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F. Darabi

Publications and source records attributed to F. Darabi.

At least 109 records · Page 6Linked to original sources
arXiv2009

Modified gravity and Space-Time-Matter theory

The correspondence between $f(R)$ theories of gravity and model theories explaining induced dark energy in a 5D Ricci-flat universe, known as the Space-Time-Matter theory (STM), is studied. It is shown that such correspondence may be used to interpret the four dimensional expressions, induced from geometry in 5D STM theories, in terms of the extra terms appearing in $f(R)$ theories of gravity. The method is demonstrated by providing an explicit example in which a given $f(R)$ is used to predict the properties of the corresponding 5D Ricci-flat universe. The accelerated expansion and the induced dark energy in a 5D Ricci-flat universe characterized by a big bounce is studied and it is shown that an arbitrary function $μ(t)$ in the 5D solutions can be rewritten, in terms of the redshift $z$, as a new arbitrary function $F(z)$ which corresponds to the 4D curvature quintessence models.

gr-qc↗
arXiv2009

Classical and Quantum Cosmology of an Accelerating Model Universe with Compactification of Extra Dimensions

We study a $(4+D)$-dimensional Kaluza-Klein cosmology with a Robertson-Walker type metric having two scale factors $a$ and $R$, corresponding to $D$-dimensional internal space and 4-dimensional universe, respectively. By introducing an exotic matter in the form of perfect fluid with an special equation of state, as the space-time part of the higher dimensional energy-momentum tensor, a four dimensional effective decaying cosmological term appears as $λ\sim R^{-m}$ with $0 \leq m\leq 2$, playing the role of an evolving dark energy in the universe. By taking $m=2$, which has some interesting implications in reconciling observations with inflationary models and is consistent with quantum tunneling, the resulting Einstein's field equations yield the exponential solutions for the scale factors $a$ and $R$. These exponential behaviors may account for the dynamical compactification of extra dimensions and the accelerating expansion of the 4-dimensional universe in terms of Hubble parameter, $H$. The acceleration of the universe may be explained by the negative pressure of the exotic matter. It is shown that the rate of compactification of higher dimensions as well as expansion of 4-dimensional universe depends on the dimension, $D$. We then obtain the corresponding Wheeler-DeWitt equation and find the general exact solutions in $D$-dimensions. A good correspondence between the solutions of classical Einstein's equations and the solutions of quantum Wheeler-DeWitt equation in any dimension, $D$, is obtained based on Hartle's point of view concerning the classical limits of quantum cosmology.

gr-qc↗
arXiv2009

Is magnetic flux quantized inside a solenoid?

In some textbooks on quantum mechanics, the description of flux quantization in a superconductor ring based on the Aharonov-Bohm effect may lead some readers to a (wrong) conclusion that flux quantization occurs as well for a long solenoid with the same quantization condition in which the charge of cooper pair $2e$ is replaced by the charge of one electron $e$. It is shown how this confusion arises and how can one avoid it.

quant-ph↗
arXiv2008

Classical and quantum wormholes in a flat $Λ$-decaying cosmology

We study the classical and quantum wormholes for a flat {\it Euclidean} Friedmann-Robertson-Walker metric with a perfect fluid including an ordinary matter source plus a source playing the role of dark energy (decaying cosmological term). It is shown that classical wormholes exist for this model and the quantum version of such wormholes are consistent with the Hawking-Page conjecture for quantum wormholes as solutions of the Wheeler-DeWitt equation.

gr-qc↗
arXiv2008

Correspondence between modified gravity and 5D Ricci-flat cosmologies

We study the correspondence between two theoretical frameworks for describing dark energy, $f(R)$ gravity and higher-dimensional Space-Time-Matter (STM) or induced-matter theory. We show that the Hubble expansion parameter in $f(R)$ gravity can be associated with a combination of metric functions in STM theory, and consider a specific example whose properties are consistent with late-time acceleration.

gr-qc↗
arXiv2008

Casimir effect for concentric spheres in de Sitter spacetime with signature change

We revisit the Casimir effect for two concentric spherical shells in de Sitter background with a new geometric configuration, namely Euclidean signature between and Lorentzian signature outside the spheres with different cosmological constants, for a massless scalar field satisfying Dirichlet boundary conditions on the spheres. It is shown that an extra constant pressure emerges due to this signature changing configuration. Some interesting aspects of this extra term are then discussed.

gr-qc↗
arXiv2008

A 5D non compact and non Ricci flat Kaluza-Klein Cosmology

A model universe is proposed in the framework of 5-dimensional noncompact Kaluza-Klein cosmology which is not Ricci flat. The 4D part as the Robertson-Walker metric is coupled to conventional perfect fluid, and its extra-dimensional part is coupled to a dark pressure through a scalar field. It is shown that neither early inflation nor current acceleration of the 4D universe would happen if the non-vacuum states of the scalar field would contribute to 4D cosmology.

