4,230 research outputs found
Possible Contributions to the Bulk Casimir Energy in Heterotic M-theory
Some possible ways for the study of the contributions of some background
fields to the bulk Casimir energy have been probed in the framework of the 5D
heterotic M-theory
On the Masses of the Universal hypermultiplets in heterotic M-theory
The reduced 5D Heterotic M-theory has a deeply rich structure. For every
Calabi-yau compactification, there exists a gravitational hypermultiplet
and a universal hypermultiplet. In this paper
we derive the formulae for the masses of the scalar sector of the universal
hypermultiplet in the framework of 5D Heterotic
M-theory.Comment: AIP Conf. Proc. 201
Ricci-Gauss-Bonnet holographic dark energy in Chern-Simons modified gravity: A flat FLRW quintessence-dominated universe
We discuss the recently suggested Ricci-Gauss-Bonnet holographic dark energy
in Chern-Simons modified gravity. We have tested some general forms of the
scale factor , and used two physically reasonable forms which have been
proved to be consistent with observations. Both solutions predict a sign
flipping in the evolution of cosmic pressure which is positive during the
early-time deceleration and negative during the late-time acceleration. This
sign flipping in the evolution of cosmic pressure helps in explaining the
cosmic deceleration-acceleration transition, and it has appeared in other
cosmological models in different contexts. However, the current work shows a
pressure singularity which needs to be explained. The evolution of the equation
of state parameter shows the same asymptotic behavior for both
solutions indicating a quintessence-dominated universe in the far future. We
also note that goes to negative values (leaving the decelerating
dust-dominated era at ) at exactly the same time the pressure becomes
negative. Again, there is another singularity in the behavior of
which happens at the same cosmic time of the pressure singularity.Comment: 13 pages, 9 figure
Crossing the phantom divide line in universal extra dimensions
We investigate the cosmic acceleration and the evolution of dark energy
across the cosmological constant boundary in universal extra dimensions UED. We
adopt an empirical approach to solve the higher-dimensional cosmological
equations so that the deceleration parameter is consistent with
observations. The expressions for the jerk and deceleration parameters are
independent of the number of dimensions . The behavior of pressure in D
shows a positive-to-negative transition corresponding to the
deceleration-to-acceleration cosmic transition. This pressure behavior helps in
providing an explanation to the cosmic deceleration-acceleration transition
although the reason behind the transition itself remains unknown. In the
conventional D cosmology, there is a no-go theorem prevents the EoS
parameter of a single perfect fluid in FRW geometry to cross the
boundary. The current model includes a single homogenous but anisotropic
perfect fluid in a homogenous FRW metric with two different scale factors in
the ordinary D and the UED. In contrast to the conventional D cosmology,
we have found that the dark energy evolution in UED shows crossing.
however, the no-go theorem is still respected in D where the EoS parameter
doesn't cross the boundary.Comment: 9 pages, 6 figures, 1 tabl
Space-time singularities and the theory of retracts
In previous publications, we have started investigating some possible
applications to the retraction theory in gravitational physics and showed that
it can be very useful in providing proofs and explaining the topological bases.
