8,381 research outputs found
Angular momentum conservation for uniformly expanding flows
Angular momentum has recently been defined as a surface integral involving an
axial vector and a twist 1-form, which measures the twisting around of
space-time due to a rotating mass. The axial vector is chosen to be a
transverse, divergence-free, coordinate vector, which is compatible with any
initial choice of axis and integral curves. Then a conservation equation
expresses rate of change of angular momentum along a uniformly expanding flow
as a surface integral of angular momentum densities, with the same form as the
standard equation for an axial Killing vector, apart from the inclusion of an
effective energy tensor for gravitational radiation.Comment: 5 revtex4 pages, 3 eps figure
On the Definition of Averagely Trapped Surfaces
Previously suggested definitions of averagely trapped surfaces are not
well-defined properties of 2-surfaces, and can include surfaces in flat
space-time. A natural definition of averagely trapped surfaces is that the
product of the null expansions be positive on average. A surface is averagely
trapped in the latter sense if and only if its area and Hawking mass
satisfy the isoperimetric inequality , with similar inequalities
existing for other definitions of quasi-local energy.Comment: 4 page
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
Quasi-local first law of black-hole dynamics
A property well known as the first law of black hole is a relation among
infinitesimal variations of parameters of stationary black holes. We consider a
dynamical version of the first law, which may be called the first law of black
hole dynamics. The first law of black hole dynamics is derived without assuming
any symmetry or any asymptotic conditions. In the derivation, a definition of
dynamical surface gravity is proposed. In spherical symmetry it reduces to that
defined recently by one of the authors (SAH).Comment: Latex, 8 pages; version to appear in Class. Quantum Gra
Cosmic Evolution and Primordial Black Hole Evaporation
A cosmological model in which primordial black holes (PBHs) are present in
the cosmic fluid at some instant t=t_0 is investigated. The time t_0 is
naturally identified with the end of the inflationary period. The PBHs are
assumed to be nonrelativistic in the comoving fluid, to have the same mass, and
may be subject to evaporation for t>t_0. Our present work is related to an
earlier paper of Zimdahl and Pavon [Phys. Rev. D {\bf 58}, 103506 (1998)], but
in contradistinction to these authors we assume that the (negative) production
rate of the PBHs is zero. This assumption appears to us to be more simple and
more physical. Consequences of the formalism are worked out. In particular, the
four-divergence of the entropy four-vector in combination with the second law
in thermodynamics show in a clear way how the the case of PBH evaporation
corresponds to a production of entropy. Accretion of radiation onto the black
holes is neglected. We consider both a model where two different sub-fluids
interact, and a model involving one single fluid only. In the latter case an
effective bulk viscosity naturally appears in the formalism.Comment: 18 pages, LaTeX, no figures. Extended discussion of the black hole
evaporation process. Version to appear in Phys. Rev.
Upper bound for entropy in asymptotically de Sitter space-time
We investigate nature of asymptotically de Sitter space-times containing a
black hole. We show that if the matter fields satisfy the dominant energy
condition and the cosmic censorship holds in the considering space-time, the
area of the cosmological event horizon for an observer approaching a future
timelike infinity does not decrease, i.e. the second law is satisfied. We also
show under the same conditions that the total area of the black hole and the
cosmological event horizon, a quarter of which is the total Bekenstein-Hawking
entropy, is less than , where is a cosmological
constant. Physical implications are also discussed.Comment: 9 pages, REVTeX,2 figures; to be published in Phys.Rev.
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
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
Gravitational radiation from dynamical black holes
An effective energy tensor for gravitational radiation is identified for
uniformly expanding flows of the Hawking mass-energy. It appears in an energy
conservation law expressing the change in mass due to the energy densities of
matter and gravitational radiation, with respect to a Killing-like vector
encoding a preferred flow of time outside a black hole. In a spin-coefficient
formulation, the components of the effective energy tensor can be understood as
the energy densities of ingoing and outgoing, transverse and longitudinal
gravitational radiation. By anchoring the flow to the trapping horizon of a
black hole in a given sequence of spatial hypersurfaces, there is a locally
unique flow and a measure of gravitational radiation in the strong-field
regime.Comment: 5 revtex4 pages. Additional comment
Complex lapse, complex action and path integrals
Imaginary time is often used in quantum tunnelling calculations. This article
advocates a conceptually sounder alternative: complex lapse. In the ``3+1''
action for the Einstein gravitational field minimally coupled to a Klein-Gordon
field, allowing the lapse function to be complex yields a complex action which
generates both the usual Lorentzian theory and its Riemannian analogue, and in
particular allows a change of signature between the two. The action and
variational equations are manifestly well defined in the Hamiltonian
representation, with the momentum fields consequently being complex. The
complex action interpolates between the Lorentzian and Riemannian actions as
they appear formally in the respective path integrals. Thus the complex-lapse
theory provides a unified basis for a path-integral quantum theory of gravity
involving both Lorentzian and Riemannian aspects. A major motivation is the
quantum-tunnelling scenario for the origin of the universe. Taken as an
explanation for the observed quantum tunnelling of particles, the complex-lapse
theory determines that the argument of the lapse for the universe now is
extremely small but negative.Comment: 12 pages, Te
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