182 research outputs found
Relativistic gyratons in asymptotically AdS spacetime
We study the gravitational field of a spinning radiation beam-pulse (a
gyraton) in a D-dimensional asymptotically AdS spacetime. It is shown that the
Einstein equations for such a system reduce to a set of two linear equations in
a (D-2)-dimensional space. By solving these equations we obtain a metric which
is an exact solution of gravitational equations with the (negative)
cosmological constant. The explicit metrics for 4D and 5D gyratons in
asymptotically AdS spacetime are given and their properties are discussed.Comment: 10 page
Exploring AdS Waves Via Nonminimal Coupling
We consider nonminimally coupled scalar fields to explore the Siklos
spacetimes in three dimensions. Their interpretation as exact gravitational
waves propagating on AdS restrict the source to behave as a pure radiation
field. We show that the related pure radiation constraints single out a unique
self-interaction potential depending on one coupling constant. For a vanishing
coupling constant, this potential reduces to a mass term with a mass fixed in
terms of the nonminimal coupling parameter. This mass dependence allows the
existence of several free cases including massless and tachyonic sources. There
even exists a particular value of the nonminimal coupling parameter for which
the corresponding mass exactly compensates the contribution generated by the
negative scalar curvature, producing a genuinely massless field in this curved
background. The self-interacting case is studied in detail for the conformal
coupling. The resulting gravitational wave is formed by the superposition of
the free and the self-interaction contributions, except for a critical value of
the coupling constant where a non-perturbative effect relating the strong and
weak regimes of the source appears. We establish a correspondence between the
scalar source supporting an AdS wave and a pp wave by showing that their
respective pure radiation constraints are conformally related, while their
involved backgrounds are not. Finally, we consider the AdS waves for
topologically massive gravity and its limit to conformal gravity.Comment: 26 pages, 1 figure. Minor change
Gyratons on Melvin spacetime
We present and analyze new exact gyraton solutions of algebraic type II on a
background which is static, cylindrically symmetric Melvin universe of type D.
For a vanishing electromagnetic field it reduces to previously studied gyratons
on Minkowski background. We demonstrate that the solutions are member of a more
general family of the Kundt spacetimes. We show that the Einstein equations
reduce to a set of mostly linear equations on a transverse 2-space and we
discuss the properties of polynomial scalar curvature invariants which are
generally non-constant but unaffected by the presence of gyratons.Comment: 15 pages, no figures, journal version extended by appendices B and
Deeper discussion of Schr\"odinger invariant and Logarithmic sectors of higher-curvature gravity
The aim of this paper is to explore D-dimensional theories of pure gravity
whose space of solutions contains certain class of AdS-waves, including in
particular Schrodinger invariant spacetimes. This amounts to consider higher
order theories, and the natural case to start with is to analyze generic
square-curvature corrections to Einstein-Hilbert action. In this case, the
Schrodinger invariant sector in the space of solutions arises for a special
relation between the coupling constants appearing in the action. On the other
hand, besides the Schrodinger invariant configurations, logarithmic branches
similar to those of the so-called Log-gravity are also shown to emerge for
another special choice of the coupling constants. These Log solutions can be
interpreted as the superposition of the massless mode of General Relativity and
two scalar modes that saturate the Breitenlohner-Freedman bound (BF) of the AdS
space on which they propagate. These solutions are higher-dimensional analogues
of those appearing in three-dimensional massive gravities with relaxed AdS_3
asymptotic. Other sectors of the space of solutions of higher-curvature
theories correspond to oscillatory configurations, which happen to be below the
BF bound. Also, there is a fully degenerated sector, for which any wave profile
is admitted. We comment on the relation between this degeneracy and the
non-renormalization of the dynamical exponent of the Schrodinger spaces. Our
analysis also includes more general gravitational actions with non-polynomial
corrections consisting of arbitrary functions of the square-curvature
invariants. The same sectors of solutions are shown to exist for this more