In the current work, we investigate the role of the retraction theory in the
mathematical study of space-time singularities and suggest a topological
restriction on the formation of singularities. Although we present a toy model
using a 5D cosmological metric as an example, the current study applies for any
collapsing system under the influence of gravity such as a massive star or the
whole universe. After showing that the cosmological space-time can be
retracted into lower dimensional circles , we prove the
existence of a homotopy between this retraction and the identity map which
defines a deformation retract on . Since a circle doesn't deformation
retract onto a point but it does retract to a point, the defined deformation
retract stops any circle from retracting into a point which means no
singularity is formed. The paper underlines the role of algebraic topology in
the mathematical study of space-time singularities and represents a new
application of the retraction theory in mathematical physics.Comment: 11 page
Bianchi Type- cosmology in gravity with
A new class of cosmological models in modified theories of gravity
proposed by Harko et al. (2011), where the gravitational Lagrangian is given by
an arbitrary function of Ricci scalar and the trace of the stress-energy
tensor , have been investigated for a specific choice of by considering time dependent deceleration parameter. The concept
of time dependent deceleration parameter (DP) with some proper assumptions
yield the average scale factor , where
and are positive constants. For , this generates a
class of accelerating models while for , the models of universe exhibit
phase transition from early decelerating phase to present accelerating phase
which is in good agreement with the results from recent astrophysical
observations. Our intention is to reconstruct models inspired by this
special law for the deceleration parameter in connection with the theories of
modified gravity. In the present study we consider the cosmological constant
as a function of the trace of the stress energy-momentum-tensor, and
dub such a model " gravity" where we have specified a certain form
of . Such models may display better uniformity with the
cosmological observations. The statefinder diagnostic pair parameter
has been embraced to characterize different phases of the universe. We also
discuss the physical consequences of the derived models.Comment: 16 pages, 10 figures. I have removed the over la
Deformation Retract and Folding of the 5D Schwarzchild Field
In this article we introduce some types of the deformtion retracts of the
Schwarzchild space making use of Lagrangian equations. The retraction of
this space into itself and into geodesics has been presented. The relation
between folding and deformation retract of this space has been achieved. A
relation for energy conservation similar to the one obtained in four dimensions
has been obtained for the five dimensional case.Comment: 10 pages, 0 figure
Topological Origin of Holographic Principle: Application to wormholes
In this paper, we suggest a mathematical representation to the holographic
principle through the theory topological retracts. We found that the
topological retraction is the mathematical analogs of the hologram idea in
modern quantum gravity and it can be used to explore the geometry of the
hologram boundary. An example has been given on the five dimensional (5D)
wormhole space-time which we found it can retract to lower dimensional
circles . In terms of the holographic principle, the description
of this volume of space-time is encoded on the lower-dimensional circle
which is the region boundary.Comment: 8 pages and 2 figure
A stable flat entropy-corrected FRW universe
In this paper, a general entropy-corrected FRW cosmological model has been
presented in which a deceleration-to-acceleration transition occurs according
to recent observations. We found that the case for the flat universe (),
supported by observations, is the most stable one where it successfully passes
all stability tests. The stability of the model has been studied through
testing the sound speed, the classical and the new nonlinear energy conditions.
The model predicts a positive pressure during the early-time decelerating
epoch, and a negative pressure during the late-time accelerating epoch in a
good agreement with cosmic history and dark energy assumption. We have
investigated all possible values of the prefactors and in the
corrected entropy-area relation to find the best values required for a stable
flat universe. We have also made use of the evolution of the equation of state
parameters in predicting the correct values of and
. The jerk and density parameters have been calculated where a good
agreement with observations and CDM model has been obtained. Two dark
energy proposals have been investigated in this model, the entropy-corrected
holographic dark energy and the modified holographic Ricci dark energy.Comment: 17 pages, 18 figure
Globally Optimal Cooperation in Dense Cognitive Radio Networks
The problem of calculating the local and global decision thresholds in hard
decisions based cooperative spectrum sensing is well known for its mathematical
intractability. Previous work relied on simple suboptimal counting rules for
decision fusion in order to avoid the exhaustive numerical search required for
obtaining the optimal thresholds. However, these simple rules are not globally
optimal as they do not maximize the overall global detection probability by
jointly selecting local and global thresholds. Instead, they maximize the
detection probability for a specific global threshold. In this paper, a
globally optimal decision fusion rule for Primary User signal detection based
on the Neyman- Pearson (NP) criterion is derived. The algorithm is based on a
novel representation for the global performance metrics in terms of the
regularized incomplete beta function. Based on this mathematical
representation, it is shown that the globally optimal NP hard decision fusion
test can be put in the form of a conventional one dimensional convex
optimization problem. A binary search for the global threshold can be applied
yielding a complexity of O(log2(N)), where N represents the number of
cooperating users. The logarithmic complexity is appreciated because we are
concerned with dense networks, and thus N is expected to be large. The proposed
optimal scheme outperforms conventional counting rules, such as the OR, AND,
and MAJORITY rules. It is shown via simulations that, although the optimal rule
tends to the simple OR rule when the number of cooperating secondary users is
small, it offers significant SNR gain in dense cognitive radio networks with
large number of cooperating users
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