430 research outputs found
Regular black holes and black universes
We give a comparative description of different types of regular static,
spherically symmetric black holes (BHs) and discuss in more detail their
particular type, which we suggest to call black universes. The latter have a
Schwarzschild-like causal structure, but inside the horizon there is an
expanding Kantowski-Sachs universe and a de Sitter infinity instead of a
singularity. Thus a hypothetic BH explorer gets a chance to survive. Solutions
of this kind are naturally obtained if one considers static, spherically
symmetric distributions of various (but not all) kinds of phantom matter whose
existence is favoured by cosmological observations. It also looks possible that
our Universe has originated from phantom-dominated collapse in another universe
and underwent isotropization after crossing the horizon. An explicit example of
a black-universe solution with positive Schwarzschild mass is discussed.Comment: 13 pages, 1 figure. 6 referenses and some discussion added, misprints
correcte
Collapse of non-spherically symmetric scalar field distributions
In the present work the collapse scenario of some exact non-spherical models
with a minimally coupled scalar field is studied. Scalar field collapse with
planar as well as toroidal, cylindrical and pseudoplanar symmetries have been
investigated. It is shown that the scalar field may have collapsing modes even
if it has the equation of state corresponding to that of a dark energy.Comment: 13 pages, 9 figures; Accepted for publication in Gen Relativ Gravit
(2011
The universe formation by a space reduction cascade with random initial parameters
In this paper we discuss the creation of our universe using the idea of extra
dimensions. The initial, multidimensional Lagrangian contains only metric
tensor. We have found many sets of the numerical values of the Lagrangian
parameters corresponding to the observed low-energy physics of our universe.
Different initial parameters can lead to the same values of fundamental
constants by the appropriate choice of a dimensional reduction cascade. This
result diminishes the significance of the search for the 'unique' initial
Lagrangian. We also have obtained a large number of low-energy vacua, which is
known as a 'landscape' in the string theory.Comment: 17 pages, 1 figur
Gravitational Geons on the Brane
In this paper, we examine the possibility of static, spherically symmetric
gravitational geons on a 3 dimensional brane embedded in a 4+1 dimensional
space-time. We choose a specific g_tt for the brane-world space-time metric. We
then calculate g_rr analytically in the weak field limit and numerically for
stronger fields. We show that the induced field equations on the brane do admit
gravitational geon solutions.Comment: 14 pages with 9 figures. To appear in General Relativity and
Gravitatio
Quantum corrections to the entropy of charged rotating black holes
Hawking radiation from a black hole can be viewed as quantum tunneling of
particles through the event horizon. Using this approach we provide a general
framework for studying corrections to the entropy of black holes beyond
semiclassical approximations. Applying the properties of exact differentials
for three variables to the first law thermodynamics, we study charged rotating
black holes and explicitly work out the corrections to entropy and horizon area
for the Kerr-Newman and charged rotating BTZ black holes. It is shown that the
results for other geometries like the Schwarzschild, Reissner-Nordstr\"{o}m and
anti-de Sitter Schwarzschild spacetimes follow easily
Wormhole Geometries In Gravity
We study wormhole solutions in the framework of f (R,T) gravity where R is
the scalar curvature, and T is the trace of the stress-energy tensor of the
matter. We have obtained the shape function of the wormhole by specifying an
equation of state for the matter field and imposing the flaring out condition
at the throat. We show that in this modified gravity scenario, the matter
threading the wormhole may satisfy the energy conditions, so it is the
effective stress-energy that is responsible for violation of the null energy
condition.Comment: 9 pages, 4 figures, published version, references adde
Gravitational Coupling and Dynamical Reduction of The Cosmological Constant
We introduce a dynamical model to reduce a large cosmological constant to a
sufficiently small value. The basic ingredient in this model is a distinction
which has been made between the two unit systems used in cosmology and particle
physics. We have used a conformal invariant gravitational model to define a
particular conformal frame in terms of large scale properties of the universe.
It is then argued that the contributions of mass scales in particle physics to
the vacuum energy density should be considered in a different conformal frame.
