Subject: ASGRG Newsletter #12 ****************************************************************************** AUSTRALASIAN SOCIETY FOR GENERAL RELATIVITY AND GRAVITATION Electronic Newsletter -- #12, Spring 2003 ****************************************************************************** Items for this newsletter should be emailed to the editor: asgrg *AT* hotmail *DOT* com The deadline for the next issue is 30 April, 2004. ****************************************************************************** CONTENTS: * ACGRG4, Monash University, 7-9 January, 2004 * BIENNIAL GENERAL MEETING OF THE ASGRG 8 January 2004 * MEMBERSHIP DETAILS ONLINE at http://www.physics.adelaide.edu.au/ASGRG/members.html * SUBSCRIPTIONS * FORTHCOMING MEETINGS * MEMBERS' ABSTRACTS at gr-qc, June 2003 - November 2003 ****************************************************************************** 4TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG4) Monash University, Melbourne, 7-9 January 2004 ACGRG4 is the fourth in a series of biennial conferences run by the ASGRG with the aim of bringing together researchers from around the world to discuss new findings in mathematical, theoretical, numerical and experimental gravitation, to make contacts and consolidate ideas. ACGRG4 will be held on the Clayton campus of Monash University, Melbourne from Wednesday January 7 to Friday January 9, 2004. Clayton campus is the largest of Monash University's seven campuses, and is situated about 20 km from the centre of Melbourne and 40 km from Melbourne's Tullamarine International Airport. Invited speakers will include: Mike Ashley (Penn State) Matthew Bailes (Swinburne) Jordan Camp (LISA) Michael Hall (ANU) David McClelland (ANU/ACIGA) Joey Medved (Victoria University of Wellington) Robin Tucker (Lancaster) Registration fees for ASGRG members are AUS$190 and AUS$120 for students. (Fees for non-members are AUS$250 and AUS$150 for students.) Registration will begin on Tuesday evening, January 6. The conference dinner will be on Thursday January 8 and will cost AUS$50 (AUS$30 for students). A one-day excursion to the Yarra Valley Vineyards is planned for Saturday January 10, at an extra cost of AUS$70. This will include wine-tasting and a full 3-course meal at a leading vineyard. The deadlines for registration or submission of abstracts have now passed. However, late applications may be considered. If you wish to register and have not done so yet, please contact the ASGRG Secretary, Dr Malcolm Anderson, at manderso@fos.ubd.edu.bn or at asgrg *AT* hotmail *DOT* com The homepage for ACGRG4 can be found at http://www.physics.adelaide.edu.au/ASGRG/ACGRG4/ ****************************************************************************** ASGRG BIENNIAL GENERAL MEETING The 2004 Biennial General Meeting of the ASGRG will be held in conjunction with ACGRG4, on the evening of Thursday January 8 (before the Conference Dinner). All ASGRG Executive Committee positions will be filled by election at the BGM. The outgoing Executive Committee members are: President - David McClelland Treasurer - Susan Scott Secretary - Malcolm Anderson Officer - David Wiltshire Officer - Peter Szekeres Co-Opted Committee Members - Tony Lun, John Steele CALL FOR NOMINATIONS Nominations are invited for all Executive Committee positions. These should be submitted to the ASGRG Secretary, Dr Malcolm Anderson, at manderso@fos.ubd.edu.bn or at asgrg *AT* hotmail *DOT* com Nominees must be ASGRG members, and must be nominated and seconded by ASGRG members. Nominations close at midday on Wednesday January 7. So far, two nominations have been submitted: President - Susan Scott (nominated by David McClelland, seconded by Antony Searle) Treasurer - Antony Searle (nominated by David McClelland, seconded by Susan Scott) ****************************************************************************** MEMBERSHIP DETAILS ONLINE: Due to requests from members, David Wiltshire has written some HTML scripts which generate membership details online from our records. If you click on http://www.physics.adelaide.edu.au/ASGRG/members.html you will find a members' list. Clicking on individual members gives their current contact details. By following a further link private details of the subscription status of any member will be sent to their registered email. This feature should enable us to update our records more frequently in response to members' input, and to allow members to keep track of their subscriptions. ****************************************************************************** SUBSCRIPTIONS: The membership script programs are intended to be run automatically once a year, at the end of July, to give members other than life members details of their current subscription status. The new version of the subscription form, at http://www2.phys.canterbury.ac.nz/ASGRG/subsform.html has been simplified so that it does not need to be updated each