GRAVITY THEORY SEMINARS 2008
Abstracts:
Thurs., Feb. 14, 4:00pm, Room 1201
Juan Maldacena, Institute for Advanced Study
``AdS/CFT duality: recent developments''
TBA
Thurs., Feb. 28, 2:00pm, Room 4102
Sam Waldman, LIGO Caltech
``Enhancing and Advancing LIGO: 4 years of detector upgrades''
After successfully completing 1 year of coincident data taking at design sensitivity, commissioning has begun on the Enhanced LIGO upgrades to the 4 km detectors. The incremental upgrades, including increased power to 35~W and a new DC readout scheme, are designed to achieve a factor of 2+ improvement in sensitivity while validating key Advanced LIGO technologies. The Advanced LIGO program begins commissioning in 2010, implementing a new optical scheme, higher power, and improved seismic isolation. Together, these improvements will increase the sensitive band to 10 - 1,000~Hz, improve the strain sensitivity to ~< 4 * 10^{-24} Hz^ {-1/2}, and extend the NS/NS inspiral range to 170~Mpc.
Thurs., Mar. 6, 2:00pm, Room 4102
Etienne Racine, University of Maryland
``Gaussianity of LISA's confusion background''
Data analysis for the proposed Laser Interferometer Space Antenna (LISA) will be complicated by the huge number of sources in the LISA band. Throughout much of the band, galactic white dwarf binaries (GWDBs) are sufficiently dense in frequency space that it will be impossible to resolve most of them, and "confusion noise" from the unresolved Galactic binaries will dominate over instrumental noise in determining LISA's sensitivity to other sources in that band. Confusion noise from unresolved extreme-mass-ratio inspirals (EMRIs) could also contribute significantly to LISA's total noise curve. To date, estimates of the effect of LISA's confusion noise on matched-filter searches and their detection thresholds have generally approximated the noise as Gaussian, based on the Central Limit Theorem. However in matched-filter searches, the appropriate detection threshold for a given class of signals may be located rather far out on the tail of the signal-to-noise probability distribution, where a priori it is unclear whether the Gaussian approximation is reliable. Using the Edgeworth expansion and the theory of large deviations, we investigate the probability distribution of the usual matched-filter detection statistic, far out on the tail of the distribution. We apply these tools to four somewhat idealized versions of specific LISA searches.
Thurs., Mar. 13, 4:00pm, Room 1201
Alex Maloney, McGill University
``Phases of Quantum Gravity in Three Dimensions''
Quantum gravity in a world with three dimensions (that is, with two spatial dimensions and one time dimension) provides a fascinating theoretical laboratory where we can test precisely our ideas about quantum general relativity. This theory has the remarkable property that when the cosmological constant is negative Einstein's theory of gravity can -- with a little help from string theory -- be quantized. So we can begin to understand the structure of space-time beyond the classical approximation. Many objects which appear quite mysterious, such as black holes and big bang or big crunch singularities, can be understood precisely at the quantum level. The Lee-Yang theory of phase transitions and the monster group both make appearances in this story.
Fri., Mar. 14, 3:30pm, Room 4102
Alex Maloney, McGill University
``The Sum Over Geometries in Three Dimensions''
We consider pure three-dimensional quantum gravity with a negative cosmological constant. The sum of known contributions to the partition function from classical geometries can be computed exactly, including quantum corrections. However, the result is not physically sensible, and if the model does exist, there are some additional contributions. One possibility is that complex geometries need to be included, leading to a holomorphically factorized partition function. We analyze the subleading corrections to the Bekenstein-Hawking entropy and show that these can be correctly reproduced in such a holomorphically factorized theory. We also consider the Hawking-Page phase transition between a thermal gas and a black hole and show that it is a phase transition of Lee-Yang type, associated with a condensation of zeros in the complex temperature plane. Finally, we analyze pure three-dimensional supergravity, with similar results.
Thurs., Mar. 27, 2:00pm, Room 4102
Dierdre Shoemaker, Pennsylvania State University
``Binary Black Hole Encounters, Bursts and Maximal Spin''
I present results from a series of numerical relativity investigations of binary black hole encounters ranging from almost direct infall to numerous orbits before infall. These encounters exhibit multiple bursts of radiation in the merger process that may be relevant to gravitational wave observations. In addition to these bursts of radiation, we studied the spin of the final black hole, an important parameter in astrophysical processes involving black holes. The final spin in the coalescence of non-spinning black holes is determined by the ``residual'' orbital angular momentum of the binary. This residual momentum consists of the orbital angular momentum that the binary is not able to shed in the process of merging. Our results show that there is a maximum rate of spin per mass of the final black hole and when that maximum occurs in terms of the orbital angular momentum.
