Equatorial magnetic helicity flux in simulations with different gauges (2009)
Mitra, Dhrubaditya, Candelaresi, Simon, Chatterjee, Piyali, Tavakol, Reza, Brandenburg, Axel
We use direct numerical simulations of forced MHD turbulence with a forcing function that produces two different signs of kinetic helicity in the upper and lower parts of the domain. We show that the...
Convective dynamos in spherical wedge geometry (2009)
Käpylä, Petri J., Korpi, Maarit J., Brandenburg, Axel, Mitra, Dhrubaditya, Tavakol, Reza
Large-scale dynamos have recently been obtained in local numerical simulations with turbulent convection in a number of setups and substantial effort has subsequently gone into distinguishing the...
Stability of the Einstein static universe in presence of vacuum energy (2009)
Carneiro, Saulo, Tavakol, Reza
The Einstein static universe has played a central role in a number of emergent scenarios recently put forward to deal with the singular origin of the standard cosmological model. Here we study the...
On vacuum density, the initial singularity and dark energy (2009)
Carneiro, Saulo, Tavakol, Reza
Standard cosmology poses a number of important questions. Apart from its singular origin, it possesses early and late accelerating phases required to account for observations. The vacuum energy has...
Cosmological scaling solutions in generalised Gauss-Bonnet gravity theories (2009)
Uddin, Kotub, Lidsey, James E., Tavakol, Reza
The conditions for the existence and stability of cosmological power-law scaling solutions are established when the Einstein-Hilbert action is modified by the inclusion of a function of the...
Chameleon scalar fields in relativistic gravitational backgrounds (2009)
Tsujikawa, Shinji, Tamaki, Takashi, Tavakol, Reza
We study the field profile of a scalar field $\phi$ that couples to a matter fluid (dubbed a chameleon field) in the relativistic gravitational background of a spherically symmetric spacetime....
Oscillatory migrating magnetic fields in helical turbulence in spherical domains (2009)
Mitra, Dhrubaditya, Tavakol, Reza, Käpylä, Petri J., Brandenburg, Axel
We perform direct numerical simulations of the compressible magnetohydrodynamical equations in a spherical shell domain with random helical forcing which has negative (positive) helicity in the...
Turbulent dynamos in spherical shell segments of varying geometrical extent (2008)
Mitra, Dhrubaditya, Tavakol, Reza, Brandenburg, Axel, Moss, David
We use three-dimensional direct numerical simulations of the helically forced magnetohydrodynamic equations in spherical shell segments in order to study the effects of changes in the geometrical...
Alpha effect and diffusivity in helical turbulence with shear (2008)
Mitra, Dhrubaditya, Käpylä, Petri J., Tavakol, Reza, Brandenburg, Axel
We study the dependence of turbulent transport coefficients, such as the components of the $\alpha$ tensor ($\alpha_{ij}$) and the turbulent magnetic diffusivity tensor ($\eta_{ij}$), on shear and...
Recollapsing quantum cosmologies and the question of entropy (2008)
Bojowald, Martin, Tavakol, Reza
Recollapsing homogeneous and isotropic models present one of the key ingredients for cyclic scenarios. This is considered here within a quantum cosmological framework in presence of a free scalar...
Constraints on scalar-tensor models of dark energy from observational and local gravity tests (2008)
Tsujikawa, Shinji, Uddin, Kotub, Mizuno, Shuntaro, Tavakol, Reza, Yokoyama, Jun'ichi
We construct a family of viable scalar-tensor models of dark energy (DE) which possess a phase of late-time acceleration preceded by a standard matter era, while at the same time satisfying the local...
Loop Quantum Cosmology: Effective theories and oscillating universes (2008)
Bojowald, Martin, Tavakol, Reza
Despite its great successes in accounting for the current observations, the so called `standard' model of cosmology faces a number of fundamental unresolved questions. Paramount among these are those...
Reconstructing the dark energy (2008)
We put forward a reconstruction scheme for dark energy which employs a number of observationally motivated geometrical constraints expected to be satisfied by the luminosity-distance relation. This...
Density perturbations in f(R) gravity theories in metric and Palatini formalisms (2007)
Tsujikawa, Shinji, Uddin, Kotub, Tavakol, Reza
We make a detailed study of matter density perturbations in both metric and Palatini formalisms in theories whose Lagrangian density is a general function, f(R), of the Ricci scalar. We derive these...
Eurico Covas, Reza Tavakol, Andrew Tworkowski, Axel Brandenburg, John Brooke, David Moss
A&A manuscript no. (will be inserted by hand later) Your thesaurus codes are:
Astronomy And Astrophysics (2007)
Axisymmetric Mean, Eurico Covas, Reza Tavakol, Andrew Tworkowski, Axel Br
.<F3.586e+05> We study axisymmetric mean field spherical and spherical shell dynamo models, with both dynamic and algebraic <F3.795e+05><F3.586e+05> #-quenchings. Our results show...
AND ASTROPHYSICS The influence of geometry and topology on axisymmetric mean-field dynamos (2007)
Eurico Covas, Reza Tavakol, Andrew Tworkowski, Axel Br, John Brooke, David Moss
Abstract. We study the changes in the dynamical behaviour of axisymmetric spherical mean-field dynamo models produced by changes in their geometry and topology, by considering a two parameter family...
John M. Brooke, Reza Tavakol, Andrew Tworkowski
A&A manuscript no. (will be inserted by hand later) Your thesaurus codes are:
Geodesic motion in the neighbourhood of submanifolds embedded in warped product spaces (2007)
Dahia, Fabio, Romero, Carlos, Silva, Lucio F. P., Tavakol, Reza
We study the classical geodesic motions of nonzero rest mass test particles and photons in (3+1+n)- dimensional warped product spaces. An important feature of these spaces is that they allow a...
