GRAVITY THEORY SEMINARS 2007
``Modeling Gravitational Attraction: Boson Stars, Accretion, and Critical Collapse''
A number of interesting gravitational problems are investigated using a distributed AMR infrastructure in three dimensions without assumptions of spatial symmetry. Collisions of compact objects are modeled using stable configurations of complex scalar field called boson stars. Different end states are achieved with different configurations of pairs of boson stars. Combining a high resolution shock capturing finite difference MHD code with a gravity code using black hole excision, we study fluid accretion around a black hole. Last, we examine the threshold of black hole formation in vacuum and scalar field collapse.
``Progress in Post-Newtonian Initial Data for Numerical Relativity''
Physically realistic initial data for the black-hole-binary problem is expected to accord with predictions from post-Newtonian theory at sufficiently large separations. Schemes for using this idea in obtaining 3+1 initial data for black-hole binaries puncture data have been proposed by Tichy et al (2003). However in numerical applications based on post-Newtonian results, some difficulties have been encountered in the far-field region. We report on progress in extending these schemes up to 2.5 pN order to obtain globally well-behaved data sets that encode the past radiation history of the binary.
``Gravitational polarization and the phenomenology of MOND''
The modified Newtonian dynamics (MOND) has been proposed as an alternative to the dark matter paradigm; the philosophy behind is that there is no dark matter and we witness a violation of the Newtonian law of dynamics. In this seminar, we interpret differently the phenomenology of MOND, as resulting from an effect of "gravitational polarization", of some cosmic fluid made of dipole moments, aligned in the gravitational field, and representing a new form of dark matter. We propose two models for describing the dipolar dark matter: One is quasi Newtonian and results from the analogy with the electric polarization in electrostatics, the other is relativistic and is based on an action principle in general relativity.
``Lunar laser ranging''
Over the past 35 years, laser ranging to retroreflector arrays placed on the lunar surface by the Apollo astronauts and the Soviet Luna missions have dramatically increased our understanding of gravitational physics along with Earth and Moon geophysics, geodesy, and dynamics. In this lecture I will review the science and instrumentation of lunar laser ranging and discuss several ways to further improve upon these measurements.
``Spontaneous symmetry breaking via gravitation''
Spontaneous symmetry breaking has been an important idea in both particle and condensed matter physics. In this talk we show how spontaneous symmetry breaking can be incorporated in a theory of gravity. Together with my collaborators, Luca Fabbri (INFN, University of Bologna, Italy) and M.B. Paranjape (University of Montreal), we showed that a conformally invariant theory -- one that does not start off looking like General Relativity -- yields Einstein gravity solutions in the low energy or long-distance regime when the conformal symmetry is spontaneously broken. I will explain what is conformal symmetry, what it means for a theory to be spontaneously broken and how both can be incorporated into a theory of gravity.
``Why is it so hard to measure the strength of gravity?''
Newton's gravitational constant G is one of the most fundamental and universal constants in nature. The value of "big G" tells us how much gravitational force acts between masses separated by a known distance. Yet unlike most other physical constants, the value of G is not precisely known. Since the first laboratory measurement by Cavendish over 200 years ago, the reduction in uncertainty in G has been only about an order of magnitude per century! A new value of G was accepted by the CODATA committee in 1986 based on measurements by Luther and Towler. However, over the next several years other measurements were made that lied far outside the 128 PPM uncertainty of the 1986 value, bringing this value of G into question. Recognizing this discrepancy, the 1998 CODATA committee increased the uncertainty on the accepted value to 1500 PPM. This situation was an embarrassment to modern physics. Recently, a number of groups around the world performed measurements of G with unprecedented sensitivity and reasonable agreement. In 2002 the CODATA committee recommended a new value for G=6.6742(10)x 10-11 m3 kg-1 s-2, conservatively lowering the uncertainty to 150 PPM. In this talk, I will discuss this controversy over the value of big G and describe how recent measurements have overcome the systematic errors that plagued previous results.
``Polarization of the Cosmic Microwave Background: Are These Guys Serious?''
