future GPU architectures.
"Inflationary bootstrap relations, blinking black holes"
First, I will discuss a way to test the essential idea underlying the inflationary paradigm: that the universe underwent a brief period of accelerated expansion followed by a long period of decelerated expansion. This idea is encapsulated in a single equation (the "e-fold constraint"), and I explain how forthcoming observations can point to its validity. Second, I will discuss a way that one might try to detect the strong bending of light rays in the vicinity of a black hole.
"Nonlocality from Planck-scale discreteness: problem and opportunity for quantum gravity"
In this informal talk, I will explain why I expect quantum gravity to exhibit a radical nonlocality on Planckian scales. Two questions then are how an approximate locality can emerge on mesoscopic scales and where one would expect the underlying nonlocality to begin to show up. I will address these questions in the context of a scalar field on a causal set, and one outcome of the discussion will be the possible existence of an intermediate length scale where the discreteness has disappeared but a residual nonlocality remains. An effective field theory that could describe this regime would provide a qualitatively new sort of phenomenology for quantum gravity.
"Black hole mergers and electromagnetic counterparts"
The anticipated detection of the gravitational waves (GWs) by the future Laser Interferometer Space Antenna (LISA) will constitute a milestone for fundamental physics and astrophysics. In this talk, I will discuss LISA's capability of providing an advance warning of supermassive black hole mergers (SMBH) and show how the size and geometry of the localization volume evolves during the observation. While the GW signatures themselves will provide a treasure trove of information, if the source can be securely identified in electromagnetic (EM) bands, this would open up entirely new scientific opportunities, to probe fundamental physics, astrophysics, and cosmology. I will describe several mechanisms that might produce EM variability during a SMBH merger. In particular, the binary may produce a roughly periodic variable electromagnetic flux, due to the orbital motion prior to coalescence, a transient signal caused by shocks in the circumbinary disk when the SMBH binary recoils and "shakes" the disk, or a prompt EM flare caused by the viscous dissipation of GWs in the ambient gas. I will discuss whether these time-variable EM signatures may be detectable using a LISA-triggered EM counterpart search campaign.
"The Born rule dies"
The Born rule may be stated mathematically as the rule that probabilities in quantum theory are expectation values of a complete orthogonal set of projection operators. This rule works for single laboratory settings in which the observer can distinguish all the different possible outcomes corresponding to the projection operators. However, theories of inflation suggest that the universe may be so large that any laboratory, no matter how precisely it is defined by its internal state, may exist in a large number of very distantly separated copies throughout the vast universe. In this case, no observer within the universe can distinguish all possible outcomes for all copies of the laboratory. Then normalized probabilities for the local outcomes that can be locally distinguished cannot be given by the expectation values of any projection operators. Thus the Born rule dies and must be replaced by another rule for observational probabilties in cosmology. The freedom of what this new rule is to be is the measure problem in cosmology. A particular volume-averged form is proposed.
"Probing the puncture for black hole simulations"
With the puncture method for black hole simulations, an infinite surface is compactified to a single point on the numerical grid. There are potential problems that can arise from such a procedure. For example, in the simple test case of a scalar field propagating on a cylinder, compactification leads to a resolution dependent time delay for waves reflected from the puncture. This does not appear to be a problem for Einstein gravity when one uses the BSSN formulation, 1+log slicing, and gamma-driver shift. Numerical studies of a single Schwarzschild black hole show that waves reflected from the puncture are "well behaved", with no artificial resolution or stencil dependence. The numerical code used for this study is based on a "cartoon" scheme in which spherically symmetric data is evolved using standard Cartesian finite difference stencils. The waves are defined by the characteristic fields in the frozen coefficients approximation. These fields propagate as perturbations in the stationary 1+log "trumpet slice" of the Schwarzschild geometry.
"Post-Newtonian calculation of the gravitational self-force for black hole binaries"
The detection and analysis of the gravitational radiation from black hole binaries by the VIRGO, LIGO and LISA observatories requires very accurate theoretical predictions, used as templates. There are two approximation schemes to perform such calculations in General Relativity: (i) the post-Newtonian theory, well suited to describe the inspiraling of arbitrary mass ratio compact binaries in the slow motion regime (v/c << 1), and (ii) the self-force approach, which gives a very accurate description of extreme mass ratio binaries. It is crucial to compare both formalisms in their common domain of validity: the slow motion regime of an extreme mass ratio binary. We present post-Newtonian calculations carried out at 3PN order (i.e. (v/c)^6 beyond the Newtonian approximation) for the self-force of an extreme mass ratio black hole binary on circular orbit. Our result is in perfect agreement with the numerical prediction of the self-force.