year. Given that our annual fee is modest, members are encouraged to pay for multiple years, and to fill in the years they are paying for. E.g., when the July 2003 - June 2004 subscriptions are requested, if you wish to pay for July 2004 - June 2005 at the same time, it may simplify matters. ****************************************************************************** FORTHCOMING MEETINGS December 14-15, 2003: Inaugural Meeting of the Center for Gravitational Wave Astronomy Brownsville, Texas, USA http://cgwa.phys.utb.edu/ December 17-20, 2003: 8th Annual Gravitational Wave Data Analysis Workshop (GWDAW-8) Milwaukee, Wisconsin, USA http://www.lsc-group.phys.uwm.edu/gwdaw8/ January 4-10, 2004: Miami Waves 2004: Conference on Geometric Analysis, Nonlinear Wave Equations and General Relativity Miami, Florida, USA http://www.math.miami.edu/anno/waves/index.htm January 5-10, 2004: International Conference on Gravitation and Cosmology (ICGC-2004) Cochin University of Science and Technology, Kochi, India http://www.cusat.ac.in/icgc04 January 7-9, 2004: 4th Conference of the ASGRG (ACGRG4) Monash University, Melbourne, Australia http://www.physics.adelaide.edu.au/ASGRG/ACGRG4/ February 12-14, 2004: Gravitational Wave Winter School YITP, Kyoto, Japan http://oberon.nagaokaut.ac.jp/gwws/index.html March 1-5, 2004: 319th WE-Heraeus-Seminar "Mathematical Relativity: New Ideas and Developments" Physikzentrum, Bad Honnef, Germany http://www.tat.physik.uni-tuebingen.de/%7Eheraeus/ April 6-7, 2004: 4th British Gravity Meeting (Britgrav IV) University of Nottingham, United Kingdom http://www.maths.nottingham.ac.uk/britgrav4/ April 15-17, 2004: International Workshop on Global Analysis Cankaya University, Ankara, Turkey http://www.cankaya.edu.tr/iwga/ May 3-7, 2004: Non-Perturbative Quantum Gravity: Loops and Spinfoams Marseille, France June 28-July 3, 2004: 6th Alexander Friedmann International Seminar on Gravitation and Cosmology Cargese Institute, Corsica, France http://www.fisica.ufpb.br/~jfonseca/friedmann/ July 7-12, 2004: The 5th LISA Symposium ESTEC, The Netherlands July 18-24, 2004: 17th International Conference of the ISGRG (GR 17) Dublin, Ireland http://www.gr17.com/ ****************************************************************************** MEMBERS' ABSTRACTS at gr-qc, December 2002 - May 2003 We list here all new abstracts that we are aware of that have been submitted by our members to gr-qc, or which are cross-linked at gr-qc. (We have not searched for abstracts on other Los Alamos archives which are not crosslinked to gr-qc.) If you do not send your papers to gr-qc but would like to have them noted in the newsletters, please send them to the Editor. ----------------------------------------------------------------------------- Paper: gr-qc/0309070 From: Michael Ashley Date: Mon, 15 Sep 2003 20:52:38 GMT (14kb) Curvature singularities and abstract boundary singularity theorems for space-time Authors: Michael J. S. L. Ashley (Centre for Gravitational Wave Physics - The Pennsylvania State University), Susan M. Scott (The Australian National University) Comments: 11 pages, accepted for publication in Contemporary Mathematics The abstract boundary construction of Scott and Szekeres is a general and flexible way to define singularities in General Relativity. The abstract boundary construction alsoproves of great utility when applied to questions about more general boundary features of space-time. Within this construction an essential singularity is a non-regular boundary point which is accessible by a curve of interest (e.g. a geodesic) within finite (affine) parameter distance and is not removable. Ashley and Scott proved the first theorem linking abstract boundary essential singularities with the notion of causal geodesic incompleteness for strongly causal, maximally extended space-times. The relationship between this result and the classical singularity theorems of Penrose and Hawking has enabled us to obtain abstract boundary singularity theorems. This paper describes essential singularity results for maximally extended space-times and presents our recent efforts to establish a relationship between the strong curvature singularities of Tipler and Krolak and abstract boundary essential singularities. ----------------------------------------------------------------------------- Paper: math.DG/0307278 From: Piotr Chrusciel Date: Mon, 21 Jul 2003 13:12:10 GMT (125kb) Boundary value problems for Dirac--type equations, with applications Authors: P.T. Chrusciel, R. Bartnik Comments: 86 A4 pages, various style files Subj-class: Differential Geometry; Mathematical Physics MSC-class: 35J55; 58J32; 83C40 We prove regularity for a class of boundary value problems for first order elliptic systems, with boundary conditions determined by spectral decompositions, under coefficient differentiability conditions weaker than previously known. We establish Fredholm