891 research outputs found
Polycritical Gravities
We present higher-derivative gravities that propagate an arbitrary number of
gravitons of different mass on (A)dS backgrounds. These theories have multiple
critical points, at which the masses degenerate and the graviton energies are
non-negative. For six derivatives and higher there are critical points with
positive energy.Comment: Version to be publishe
On quasi-local Hamiltonians in General Relativity
We analyse the definition of quasi-local energy in GR based on a Hamiltonian
analysis of the Einstein-Hilbert action initiated by Brown-York. The role of
the constraint equations, in particular the Hamiltonian constraint on the
timelike boundary, neglected in previous studies, is emphasized here. We argue
that a consistent definition of quasi-local energy in GR requires, at a
minimum, a framework based on the (currently unknown) geometric well-posedness
of the initial boundary value problem for the Einstein equations.Comment: 9 page
Alternatives to standard puncture initial data for binary black hole evolution
Standard puncture initial data have been widely used for numerical binary
black hole evolutions despite their shortcomings, most notably the inherent
lack of gravitational radiation at the initial time that is later followed by a
burst of spurious radiation. We study the evolution of three alternative
initial data schemes. Two of the three alternatives are based on post-Newtonian
expansions that contain realistic gravitational waves. The first scheme is
based on a second-order post-Newtonian expansion in Arnowitt, Deser, and Misner
transverse-traceless (ADMTT) gauge that has been resummed to approach standard
puncture data at the black holes. The second scheme is based on asymptotic
matching of the 4-metrics of two tidally perturbed Schwarzschild solutions to a
first-order post-Newtonian expansion in ADMTT gauge away from the black holes.
The final alternative is obtained through asymptotic matching of the 4-metrics
of two tidally perturbed Schwarzschild solutions to a second-order
post-Newtonian expansion in harmonic gauge away from the black holes. When
evolved, the second scheme fails to produce quasicircular orbits (and instead
leads to a nearly head-on collision). This failure can be traced back to
inaccuracies in the extrinsic curvature due to low order matching. More
encouraging is that the latter two alternatives lead to quasicircular orbits
and show gravitational radiation from the onset of the evolution, as well as a
reduction of spurious radiation. Current deficiencies compared to standard
punctures data include more eccentric trajectories during the inspiral and
larger constraint violations, since the alternative data sets are only
approximate solutions of Einstein's equations. The eccentricity problem can be
ameliorated by adjusting the initial momentum parameters.Comment: 11 pages, 11 figures, 1 appendix, typos corrected, removed duplicate
reference, matches published versio
Unified First Law and Thermodynamics of Apparent Horizon in FRW Universe
In this paper we revisit the relation between the Friedmann equations and the
first law of thermodynamics. We find that the unified first law firstly
proposed by Hayward to treat the "outer"trapping horizon of dynamical black
hole can be used to the apparent horizon (a kind of "inner" trapping horizon in
the context of the FRW cosmology) of the FRW universe. We discuss three kinds
of gravity theorties: Einstein theory, Lovelock thoery and scalar-tensor
theory. In Einstein theory, the first law of thermodynamics is always satisfied
on the apparent horizon. In Lovelock theory, treating the higher derivative
terms as an effective energy-momentum tensor, we find that this method can give
the same entropy formula for the apparent horizon as that of black hole
horizon. This implies that the Clausius relation holds for the Lovelock theory.
In scalar-tensor gravity, we find, by using the same procedure, the Clausius
relation no longer holds. This indicates that the apparent horizon of FRW
universe in the scalar-tensor gravity corresponds to a system of
non-equilibrium thermodynamics. We show this point by using the method
developed recently by Eling {\it et al.} for dealing with the gravity.Comment: v2: revtex, 23 pages, references added, minor changes, to appear in
PR
Dynamical evolution of scalar perturbation in Ho\v{r}ava-Lifshitz black-hole spacetimes
We study the dynamical evolution of a massless scalar perturbation in the
Ho\v{r}ava-Lifshitz black-hole spacetimes with the coupling constants
, and , respectively. Our calculation
shows that, for the three cases, the scalar perturbations decay without any
oscillation in which the decay rate imprints the parameter of the
Ho\v{r}ava-Lifshitz black hole. The results are quite different from those in
the Schwarzschild AdS black hole and can help us understand more about the
Ho\v{r}ava-Lifshitz gravity.Comment: 14 pages, 5 figure
Remarks on the consistency of minimal deviations from General Relativity
We study the consequences of the modification of the phase space structure of
General Relativity imposed by breaking the full diffeomorphism invariance but
retaining the time foliation preserving diffeomorphisms. We examine the
different sectors in phase space that satisfy the new structure of constraints.
