22,103 research outputs found
Centre Commissioned External Review (CCER) of the IWMI-TATA Water Policy Research Program
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On the Role of Initial Data in the Gravitational Collapse of Inhomogeneous Dust
We consider here the gravitational collapse of a spherically symmetric
inhomogeneous dust cloud described by the Tolman-Bondi models. By studying a
general class of these models, we find that the end state of the collapse is
either a black hole or a naked singularity, depending on the parameters of the
initial density distribution, which are , the initial central density
of the massive body, and , the initial boundary. The collapse ends in a
black hole if the dimensionless quantity constructed out of this
initial data is greater than 0.0113, and it ends in a naked singularity if
is less than this number. A simple interpretation of this result can be
given in terms of the strength of the gravitational potential at the starting
epoch of the collapse.Comment: Original title changed, numerical range of naked singularity
corrected. Plain Tex File. 14 pages. To appear in Physical Review
Quantum evaporation of a naked singularity
We investigate here quantum effects in gravitational collapse of a scalar
field model which classically leads to a naked singularity. We show that
non-perturbative semi-classical modifications near the singularity, based on
loop quantum gravity, give rise to a strong outward flux of energy. This leads
to the dissolution of the collapsing cloud before the singularity can form.
Quantum gravitational effects thus censor naked singularities by avoiding their
formation. Further, quantum gravity induced mass flux has a distinct feature
which may lead to a novel observable signature in astrophysical bursts.Comment: 4 pages, 2 figures. Minor changes to match published version in
Physical Review Letter
Gravitational collapse of an isentropic perfect fluid with a linear equation of state
We investigate here the gravitational collapse end states for a spherically
symmetric perfect fluid with an equation of state . It is shown that
given a regular initial data in terms of the density and pressure profiles at
the initial epoch from which the collapse develops, the black hole or naked
singularity outcomes depend on the choice of rest of the free functions
available, such as the velocities of the collapsing shells, and the dynamical
evolutions as allowed by Einstein equations. This clarifies the role that
equation of state and initial data play towards determining the final fate of
gravitational collapse.Comment: 7 Pages, Revtex4, To appear in Classical and Quantum Gravit
The Linearly Independent Non Orthogonal yet Energy Preserving (LINOEP) vectors
It is well known that, in any inner product space, a set of linearly
independent (LI) vectors can be transformed to a set of orthogonal vectors,
spanning the same space, by the Gram-Schmidt Orthogonalization Method (GSOM).
In this paper, we propose a transformation from a set of LI vectors to a set of
LI non orthogonal yet energy (square of the norm) preserving (LINOEP) vectors
in an inner product space and we refer it as LINOEP method. We also show that
there are various solutions to preserve the square of the norm.Comment: 6 pages, 2 figure
The Final Fate of Spherical Inhomogeneous Dust Collapse
We examine the role of the initial density and velocity distribution in the
gravitational collapse of a spherical inhomogeneous dust cloud. Such a collapse
is described by the Tolman-Bondi metric which has two free functions: the
`mass-function' and the `energy function', which are determined by the initial
density and velocity profile of the cloud. The collapse can end in a black-hole
or a naked singularity, depending on the initial parameters characterizing
these profiles. In the marginally bound case, we find that the collapse ends in
a naked singularity if the leading non-vanishing derivative of the density at
the center is either the first one or the second one. If the first two
derivatives are zero, and the third derivative non-zero, the singularity could
either be naked or covered, depending on a quantity determined by the third
derivative and the central density. If the first three derivatives are zero,
the collapse ends in a black hole. In particular, the classic result of
Oppenheimer and Snyder, that homogeneous dust collapse leads to a black hole,
is recovered as a special case. Analogous results are found when the cloud is
not marginally bound, and also for the case of a cloud starting from rest. We
also show how the strength of the naked singularity depends on the density and
velocity distribution. Our analysis generalizes and simplifies the earlier work
of Christodoulou and Newman [4,5] by dropping the assumption of evenness of
density functions. It turns out that relaxing this assumption allows for a
smooth transition from the naked singularity phase to the black-hole phase, and
also allows for the occurrence of strong curvature naked singularities.Comment: 23 pages; Plain Tex; TIFR-TAP preprin
Mappings preserving locations of movable poles: a new extension of the truncation method to ordinary differential equations
The truncation method is a collective name for techniques that arise from
truncating a Laurent series expansion (with leading term) of generic solutions
of nonlinear partial differential equations (PDEs). Despite its utility in
finding Backlund transformations and other remarkable properties of integrable
PDEs, it has not been generally extended to ordinary differential equations
(ODEs). Here we give a new general method that provides such an extension and
show how to apply it to the classical nonlinear ODEs called the Painleve
equations. Our main new idea is to consider mappings that preserve the
locations of a natural subset of the movable poles admitted by the equation. In
this way we are able to recover all known fundamental Backlund transformations
for the equations considered. We are also able to derive Backlund
transformations onto other ODEs in the Painleve classification.Comment: To appear in Nonlinearity (22 pages
Views and attitudes towards blood donation: a qualitative investigation of Indian non-donors living in England
OBJECTIVE:
To explore the views and attitudes of Indians living in England on blood donation.
BACKGROUND:
In light of the predicted shortages in blood supply, it is vital to consider ways in which to maximise donation rates. These include addressing the issue of lower donation rates among ethnic minorities, including Indians. However research specifically among minority ethnicities in UK is sparse.
SETTING:
General practice in North London.
PARTICIPANTS: A convenience sample of 12 non-donor Indians living in England.
METHODS: This is a qualitative investigation involving semistructured interviews. Themes derived were analysed using thematic framework analysis.
RESULTS:
Five key themes emerged from the data, and these concerned participantsâ perspectives regarding attitudes towards blood, blood donation as a âgood thingâ, donation disincentives, the recipient matters and the donor matters.
CONCLUSION:
A variety of attitudes were presented, but were generally positive, and blood was conceptualised in a manner previously found to be consistent with donation. However, lack of awareness and accessibility were prominent barriers, indicating the need for improvement in these capacities. In contrast to this, blood was also greatly associated with family and acted as a symbol of kinship: this âemotional chargeâ often acted to dissuade participants from separating with their blood through donation. Possibly due to this, there was also a strong preference for donated blood to be distributed within the family, as opposed to strangers. This presents a potential barrier to blood donation for some Indians within the current system in which donations are given to unknown recipients
Preparation, characterization, and electrical properties of epitaxial NbO2 thin film lateral devices
Epitaxial NbO2 (110) films, 20 nm thick, were grown by pulsed laser
deposition on Al2O3 (0001) substrates. The Ar/O2 total pressure during growth
was varied to demonstrate the gradual transformation between NbO2 and Nb2O5
phases, which was verified using x-ray diffraction, x-ray photoelectron
spectroscopy, and optical absorption measurements. Electric resistance
threshold switching characteristics were studied in a lateral geometry using
interdigitated Pt top electrodes in order to preserve the epitaxial crystalline
quality of the films. Volatile and reversible transitions between high and low
resistance states were observed in epitaxial NbO2 films, while irreversible
transitions were found in case of Nb2O5 phase. Electric field pulsed current
measurements confirmed thermally-induced threshold switching.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in Journal of Physics D: Applied Physics. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The Version of Record is
available online at http://dx.doi.org/10.1088/0022-3727/48/33/33530
Study of spur dikes
In order to study the variables governing the depth of scour around obstructions such as spur dikes, tests were carried out. The salient features of these tests are summarized herein
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