44 research outputs found
Critical Collapse of an Ultrarelativistic Fluid in the Limit
In this paper we investigate the critical collapse of an ultrarelativistic
perfect fluid with the equation of state in the limit of
. We calculate the limiting continuously self similar (CSS)
solution and the limiting scaling exponent by exploiting self-similarity of the
solution. We also solve the complete set of equations governing the
gravitational collapse numerically for and
compare them with the CSS solutions. We also investigate the supercritical
regime and discuss the hypothesis of naked singularity formation in a generic
gravitational collapse. The numerical calculations make use of advanced methods
such as high resolution shock capturing evolution scheme for the matter
evolution, adaptive mesh refinement, and quadruple precision arithmetic. The
treatment of vacuum is also non standard. We were able to tune the critical
parameter up to 30 significant digits and to calculate the scaling exponents
accurately. The numerical results agree very well with those calculated using
the CSS ansatz. The analysis of the collapse in the supercritical regime
supports the hypothesis of the existence of naked singularities formed during a
generic gravitational collapse.Comment: 23 pages, 16 figures, revised version, added new results of
investigation of a supercritical collapse and the existence of naked
singularities in generic gravitational collaps
Stationary strings and branes in the higher-dimensional Kerr-NUT-(A)dS spacetimes
We demonstrate complete integrability of the Nambu-Goto equations for a
stationary string in the general Kerr-NUT-(A)dS spacetime describing the
higher-dimensional rotating black hole. The stationary string in D dimensions
is generated by a 1-parameter family of Killing trajectories and the problem of
finding a string configuration reduces to a problem of finding a geodesic line
in an effective (D-1)-dimensional space. Resulting integrability of this
geodesic problem is connected with the existence of hidden symmetries which are
inherited from the black hole background. In a spacetime with p mutually
commuting Killing vectors it is possible to introduce a concept of a
-brane, that is a p-brane with the worldvolume generated by these fields
and a 1-dimensional curve. We discuss integrability of such -branes in the
Kerr-NUT-(A)dS spacetime.Comment: 8 pages, no figure
Scattering of a Long Cosmic String by a Rotating Black Hole
The scattering of a straight, infinitely long string by a rotating black hole
is considered. We assume that a string is moving with velocity v and that
initially the string is parallel to the axis of rotation of the black hole. We
demonstrate that as a result of scattering, the string is displaced in the
direction perpendicular to the velocity by an amount kappa(v,b), where b is the
impact parameter. The late-time solution is represented by a kink and
anti-kink, propagating in opposite directions at the speed of light, and
leaving behind them the string in a new ``phase''. We present the results of
the numerical study of the string scattering and their comparison with the
weak-field approximation, valid where the impact parameter is large, b/M >> 1,
and also with the scattering by a non-rotating black hole which was studied in
earlier works.Comment: 27 pages, 14 figures, to be published in Classical and Quantum
Gravit
Reissner-Nordstrom Black Holes and Thick Domain Walls
We solve numerically equations of motion for real self-interacting scalar
fields in the background of Reissner-Nordstrom black hole and obtained a
sequence of static axisymmetric solutions representing thick domain walls
charged black hole systems. In the case of extremal Reissner-Nordstrom black
hole solution we find that there is a parameter depending on the black hole
mass and the width of the domain wall which constitutes the upper limit for the
expulsion to occur.Comment: 18 pages, 10 figures, accepted for Phys. Rev.
Environmental movements in space-time: the Czech and Slovak republics from Stalinism to post-socialism
To demonstrate the role of space and time in social movements, the paper analyses the evolution and context of the environmental movement in the Czech and Slovak republics from 1948 to 1998. It shows that the movement's identity was formed under socialism and that political opportunity and resource availability changed markedly over time, as did its organisational and spatial structure. The movement played a significant part in the collapse of the socialist regime, but in the 1990s was marginalised in the interests of building a market economy and an independent Slovakia. Nevertheless a diverse and flexible range of groups existed by the late 1990s. The successive space-times allow analysis of the multiple and changing variables that influence the geography of social movements
Harnessing the potential of ligninolytic enzymes for lignocellulosic biomass pretreatment
Abundant lignocellulosic biomass from various industries provides a great potential feedstock for the production of value-added products such as biofuel, animal feed, and paper pulping. However, low yield of sugar obtained from lignocellulosic hydrolysate is usually due to the presence of lignin that acts as a protective barrier for cellulose and thus restricts the accessibility of the enzyme to work on the cellulosic component. This review focuses on the significance of biological pretreatment specifically using ligninolytic enzymes as an alternative method apart from the conventional physical and chemical pretreatment. Different modes of biological pretreatment are discussed in this paper which is based on (i) fungal pretreatment where fungi mycelia colonise and directly attack the substrate by releasing ligninolytic enzymes and (ii) enzymatic pretreatment using ligninolytic enzymes to counter the drawbacks of fungal pretreatment. This review also discusses the important factors of biological pretreatment using ligninolytic enzymes such as nature of the lignocellulosic biomass, pH, temperature, presence of mediator, oxygen, and surfactant during the biodelignification process