In this manner, a decaying mechanism is presented in which the conformal factor
appears as a dynamical field and plays a key role to relax a large effective
cosmological constant. Moreover, we argue that this model also provides a
possible explanation for the coincidence problem.Comment: To appear in GR
Cosmological Consequences of String-forming Open Inflation Models
We present a study of open inflation cosmological scenarios in which cosmic
strings form betwen the two inflationary epochs. It is shown that in these
models strings are stretched outside the horizon due to the inflationary
expansion but must necessarily re-enter the horizon before the epoch of equal
matter and radiation densities. We determine the power spectrum of cold dark
matter perturbations in these hybrid models, finding good agreement with
observations for values of and comparable
contributions from the active and passive sources to the CMB. Finally, we
briefly discuss other cosmological consequences of these models.Comment: 11 LaTeX pages with 3 eps figure
Scalar brane backgrounds in higher order curvature gravity
We investigate maximally symmetric brane world solutions with a scalar field.
Five-dimensional bulk gravity is described by a general lagrangian which yields
field equations containing no higher than second order derivatives. This
includes the Gauss-Bonnet combination for the graviton. Stability and
gravitational properties of such solutions are considered, and we particularily
emphasise the modifications induced by the higher order terms. In particular it
is shown that higher curvature corrections to Einstein theory can give rise to
instabilities in brane world solutions. A method for analytically obtaining the
general solution for such actions is outlined. Genericaly, the requirement of a
finite volume element together with the absence of a naked singularity in the
bulk imposes fine-tuning of the brane tension. A model with a moduli scalar
field is analysed in detail and we address questions of instability and
non-singular self-tuning solutions. In particular, we discuss a case with a
normalisable zero mode but infinite volume element.Comment: published versio
XANES evidence for sulphur speciation in Mn-, Ni- and W-bearing silicate melts
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 73 (2009): 6847-6867, doi:10.1016/j.gca.2009.08.013.S K edge XANES and Mn, W and Ni XANES and EXAFS spectra of silicate glasses synthesised at 1400° C and 1 bar with compositions in the CaO-MgO-Al2O3-SiO2-S plus MnO,
NiO, or WO3 systems were used to investigate sulphur speciation in silicate glasses.
S K-edge spectra comprised a composite peak with an edge between 2470 and 2471.4 eV,
which was attributed to S2-, and a peak of variable height with an edge at 2480.2 to 2480.8
eV, which is consistent with the presence of S6+. The latter peak was attributed to sample
oxidation during sample storage. W-rich samples produced an additional lower energy peak
at 2469.8 eV that is tentatively attributed to the existence of S 3p orbitals hybridised with
the W 5d states.
Deconvolution of the composite peak reveals that the composite peak for Mn-bearing
samples fits well to a model that combines three Lorentzians at 2473.1, 2474.9 and 2476.2
eV with an arctan edge step. The composite peak for W-bearing samples fits well to the
same combination plus an additional Lorentzian at 2469.8 eV. The ratio of the proportions
of the signal accounted for by peaks at 2473.1eV and 2476.2eV correlates with Mn:Ca molar
ratios, but not with W:Ca ratios. Spectra from Ni-bearing samples were qualitatively similar
but S levels were too low to allow robust quantification of peak components. Some part of
the signal accounted for by the 2473.1 eV peak was therefore taken to record the formation
of Mn-S melt species, while the 2469.8 peak is interpreted to record the formation of W-S
melt species. The 2474.9 eV and 2476.2 eV peaks were taken to be dominated by Ca-S
and Mg-S interactions. However, a 1:1 relationship between peak components and specific
energy transitions is not proposed. This interpretation is consistent with known features of
the lower parts of the conduction band in monosulphide minerals and indicates a similarity
between sulphur species in the melts and the monosulphides. S XANES spectra cannot be
reproduced by a combination of the spectra of the component element monosulphides.
Mn-, W- and Ni- XANES and EXAFS for synthetic glasses without sulphide exsolution
did not show any sensitivity to the presence of sulphur, which is unsurprising as S:O ratios
were sufficiently low that metals would be mostly co-ordinated by O. Mn EXAFS spectra
were consistent with divalent Mn in 5 co-ordinated Mn-O melt species. W spectra were
consistent with tetrahedrally co-ordinated hexavalent W, most likely in scheelite-like melt
species, and Ni spectra were consistent with [4] co-ordinated divalent Ni. These results
indicate lower coordinations for bothWand Ni than those inferred by some previous workers.
Cation coordination may reflect the proportion of non-bridging oxygens, which is lower in
the Ca-rich and Al-poor samples investigated here than for previous studies.This work was performed with
814 support from the Australian Synchrotron Research Program (ASRP), which is funded by the
815 Commonwealth of Australia under the Major National Research Facilities Program
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