The polarization of the cosmic microwave background (CMB) could contain the oldest information in the universe, dating from an inflationary epoch just after the Big Bang. Detecting this signal presents an experimental challenge, as it is both faint and hidden behind complicated foregrounds. The rewards, however, are great, as a positive detection would not only establish inflation as a physical reality but also provide a model-independent measurement of the relevant energy scale. I will present the scientific motivation behind measurements of the CMB polarization and discuss how recent experimental progress could lead to a detection in the not-very-distant future.
``Thermal noise and drift in precise interferometry''
Thermal noise plays a very important role in precise interferometry. The mirror thermal noise, which is thermal fluctuation of the mirror surface, isa sensitivity-limiting factor for interferometric gravitational wave (GW) detectors. Its clear understanding and reduction are the most urgent issues in developing new GW detectors. In this talk, I will present a direct measurement of mirror thermal noise with 4 different mirror substrate materials. Comparing the results with their corresponding theories, I will address remaining problemsto be solved for future GW detectors. Thermal noise has been realized to be important not just in GW detectors but also in wider interferometric measurements. We show that thermal noise of a rigid reference cavity can explain the mysterious noise floor in its frequency stability. Rigid cavities are used in various physics fields, as well as LIGO. While the thermal noise of mirror and pendulum limit the length stability of GW detectors' suspended cavity, thermal noise of mirrors and spacers limit that of the rigid cavity. I will compare our calculations and experimental results, suggesting some possible ways to improve current frequency stabilities. As a conjunction with precise interferometry, I will also mention the platform (suspension-point) interferometer and other projects, which I am working on at NASA. The interferometry testbed is designed to suppress thermal drift, seismic noise, and other environmental noise at longer timescales. It will be applied tointerferometric measurements in future space missions, such as LISA, JWST, and TPF-C. It may be of interest to advanced LIGO as well.
``Mach's Holographic Principle''
Mach's principle is the profound claim that the matter distribution in the universe determines the inertial frames. I will propose a precise formulation of Mach's principle that can actually be realized within general relativity, without modifying either Einstein's equations or discarding any solutions. The proposal draws on ideas from the black hole membrane paradigm and is consistent with the AdS/CFT correspondence.
``X-rays from Compact Stars: Probing Fundamental Physics''
X-ray emission from neutron stars and black holes provides our most intimate view of these extreme objects, and contains clues to the behavior of matter and gravitational fields at the present limits of physical theory. I will survey recent advances in our observational understanding of the X-ray properties of accreting compact objects (black holes and neutron stars) and describe how such observations can be used to test our current understanding of physics.
``LISA: The Science and the Instrument''
By mapping the gravitational wave sky over a wide range of low frequencies, the Laser Interferometer Space Antenna (LISA) mission will: (1) understand the formation and growth of massive black holes, (2) trace the merger history of black holes and their host galaxies, (3) survey binaries of stellar-mass compactobjects, (4) test theories of relativity, and (5) probe new physics and cosmology. LISA will be capable of acquiring a great deal of highly accurate astrophysical information about very many binary systems involving massive blackholes, intermediate-mass black holes, and stellar-mass compact objects and aboutastrophysical foregrounds and backgrounds. The expected science return will be summarized, the measurement concept will be explained, and the baseline architecture will be described.
``LISA Science Out to z~10''
The most interesting characterization of a gravitational wave detector's performance is the accuracy with which astrophysical source parameters can be estimated. In the most general circumstance, 17 parameters of an inspiraling binary system can be estimated by fitting waveforms. LISA can measure parameters like mass and spin to better than 1% even for high redshift sources. The most difficult parameter to estimate accurately is almost always luminosity distance. Even so, LISA can measure luminosity distance of even intermediate-mass systems (total mass~1e4 M_sun) out to z~10 with distance accuracies approaching 25% in many cases. For more massive systems at much lower redshifts, relativity can be severely tested. The science that can be done with LISA data will be described.