"Gravitational wave bursts from vortex avalanches in pulsar glitches"
We estimate the burst gravitational wave signal from a glitching pulsar. The glitches are modeled as a nonaxisymmetric rearrangement of pinned superfluid vortices in the inner crust of the star, resulting in the reorganization of the superfluid velocity field, a time-varying current quadrupole moment, and hence a gravitational wave signal. We present two alternative models for the collective motion of vortices during a glitch: an avalanche process, in which stress reservoirs relax via a domino effect (like tectonic plates), and a coherent noise process, in which vortices respond stochastically to a global stimulus. We calculate the amplitude, polarization, and frequency content of the burst signal from a single glitch, and cross-correlate the waveform with standard templates in burst pipelines. We also set out the conditions for the signal to be detectable by Advanced LIGO. For both the avalanche and coherent noise processes the glitch sizes are observed to follow a power law with an index that varies between pulsars, and the waiting times between successive glitches have a Poissonian distribution, as observed in radio timing data. Based on these statistical distributions, we estimate the combined stochastic gravitational wave signal emanating from a realistically distributed pulsar population in a Milky-Way-type galaxy.
"The trace anomaly and cosmological horizon fluctuations"
"The quantum Big Bounce for an inhomogeneous cosmological model"
We present an analysis of the quantum dynamics of an inhomogeneous and anisotropic universe making use of Loop Quantum Cosmology techniques. This study is performed in the context of the classical effective dynamics and different regimes, from the homogeneous to the inhomogeneities dominated universe, are considered. Combining both analytical treatments and numerical tools we show that, for all these cases, the classical initial singularity is replaced by a quantum bounce that joins two large classical universes. The behavior of the inhomogeneities through the bounce is also analyzed.
"Electroweak baryogensis: Theoretical progress and experimental tests"
Explaining the origin of the visible matter of the universe is one of the outstanding problems at the interface of particle and nuclear physics with cosmology. It is possible that new physics at the electroweak scale may provide the necessary ingredients for successful baryogenesis. In this talk, I discuss recent theoretical developments in electroweak baryogenesis and their implications for experimental tests using both "table top" searches for permanent electric dipole moments and collider searches for new particles.
"HPC Phase VI - The final convergence"
Since 2007, high performance computing has been at the beginnings of the most dramatic change in form and function in the last decade and half. Since the advent of the killer micro and the MPPs and commodity Clusters it spawned supported by message-passing programming techniques, most notably MPI, HPC has been on an exponential curve augmenting performance at historic rates through incremental changes to feature size, clock rate, and architectural complexity. But as always happens with S-curves, HPC is turning towards its final asymptote and is undergoing what may prove to be its 6th and potential final phase change. Most visible is the adoption of multicore heterogeneous system architectures driven by constraints in power, complexity, clock rate, and reliability while continuing to exploit improvements in feature size to achieve growth in performance. To realize this goal and the achievement of Exascale performance by the end of the next decade within practical limitations critical advances in efficiency, scalability, energy, and programmability will be required. In all previous such metamorphoses in HPC, the underlying principles of a new execution model was used to guide the codesign of new architectures, programming methods, and system software. Such is the case for the emerging HPC Phase VI. This presentation will discuss the likely elements the new execution model based on the exploratory ParalleX model of computation, and describe key attributes of architecture, operating, and runtime system software, and programming methods that are likely to gain ascendency over the next decade. Results from recent experiments with HPX prototype runtime system will be presented.
"Modeling black hole accretion"
For almost the past 20 years, the paradigm for black hole accretion has highlighted the central role of magnetohydrodynamic (MHD) turbulence. However, only in recent years have high-resolution simulations started to explore the subtle nature of MHD turbulent disks. After motivating the astrophysical importance of understanding black hole accretion, I will discuss results from a series of simulations focusing on geometrically-thin accretion disks. I shall focus on the dynamics of the disk and the transition to the plunging flow close to the black hole, both of which are important issues to understand if one wishes to explore strong-gravity physics using electromagnetic observations of accreting black holes.
"General relativistic simulations of binary neutron stars: Gravitational waves and matter dynamics"
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. I will present results obtained with the use of the fully general relativistic magnetohydrodynamic code Whisky in simulating binary neutron stars that inspiral, merge and eventually form a black hole surrounded by a hot accretion disk. I will describe in particular the gravitational waves that are emitted by both equal and unequal mass systems and the effects that magnetic fields can have on them. I will also discuss the properties of the tori that can be formed by these systems.
