7,688 research outputs found
Energy of gravitational radiation in plane-symmetric space-times
Gravitational radiation in plane-symmetric space-times can be encoded in a
complex potential, satisfying a non-linear wave equation. An effective energy
tensor for the radiation is given, taking a scalar-field form in terms of the
potential, entering the field equations in the same way as the matter energy
tensor. It reduces to the Isaacson energy tensor in the linearized,
high-frequency approximation. An energy conservation equation is derived for a
quasi-local energy, essentially the Hawking energy. A transverse pressure
exerted by interacting low-frequency gravitational radiation is predicted.Comment: 7 REVTeX4 page
Late Miocene to early Pliocene stratigraphic record in northern Taranaki Basin: Condensed sedimentation ahead of Northern Graben extension and progradation of the modern continental margin
The middle Pliocene-Pleistocene progradation of the Giant Foresets Formation in Taranaki Basin built up the modern continental margin offshore from western North Island. The late Miocene to early Pliocene interval preceding this progradation was characterised in northern Taranaki Basin by the accumulation of hemipelagic mudstone (Manganui Formation), volcaniclastic sediments (Mohakatino Formation), and marl (Ariki Formation), all at bathyal depths. The Manganui Formation has generally featureless wireline log signatures and moderate to low amplitude seismic reflection characteristics. Mohakatino Formation is characterised by a sharp decrease in the GR log value at its base, a blocky GR log motif reflecting sandstone packets, and erratic resistivity logs. Seismic profiles show bold laterally continuous reflectors. The Ariki Formation has a distinctive barrel-shaped to blocky GR log motif. This signature is mirrored by the SP log and often by an increase in resistivity values through this interval. The Ariki Formation comprises (calcareous) marl made up of abundant planktic foraminifera, is 109 m thick in Ariki-1, and accumulated over parts of the Western Stable Platform and beneath the fill of the Northern Graben. It indicates condensed sedimentation reflecting the distance of the northern region from the contemporary continental margin to the south
A Cosmological Constant Limits the Size of Black Holes
In a space-time with cosmological constant and matter satisfying
the dominant energy condition, the area of a black or white hole cannot exceed
. This applies to event horizons where defined, i.e. in an
asymptotically deSitter space-time, and to outer trapping horizons (cf.
apparent horizons) in any space-time. The bound is attained if and only if the
horizon is identical to that of the degenerate `Schwarzschild-deSitter'
solution. This yields a topological restriction on the event horizon, namely
that components whose total area exceeds cannot merge. We
discuss the conjectured isoperimetric inequality and implications for the
cosmic censorship conjecture.Comment: 10 page
Unified first law of black-hole dynamics and relativistic thermodynamics
A unified first law of black-hole dynamics and relativistic thermodynamics is
derived in spherically symmetric general relativity. This equation expresses
the gradient of the active gravitational energy E according to the Einstein
equation, divided into energy-supply and work terms. Projecting the equation
along the flow of thermodynamic matter and along the trapping horizon of a
blackhole yield, respectively, first laws of relativistic thermodynamics and
black-hole dynamics. In the black-hole case, this first law has the same form
as the first law of black-hole statics, with static perturbations replaced by
the derivative along the horizon. There is the expected term involving the area
and surface gravity, where the dynamic surface gravity is defined as in the
static case but using the Kodama vector and trapping horizon. This surface
gravity vanishes for degenerate trapping horizons and satisfies certain
expected inequalities involving the area and energy. In the thermodynamic case,
the quasi-local first law has the same form, apart from a relativistic factor,
as the classical first law of thermodynamics, involving heat supply and
hydrodynamic work, but with E replacing the internal energy. Expanding E in the
Newtonian limit shows that it incorporates the Newtonian mass, kinetic energy,
gravitational potential energy and thermal energy. There is also a weak type of
unified zeroth law: a Gibbs-like definition of thermal equilibrium requires
constancy of an effective temperature, generalising the Tolman condition and
the particular case of Hawking radiation, while gravithermal equilibrium
further requires constancy of surface gravity. Finally, it is suggested that
the energy operator of spherically symmetric quantum gravity is determined by
the Kodama vector, which encodes a dynamic time related to E.Comment: 18 pages, TeX, expanded somewhat, to appear in Class. Quantum Gra
Late Miocene to early Pliocene biofacies of Wanganui and Taranaki Basins, New Zealand: Applications to paleoenvironmental and sequence stratigraphic analysis
The Matemateaonga Formation is late Miocene to early Pliocene (upper Tongaporutuan to lower Opoitian New Zealand Stages) in age. The formation comprises chiefly shellbeds, siliciclastic sandstone, and siltstone units and to a lesser extent non-marine and shallow marine conglomerate and rare paralic facies. The Matemateaonga Formation accumulated chiefly in shelf paleoenvironments during basement onlap and progradation of a late Miocene to early Pliocene continental margin wedge in the Wanganui and Taranaki Basins. The formation is strongly cyclothemic, being characterised by recurrent vertically stacked facies successions, bounded by sequence boundaries. These facies accumulated in a range of shoreface to mid-outer shelf paleoenvironments during conditions of successively oscillating sea level. This sequential repetition of facies and the biofacies they enclose are the result of sixth-order glacio-eustatic cyclicity. Macrofaunal associations have been identified from statistical analysis of macrofossil occurrences collected from multiple sequences. Each association is restricted to particular lithofacies and stratal positions and shows a consistent order and/or position within the sequences. This pattern of temporal paleoecologic change appears to be the result of lateral, facies-related shifting of broad biofacies belts, or habitat-tracking, in response to fluctuations of relative sea level, sediment flux, and other associated paleoenvironmental variables. The associations also show strong similarity in terms of their generic composition to biofacies identified in younger sedimentary strata and the modern marine benthic environment in New Zealand
Is the gravitational action additive?