properties for Dirac-type equations with these boundary conditions. Our results include sharp solvability criteria, over both compact and non-compact manifolds; weighted Poincare and Schroedinger-Lichnerowicz inequalities provide asymptotic control in the non-compact case. One application yields existence of solutions for the Witten equation with a spectral boundary condition used by Herzlich in his proof of a geometric lower bound for the ADM mass of asymptotically flat 3-manifolds. ---------------------------------------------------------------------------- Paper: physics/0309016 Quantum-Foam In-Flow Theory of Gravity and the Global Positioning System (GPS) Authors: Reginald T. Cahill (Flinders University) Comments: 25 pages, 1 eps figure Subj-class: General Physics It is shown that a new quantum-foam in-flow theory of gravity is mathematically equivalent to the General Relativity theory of gravity for the operation of the Global Positioning System (GPS). The differences between the two theories become experimentally evident in other situations such as in the so-called `dark matter' effect, in the observation of absolute motion and ipso facto in the observation of the in-flow motion into the Sun, and in the observation of a new class of gravitational waves, effects which are present in existing experimental observations, but are not within General Relativity. This new theory of gravity arises within the information-theoretic Process Physics. ----------------------------------------------------------------------------- Paper: gr-qc/0306096 From: Neil J. Cornish Date (v1): Fri, 20 Jun 2003 19:54:14 GMT (26kb) Date (revised v2): Sat, 21 Jun 2003 00:59:50 GMT (26kb) The Effects of Orbital Motion on LISA Time Delay Interferometry Authors: Neil J. Cornish, Ronald W. Hellings Comments: 12 pages, 2 figures Journal-ref: Class.Quant.Grav. 20 (2003) 4851-4860 In an effort to eliminate laser phase noise in laser interferometer spaceborne gravitational wave detectors, several combinations of signals have been found that allow the laser noise to be canceled out while gravitational wave signals remain. This process is called time delay interferometry (TDI). In the papers that defined the TDI variables, their performance was evaluated in the limit that the gravitational wave detector is fixed in space. However, the performance depends on certain symmetries in the armlengths that are available if the detector is fixed in space, but that will be broken in the actual rotating and flexing configuration produced by the LISA orbits. In this paper we investigate the performance of these TDI variables for the real LISA orbits. First, addressing the effects of rotation, we verify Daniel Shaddock's result that the Sagnac variables will not cancel out the laser phase noise, and we also find the same result for the symmetric Sagnac variable. The loss of the latter variable would be particularly unfortunate since this variable also cancels out gravitational wave signal,allowing instrument noise in the detector to be isolated and measured. Fortunately, we have found a set of more complicated TDI variables, which we call Delta-Sagnac variables, one of which accomplishes the same goal as the symmetric Sagnac variable to good accuracy. Finally, however, as we investigate the effects of the flexing of the detector arms due to non-circular orbital motion, we show that all variables, including the interferometer variables, which survive the rotation-induced loss of direction symmetry, will not completely cancel laser phase noise when the armlengths are changing with time. This unavoidable problem will place a stringent requirement on laser stability of 5 Hz per root Hz. ----------------------------------------------------------------------------- Paper: astro-ph/0310233 From: Neil J. Cornish Date: Wed, 8 Oct 2003 20:27:22 GMT (16kb) Constraining the Topology of the Universe Authors: Neil J. Cornish, David N. Spergel, Glenn D. Starkman, Eiichiro Komatsu Comments: Submitted to PRL The first year data from the Wilkinson Microwave Anisotropy Probe are used to place stringent constraints on the topology of the Universe. We search for pairs of circles on the sky with similar temperature patterns along each circle. We restrict the search to back-to-back circle pairs, and to nearly back-to-back circle pairs, as this covers the majority of the topologies that one might hope to detect in a nearly flat universe. We do not find any matched circles with radius greater than 25 degrees. For a wide class of models, the non-detection rules out the possibility that we live in a universe with topology scale smaller than 24 Gpc. ----------------------------------------------------------------------------- Paper: gr-qc/0311069 From: Neil J. Cornish Date: Fri, 21 Nov 2003 01:14:33 GMT (80kb) Forward Modeling of Space-borne Gravitational Wave Detectors Authors: Louis J. Rubbo, Neil J. Cornish, Olivier