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 $\Delta V/\Delta t = N_{\rm sur}-N_{\rm bulk}$ where $N_{\rm bulk}$ and $N_{\rm sur}$ 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 $\Delta V /\Delta t=N_{\rm sur}-N_{\rm bulk}-N_{\rm extr}$ where $N_{\rm extr}$ is referred to the degree of freedom related to the extrinsic geometry of the brane while $N_{\rm sur}$ and $N_{\rm bulk}$ 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 $dV/dt=N_{sur}-N_{bulk}$ where $N_{sur}$ and $N_{bulk}=-N_{em}+N_{de}$ are the degrees of freedom assigned to the surface area and the matter-energy content inside the bulk such that the indexes $"em"$ and $"de"$ 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 $dV/dt=N_{sur}+N_{em}-N_{de}$ including the number of degrees of freedom related to this spin interactions is obtained through the modification in $N_{bulk}$ term as $N_{bulk}=-N_{em}+N_{spin}+N_{de}$ leading to $dV /d t=N_{sur}+N_{em}-N_{spin} -N_{de}$ in which $N_{spin}$ 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 $\frac{3}{4\pi}$.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 $\mathcal{V}(\phi)$, 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