893 research outputs found
3+1 Approach to the Long Wavelength Iteration Scheme
Large-scale inhomogeneities and anisotropies are modeled using the Long
Wavelength Iteration Scheme. In this scheme solutions are obtained as
expansions in spatial gradients, which are taken to be small. It is shown that
the choice of foliation for spacetime can make the iteration scheme more
effective in two respects: (i) the shift vector can be chosen so as to dilute
the effect of anisotropy on the late-time value of the extrinsic curvature of
the spacelike hypersurfaces of the foliation; and (ii) pure gauge solutions
present in a similar calculation using the synchronous gauge vanish when the
spacelike hypersurfaces have extrinsic curvature with constant trace. We
furthermore verify the main conclusion of the synchronous gauge calculation
which is large-scale inhomogeneity decays if the matter--considered to be that
of a perfect-fluid with a barotropic equation of state--violates the
strong-energy condition. Finally, we obtain the solution for the lapse function
and discuss its late-time behaviour. It is found that the lapse function is
well-behaved when the matter violates the strong energy condition.Comment: 21 pages, TeX file, already publishe
Collapse of a Circular Loop of Cosmic String
We study the collapse of a circular loop of cosmic string. The gravitational
field of the string is treated using the weak field approximation. The
gravitational radiation from the loop is evaluated numerically. The memtric of
the loop near the point of collapse is found analytically.Comment: 15 page
Relativistic Two-stream Instability
We study the (local) propagation of plane waves in a relativistic,
non-dissipative, two-fluid system, allowing for a relative velocity in the
"background" configuration. The main aim is to analyze relativistic two-stream
instability. This instability requires a relative flow -- either across an
interface or when two or more fluids interpenetrate -- and can be triggered,
for example, when one-dimensional plane-waves appear to be left-moving with
respect to one fluid, but right-moving with respect to another. The dispersion
relation of the two-fluid system is studied for different two-fluid equations
of state: (i) the "free" (where there is no direct coupling between the fluid
densities), (ii) coupled, and (iii) entrained (where the fluid momenta are
linear combinations of the velocities) cases are considered in a
frame-independent fashion (eg. no restriction to the rest-frame of either
fluid). As a by-product of our analysis we determine the necessary conditions
for a two-fluid system to be causal and absolutely stable and establish a new
constraint on the entrainment.Comment: 15 pages, 2 eps-figure
Generation of scalar-tensor gravity effects in equilibrium state boson stars
Boson stars in zero-, one-, and two-node equilibrium states are modeled
numerically within the framework of Scalar-Tensor Gravity. The complex scalar
field is taken to be both massive and self-interacting. Configurations are
formed in the case of a linear gravitational scalar coupling (the Brans-Dicke
case) and a quadratic coupling which has been used previously in a cosmological
context. The coupling parameters and asymptotic value for the gravitational
scalar field are chosen so that the known observational constraints on
Scalar-Tensor Gravity are satisfied. It is found that the constraints are so
restrictive that the field equations of General Relativity and Scalar-Tensor
gravity yield virtually identical solutions. We then use catastrophe theory to
determine the dynamically stable configurations. It is found that the maximum
mass allowed for a stable state in Scalar-Tensor gravity in the present
cosmological era is essentially unchanged from that of General Relativity. We
also construct boson star configurations appropriate to earlier cosmological
eras and find that the maximum mass for stable states is smaller than that
predicted by General Relativity, and the more so for earlier eras. However, our
results also show that if the cosmological era is early enough then only states
with positive binding energy can be constructed.Comment: 20 pages, RevTeX, 11 figures, to appear in Class. Quantum Grav.,
comments added, refs update
Scaling of curvature in sub-critical gravitational collapse
We perform numerical simulations of the gravitational collapse of a
spherically symmetric scalar field. For those data that just barely do not form
black holes we find the maximum curvature at the position of the central
observer. We find a scaling relation between this maximum curvature and
distance from the critical solution. The scaling relation is analogous to that
found by Choptuik for black hole mass for those data that do collapse to form
black holes. We also find a periodic wiggle in the scaling exponent.Comment: Revtex, 2 figures, Discussion modified, to appear in Phys. Rev.