Thurs., Apr. 3, 4:00pm, Room 1201
Richard Easther, Yale University
``The Observational Fingerprints of Inflation''
Inflation ensures that the early universe is homogeneous and isotropic, and generates small perturbations whose amplitude is almost independent of their wavelength. However, we know very little about the physical mechanism that drives the inflationary era, and there are many competing proposals. Since inflation renders the early universe almost featureless, the differences between inflationary scenarios are necessarily small, but will be of crucial importance when testing competing models. I will first describe how several key predictions of inflation have been tested and verified by cosmological observations. I will then discuss possible ``fingerprints'' of specific inflationary models, including the detailed form of the primordial perturbation spectrum, gravitational waves (produced both during and immediately after inflation), and primordial fluctuations with non-Gaussian statistics, and the possibility of exploiting these in upcoming and proposed experiments.
Thurs., Apr. 10, 2:00pm, Room 4102
Thomas Sotiriou, University of Maryland
``f(R) Gravity''
Modified gravity theories have received increased attention lately due to combined motivation coming from both high-energy physics and cosmology and astrophysics. Among numerous alternatives to Einstein's theory of gravity, theories which include higher order curvature invariants, and specifically the particular class of f(R) theories, have a long history. In the last 5 years there has been a new stimulus for their study, leading to a number of interesting results. An introduction to f(R) theories of gravity will be given and their consequences and constraints will be discussed.
Thurs., Apr. 17, 2:00pm, Room 4102
Sergei Dubovsky, Harvard University
``Superluminal travel in two dimensions''
TBA
Tues., Apr. 22, 2:00pm, Room 1201
Joseph Samuel, Raman Research Institute
``Surface tension and the cosmological constant''
One of the few predictions from quantum gravity models is Sorkin's observation that the cosmological constant has quantum fluctuations originating in the fundamental discreteness of spacetime at the Planck scale. Here we present a compelling analogy between the cosmological constant of the universe and the surface tension of fluid membranes. The discreteness of spacetime on the Planck scale translates into the discrete molecular structure of a fluid membrane. We propose an analog quantum gravity experiment which realises Sorkin's idea in the laboratory. We also notice that the analogy sheds light on the cosmological constant problem, suggesting a mechanism for dynamically generating a vanishingly small cosmological constant. We emphasize the generality of Sorkin's idea and suggest that similar effects occur generically in quantum gravity models.
Thurs., Apr. 24, 4:00pm, Room 1201
W.G Unruh, University of British Columbia
``Where do the particles come from?''
When Hawking discovered black hole evaporation, the quantum emission of particles by black holes, left open was the origin of the particles. He presented the picture of pair creation, of virtual particle pairs ripped apart by the gravitiational field near the horizon, but no calculation to support this picture. Instead the closest picture was of a horizon splitting at absurdly high energies (trans Plankian problem). Using dumb holes, the acoustic or other analog of black holes, I will argue, both numerically and analytically, that the creation process is a low energy process which happens well outside the horizon.
Thurs., May 8, 2:00pm, Room 4102
Eric Poisson, University of Guelph
``The gravitational self-force''
The gravitational self-force describes the effects of a particle's own field on its motion. While the motion is geodesic in the test-mass limit, for a finite-mass body the gravitational perturbation created by the body produces an acceleration, and the object is said to move under the influence of its own self-force. I will first describe the astrophysical context of the work now being done to compute the gravitational self-force. Next I will review the foundations of the self-force, and describe how to obtain the equations of motion for a small black hole moving in an external universe. This calculation produces a prescription that can be applied to the case of point particles, and I will explain how an infinite retarded field can be unambiguously decomposed into a singular piece that exerts no force, and a smooth remainder that is responsible for the acceleration. I will describe the recent effort, by a number of workers, to compute the self-force in the case of a small mass moving in the field of a much more massive black hole. And finally, I will discuss open issues and future work.
Mon., May 19, 2:00pm, Room 4102
James Overduin, University of Waterloo / Stanford University
``From Gravity Probe B to STEP: Testing Einstein in Space''
TBA
Fri., Sep 5, 1:30pm, Room 4102
Raman Sundrum, Johns Hopkins University and University of Maryland
``Lorentz violation and superluminality via AdS/CFT duality''
I describe a UV complete deformed conformal field theory (CFT) whose AdS/CFT dual describes Lorentz-violation and superluminality in a gravitating AdS_5 spacetime. This is a concrete attempt to wrestle with the possibility of Lorentz violation in Nature and its compatibility with General Relativity, holographic principles, and UV completeness. I will give the working rules of AdS/CFT and their plausibility for those new to it.