general family of theories. We finally consider the Chern-Simons modified
gravity in four dimensions, for which we derive both the Schrodinger invariant
as well as the logarithmic sectors.Comment: This paper is dedicated to the memory of Laurent Houar
Local freedom in the gravitational field
In a cosmological context, the electric and magnetic parts of the Weyl
tensor, E_{ab} and H_{ab}, represent the locally free curvature - i.e. they are
not pointwise determined by the matter fields. By performing a complete
covariant decomposition of the derivatives of E_{ab} and H_{ab}, we show that
the parts of the derivative of the curvature which are locally free (i.e. not
pointwise determined by the matter via the Bianchi identities) are exactly the
symmetrised trace-free spatial derivatives of E_{ab} and H_{ab} together with
their spatial curls. These parts of the derivatives are shown to be crucial for
the existence of gravitational waves.Comment: New results on gravitational waves included; new references added;
revised version (IOP style) to appear Class. Quantum Gra
Inhomogeneous High Frequency Expansion-Free Gravitational Waves
We describe a natural inhomogeneous generalization of high frequency plane
gravitational waves. The waves are high frequency waves of the Kundt type whose
null propagation direction in space-time has vanishing expansion, twist and
shear but is not covariantly constant. The introduction of a cosmological
constant is discussed in some detail and a comparison is made with high
frequency gravity waves having wave fronts homeomorphic to 2-spheres.Comment: 18 pages, Latex file, accepted for publication in Physical Review
The Finiteness Requirement for Six-Dimensional Euclidean Einstein Gravity
The finiteness requirement for Euclidean Einstein gravity is shown to be so
stringent that only the flat metric is allowed. We examine counterterms in 4D
and 6D Ricci-flat manifolds from general invariance arguments.Comment: 15 pages, Introduction is improved, many figures(eps
PP-waves in AdS Gauged Supergravities and Supernumerary Supersymmetry
Purely gravitational pp-waves in AdS backgrounds are described by the
generalised Kaigorodov metrics, and they generically preserve 1/4 of the
maximum supersymmetry allowed by the AdS spacetimes. We obtain 1/2
supersymmetric purely gravitational pp-wave solutions, in which the Kaigorodov
component is set to zero. We construct pp-waves in AdS gauged supergravities
supported by a vector field. We find that the solutions in D=4 and D=5 can then
preserve 1/2 of the supersymmetry. Like those in ungauged supergravities, the
supernumerary supersymmetry imposes additional constraints on the harmonic
function associated with the pp-waves. These new backgrounds provide
interesting novel features of the supersymmetry enhancement for the dual
conformal field theory in the infinite-momentum frame.Comment: Latex, 27 pages, minor typos corrected and reference added, to appear
in Phys. Rev.
Brane Waves
In brane-world cosmology gravitational waves can propagate in the higher
dimensions (i.e., in the `bulk'). In some appropriate regimes the bulk
gravitational waves may be approximated by plane waves. We systematically study
five-dimensional gravitational waves that are algebraically special and of type
N. In the most physically relevant case the projected non-local stress tensor
on the brane is formally equivalent to the energy-momentum tensor of a null
fluid. Some exact solutions are studied to illustrate the features of these
branes; in particular, we show explicity that any plane wave brane can be
embedded into a 5-dimensional Siklos spacetime. More importantly, it is
possible that in some appropriate regime the bulk can be approximated by
gravitational plane waves and thus may act as initial conditions for the
gravitational field in the bulk (thereby enabling the field equations to be
integrated on the brane).Comment: 9 pages v3:revised version, to appear in CQ
Non-expanding impulsive gravitational waves
We investigate a class of impulsive gravitational waves which propagate
either in Minkowski or in the (anti-)de Sitter background. These waves are
constructed as impulsive members of the Kundt class of
non-twisting, non-expanding type N solutions of vacuum Einstein equations with
a cosmological constant . We show that the only non-trivial waves of
this type in Minkowski spacetime are impulsive pp-waves. For we
demonstrate that the canonical subclasses of , which are
invariantly different for smooth profiles, are all locally equivalent for
impulsive profiles. Also, we present coordinate system for these impulsive
solutions which is explicitly continuous.Comment: 12 pages, to appear in Class. Quantum Gra
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