25 research outputs found
Critical Collapse of the Massless Scalar Field in Axisymmetry
We present results from a numerical study of critical gravitational collapse
of axisymmetric distributions of massless scalar field energy. We find
threshold behavior that can be described by the spherically symmetric critical
solution with axisymmetric perturbations. However, we see indications of a
growing, non-spherical mode about the spherically symmetric critical solution.
The effect of this instability is that the small asymmetry present in what
would otherwise be a spherically symmetric self-similar solution grows. This
growth continues until a bifurcation occurs and two distinct regions form on
the axis, each resembling the spherically symmetric self-similar solution. The
existence of a non-spherical unstable mode is in conflict with previous
perturbative results, and we therefore discuss whether such a mode exists in
the continuum limit, or whether we are instead seeing a marginally stable mode
that is rendered unstable by numerical approximation.Comment: 11 pages, 8 figure
Generalized harmonic formulation in spherical symmetry
In this pedagogically structured article, we describe a generalized harmonic
formulation of the Einstein equations in spherical symmetry which is regular at
the origin. The generalized harmonic approach has attracted significant
attention in numerical relativity over the past few years, especially as
applied to the problem of binary inspiral and merger. A key issue when using
the technique is the choice of the gauge source functions, and recent work has
provided several prescriptions for gauge drivers designed to evolve these
functions in a controlled way. We numerically investigate the parameter spaces
of some of these drivers in the context of fully non-linear collapse of a real,
massless scalar field, and determine nearly optimal parameter settings for
specific situations. Surprisingly, we find that many of the drivers that
perform well in 3+1 calculations that use Cartesian coordinates, are
considerably less effective in spherical symmetry, where some of them are, in
fact, unstable.Comment: 47 pages, 15 figures. v2: Minor corrections, including 2 added
references; journal version
Tips for implementing multigrid methods on domains containing holes
As part of our development of a computer code to perform 3D `constrained
evolution' of Einstein's equations in 3+1 form, we discuss issues regarding the
efficient solution of elliptic equations on domains containing holes (i.e.,
excised regions), via the multigrid method. We consider as a test case the
Poisson equation with a nonlinear term added, as a means of illustrating the
principles involved, and move to a "real world" 3-dimensional problem which is
the solution of the conformally flat Hamiltonian constraint with Dirichlet and
Robin boundary conditions. Using our vertex-centered multigrid code, we
demonstrate globally second-order-accurate solutions of elliptic equations over
domains containing holes, in two and three spatial dimensions. Keys to the
success of this method are the choice of the restriction operator near the
holes and definition of the location of the inner boundary. In some cases (e.g.
two holes in two dimensions), more and more smoothing may be required as the
mesh spacing decreases to zero; however for the resolutions currently of
interest to many numerical relativists, it is feasible to maintain second order
convergence by concentrating smoothing (spatially) where it is needed most.
This paper, and our publicly available source code, are intended to serve as
semi-pedagogical guides for those who may wish to implement similar schemes.Comment: 18 pages, 11 figures, LaTeX. Added clarifications and references re.
scope of paper, mathematical foundations, relevance of work. Accepted for
publication in Classical & Quantum Gravit
Numerical Relativity Using a Generalized Harmonic Decomposition
A new numerical scheme to solve the Einstein field equations based upon the
generalized harmonic decomposition of the Ricci tensor is introduced. The
source functions driving the wave equations that define generalized harmonic
coordinates are treated as independent functions, and encode the coordinate
freedom of solutions. Techniques are discussed to impose particular gauge
conditions through a specification of the source functions. A 3D, free
evolution, finite difference code implementing this system of equations with a
scalar field matter source is described. The second-order-in-space-and-time
partial differential equations are discretized directly without the use first
order auxiliary terms, limiting the number of independent functions to
fifteen--ten metric quantities, four source functions and the scalar field.
This also limits the number of constraint equations, which can only be enforced
to within truncation error in a numerical free evolution, to four. The
coordinate system is compactified to spatial infinity in order to impose
physically motivated, constraint-preserving outer boundary conditions. A
variant of the Cartoon method for efficiently simulating axisymmetric
spacetimes with a Cartesian code is described that does not use interpolation,
and is easier to incorporate into existing adaptive mesh refinement packages.
Preliminary test simulations of vacuum black hole evolution and black hole
formation via scalar field collapse are described, suggesting that this method
may be useful for studying many spacetimes of interest.Comment: 18 pages, 6 figures; updated to coincide with journal version, which
includes some expanded discussions and a new appendix with a stability
analysis of a simplified problem using the same discretization scheme
described in the pape
Gravitational collapse in 2+1 dimensional AdS spacetime
We present results of numerical simulations of the formation of black holes
from the gravitational collapse of a massless, minimally-coupled scalar field
in 2+1 dimensional, axially-symmetric, anti de-Sitter (AdS) spacetime. The
geometry exterior to the event horizon approaches the BTZ solution, showing no
evidence of scalar `hair'. To study the interior structure we implement a
variant of black-hole excision, which we call singularity excision. We find
that interior to the event horizon a strong, spacelike curvature singularity
develops. We study the critical behavior at the threshold of black hole
formation, and find a continuously self-similar solution and corresponding
mass-scaling exponent of approximately 1.2. The critical solution is universal
to within a phase that is related to the angle deficit of the spacetime.Comment: 31 pages, 20 figures, LaTeX. Replaced with version to be published in
Phys. Rev.