Thursday, Sept. 11, 4:00 pm, Room 4102
Christoph Schmid,
ETH Zurich
"Mach's Principle: Exact frame-dragging by energy currents in perturbed Friedmann-Robertson-Walker universes"
We show that there is exact dragging of the axis directions of local inertial frames by a weighted average of the cosmological energy currents via gravitomagnetism for all linear perturbations of all Friedmann-Robertson-Walker universes and for all energy-momentum-stress tensors. Hence the postulate formulated by Ernst Mach about the physical cause for the time-evolution of inertial axes is shown to hold for linear perturbations.
Friday, Sept. 12, 1:30 pm, Room 1201
Aneesh Manohar, UC San Diego
"Electroweak corrections at LHC energies"
The theory of radiative corrections to high energy scattering processes, found in field theory textbooks, is briefly reviewed. These corrections can be obtained more efficiently by using soft-collinear effective theory. This allows one to compute electroweak radiative corrections in the standard model due to W and Z exchange. At LHC energies, these particles are essentially massless, and the electroweak corrections are large, changing the cross-section by 25-40%. The results apply to the basic LHC scattering processes such as jet production and Drell-Yan. The corrections are typically not included in standard parton shower Monte-Carlos.
Friday, Sept. 19, 1:30 pm, Room 4102
Sergey Ketov,
Tokyo Metropolitan University and CPST/EP, University
of
Maryland
"Superstring-induced quartic curvature gravity and geometrical inflation"
Friday, Sept. 26, 1:30 pm, Room 1201
Christopher Herzog,
Princeton University
"Holographic superfluidity and superconductivity"
I will describe a simple gravitational model in 3+1 dimensions that is dual to a strongly interacting field theory in 2+1 dimensions with remarkable properties. The gravity model consists of a charged black hole that acquires scalar hair when the Hawking temperature becomes sufficiently low. The gravitational instability is dual to a phase transition in the field theory between a normal and a superconducting (or superfluid) state. From the gravity model, we calculate the thermodynamic and transport properties of the field theory, yielding intringuingly familiar results. These methods hold promise for understanding strongly interacting real world condensed matter systems.
Friday, Oct. 3, 1:30 pm, Room 4102
Peter Shawhan,
University of Maryland
"Recent results from LIGO"
After many years of construction and commissioning, the LIGO gravitational-wave detectors reached their design sensitivity levels in 2005 and began a "science run" that lasted until late 2007. The GEO 600 and Virgo detectors in Europe also collected data for part of that time. The first few analysis results from that run have been released, taking advantage of the unprecedented amplitude sensitivity and observation time to arrive at some astrophysically interesting results. I will describe these and also summarize the plans for the next science run.
Friday, Oct. 10, 1:30 pm, Room 1201
Avi Loeb,
Institute for Theory and Computation (ITC), Harvard University
"Exploring new physics in the early universe and around black holes"
The Universe offers environments with extreme physical conditions that cannot be realized in laboratories on Earth. These environments provide unprecedented tests for extensions of the Standard Model. I will describe two such "astrophysical laboratories", which are likely to represent new frontiers in observational astrophysics over the next decade. One provides a novel probe of the initial conditions from inflation and the nature of the dark matter, based on 3D mapping of the distribution of cosmic hydrogen through its resonant 21 cm line. The second allows to constrain the metric around supermassive black holes based on direct imaging or the detection of gravitational waves. I will describe past and future observations of these environments and some related theoretical work.
Friday, Oct. 17, 2:00 pm, Room 4102
Frank Herrmann,
University of Maryland
"Numerical simulations of black holes"
Numerical Relativity has made remarkable progress in the last years. In 2005 stable binary black hole simulations became first possible and since then a number of different aspects of this system have been studied. I will review different techniques in use today and describe comparison studies to post-Newtonian methods as well as predictions for the recoil the single black hole formed in a generic merger receives.