Poujade Comments: 14 Pages, 14 Figures, RevTex 4 Planning is underway for several space-borne gravitational wave observatories to be built in the next ten to twenty years. Realistic and efficient forward modeling will play a key role in the design and operation of these observatories. Space-borne interferometric gravitational wave detectors operate very differently from their ground based counterparts. Complex orbital motion, virtual interferometry, and finite size effects complicate the description of space-based systems, while nonlinear control systems complicate the description of ground based systems. Here we explore the forward modeling of space-based gravitational wave detectors and introduce an adiabatic approximation to the detector response that significantly extends the range of the standard low frequency approximation. The adiabatic approximation will aid in the development of data analysis techniques, and improve the modeling of astrophysical parameter extraction. ----------------------------------------------------------------------------- Paper: gr-qc/0306077 From: Benedict Cusack Date (v1): Tue, 17 Jun 2003 07:14:29 GMT (428kb) Date (revised v2): Fri, 8 Aug 2003 06:49:24 GMT (429kb) Study of an electro-optic modulator capable of generating simultaneous amplitude and phase modulations Authors: Benedict J Cusack, Benjamin S Sheard, Daniel A Shaddock, Malcolm B Gray, Ping Koy Lam, Stan E Whitcomb Comments: 12 pages, 8 figures, 1 table We report on the analysis and prototype-characterization of a dual- electrode electro-optic modulator that can generate both amplitude and phase modulations with a selectable relative phase, termed a universally tunable modulator (UTM). All modulation states can be reached by tuning only theelectrical inputs, facilitating real-time tuning, and the device is shown to have good suppression and stability properties. A mathematical analysis is presented, including the development of a geometric phase representation for modulation. The experimental characterization of the device shows that relative suppressions of 38 dB, 39 dB and 30 dB for phase, single-sideband and carrier-suppressed modulations, respectively, can be obtained, as well as showing the device is well-behaved when scanning continuously through the parameter space of modulations. Uses for the device are discussed, including the tuning of lock points in optical locking schemes, single sideband applications, modulation fast-switching applications, and applications requiring combined modulations. ----------------------------------------------------------------------------- Paper: gr-qc/0307007 From: Adrian P. Gentle Date (v1): Tue, 1 Jul 2003 16:53:41 GMT (10kb) Date (revised v2): Wed, 29 Oct 2003 16:50:17 GMT (10kb) The constraints as evolution equations for numerical relativity Authors: Adrian P. Gentle, Nathan D. George, Arkady Kheyfets, Warner A. Miller Comments: 10 pages, updated to match published version The Einstein equations have proven surprisingly difficult to solve numerically. A standard diagnostic of the problems which plague the field is the failure of computational schemes to satisfy the constraints, which are known to be mathematically conserved by the evolution equations. We describe a new approach to rewriting the constraints as first-order evolution equations, thereby guaranteeing that they are satisfied to a chosen accuracy by any discretization scheme. This introduces a set of four subsidiary constraints which are far simpler than the standard constraint equations, and which should be more easily conserved in computational applications. We explore the manner in which the momentum constraints are already incorporated in several existing formulations of the Einstein equations, and demonstrate the ease with which our new constraint- conserving approach can be incorporated into these schemes. ----------------------------------------------------------------------------- Paper: gr-qc/0308024 From: Antony C. Searle Date: Sat, 9 Aug 2003 07:03:58 GMT (234kb) Spectral Line Removal in the LIGO Data Analysis System (LDAS) Authors: Antony C. Searle, Susan M. Scott, David E. McClelland Comments: 11 pages, 5 figures, to be published in CQG GWDAW02 proceedings Journal-ref: Class.Quant.Grav. 20 (2003) S721-S730 High power in narrow frequency bands, spectral lines, are a feature of an interferometric gravitational wave detector's output. Some lines are coherent between interferometers, in particular, the 2 km and 4 km LIGO Hanford instruments. This is of concern to data analysis techniques, such as the stochastic background search, that use correlations between instruments to detect gravitational radiation. Several techniques of `line removal' have been proposed. Where a line is attributable to a measurable environmental disturbance, a simple linear model may be fitted to predict, and subsequently subtract away, that line. This