8,025 research outputs found
Open-Closed Duality at Tree Level
We study decay of unstable D-branes in string theory in the presence of
electric field, and show that the classical open string theory results for
various properties of the final state agree with the properties of closed
string states into which the system is expected to decay. This suggests a
duality between tree level open string theory on unstable D-branes and closed
strings at high density.Comment: LaTeX file, 10 page
Vanishing Cosmological Constant in Modified Gauss-Bonnet Gravity with Conformal Anomaly
We consider dark energy cosmology in a de Sitter universe filled with quantum
conformal matter. Our model represents a Gauss-Bonnet model of gravity with
contributions from quantum effects. To the General Relativity action an
arbitrary function of the GB invariant, f(G), is added, and taking into account
quantum effects from matter the cosmological constant is studied. For the
considered model the conditions for a vanishing cosmological constant are
considered. Creation of a de Sitter universe by quantum effects in a GB
modified gravity is discussed.Comment: 8 pages latex, 1 figure. To appear in Int. J. Mod. Phys.
Brane-Antibrane Systems at Finite Temperature and Phase Transition near the Hagedorn Temperature
In order to study the thermodynamic properties of brane-antibrane systems, we
compute the finite temperature effective potential of tachyon T in this system
on the basis of boundary string field theory. At low temperature, the minimum
of the potential shifts towards T=0 as the temperature increases. In the
D9-antiD9 case, the sign of the coefficient of |T|^2 term of the potential
changes slightly below the Hagedorn temperature. This means that a phase
transition occurs near the Hagedorn temperature. On the other hand, the
coefficient is kept negative in the Dp-antiDp case with p <= 8, and thus a
phase transition does not occur. This leads us to the conclusion that only a
D9-antiD9 pair and no other (lower dimensional) brane-antibrane pairs are
created near the Hagedorn temperature. We also discuss a phase transition in
NS9B-antiNS9B case as a model of the Hagedorn transition of closed strings.Comment: 28 pages, 3 figures, minor errors correcte
Decay of Unstable D-branes with Electric Field
Using the techniques of two dimensional conformal field theory we construct
time dependent classical solutions in open string theory describing the decay
of an unstable D-brane in the presence of background electric field, and
explicitly evaluate the time dependence of the energy momentum tensor and the
fundamental string charge density associated with this solution. The final
decay product can be interpreted as a combination of stretched fundamental
strings and tachyon matter.Comment: 35 pages, LaTe
Security and Privacy Issues in Wireless Mesh Networks: A Survey
This book chapter identifies various security threats in wireless mesh
network (WMN). Keeping in mind the critical requirement of security and user
privacy in WMNs, this chapter provides a comprehensive overview of various
possible attacks on different layers of the communication protocol stack for
WMNs and their corresponding defense mechanisms. First, it identifies the
security vulnerabilities in the physical, link, network, transport, application
layers. Furthermore, various possible attacks on the key management protocols,
user authentication and access control protocols, and user privacy preservation
protocols are presented. After enumerating various possible attacks, the
chapter provides a detailed discussion on various existing security mechanisms
and protocols to defend against and wherever possible prevent the possible
attacks. Comparative analyses are also presented on the security schemes with
regards to the cryptographic schemes used, key management strategies deployed,
use of any trusted third party, computation and communication overhead involved
etc. The chapter then presents a brief discussion on various trust management
approaches for WMNs since trust and reputation-based schemes are increasingly
becoming popular for enforcing security in wireless networks. A number of open
problems in security and privacy issues for WMNs are subsequently discussed
before the chapter is finally concluded.Comment: 62 pages, 12 figures, 6 tables. This chapter is an extension of the
author's previous submission in arXiv submission: arXiv:1102.1226. There are
some text overlaps with the previous submissio
Exact Analysis of Scaling and Dominant Attractors Beyond the Exponential Potential
By considering the potential parameter as a function of another
potential parameter [47], We successfully extend the analysis of
two-dimensional autonomous dynamical system of quintessence scalar field model
to the analysis of three-dimension, which makes us be able to research the
critical points of a large number of potentials beyond the exponential
potential exactly. We find that there are ten critical points in all, three
points } are general points which are possessed by all
quintessence models regardless of the form of potentials and the rest points
are closely connected to the concrete potentials. It is quite surprising that,
apart from the exponential potential, there are a large number of potentials
which can give the scaling solution when the function
equals zero for one or some values of
and if the parameter also satisfies the condition
Eq.(16) or Eq.(17) at the same time. We give the differential equations to
derive these potentials from . We also find that, if some
conditions are satisfied, the de-Sitter-like dominant point and the
scaling solution point (or ) can be stable simultaneously but
and can not be stable simultaneity. Although we survey scaling
solutions beyond the exponential potential for ordinary quintessence models in
standard general relativity, this method can be applied to other extensively
scaling solution models studied in literature[46] including coupled
quintessence, (coupled-)phantom scalar field, k-essence and even beyond the
general relativity case . we also discuss the disadvantage
of our approach.Comment: 16 pages,no figure, this new revision has taken the suggestions from
CQG referees and has been accepted for publication in Classical and Quantum
Gravit
Cosmological Scaling Solutions of Multiple Tachyon Fields with Inverse Square Potentials
We investigate cosmological dynamics of multiple tachyon fields with inverse
square potentials. A phase-space analysis of the spatially flat FRW models
shows that there exists power-law cosmological scaling solutions. We study the
stability of the solutions and find that the potential-kinetic-scaling solution
is a global attractor. However, in the presence of a barotropic fluid the
solution is an attractor only in one region of the parameter space and the
tracking solution is an attractor in the other region. We briefly discuss the
physical consequences of these results.Comment: 10 pages, 1 figure, LaTeX2
Non-minimally Coupled Tachyonic Inflation in Warped String Background
We show that the non-minimal coupling of tachyon field to the scalar
curvature, as proposed by Piao et al, with the chosen coupling parameter does
not produce the effective potential where the tachyon field can roll down from
T=0 to large along the slope of the potential. We find a correct choice of
the parameters which ensures this requirement and support slow-roll inflation.
However, we find that the cosmological parameter found from the analysis of the
theory are not in the range obtained from observations. We then invoke warped
compactification and varying dilaton field over the compact manifold, as
proposed by Raeymaekers, to show that in such a setup the observed parameter
space can be ensured.Comment: minor typos corrected and references adde
Localization of the SFT inspired Nonlocal Linear Models and Exact Solutions
A general class of gravitational models driven by a nonlocal scalar field
with a linear or quadratic potential is considered. We study the action with an
arbitrary analytic function , which has both simple and double roots.
The way of localization of nonlocal Einstein equations is generalized on models
with linear potentials. Exact solutions in the Friedmann-Robertson-Walker and
Bianchi I metrics are presented.Comment: 20 pages, 3 figures, published in the proceedings of the VIII
International Workshop "Supersymmetries and Quantum Symmetries" (SQS'09),
Dubna, Russia, July 29 - August 3, 2009, http://theor.jinr.ru/~sqs09
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