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.
******************************************************************************