technique has been implemented (as the command oelslr) in the LIGO Data Analysis System (LDAS). We demonstrate its application to LIGO S1 data. ----------------------------------------------------------------------------- Paper: gr-qc/0306125 From: Daniel A. Shaddock Date: Sun, 29 Jun 2003 18:28:42 GMT (224kb) Operating LISA as a Sagnac interferometer Authors: Daniel A. Shaddock Comments: 8 pages, 2 figures A phase-locking configuration for LISA is proposed that provides a significantly simpler mode of operation. The scheme provides one Sagnac signal readout inherently insensitive to laser frequency noise and optical bench motion for a non-rotating LISA array. This Sagnac output is also insensitive to clock noise, requires no time shifting of data, nor absolute arm length knowledge. As all measurements are made at one spacecraft, neither clock synchronization nor exchange of phase information between spacecraft is required. The phase-locking configuration provides these advantages for only one Sagnac variable yet retains compatibility with the baseline approach for obtaining the other TDI variables. The orbital motion of the LISA constellation is shown to produce a 14 km path length difference between the counter-propagating beams in the Sagnac interferometer. With this length difference a laser frequency noise spectral density of 1 Hz/$\sqrt{\rm Hz}$ would consume the entire optical path noise budget of the Sagnac variables. A significant improvement of laser frequency stability (currently at 30 Hz/$\sqrt{\rm Hz}$) would be needed for full-sensitivity LISA operation in the Sagnac mode. Alternatively, an additional level of time-delay processing could be applied to remove the laser frequency noise. The new time-delayed combinations of the phase measurements are presented. ----------------------------------------------------------------------------- Paper: gr-qc/0307080 From: Daniel A. Shaddock Date: Wed, 16 Jul 2003 21:11:58 GMT (531kb) Data Combinations Accounting for LISA Spacecraft Motion Authors: Daniel A. Shaddock, Massimo Tinto, Frank B. Estabrook, J. W. Armstrong Comments: 10 pages, 3 figures Journal-ref: Phys.Rev. D68 (2003) 061303 LISA is an array of three spacecraft in an approximately equilateral triangle configuration which will be used as a low-frequency gravitational wave detector. We present here new generalizations of the Michelson- and Sagnac-type time-delay interferometry data combinations. These combinations cancel laser phase noise in the presence of different up and down propagation delays in each arm of the array, and slowly varying systematic motion of the spacecraft. The gravitational wave sensitivities of these generalized combinations are the same as previously computed for the stationary cases, although the combinations are now more complicated. We introduce a diagrammatic representation to illustrate that these combinations are actually synthesized equal-arm interferometers. ----------------------------------------------------------------------------- Paper: gr-qc/0306038 From: Matt Visser Date: Tue, 10 Jun 2003 03:58:24 GMT (13kb) Bounds on the interior geometry and pressure profile of static fluid spheres Authors: Damien Martin (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) Comments: 20 pages. Uses: iopart.cls setstack.sty Journal-ref: Class.Quant.Grav. 20 (2003) 3699-3716 It is a famous result of relativistic stellar structure that (under mild technical conditions) a static fluid sphere satisfies the Buchdahl--Bondi bound 2M/R <= 8/9; the surprise here being that the bound is not 2M/R <= 1. In this article we provide further generalizations of this bound by placing a number of constraints on the interior geometry (the metric components), on the local acceleration due to gravity, on various combinations of the internal density and pressure profiles, and on the internal compactness 2m(r)/r of static fluid spheres. We do this by adapting the standard tool of comparing the generic fluid sphere with a Schwarzschild interior geometry of the same mass and radius -- in particular we obtain several results for the pressure profile (not merely the central pressure) that are considerably more subtle than might first be expected. ----------------------------------------------------------------------------- Paper: gr-qc/0306109 From: Matt Visser Date: Tue, 24 Jun 2003 00:50:38 GMT (6kb) Algorithmic construction of static perfect fluid spheres Authors: Damien Martin (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) Comments: 5 pages. Uses: revtex4 Perfect fluid spheres, both Newtonian and relativistic, have attracted considerable attention as the first step in developing realistic stellar models (or models for fluid planets). Whereas there have been some early hints on how one might find general solutions to the perfect fluid constraint in the absence of a specific equation of state, explicit and fully general