140 research outputs found
Vacuum thin shell solutions in five-dimensional Lovelock gravity
Junction conditions for vacuum solutions in five-dimensional
Einstein-Gauss-Bonnet gravity are studied. We focus on those cases where two
spherically symmetric regions of space-time are joined in such a way that the
induced stress tensor on the junction surface vanishes. So a spherical vacuum
shell, containing no matter, arises as a boundary between two regions of the
space-time. Such solutions are a generalized kind of spherically symmetric
empty space solutions, described by metric functions of the class . New
global structures arise with surprising features. In particular, we show that
vacuum spherically symmetric wormholes do exist in this theory. These can be
regarded as gravitational solitons, which connect two asymptotically (Anti)
de-Sitter spaces with different masses and/or different effective cosmological
constants. We prove the existence of both static and dynamical solutions and
discuss their (in)stability under perturbations that preserve the symmetry.
This leads us to discuss a new type of instability that arises in
five-dimensional Lovelock theory of gravity for certain values of the coupling
of the Gauss-Bonnet term.Comment: 9 pages. This is an extended version of the authors' contribution to
the Proceedings of the Marcel Grossmann Meeting, held in Paris, 12-18 July
200
Shock waves and Birkhoff's theorem in Lovelock gravity
Spherically symmetric shock waves are shown to exist in Lovelock gravity.
They amount to a change of branch of the spherically symmetric solutions across
a null hypersurface. The implications of their existence for the status of
Birkhoff's theorem in the theory is discussed.Comment: 9 pages, no figures, clarifying changes made in the text of section
III and references adde
On the stationarity of linearly forced turbulence in finite domains
A simple scheme of forcing turbulence away from decay was introduced by
Lundgren some time ago, the `linear forcing', which amounts to a force term
linear in the velocity field with a constant coefficient. The evolution of
linearly forced turbulence towards a stationary final state, as indicated by
direct numerical simulations (DNS), is examined from a theoretical point of
view based on symmetry arguments. In order to follow closely the DNS the flow
is assumed to live in a cubic domain with periodic boundary conditions. The
simplicity of the linear forcing scheme allows one to re-write the problem as
one of decaying turbulence with a decreasing viscosity. Scaling symmetry
considerations suggest that the system evolves to a stationary state, evolution
that may be understood as the gradual breaking of a larger approximate symmetry
to a smaller exact symmetry. The same arguments show that the finiteness of the
domain is intimately related to the evolution of the system to a stationary
state at late times, as well as the consistency of this state with a high
degree of isotropy imposed by the symmetries of the domain itself. The
fluctuations observed in the DNS for all quantities in the stationary state can
be associated with deviations from isotropy. Indeed, self-preserving isotropic
turbulence models are used to study evolution from a direct dynamical point of
view, emphasizing the naturalness of the Taylor microscale as a self-similarity
scale in this system. In this context the stationary state emerges as a stable
fixed point. Self-preservation seems to be the reason behind a noted similarity
of the third order structure function between the linearly forced and freely
decaying turbulence, where again the finiteness of the domain plays an
significant role.Comment: 15 pages, 7 figures, changes in the discussion at the end of section
VI, formula (60) correcte
Cosmological Evolution of a Purely Conical Codimension-2 Brane World
We study the cosmological evolution of isotropic matter on an infinitely thin
conical codimension-two brane-world. Our analysis is based on the boundary
dynamics of a six-dimensional model in the presence of an induced gravity term
on the brane and a Gauss-Bonnet term in the bulk. With the assumption that the
bulk contains only a cosmological constant Lambda_B, we find that the isotropic
evolution of the brane-universe imposes a tuned relation between the energy
density and the brane equation of state. The evolution of the system has fixed
points (attractors), which correspond to a final state of radiation for
Lambda_B=0 and to de Sitter state for Lambda_B>0. Furthermore, considering
anisotropic matter on the brane, the tuning of the parameters is lifted, and
new regions of the parametric space are available for the cosmological
evolution of the brane-universe. The analysis of the dynamics of the system
shows that, the isotropic fixed points remain attractors of the system, and for
values of Lambda_B which give acceptable cosmological evolution of the equation
of state, the line of isotropic tuning is a very weak attractor. The initial
conditions, in this case, need to be fine tuned to have an evolution with
acceptably small anisotropy.Comment: 20 pages, 4 figures, typo correcte
A supersymmetric D-brane Model of Space-Time Foam
We present a supersymmetric model of space-time foam with two stacks of eight
D8-branes with equal string tensions, separated by a single bulk dimension