An Axisymmetric Gravitational Collapse Code
We present a new numerical code designed to solve the Einstein field
equations for axisymmetric spacetimes. The long term goal of this project is to
construct a code that will be capable of studying many problems of interest in
axisymmetry, including gravitational collapse, critical phenomena,
investigations of cosmic censorship, and head-on black hole collisions. Our
objective here is to detail the (2+1)+1 formalism we use to arrive at the
corresponding system of equations and the numerical methods we use to solve
them. We are able to obtain stable evolution, despite the singular nature of
the coordinate system on the axis, by enforcing appropriate regularity
conditions on all variables and by adding numerical dissipation to hyperbolic
equations.Comment: 19 pages, 9 figure
Modified general relativity as a model for quantum gravitational collapse
We study a class of Hamiltonian deformations of the massless
Einstein-Klein-Gordon system in spherical symmetry for which the Dirac
constraint algebra closes. The system may be regarded as providing effective
equations for quantum gravitational collapse. Guided by the observation that
scalar field fluxes do not follow metric null directions due to the
deformation, we find that the equations take a simple form in characteristic
coordinates. We analyse these equations by a unique combination of numerical
methods and find that Choptuik's mass scaling law is modified by a mass gap as
well as jagged oscillations. Furthermore, the results are universal with
respect to different initial data profiles and robust under changes of the
deformation.Comment: 22 pages, 4 figure
AMR, stability and higher accuracy
Efforts to achieve better accuracy in numerical relativity have so far
focused either on implementing second order accurate adaptive mesh refinement
or on defining higher order accurate differences and update schemes. Here, we
argue for the combination, that is a higher order accurate adaptive scheme.
This combines the power that adaptive gridding techniques provide to resolve
fine scales (in addition to a more efficient use of resources) together with
the higher accuracy furnished by higher order schemes when the solution is
adequately resolved. To define a convenient higher order adaptive mesh
refinement scheme, we discuss a few different modifications of the standard,
second order accurate approach of Berger and Oliger. Applying each of these
methods to a simple model problem, we find these options have unstable modes.
However, a novel approach to dealing with the grid boundaries introduced by the
adaptivity appears stable and quite promising for the use of high order
operators within an adaptive framework
Are moving punctures equivalent to moving black holes?
When simulating the inspiral and coalescence of a binary black-hole system,
special care needs to be taken in handling the singularities. Two main
techniques are used in numerical-relativity simulations: A first and more
traditional one ``excises'' a spatial neighbourhood of the singularity from the
numerical grid on each spacelike hypersurface. A second and more recent one,
instead, begins with a ``puncture'' solution and then evolves the full
3-metric, including the singular point. In the continuum limit, excision is
justified by the light-cone structure of the Einstein equations and, in
practice, can give accurate numerical solutions when suitable discretizations
are used. However, because the field variables are non-differentiable at the
puncture, there is no proof that the moving-punctures technique is correct,
particularly in the discrete case. To investigate this question we use both
techniques to evolve a binary system of equal-mass non-spinning black holes. We
compare the evolution of two curvature 4-scalars with proper time along the
invariantly-defined worldline midway between the two black holes, using
Richardson extrapolation to reduce the influence of finite-difference
truncation errors. We find that the excision and moving-punctures evolutions
produce the same invariants along that worldline, and thus the same spacetimes
throughout that worldline's causal past. This provides convincing evidence that
moving-punctures are indeed equivalent to moving black holes.Comment: 4 pages, 3 eps color figures; v2 = major revisions to introduction &
conclusions based on referee comments, but no change in analysis or result
Scalar field spacetimes and the AdS/CFT conjecture
We describe a class of asymptotically AdS scalar field spacetimes, and
calculate the associated conserved charges for three, four and five spacetime
dimensions using the conformal and counter-term prescriptions. The energy
associated with the solutions in each case is proportional to , where is a constant and is a scalar charge. In five spacetime
dimensions, the counter-term prescription gives an additional vacuum (Casimir)
energy, which agrees with that found in the context of AdS/CFT correspondence.
We find a surprising degeneracy: the energy of the ``extremal'' scalar field
solution equals the energy of pure AdS. This result is discussed in light
of the AdS/CFT conjecture.Comment: 5 pages, Latex, additional commentary on results, version to appear
in Phys. Rev.