The gravitational action is not always additive in the usual sense. We
provide a general prescription for the change in action that results when
different portions of the boundary of a spacetime are topologically identified.
We discuss possible implications for the superposition law of quantum gravity.
We present a definition of `generalized additivity' which does hold for
arbitrary spacetime composition.Comment: 20 pages LaTeX file, report numbers UMD-PP 94-100 and Alberta Thy
10-9
Classification of spacelike surfaces in spacetime
A classification of 2-dimensional surfaces imbedded in spacetime is
presented, according to the algebraic properties of their shape tensor. The
classification has five levels, and provides among other things a refinement of
the concepts of trapped, umbilical and extremal surfaces, which split into
several different classes. The classification raises new important questions
and opens many possible new lines of research. These, together with some
applications and examples, are briefly considered.Comment: 42 pages, 10 tables, many diagram
Production and decay of evolving horizons
We consider a simple physical model for an evolving horizon that is strongly
interacting with its environment, exchanging arbitrarily large quantities of
matter with its environment in the form of both infalling material and outgoing
Hawking radiation. We permit fluxes of both lightlike and timelike particles to
cross the horizon, and ask how the horizon grows and shrinks in response to
such flows. We place a premium on providing a clear and straightforward
exposition with simple formulae.
To be able to handle such a highly dynamical situation in a simple manner we
make one significant physical restriction, that of spherical symmetry, and two
technical mathematical restrictions: (1) We choose to slice the spacetime in
such a way that the space-time foliations (and hence the horizons) are always
spherically symmetric. (2) Furthermore we adopt Painleve-Gullstrand coordinates
(which are well suited to the problem because they are nonsingular at the
horizon) in order to simplify the relevant calculations.
We find particularly simple forms for surface gravity, and for the first and
second law of black hole thermodynamics, in this general evolving horizon
situation. Furthermore we relate our results to Hawking's apparent horizon,
Ashtekar et al's isolated and dynamical horizons, and Hayward's trapping
horizons. The evolving black hole model discussed here will be of interest,
both from an astrophysical viewpoint in terms of discussing growing black
holes, and from a purely theoretical viewpoint in discussing black hole
evaporation via Hawking radiation.Comment: 25 pages, uses iopart.cls V2: 5 references added; minor typos; V3:
some additional clarifications, additional references, additional appendix on
the Viadya spacetime. This version published in Classical and Quiantum
Gravit
Actions for signature change
This is a contribution on the controversy about junction conditions for
classical signature change. The central issue in this debate is whether the
extrinsic curvature on slices near the hypersurface of signature change has to
be continuous ({\it weak} signature change) or to vanish ({\it strong}
signature change). Led by a Lagrangian point of view, we write down eight
candidate action functionals ,\dots as possible generalizations of
general relativity and investigate to what extent each of these defines a
sensible variational problem, and which junction condition is implied. Four of
the actions involve an integration over the total manifold. A particular
subtlety arises from the precise definition of the Einstein-Hilbert Lagrangian
density . The other four actions are constructed as sums of
integrals over singe-signature domains. The result is that {\it both} types of
junction conditions occur in different models, i.e. are based on different
first principles, none of which can be claimed to represent the ''correct''
one, unless physical predictions are taken into account. From a point of view
of naturality dictated by the variational formalism, {\it weak} signature
change is slightly favoured over {\it strong} one, because it requires less
{\it \`a priori} restrictions for the class of off-shell metrics. In addition,
a proposal for the use of the Lagrangian framework in cosmology is made.Comment: 36 pages, LaTeX, no figures; some corrections have been made, several
Comments and further references are included and a note has been added
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