containing D0-brane particles that represent quantum fluctuations in the
space-time foam. The ground state configuration with static D-branes has zero
vacuum energy. However, gravitons and other closed-string states propagating
through the bulk may interact with the D0-particles, causing them to recoil and
the vacuum energy to become non zero. This provides a possible origin of dark
energy. Recoil also distorts the background metric felt by energetic massless
string states, which travel at less than the usual (low-energy) velocity of
light. On the other hand, the propagation of chiral matter anchored on the D8
branes is not affected by such space-time foam effects.Comment: 33 pages, latex, five figure
On CPT Symmetry: Cosmological, Quantum-Gravitational and other possible violations and their phenomenology
I discuss various ways in which CPT symmetry may be violated, and their
phenomenology in current or immediate future experimental facilities, both
terrestrial and astrophysical. Specifically, I discuss first violations of CPT
symmetry due to the impossibility of defining a scattering matrix as a
consequence of the existence of microscopic or macroscopic space-time
boundaries, such as Planck-scale Black-Hole (event) horizons, or cosmological
horizons due to the presence of a (positive) cosmological constant in the
Universe. Second, I discuss CPT violation due to breaking of Lorentz symmetry,
which may characterize certain approaches to quantum gravity, and third, I
describe models of CPT non invariance due to violations of locality of
interactions. In each of the above categories I discuss experimental
sensitivities. I argue that the majority of Lorentz-violating cases of CPT
breaking, with minimal (linear) suppression by the Planck-mass scale, are
already excluded by current experimental tests. There are however some
(stringy) models which can evade these constraints.Comment: 27 pages latex, Conference talk Beyond the Desert 200
Brane Universes with Gauss-Bonnet-Induced-Gravity
The DGP brane world model allows us to get the observed late time
acceleration via modified gravity, without the need for a ``dark energy''
field. This can then be generalised by the inclusion of high energy terms, in
the form of a Gauss-Bonnet bulk. This is the basis of the
Gauss-Bonnet-Induced-Gravity (GBIG) model explored here with both early and
late time modifications to the cosmological evolution. Recently the simplest
GBIG models (Minkowski bulk and no brane tension) have been analysed. Two of
the three possible branches in these models start with a finite density
``Big-Bang'' and with late time acceleration. Here we present a comprehensive
analysis of more general models where we include a bulk cosmological constant
and brane tension. We show that by including these factors it is possible to
have late time phantom behaviour.Comment: 12 pages, 19 figures. Minor modifications to text, comments on
phantom behaviour added. References added. As submitted to JCA
Validity of the Generalized Second Law of Thermodynamics of the Universe Bounded by the Event Horizon in Holographic Dark Energy Model
In this letter, we investigate the validity of the generalized second law of
thermodynamics of the universe bounded by the event horizon in the holographic
dark energy model. The universe is chosen to be homogeneous and isotropic and
the validity of the first law has been assumed here. The matter in the universe
is taken in the form of non-interacting two fluid system- one component is the
holographic dark energy model and the other component is in the form of dust.Comment: 8 page
Asymptotically Lifshitz wormholes and black holes for Lovelock gravity in vacuum
Static asymptotically Lifshitz wormholes and black holes in vacuum are shown
to exist for a class of Lovelock theories in d=2n+1>7 dimensions, selected by
requiring that all but one of their n maximally symmetric vacua are AdS of
radius l and degenerate. The wormhole geometry is regular everywhere and
connects two Lifshitz spacetimes with a nontrivial geometry at the boundary.
The dynamical exponent z is determined by the quotient of the curvature radii
of the maximally symmetric vacua according to n(z^2-1)+1=(l/L)^2, where L
corresponds to the curvature radius of the nondegenerate vacuum. Light signals
are able to connect both asymptotic regions in finite time, and the
gravitational field pulls towards a fixed surface located at some arbitrary
proper distance to the neck. The asymptotically Lifshitz black hole possesses
the same dynamical exponent and a fixed Hawking temperature given by T=z/(2^z
pi l). Further analytic solutions, including pure Lifshitz spacetimes with a
nontrivial geometry at the spacelike boundary, and wormholes that interpolate
between asymptotically Lifshitz spacetimes with different dynamical exponents
are also found.Comment: 19 pages, 1 figur
Intermediate inflation in Gauss-Bonnet braneworld
In this article we study an intermediate inflationary universe models using
the Gauss-Bonnet brane. General conditions required for these models to be
realizable are derived and discussed. We use recent astronomical observations
to constraint the parameters appearing in the model.Comment: 16 pages, 2 figures, accepted for publication in European Physical
Journal
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