707 research outputs found
Signature Change by GUP
We revisit the issue of continuous signature transition from Euclidean to
Lorentzian metrics in a cosmological model described by FRW metric minimally
coupled with a self interacting massive scalar field. Then, using a
noncommutative phase space of dynamical variables deformed by Generalized
Uncertainty Principle (GUP) we show that the signature transition occurs even
for a model described by FRW metric minimally coupled with a free massless
scalar field accompanied by a cosmological constant. This indicates that the
continuous signature transition might have been easily occurred at early
universe just by a free massless scalar field, a cosmological constant and a
noncommutative phase space deformed by GUP, without resorting to a massive
scalar field having an {\it ad hoc} complicate potential. We also study the
quantum cosmology of the model and obtain a solution of Wheeler-DeWitt equation
which shows a good correspondence with the classical path.Comment: 14 pages, to be published in IJMP
Emergent Universe in the Braneworld Scenario
According to Padmanabhan's proposal, the difference between the surface
degrees of freedom and the bulk degrees of freedom in a region of space may
result in the acceleration of Universe expansion through the relation where and
are referred to the degrees of freedom related to the matter and energy content
inside the bulk and surface area, respectively \cite{Pad1}. In this paper, we
study the dynamical effect of the extrinsic geometrical embedding of an
arbitrary four dimensional brane in a higher dimensional bulk space and
investigate the corresponding degrees of freedom. Considering the modification
of Friedmann equations arising from a general braneworld scenario, we obtain a
correction term in Padmanabhan's relation, denoting the number of degrees of
freedom related to the extrinsic geometry of the brane embedded in higher
dimensional spacetime as where is referred to the degree of freedom related to the
extrinsic geometry of the brane while and are as
well as before. Finally, we study the validity of the first and second laws of
thermodynamics for this general braneworld scenario in the state of thermal
equilibrium and in the presence of confined matter fields to the brane with the
induced geometric matter fields.Comment: 16 pages, Major revisio
Braneworld Black Holes and Entropy Bounds
The Bousso's D-bound entropy for the various possible black hole solutions on
a 4-dimensional brane is checked. It is found that the D-bound entropy here is
apparently different from that of obtained for the 4-dimensional black hole
solutions. This difference is interpreted as the extra loss of information,
associated to the extra dimension, when an extra-dimensional black hole is
moved outward the observer's cosmological horizon. Also, it is discussed that
N-bound entropy is hold for the possible solutions here. Finally, by adopting
the recent Bohr-like approach to black hole quantum physics for the excited
black holes, the obtained results are written also in terms of the black hole
excited states.Comment: 13 pages, accepted for publication in Physics Letters
Emergent Cosmos in Einstein-Cartan Theory
Based on the Padmanabhan's proposal, the accelerated expansion of the
universe can be driven by the difference between the surface and bulk degrees
of freedom in a region of space, described by the relation
where and are the
degrees of freedom assigned to the surface area and the matter-energy content
inside the bulk such that the indexes and represent
energy-momentum and dark energy, respectively. In the present work, the
dynamical effect of the Weyssenhoff perfect fluid with intrinsic spin and its
corresponding spin degrees of freedom in the framework of Einstein-Cartan (EC)
theory are investigated. Based on the modification of Friedmann equations due
to the spin-spin interactions, a correction term for the Padmanabhan's original
relation including the number of degrees of
freedom related to this spin interactions is obtained through the modification
in term as leading to in which is the corresponding
degrees of freedom related to the intrinsic spin of the matter content of the
universe. Moreover, the validity of the unified first law and the generalized
second law of thermodynamics for the Einstein-Cartan cosmos are investigated.
Finally, by considering the covariant entropy conjecture and the bound
resulting from the emergent scenario, a total entropy bound is obtained. Using
this bound, it is shown that the for the universe as an expanding
thermodynamical system, the total effective Komar energy never exceeds the
square of the expansion rate with a factor of .Comment: 12 Pages, Accepted for Publication in Eur. Phys. J.
D-bound and Bekenstein Bound for the Surrounded Vaidya Black Hole
We study the Vaidya black hole surrounded by the exotic quintessence-like,
phantom-like and cosmological constant-like fields by means of entropic
considerations. Explicitly, we show that for this thermodynamical system, the
requirement for the identification of D-bound and Bekenstein entropy bound can
be considered as a thermodynamical criterion by which one can rule out the
quintessence-like and phantom-like fields, and prefer the cosmological constant
as a vi{\th}able cosmological field.Comment: 12 pages, minor revisio
A Generalization to the Rastall Theory and Cosmic Eras
A generalized version for the Rastall theory is proposed showing the
agreement with the cosmic accelerating expansion. In this regard, a coupling
between geometry and the pressureless matter fields is derived which may play
the role of dark energy responsible for the current accelerating expansion
phase. Moreover, our study also shows that the radiation field may not be
coupled to the geometry in a non-minimal way which represents that the ordinary
energy-momentum conservation law is respected by the radiation source. It is
also shown that the primary inflationary era may be justified by the ability of
the geometry to couple to the energy-momentum source in an empty flat FRW
universe. In fact, this ability is independent of the existence of the
energy-momentum source and may compel the empty flat FRW universe to expand
exponentially. Finally, we consider a flat FRW universe field by a spatially
homogeneous scalar field evolving in potential , and study
the results of applying the slow-roll approximation to the system which may
lead to an inflationary phase for the universe expansion.Comment: 11 page
Einstein and Rastall Theories of Gravitation in Comparison
We profit by a recent paper of Visser claiming that Rastall gravity is
equivalent to Einstein gravity to compare the two gravitational theories in a
general way. Our conclusions are different from Visser's ones. We indeed argue
that these two theories are not equivalent. In fact, Rastall theory of gravity
is an "open" theory when compared to Einstein general theory of relativity.
Thus, it is ready to accept the challenges of observational cosmology and
quantum gravity.Comment: 8 pages, comment on the paper arXiv:1711.11500, "Rastall gravity is
equivalent to Einstein gravity", by Matt Visser. Final version matching the
paper published in the European Physical Journal
Introducing box-plate beam-to-column moment connections
Nowadays, using high-ductility structures in the construction and use of significant buildings is highly appreciated. To use more ductile structures, effort has been made in this research to introduce box-plate beam-to-column connections. They underwent hysteretic loading and it was found from their moment-rotation curves that the bending capacity and ductility of the box-plate connection were more than ordinary rigid connection, and those of the latter were more than those of the normal typical one. It was also shown that stress concentration in box-plate connections disappears over the top and bottom flange plates
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