solutions of the perfect fluid constraint have only very recently been developed. In this article we present a version of Lake's algorithm [Phys. Rev. D 67 (2003) 104015; gr-qc/0209104] wherein: (1) we re-cast the algorithm in terms of variables with a clear physical meaning -- the average density and the locally measured acceleration due to gravity, (2) we present explicit and fully general formulae for the mass profile and pressure profile, and (3) we present an explicit closed-form expression for the central pressure. ----------------------------------------------------------------------------- Paper: cond-mat/0307491 From: Matt Visser Date (v1): Sun, 20 Jul 2003 23:01:39 GMT (42kb) Date (revised v2): Thu, 14 Aug 2003 04:28:20 GMT (42kb) Probing semiclassical analogue gravity in Bose--Einstein condensates with widely tunable interactions Authors: Carlos Barcelo (University of Portsmouth), Stefano Liberati (University of Maryland), Matt Visser (Victoria University of Wellington) Comments: 18 pages; uses revtex4. V2: Added brief discussion of "Bose-Nova" phenomenon, and appropriate references Bose-Einstein condensates (BEC) have recently been the subject of considerable study as possible analogue models of general relativity. In particular it was shown that the propagation of phase perturbations in a BEC can, under certain conditions, closely mimic the dynamics of scalar quantum fields in curved spacetimes. In two previous articles [gr-qc/0110036, gr-qc/0305061] we noted that a varying scattering length in the BEC corresponds to a varying speed of light in the "effective metric". Recent experiments have indeed achieved a controlled tuning of the scattering length in Rubidium 85. In this article we shall discuss the prospects for the use of this particular experimental effect to test some of the predictions of semiclassical quantum gravity, for instance, particle production in an expanding universe. We stress that these effects are generally much larger than the Hawking radiation expected from causal horizons, and so there are much better chances for their detection in the near future. ----------------------------------------------------------------------------- Paper: gr-qc/0309072 From: Matt Visser Date (v1): Tue, 16 Sep 2003 03:39:57 GMT (12kb) Date (revised v2): Tue, 23 Sep 2003 20:35:03 GMT (12kb) Heuristic approach to the Schwarzschild geometry Authors: Matt Visser (Victoria University of Wellington) Comments: 12 pages; uses iopart.cls setstack.sty V2: references added; some clarifying comments; no physics changes In this article I will present a simple Newtonian heuristic for "deriving" a weak-field approximation for the spacetime geometry of a point particle. The heuristic is based on Newtonian gravity, the notion of local inertial frames [the Einstein equivalence principle], plus the use of Galilean coordinate transformations to connect the freely falling local inertial frames back to the "fixed stars". Because of the heuristic and quasi-Newtonian manner in which the spacetime geometry is obtained, we are only justified in expecting it to be a weak-field approximation to the true spacetime geometry. However, in the case of a spherically symmetric point mass the result is an exact solution of the vacuum Einstein field equations -- it is the Schwarzschild geometry in Painleve-Gullstrand coordinates. This result is much stronger than the well-known result of Michell and Laplace whereby a Newtonian argument correctly estimates the value of the Schwarzschild radius -- using the heuristic of this article one obtains the entire Schwarzschild geometry. Unfortunately the heuristic construction does not seem to generalize; it does not give the correct result for the Reissner--Nordstrom geometry (though it gets rather close), and does not seem capable of generating the Kerr geometry. Thus it is at this stage still somewhat unclear as to whether there is anything deeper to the heuristic than a remarkable but fortuitous coincidence. ---------------------------------------------------------------------------- Paper: gr-qc/0309109 From: Matt Visser Date (v1): Tue, 23 Sep 2003 04:08:40 GMT (9kb) Date (revised v2): Mon, 29 Sep 2003 22:25:25 GMT (10kb) Jerk and the cosmological equation of state Authors: Matt Visser (Victoria University of Wellington) Comments: 10 pages; uses iopart.cls setstack.sty V2: six additional references, some clarifying comments and discussion, no physics changes Linearizing the cosmological equation of state around the current epoch is the simplest model one can consider that does not make any "a priori" restrictions on the nature of the cosmological fluid. Most popular cosmological models attempt to be "predictive", in the sense that once some a priori equation of state is chosen the Friedmann equations are used to determine the evolution of the FRW scale factor a(t). In contrast, a "retrodictive" approach might usefully take