``The next great step in space-based gamma-ray astrophysics: the Gamma-ray Large Area Space Telescope (GLAST)''
The Gamma-ray Large Area Space Telescope, GLAST, is an upcoming mission to measure the cosmic gamma-ray flux in the energy range 20 MeV to >300 GeV, with supporting measurements for gamma-ray bursts from 10 keV to 25 MeV. With its launch in late 2007, GLAST will open a new and important window on a wide variety of high-energy phenomena, including black holes and active galactic nuclei; gamma-ray bursts; the origin of cosmic rays and supernova remnants; and searches for hypothetical new phenomena such as supersymmetric dark matter annihilations. Starting with an introduction to gamma-ray astrophysics and GLAST science opportunities, the lecture will include a description of the instruments, the collaboration of particle physicists and astrophysicists, and the mission status.
``Black Holes and Their Echoes in the Universe''
The detection and characterization of gravitational waves is a formidable undertaking, requiring innovative engineering, powerful data analysis tools as well as careful theoretical and numerical modeling. Binary black holes are expected to be one of the primary sources of gravitational radiation. The progress in numerical relativity is moving toward longer waveforms from inspiraling binaries to an exploration of the parameter space. I will discuss aspects of numerical simulations of binary black holes in connection with spins, gravitational recoil and eccentricities that have been recently obtained and have direct relevance to gravitational wave data analysis and astrophysics.
``Development of LIGO: A View From Washington''
LIGO is an audacious project attempting both to confirm the essence of dynamical gravitation, and to harness gravitational waves as a new probe of the cosmos. Achieving its already-demonstrated sensitivity required many technologies to advance many orders of magnitude beyond the state of the art before its initiation. The development of LIGO transformed Gravitational Physics from a small-scale individual-investigator effort into a major new international Big Science collaboration. For three decades, the participant community experienced all the struggle and pain that normally accompanies such a transition. It has been a high-risk, high-reward gamble, always full of high promise that has yet to pay off. This talk will explore the development of LIGO as seen from the perspective of its patron in Washington. Construction of this new facility required a 100-fold expansion of the annual budget for research in this subfield. In the face of this challenge and opportunity, the U.S. Government invested scarce research funds with vision and patience, and managed a very long-term, new, risky, and expensive investment with some wisdom.
``Numerical Simulations of General Gravitational Singularities''
The singularity theorems of general relativity tell us that gravitational collapse results in some sort of catastrophic behavior in the center of the collapsed object. However, it is only recently that computer simulations have shown the exact nature of these singularities. This talk presents the numerical methods and results of the simulations.
"Viscosity, Black Holes, and Relativistic Heavy Ion Collisions"
Viscosity is a very old concept which was introduced to physics by Navier in the 19th century. However, in strongly coupled systems viscosity is extremely difficult to compute ab initio. In this talk I will describe some recent surprising developments in string theory which allow one to compute, easily and conveniently, the viscosity in a class of strongly interacting relativistic quantum field theories. I will describe efforts to measure the viscosity and other physical properties of the quark gluon plasma created at the Relativistic Heavy Ion Collider, and mention possible connections to the string-theory calculations.
``LOOC, UP in the Sky! Searching for Optical Counterparts of Gravitational-wave Burst Candidates''
The LIGO-VIRGO network of interferometers is currently performing sensitive searches for gravitational radiation. Gravitational wave bursts (short duration signals) are expected to be associated with highly energetic astrophysical processes. With such high energies present, it is likely these astrophysical events will have signatures in the EM spectrum as well as in gravitational radiation. The proposed LOOC UP project will use near real time data from the LIGO-VIRGO network, as well as optical telescopes, to actively seek the optical counterparts to gravitational burst event candidates. In this talk, I will discuss the motivation, potential benefits, and mechanics of such a search. I will also discuss pilot studies performed this past summer.
``Excision Without Excision: Why the Binary Black Hole Codes Work''
The binary black hole problem, analysing the interaction and final merger of two black holes to form a single one, is a computation that needs to be done numerically. The attempts over the last 20 years have been spectacularly unsuccessful. Everything has changed in the last two years and now people routinely model, in a single run, all three elements of the merger, the inward spiral, the merger itself, and the subsequent ringdown of the final black hole. The current favored technique is called the `moving puncture' method. I will try, from an outsider's perspective, to give an outline of this moving puncture approach. Further, I will explain why, in my mind, the moving puncture method works in the face of apparently intractable difficulties.