"Topological effects in classical and quantum gravity"
We consider some novel topological effects, with potentially observable consequences, in classical and quantum gravity. In one scenario, we find that the topology of extra dimensions generically breaks global Lorentz symmetry, leading to distinct experimental signatures in the context of brane worlds. In a different scenario, we show that topological terms in the gravitational action, such as those expected in heterotic string theory, can greatly enhance the instability of four-dimensional de Sitter space by favoring the nucleation of primordial black holes.
"Gravitational radiation reaction and the inspiral of an extreme mass ratio black hole binary system"
A small black hole, m, orbiting a much more massive hole, M, travels along a path through spacetime which is most
easily described as free-fall motion in M's geometry perturbed by the presence of m. This motion reflects the
gravitational "self-force" on m, which includes both gravitational radiation reaction as well as more conservative
A novel technique uses currently available methods of numerical relativity to determine (1) the perturbation in the geometry, (2) the self-force acting back on m, and (3) the signature of the self-force on the gravitational waves emitted during this extreme mass ratio inspiral.
"Baryons and holography"
The duality between certain strongly coupled gauge theories and classical gravity in higher dimensions has inspired many attempts to model QCD. This talk focuses on attempts to describe baryons in the context of such models. A set of "rule of the game" are proposed as minimum conditions for sensible holographic models of baryons. It is shown that these rules are highly constraining and rule out large classes of possible models. However, it is possible to construct models which satisfy these rules.
"High performance computing challenges in black hole simulations"
As part of the "UM-Sun Microsystems mini-workshop on data, scalable charge and HPC with GPUs."
The presentation will describe the huge computational and storage demands to numerically study the binary black hole problem and map its configuration space, a key problem for gravitational wave observatories already taking data at design sensitivity. Both "standard" and heterogeneous large-scale parallel simulations of binary black holes will be discussed.
"Hamiltonian of a spinning test-particle in curved spacetime"
We compute the unconstrained Hamiltonian of a spinning test-partigle in a curved spacetime at linear order in the particle's spin. The equations of motion of this unconstrained Hamiltonian coincide with the Mathisson-Papapetrou-Pirani equations. We then use the formalism of Dirac brackets to derive the constrained Hamiltonian and the corresponding phase-space algebra in the Newton-Wigner spin supplementary condition, suitably generalized to curved spacetime, and find that the phase-space algebra $(q,p,S)$ is canonical at linear order in the particle spin. We provide explicit expressions for this Hamiltonian in static spherically symmetric spacetimes, as well as in stationary axi-symmetric ones, and show how these could be useful to build a novel effective-one-body model for spinning black hole binaries.
"Modeling non-linearities in the ringdown of colliding
I will present a new covariant and gauge invariant formalism for arbitrary second order perturbations of (non-rotating) black holes. Next, I will discuss a numerical implementation of this formalism and studies of the gravitational radiation generated by the self-coupling of linear perturbations. Finally, I will review some ongoing comparisons with full non-linear simulations of colliding black holes. One of the goals of this ongoing effort is to gain further insights into the late stages of colliding black holes and its analytical modeling. A second motivation comes from the possibility of LISA being able to detect a second ringdown mode.
"Making strings in non-Abelian gauge theories"
I review advances in understanding color confinement from the inception of the idea of the Dual Meissner Effect (Nambu, Mandelstam, 't Hooft mid-1970's) till present.
"Non-Gaussianity of the primordial
After introducing an effective field theory description for inflationary perturbations, I will show how inflation can generate a potentially large and detectable amount of non-Gaussianity in the primordial density perturbations. Non-Gaussianities are probes of the interactions of the inflaton and therefore contain an unprecedented amount of information on inflation. From the observational point of view, this signal can be well investigated by analyzing the cosmological data in search for three different kind of non-Gaussianities. I will present the results of this analysis in the WMAP 5yr data.
"The Feynman propagator on a causal set"
How sure are you that spacetime is continuous? In one approach to quantum gravity, causal set theory, spacetime is modeled as a discrete structure: a causal set. This talk begins with a brief introduction to causal sets, then describes a new approach to modeling a quantum scalar field on a causal set. We obtain the Feynman propagator for the field by a novel procedure starting with the Pauli-Jordan commutation function. The candidate Feynman propagator is shown to agree with the continuum result. This model opens the door to physical predictions for scalar matter on a causal set.
|Home People Ph.D. Program Seminars Available Positions Related Links|
Back to the UM Physics Department Home Page
Back to the University of Maryland Home Page