752 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
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
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
{\delta}N formalism
Precise understanding of nonlinear evolution of cosmological perturbations
during inflation is necessary for the correct interpretation of measurements of
non-Gaussian correlations in the cosmic microwave background and the
large-scale structure of the universe. The "{\delta}N formalism" is a popular
and powerful technique for computing non-linear evolution of cosmological
perturbations on large scales. In particular, it enables us to compute the
curvature perturbation, {\zeta}, on large scales without actually solving
perturbed field equations. However, people often wonder why this is the case.
In order for this approach to be valid, the perturbed Hamiltonian constraint
and matter-field equations on large scales must, with a suitable choice of
coordinates, take on the same forms as the corresponding unperturbed equations.
We find that this is possible when (1) the unperturbed metric is given by a
homogeneous and isotropic Friedmann-Lema\^itre-Robertson-Walker metric; and (2)
on large scales and with a suitable choice of coordinates, one can ignore the
shift vector (g0i) as well as time-dependence of tensor perturbations to
gij/a2(t) of the perturbed metric. While the first condition has to be assumed
a priori, the second condition can be met when (3) the anisotropic stress
becomes negligible on large scales. However, in order to explicitly show that
the second condition follows from the third condition, one has to use
gravitational field equations, and thus this statement may depend on the
details of theory of gravitation. Finally, as the {\delta}N formalism uses only
the Hamiltonian constraint and matter-field equations, it does not a priori
respect the momentum constraint. We show that the violation of the momentum
constraint only yields a decaying mode solution for {\zeta}, and the violation
vanishes when the slow-roll conditions are satisfied.Comment: 10 page
Extreme throat initial data set and horizon area--angular momentum inequality for axisymmetric black holes
We present a formula that relates the variations of the area of extreme
throat initial data with the variation of an appropriate defined mass
functional. From this expression we deduce that the first variation, with fixed
angular momentum, of the area is zero and the second variation is positive
definite evaluated at the extreme Kerr throat initial data. This indicates that
the area of the extreme Kerr throat initial data is a minimum among this class
of data. And hence the area of generic throat initial data is bounded from
below by the angular momentum. Also, this result strongly suggests that the
inequality between area and angular momentum holds for generic asymptotically
flat axially symmetric black holes. As an application, we prove this inequality
in the non trivial family of spinning Bowen-York initial data.Comment: 11 pages. Changes in presentation and typos correction
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