Cosmological Probes of Dark Energy
One of the great mysteries of modern cosmology is the origin and nature of dark energy - a smooth component that contributes about 75% of the total energy density in the universe and causes its accelerated expansion. Although discovered less than a decade ago, dark energy domination has recently been confirmed via several independent cosmological probes; nevertheless, there are no good theoretical leads as to its physical provenance. In this talk I examine critically various approaches to measure the macroscopic properties of dark energy, and describe accurate yet general methods to model the expansion history of the universe in the presence of dark energy. I discuss the importance of controlling the systematic errors in upcoming surveys, and show examples from some recent work. Finally, I review upcoming surveys specifically designed to help understand the origin and physical nature of dark energy.
``Gravitational Recoil from Binary Black Holes: Computational Methods and Astrophysical Applications''
The asymmetric emission of gravitational radiation from an inspiraling binary black hole system will produce a net momentum flux in one direction, resulting in the final black hole receiving a kick in the opposite direction. Recent advances in numerical relativity, as well as analytic methods, have allowed us to predict accurately what the final recoil velocity should be. Including the effects of black hole spin, there appear to be certain configurations that lead to extraordinarily large kicks, upwards of 3000 km/s. We will discuss the methods used to calculate these kicks as well as some of their astrophysical implications.
``High-accuracy comparison of numerical relativity simulations with post-Newtonian expansions''
Numerical simulations of 15 orbits of an equal-mass binary black hole system will be presented. Gravitational waveforms from these simulations, covering more than 30 cycles and ending about 1.5 cycles before merger, are compared with those from quasi-circular zero-spin post-Newtonian (PN) formulae. Matching numerical results to PN waveforms early in the run yields excellent agreement (within 0.05 radians) over the first 15 cycles, thus validating the numerical simulation and establishing a regime where PN theory is accurate. In the last 15 cycles to merger, however, generic time-domain Taylor approximants build up phase differences of several radians. But, apparently by coincidence, one specific post-Newtonian approximant, TaylorT4 at 3.5PN order, agrees much better with the numerical simulations, with accumulated phase differences of less than 0.05 radians over the 30-cycle waveform.
``Inspirals of point particles into black holes via two-timescale expansions''
The inspiral of stellar mass compact objects into massive black holes are an important source for future gravitational wave detectors such as LISA and Advanced LIGO. Detection of these sources and extracting information from the signal relies on accurate theoretical models of the binary dynamics. We analyze this problem using a two-timescale expansion, which provides a rigorous derivation of the prescription for computing the leading order waveform. As shown by Mino, this leading order waveform, which we call the adiabatic waveform, requires only the radiative self force. The two-timescale method also lays the foundations for calculating the post-adiabatic corrections needed for measurement templates. We show that the leading order post-adiabatic corrections (terms in the phase that scale as the square root of the mass ratio) are due to transient resonances that occur during an inspiral when the ratio of the radial and azimuthal frequencies is a low order rational number. This effect is not seen in post-Newtonian expansions. At the next, subleading order (order unity terms in the phase), there are phase corrections due to the conservative and dissipative pieces of the first order self force, and the dissipative piece of the second order self force. The resonant phase shifts depend on the subleading order terms. Therefore, going beyond the adiabatic approximation would require computation of the dissipative piece of the second order self force. It is not clear if this will be tractable analytically.
``Spin Foam Models of 4D Gravity''
In this talk I will review the construction based on a spacetime approach of 4 dimensional quantum gravity amplitudes associated with loop quantum gravity, i-e spin foam models. I will present recent developpements in this field that resolve some of the key difficulties associated with previous models and allows us to incorporate the Immirzi parameter and shed new light on the relationship between the canonical and the spin foam framework.
``Black Hole Rigidity''
We prove that for any spacetime dimension, stationary black hole spacetimes which are analytic solutions of the vacuum Einstein equations must be axisymmetric. This is joint work with Vincent Moncrief.
``Preferred Directions in Inflation"
``Possible and Impossible in Quantum Gravity''
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