For some sectors we find an infinite tower of constraints. In spite of that, we
also show that these sectors allow for solutions, among them some well known
families of black hole and cosmologies which fulfill all the constraints. We
raise some physical concerns on the consequences of an absolute Galilean time,
on the thermodynamical pathologies of such models and on their unusual vacuum
structure.Comment: latex 28 pages, 1 figure. Added comments and a reference. Text
improved
Gravity from the extension of spatial diffeomorphisms
The possibility of the extension of spatial diffeomorphisms to a larger
family of symmetries in a class of classical field theories is studied. The
generator of the additional local symmetry contains a quadratic kinetic term
and a potential term which can be a general (not necessarily local) functional
of the metric. From the perspective of the foundation of Einstein's gravity our
results are positive: The extended constraint algebra is either that of
Einstein's gravity, or ultralocal gravity. If our goal is a simple modification
of Einstein's gravity that for example makes it perturbatively renormalizable,
as has recently been suggested, then our results show that there is no such
theory within this class.Comment: 34 page
Binary black-hole evolutions of excision and puncture data
We present a new numerical code developed for the evolution of binary
black-hole spacetimes using different initial data and evolution techniques.
The code is demonstrated to produce state-of-the-art simulations of orbiting
and inspiralling black-hole binaries with convergent waveforms. We also present
the first detailed study of the dependence of gravitational waveforms resulting
from three-dimensional evolutions of different types of initial data. For this
purpose we compare the waveforms generated by head-on collisions of superposed
Kerr-Schild, Misner and Brill-Lindquist data over a wide range of initial
separations.Comment: 21 pages, 13 figures, final version accepted for publication in PR
Gravitational GUT Breaking and the GUT-Planck Hierarchy
It is shown that non-renormalizable gravitational interactions in the Higgs
sector of supersymmetric grand unified theories (GUT's) can produce the
breaking of the unifying gauge group at the GUT scale ~GeV. Such a breaking offers an attractive alternative to the
traditional method where the superheavy GUT scale mass parameters are added ad
hoc into the theory. The mechanism also offers a natural explanation for the
closeness of the GUT breaking scale to the Planck scale. A study of the minimal
SU(5) model endowed with this mechanism is presented and shown to be
phenomenologically viable. A second model is examined where the Higgs doublets
are kept naturally light as Goldstone modes. This latter model also achieves
breaking of at but cannot easily satisfy the current
experimental proton decay bound.Comment: 11 pages, REVTeX, 1 figure included as an uuencoded Z-compressed
PostScript file. Our Web page at
http://physics.tamu.edu/~urano/research/gutplanck.html contains ready to
print PostScript version (with figures) as well as color version of plot
Physical decomposition of the gauge and gravitational fields
Physical decomposition of the non-Abelian gauge field has recently solved the
two-decade-lasting problem of a meaningful gluon spin. Here we extend this
approach to gravity and attack the century-lasting problem of a meaningful
gravitational energy. The metric is unambiguously separated into a pure
geometric term which contributes null curvature tensor, and a physical term
which represents the true gravitational effect and always vanishes in a flat
space-time. By this decomposition the conventional pseudo-tensors of the
gravitational stress-energy are easily rescued to produce definite physical
result. Our decomposition applies to any symmetric tensor, and has interesting
relation to the transverse-traceless (TT) decomposition discussed by Arnowitt,
Deser and Misner, and by York.Comment: 11 pages, no figure; significant revision, with discussion on
relations of various metric decomposition
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