Friday, Oct. 24, 2:00 pm, Room 1201
Mithat Unsal,
SLAC and Stanford University
"Topological symmetry and (de)confinement in gauge theories and spin systems"
The non-perturbative dynamics of non-supersymmetric QCD-like and chiral gauge theories remained largely elusive despite much effort over the years. Recently, novel techniques (such as center stabilizing double trace deformations) which allow us to coninuously connect the physics of these gauge theories on R^4 to very small S^1 \times R^3 are found. In most cases, the physics of small S^1 is analytically tractable. The types of topological excitations that appear in this window are far richer than anticipated earlier. For example, a type of composite referred as magnetic bion with net magnetic charge +2 is responsible for the appearance of a mass gap in large class of QCD-like theories. What makes this excitation interesting is also an exotic mechanism of pairing, induced by fermions. Some of these techniques also find useful applications to two dimensional frustrated spin-systems. In this talk, I will give an elementrary introduction to the world of gauge theories through this window.
CSCAMM Seminar
Wednesday, Nov. 5, 2:00 pm, CSIC Room 4122
Burkhard Zink,
Louisiana State University
"Numerical techniques for accretion flows around black holes"
I will discuss a research program to study the infall of stellar material into black holes, which is an important process used to explain a large class of energetic phenomena in astrophysics. To study these flows, three-dimensional general relativistic simulations of magnetohydrodynamics are needed, as well as a full treatment of Einstein's field equations. In this talk I will touch on three aspects of this program: First, the application of multi-block techniques to use adapted grid systems when modeling black holes and accretion disks. Secondly, I will describe recent efforts to develop schemes for radiation transport in these systems. And finally, I will share my perspective on the future role of many-core and GPU computing in astrophysics.
Thursday, Nov. 6, 4:00 pm, Room 4102
Aron Wall,
University of Maryland
"Towards a proof of the Generalized Second Law?"
The Generalized Second Law (GSL) states that the area of an event horizon plus the entropy of matter outside of the horizon always increases as time passes. Although widely believed, this conjecture is difficult to make precise and has only been proven for semiclassical quasi-steady black holes. This talk will attempt to pinpoint the meaning of the GSL. It will also discuss ways of making the law more local, perhaps making it easer to find a proof.
CSCAMM Seminar
Thursday, Nov. 6, 12:00 pm, CSIC Room 4122
Matthew Duez,
Cornell University
"Black hole-neutron star binaries in numerical relativity"
The merger of a neutron star with a comparable-mass black hole is a strong source of gravitational waves and a promising setup for generating a short-duration gamma ray burst. The merger may proceed quite differently depending on the mass ratio, black hole spin, and neutron star equation of state. In this talk, I present results of numerical simulations of black hole-neutron star mergers with different mass ratios and black hole spins. The Einstein equations for the spacetime metric are evolved pseudospectrally on one grid, while the hydrodynamic equations for the neutron star fluid are evolved using finite volume techniques on a second grid. The gravitational wave signals are extracted, and the post-merger accretion disk masses are estimated.
Friday, Nov. 7, 2:00 pm, Room 1201
Maxim Pospelov,
University of Victoria and Perimeter Institute
"Catalysis of Primodial Nucleosynthesis"
About 1000 seconds after the Big Bang the nuclear reactions between primordial protons and neutrons led to the formation of light nuclei, such as deuterium, helium and lithium. This epoch is known as the Big Bang Nucleosynthesis (BBN) and is well described by the combination of general relativity, quantum mechanics and particle/nuclear physics. Theoretical prediction of elemental abundances are confronted with observations, providing an important test of modern cosmology. In this talk, I explain how the nonstandard particle physics (such as e.g. supersymmetric particles) may alter the outcome of primordial nuclear reactions leading to detectable consequences. I introduce the concept of Catalyzed BBN, and show that heavy metastable charged particles can completely change lithium abundance, providing an important test of many particle physics models, and perhaps helping to resolve existing puzzles with lithium abundance.
Friday, Nov. 14, 1:30 pm, Room 4102
Ho-Jung Paik,
University of Maryland
"A short-range inverse-square law test: Progress report"
I give a progress report on our new short-range test of the inverse-square law.
Friday, Nov. 21, 2:00 pm, Room 4102
Anil Zenginoglu,
University of Maryland
"Null infinity on a grishell"
In numerical relativity, the truncation of the solution domain to simulate isolated systems leads to well-known problems related to the outer boundary and the extraction of gravitational waves. A potential solution to these problems is to introduce compactifying coordinates on spacelike slices that approach null infinity. I will review recent developments in this subject focusing on the approach that fixes the location of null infinity on the numerical grid to a coordinate shell.
Friday, Dec. 5, 2:00 pm, Room 4102
Enrique Pazos,
University of Maryland
"Recent work in numerical relativity"
"I will talk about some work in progress towards the comparison of waveforms computed using different computer codes and our own effort of constructing a multi-block domain suitable for solving the problem of binary black holes."
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