observational data concerning the scale factor, and use the Friedmann equations to infer an observed cosmological equation of state. In particular, the value and derivatives of the scale factor determined at the current epoch place constraints on the value and derivatives of the cosmological equation of state at the current epoch. I shall demonstrate that determining the linearized equation of state at the current epoch requires a measurement of the jerk -- the third derivative of the scale factor with respect to time. Since the jerk is rather difficult to measure, being related to the third term in the Taylor series expansion of the Hubble law, it becomes clear why direct observational constraints on the cosmological equation of state are so relatively weak; and are likely to remain weak for the foreseeable future. ---------------------------------------------------------------------------- Paper: gr-qc/0310009 From: Matt Visser Date (v1): Thu, 2 Oct 2003 06:06:10 GMT (15kb) Date (revised v2): Thu, 9 Oct 2003 19:53:35 GMT (16kb) Dirty black holes: Quasinormal modes Authors: A J M Medved, Damien Martin, Matt Visser Comments: 15 pages; uses iopart.cls setstack.sty; V2: one additional reference added, no physics changes In this paper, we investigate the asymptotic nature of the quasinormal modes for "dirty" black holes -- generic static and spherically symmetric spacetimes for which a central black hole is surrounded by arbitrary "matter" fields. We demonstrate that, to the leading asymptotic order, the [imaginary] spacing between modes is precisely equal to the surface gravity, independent of the specifics of the black hole system. Our analytical method is based on locating the complex poles in the first Born approximation for the scattering amplitude. We first verify that our formalism agrees, asymptotically, with previous studies on the Schwarzschild black hole. The analysis is then generalized to more exotic black hole geometries. We also extend considerations to spacetimes with two horizons and briefly discuss the degenerate-horizon scenario. ---------------------------------------------------------------------------- Paper: gr-qc/0310097 From: Matt Visser Date (v1): Mon, 20 Oct 2003 01:54:42 GMT (16kb) Date (revised v2): Wed, 29 Oct 2003 02:53:40 GMT (16kb) Dirty black holes: Quasinormal modes for "squeezed" horizons Authors: A. J. M. Medved, Damien Martin, Matt Visser Comments: 15 pages, uses iopart.cls and setstack.sty V2: Two references added. Also, the appendix now relates our computation of the Regge-Wheeler potential for gravity in a generic "dirty" black hole to the results of Karlovini [gr-qc/0111066] We consider the quasinormal modes for a class of black hole spacetimes that, informally speaking, contain a closely "squeezed'' pair of horizons. (This scenario, where the relevant observer is presumed to be "trapped'' between the horizons, is operationally distinct from near-extremal black holes with an external observer.) It is shown, by analytical means, that the spacing of the quasinormal frequencies equals the surface gravity at the squeezed horizons. Moreover, we can calculate the real part of these frequencies provided that the horizons are sufficiently close together (but not necessarily degenerate or even "nearly degenerate"). The novelty of our analysis (which extends a model-specific treatment by Cardoso and Lemos) is that we consider "dirty" black holes; that is, the observable portion of the (static and spherically symmetric) spacetime is allowed to contain an arbitrary distribution of matter. ---------------------------------------------------------------------------- Paper: gr-qc/0310107 From: David Wiltshire Date: Thu, 23 Oct 2003 03:50:24 GMT (27kb) Stable gravastars - an alternative to black holes? Authors: Matt Visser (Victoria University of Wellington), David L. Wiltshire (University of Canterbury) Comments: 19 pages, 5 figures; uses iopart.cls, setstack.sty The "gravastar" picture developed by Mazur and Mottola is one of a very small number of serious challenges to our usual conception of a "black hole". In the gravastar picture there is effectively a phase transition at/ near where the event horizon would have been expected to form, and the interior of what would have been the black hole is replaced by a segment of de Sitter space. While Mazur and Mottola were able to argue for the thermodynamic stability of their configuration, the question of dynamic stability against spherically symmetric perturbations of the matter or gravity fields remains somewhat obscure. In this article we construct a model that shares the key features of the Mazur-Mottola scenario, and which is sufficiently simple for a full dynamical analysis. We find that there are some physically reasonable equations of state for the transition layer that lead to stability. ******************************************************************************