gr-qc↗
arXiv2008

Bouncing cosmological solutions and their stability

In the present paper we consider the bouncing braneworld scenario, in which the bulk is given by a five-dimensional charged AdS black hole spacetime with matter field confined in a $D_3$ brane. Then, we study the stability of solutions with respect to homogeneous and isotropic perturbations. Specifically, the AdS black hole with zero ADM mass and charge, and open horizon is an attractor, while the charged AdS black hole with zero ADM mass and flat horizon, is a repeller.

hep-th↗
arXiv2008

Signature change, vacuum condensation and cosmological constant

We proposes an alternative model of duality symmetry, based on the previously obtained divergence theory, including an scalar field, an internal vector and a metric signature. At some small scale an effective scalar field equation has appeared whose potential acts like a Higgs one, where the metric signature plays the role of an order parameter. Non-vanishing Vacuum condensation of this Higgs field occurs once a signature change from Euclidean to Lorentzian is formed. The mass scale of Higgs field excitations around this vacuum may contribute, in the Lorentzian sector, to the cosmological constant, in agreement with observations.

gr-qc↗
arXiv2006

A quantum cosmology and discontinuous signature changing classical solutions

We revisit the classical and quantum cosmology of a universe in which a self interacting scalar field is coupled to gravity with a flat FRW type metric undergoing continuous signature transition. We arrange for quantum cosmologically allowed discontinuity in the classical solutions at the signature changing hypersurface, provided these solutions be dual in some respects. This may be of some importance in the study of early universe within the signature changing scenarios.

gr-qc↗
arXiv2006

Casimir effect in a two dimensional signature changing spacetime

We study the Casimir effect for free massless scalar fields propagating on a two-dimensional cylinder with a metric that admits a change of signature from Lorentzian to Euclidean. We obtain a nonzero pressure, on the hypersurfaces of signature change, which destabilizes the signature changing region and so alters the energy spectrum of scalar fields. The modified region and spectrum, themselves, back react on the pressure. Moreover, the central term of diffeomorphism algebra of corresponding infinite conserved charges changes correspondingly.

gr-qc↗
arXiv2005

Casimir effect for a spherical shell in de Sitter spacetime with signature change

The Casimir stress on a spherical shell in de Sitter signature changing background for massless scalar field satisfying Dirichlet boundary conditions on the shell is calculated. The Casimir stress is calculated for inside and outside of the shell with different backgrounds corresponding to different metric signatures and cosmological constants. An important contribution appears due to signature change which leads to a transient rapid expansion of the bubbles in this background.

hep-th↗
arXiv2005

Gravitational complementary principle: A new approach to quantum gravity

A new idea of quantum gravity is developed based on {\it Gravitational Complementary Principle}. This principle states that gravity has dual complement features: The quantum and classical aspects of gravity are complement and absolutely separated by the planck length into planckian and over-planckian domains, respectively. The classical Einstein equations are correct at the fundamental level at over-planckian domain and general relativity is not a low energy limit of a more fundamental theory. The quantum gravity is totally confined to the planckian domain with a new kind of ultra-short range interaction, mediated by massive (Planck mass) particles, through the virtual microscopic wormholes of the Planck scale with action $\hbar$. There is no room for gravitons or extra dimensions in this scenario. It is shown that the hierarchy problem can solve the cosmological constant problem via this new quantum gravity.

hep-ph↗
arXiv2005

Dynamical generation of space-time signature by spontaneous symmetry breaking

The problem of dynamical generation of 4-D space-time signature at small scales and its stabilization towards Lorentzian signature at large scales is studied in the context of Higgs mechanism in a two-time scenario. It is also shown that Lorentz invariance at small scales can be violated but at large space-time scales is restored.

gr-qc↗
arXiv2005

Noncommutativity in quantum cosmology and the Hierarchy problem

We study the quantum cosmology of an empty (4+1)-dimensional Kaluza-Klein cosmology with a negative cosmological constant and a FRW type metric with two scale factors, one for 4-D universe and the other for one compact extra dimension. By assuming the noncommutativity in the corresponding mini-superspace we suggest a solution for the Hierarchy problem, at the level of Wheeler-DeWitt equation.

gr-qc↗
arXiv2004

On the Quantum Aspects of Geophysics

We introduce a simple quantum mechanical justification for the formation of folded mountains. It is very appealing to develop this idea to a theory of {\it Quantum Geophysics}

physics.gen-ph↗