Spontaneous Scalarization and Boson Stars
We study spontaneous scalarization in Scalar-Tensor boson stars. We find that
scalarization does not occur in stars whose bosons have no self-interaction. We
introduce a quartic self-interaction term into the boson Lagrangian and show
that when this term is large, scalarization does occur. Strong self-interaction
leads to a large value of the compactness (or sensitivity) of the boson star, a
necessary condition for scalarization to occur, and we derive an analytical
expression for computing the sensitivity of a boson star in Brans-Dicke theory
from its mass and particle number. Next we comment on how one can use the
sensitivity of a star in any Scalar-Tensor theory to determine how its mass
changes when it undergoes gravitational evolution. Finally, in the Appendix, we
derive the most general form of the boson wavefunction that minimises the
energy of the star when the bosons carry a U(1) charge.Comment: 23 pages, 5 postscript figures. Typing errors corrected. Includes
some new text that relates the paper to several previous results. Accepted
for publication in PR
Black holes and a scalar field in an expanding universe
We consider a model of an inhomogeneous universe including a massless scalar
field, where the inhomogeneity is assumed to consist of many black holes. This
model can be constructed by following Lindquist and Wheeler, which has already
been investigated without including scalar field to show that an averaged scale
factor coincides with that of the Friedmann model. In this work we construct
the inhomogeneous universe with an massless scalar field, where we assume that
the averaged scale factor and scalar field are given by those of the Friedmann
model including a scalar field. All of our calculations are carried out in the
framework of Brans-Dicke gravity. In constructing the model of an inhomogeneous
universe, we define the mass of a black hole in the Brans-Dicke expanding
universe which is equivalent to ADM mass if the mass evolves adiabatically, and
obtain an equation relating our mass to the averaged scalar field and scale
factor. As the results we find that the mass has an adiabatic time dependence
in a sufficiently late stage of the expansion of the universe, and that the
time dependence is qualitatively diffenrent according to the sign of the
curvature of the universe: the mass increases decelerating in the closed
universe case, is constant in the flat case and decreases decelerating in the
open case. It is also noted that the mass in the Einstein frame depends on
time. Our results that the mass has a time dependence should be retained even
in the general scalar-tensor gravitiy with a scalar field potential.
Furthermore, we discuss the relation of our results to the uniqueness theorem
of black hole spacetime and gravitational memory effect.Comment: 16 pages, 3 tables, 5 figure
Brans-Dicke Boson Stars: Configurations and Stability through Cosmic History
We make a detailed study of boson star configurations in Jordan--Brans--Dicke
theory, studying both equilibrium properties and stability, and considering
boson stars existing at different cosmic epochs. We show that boson stars can
be stable at any time of cosmic history and that equilibrium stars are denser
in the past. We analyze three different proposed mass functions for boson star
systems, and obtain results independently of the definition adopted. We study
how the configurations depend on the value of the Jordan--Brans--Dicke coupling
constant, and the properties of the stars under extreme values of the
gravitational asymptotic constant. This last point allows us to extract
conclusions about the stability behaviour concerning the scalar field. Finally,
other dynamical variables of interest, like the radius, are also calculated. In
this regard, it is shown that the radius corresponding to the maximal boson
star mass remains roughly the same during cosmological evolution.Comment: 9 pages RevTeX file with nine figures incorporated (uses RevTeX and
epsf
The New Albany Shale gas play in southern Indiana
This poster was presented at the 2006 Eastern Section American Association of Petroleum Geologists, 35th Annual Meeting, in Buffalo, N.Y., October 8-11, 2006.The New Albany Shale (Devonian and Mississippian) in Indiana is mostly brownish-black organic-rich shale with lesser greenish-gray shale. The formation is 100 to 140 feet thick in southeastern Indiana and dips and thickens to the southwest into the Illinois Basin, where it attains a thickness of more than 360 feet in Posey County.
Gas production from New Albany Shale began in 1885 and drilling activity continued into the 1930s, when interest waned in favor of more lucrative opportunities elsewhere. Renewed activity, driven by higher gas prices, has been brisk since the mid-1990s, witnessed by the completion of more than 400 productive wells. The majority of these wells were drilled in Harrison County, where production typically occurs at depths from 500 to 1,100 feet and production rates generally range from 20 to 450 MCFGPD. In the past 2 years, Daviess County and surrounding areas have become the focus of New Albany exploration after the El Paso Production No. 2-10 Peterson horizontal discovery well was rumored to have tested 1.3 MMCFGPD at an approximate measured depth of 2,200 feet.
New Albany production is mostly from the organic-rich Clegg Creek Member. Gas compositions (C1-C4 and CO2) and carbon and hydrogen isotopic signatures indicate that both purely thermogenic and mixed thermogenic and biogenic gases are produced from the New Albany. Produced water ranges from brine to water diluted through recharge by modern precipitation; the brine zones contain primarily thermogenic gas and the diluted water zones contain gas of mixed thermogenic and biogenic origin
Anisotropic stresses in inhomogeneous universes
Anisotropic stress contributions to the gravitational field can arise from
magnetic fields, collisionless relativistic particles, hydrodynamic shear
viscosity, gravitational waves, skew axion fields in low-energy string
cosmologies, or topological defects. We investigate the effects of such
stresses on cosmological evolution, and in particular on the dissipation of
shear anisotropy. We generalize some previous results that were given for
homogeneous anisotropic universes, by including small inhomogeneity in the
universe. This generalization is facilitated by a covariant approach. We find
that anisotropic stress dominates the evolution of shear, slowing its decay.
The effect is strongest in radiation-dominated universes, where there is slow
logarithmic decay of shear.Comment: 7 pages Revte
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