6,755 research outputs found
Vacuum decay via Lorentzian wormholes
We speculate about the spacetime description due to the presence of
Lorentzian wormholes (handles in spacetime joining two distant regions or other
universes) in quantum gravity. The semiclassical rate of production of these
Lorentzian wormholes in Reissner-Nordstr\"om spacetimes is calculated as a
result of the spontaneous decay of vacuum due to a real tunneling
configuration. In the magnetic case it only depends on the field theoretical
fine structure constant. We predict that the quantum probability corresponding
to the nucleation of such geodesically complete spacetimes should be actually
negligible in our physical Universe
Time-Dependent Multi-Centre Solutions from New Metrics with Holonomy Sim(n-2)
The classifications of holonomy groups in Lorentzian and in Euclidean
signature are quite different. A group of interest in Lorentzian signature in n
dimensions is the maximal proper subgroup of the Lorentz group, SIM(n-2).
Ricci-flat metrics with SIM(2) holonomy were constructed by Kerr and Goldberg,
and a single four-dimensional example with a non-zero cosmological constant was
exhibited by Ghanam and Thompson. Here we reduce the problem of finding the
general -dimensional Einstein metric of SIM(n-2) holonomy, with and without
a cosmological constant, to solving a set linear generalised Laplace and
Poisson equations on an (n-2)-dimensional Einstein base manifold. Explicit
examples may be constructed in terms of generalised harmonic functions. A
dimensional reduction of these multi-centre solutions gives new time-dependent
Kaluza-Klein black holes and monopoles, including time-dependent black holes in
a cosmological background whose spatial sections have non-vanishing curvature.Comment: Typos corrected; 29 page
On boundary terms and conformal transformations in curved space-times
We intend to clarify the interplay between boundary terms and conformal
transformations in scalar-tensor theories of gravity. We first consider the
action for pure gravity in five dimensions and show that, on compactifing a la
Kaluza-Klein to four dimensions, one obtains the correct boundary terms in the
Jordan (or String) Frame form of the Brans-Dicke action. Further, we analyze
how the boundary terms change under the conformal transformations which lead to
the Pauli (or Einstein) frame and to the non-minimally coupled massless scalar
field. In particular, we study the behaviour of the total energy in
asymptotically flat space-times as it results from surface terms in the
Hamiltonian formalism.Comment: LaTeX 2e, 12 pages, no figure
An instability of higher-dimensional rotating black holes
We present the first example of a linearized gravitational instability of an
asymptotically flat vacuum black hole. We study perturbations of a Myers-Perry
black hole with equal angular momenta in an odd number of dimensions. We find
no evidence of any instability in five or seven dimensions, but in nine
dimensions, for sufficiently rapid rotation, we find perturbations that grow
exponentially in time. The onset of instability is associated with the
appearance of time-independent perturbations which generically break all but
one of the rotational symmetries. This is interpreted as evidence for the
existence of a new 70-parameter family of black hole solutions with only a
single rotational symmetry. We also present results for the Gregory-Laflamme
instability of rotating black strings, demonstrating that rotation makes black
strings more unstable.Comment: 38 pages, 13 figure
On the initial value problem for second order scalar fluctuations in Einstein static
We consider fluctuations in a perfect irrotational fluid coupled to gravity
in an Einstein static universe background. We show that the homogeneous linear
perturbations of the scalar and metric fluctuations in the Einstein static
universe must be present if the second order constraint equations are to be
integrable. I.e., the 'linearization stability' constraint forces the presence
of these homogeneous modes. Since these linear homogeneous scalar modes are
well known to be exponentially unstable, the tactic of neglecting these modes
to create a long-lived, almost Einstein universe does not work, even if all
higher order (L 1) modes are dynamically stable.Comment: 8 pages, no figures, changes made to the presentation throughout to
emphasize the linear nature of the analysis and the treatment of the
irrotational perfect fluid. Conclusions unchanged. Submitted to PR
Supersymmetry Enhancement of D-p-branes and M-branes
We examine the supersymmetry of classical D-brane and M-brane configurations
and explain the dependence of Killing spinors on coordinates. We find that one
half supersymmetry is broken in the bulk and that supersymmetry near the
D-brane horizon is restored for , for solutions in the stringy frame,
but only for in the10d canonical frame. We study the enhancement for the
case of four intersecting D-3-branes in 10 dimensions and the implication of
this for the size of the infinite throat of the near horizon geometry in
non-compactified theory. We found some indications of universality of near
horizon geometries of various intersecting brane configurations.Comment: 18 pages, late
Variable cavity volume tooling for high-performance resin infusion moulding
This article describes the research carried out by Warwick under the BAE Systems/EPSRC programme ‘Flapless Aerial Vehicles Integrated Interdisciplinary Research – FLAVIIR’. Warwick's aim in FLAVIIR was to develop low-cost innovative tooling technologies to enable the affordable manufacture of complex composite aerospace structures and to help realize the aim of the Grand Challenge of maintenance-free, low-cost unmanned aerial vehicle manufacture. This article focuses on the evaluation of a novel tooling process (variable cavity tooling) to enable the complete infusion of resin throughout non-crimp fabric within a mould cavity under low (0.1 MPa) injection pressure. The contribution of the primary processing parameters to the mechanical properties of a carbon composite component (bulk-head lug section), and the interactions between parameters, was determined. The initial mould gap (di) was identified as having the most significant effect on all measured mechanical properties, but complex interactions between di, n (number of fabric layers), and vc (mould closure rate) were observed. The process capability was low due to the manual processing, but was improved through process optimization, and delivered properties comparable to high-pressure resin transfer moulding
Mixing of Xi_c and Xi_c' Baryons
The mixing angle between the Xi_c and Xi_c' baryons is shown to be small,
with a negligible shift in the Xi_c masses.Comment: One missprint corrected. The numerator of Eq. (12) should read
{2[(Sigma_c^{*++}-Sigma_c^{++})-(Xi_c^{*+}-Xi_c^{'+})]} The correct equation
was used in the calculation so no other change is mad
Multi-black hole solutions in five dimensions
Using a recently developed generalized Weyl formalism, we construct an
asymptotically flat, static vacuum Einstein solution that describes a
superposition of multiple five-dimensional Schwarzschild black holes. The
spacetime exhibits a U(1)\times U(1) rotational symmetry. It is argued that for
certain choices of parameters, the black holes are collinear and so may be
regarded as a five-dimensional generalization of the Israel-Khan solution. The
black holes are kept in equilibrium by membrane-like conical singularities
along the two rotational axes; however, they still distort one another by their
mutual gravitational attraction. We also generalize this solution to one
describing multiple charged black holes, with fixed mass-to-charge ratio, in
Einstein-Maxwell-dilaton theory.Comment: 23 pages, 6 figure
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