Cosmological perturbations in Palatini modified gravity (2007)
Uddin, Kotub, Lidsey, James E, Tavakol, Reza
Two approaches to the study of cosmological density perturbations in modified theories of Palatini gravity have recently been discussed. These utilise, respectively, a generalisation of Birkhoff's...
Non-linear vector perturbations in a contracting universe (2007)
Mena, Filipe C., Mulryne, David J., Tavakol, Reza
A number of scalar field models proposed as alternatives to the standard inflationary scenario involve contracting phases which precede the universe's present phase of expansion. An important...
Fay, Stephane, Tavakol, Reza, Tsujikawa, Shinji
We make a systematic study of the cosmological dynamics for a number of f(R) gravity theories in Palatini formalism. We find a number of interesting results: (i) models based on theories of the type...
We put forward a new model-independent reconstruction scheme for dark energy which utilises the expected geometrical features of the luminosity-distance relation. The important advantage of this...
Anderson, Edward, Tavakol, Reza
The geodesics of a spacetime seldom coincide with those of an embedded submanifold of codimension one. We investigate this issue for higher-dimensional general relativity-like models, firstly in the...
An emergent universe from a loop (2005)
Mulryne, David J., Tavakol, Reza, Lidsey, James E., Ellis, George F. R.
Closed, singularity-free, inflationary cosmological models have recently been studied in the context of general relativity. Despite their appeal, these so called emergent models suffer from a number...
Oscillatory Universes in Loop Quantum Cosmology and Initial Conditions for Inflation (2004)
Lidsey, James E., Mulryne, David J., Nunes, N. J., Tavakol, Reza
Positively-curved, oscillatory universes are studied within the context of Loop Quantum Cosmology subject to a consistent semi-classical treatment. The semi-classical effects are reformulated in...
The role of anisotropy and inhomogeneity in Lemaitre-Tolman-Bondi collapse (2004)
Mena, Filipe C., Nolan, Brien C., Tavakol, Reza
We study the effects of shear and density inhomogeneities in the formation of naked singularities in spherically symmetric dust space--times. We find that in general neither of these physical...
Generalisations of the Einstein-Straus model to cylindrically symmetric settings (2004)
Mena, Filipe C., Tavakol, Reza, Vera, Raul
We study generalisations of the Einstein--Straus model in cylindrically symmetric settings by considering the matching of a static space-time to a non-static spatially homogeneous space-time,...
Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity (2004)
Bojowald, Martin, Lidsey, James E., Mulryne, David J., Singh, Parampreet, Tavakol, Reza
In loop quantum gravity, modifications to the geometrical density cause a self-interacting scalar field to accelerate away from a minimum of its potential. In principle, this mechanism can generate...
Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity (2004)
Bojowald,Martin, Lidsey,James E., Mulryne,David J., Singh,Parampreet, Tavakol,Reza
In loop quantum gravity, modifications to the geometrical density cause a self-interacting scalar field to accelerate away from a minimum of its potential. In principle, this mechanism can generate...
Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity (2004)
Bojowald, Martin, Lidsey, James E., Mulryne, David J., Singh, Parampreet, Tavakol, Reza
In loop quantum gravity, modifications to the geometrical density cause a self-interacting scalar field to accelerate away from a minimum of its potential. In principle, this mechanism can generate...
Covas, Eurico, Moss, David, Tavakol, Reza
Analyses of recent helioseismic data have produced ample evidence for substantial dynamical variation of the differential rotation within the solar convection zone. Given the inevitable difficulties...
PDE System Approach to Large Extra Dimensions (2003)
Anderson, Edward, Tavakol, Reza
We explore some fundational issues regarding the splitting of D-dimensional EFE's w.r.t timelike and spacelike (D-1)-dimensional hypersurfaces, first without and then with thin matter sheets such as...
Reformulation and Interpretation of SMS Braneworld (2003)
Anderson, Edward, Tavakol, Reza
We reformulate the Shiromizu, Maeda and Sasaki (SMS) braneworlds within the framework of the five-dimensional Einstein equations. In many applications of the braneworld Einstein field equations, the...
Running of the Scalar Spectral Index and Observational Signatures of Inflation (2003)
Lidsey, James E., Tavakol, Reza
Some of the consequences for inflationary cosmology of a scale dependence (running) in the tilt of the scalar perturbation spectrum are considered. In the limit where the running is itself...
BritGravII - The Second British Gravity Meeting (2002)
The second of the annual BritGrav meetings on current research in Gravitational Physics in Britain took place at the School of Mathematical Sciences of Queen Mary, University of London on June 10/11,...
Second Order Perturbations of Flat Dust FLRW Universes with a Cosmological Constant (2002)
Mena, Felipe C., Tavakol, Reza, Bruni, Marco
We summarize recent results concerning the evolution of second order perturbations in flat dust irrotational FLRW models with $\Lambda\ne 0$. We show that asymptotically these perturbations tend to...
Generalisation of the Einstein-Straus model to anisotropic settings (2002)
Mena, Filipe C., Tavakol, Reza, Vera, Raül
We study the possibility of generalising the Einstein--Straus model to anisotropic settings, by considering the matching of locally cylindrically symmetric static regions to the set of $G_4$ on $S_3$...
Effects of boundary conditions on the dynamics of the solar convection zone (2002)
Tavakol, Reza, Covas, Eurico, Moss, David, Tworkowski, Andrew
Recent analyses of the helioseismic data have produced evidence for a variety of interesting dynamical behaviour associated with torsional oscillations. What is not so far clear is whether these...
Robustness of the Quintessence Scenario in Particle Cosmologies (2002)
The Erwin, Schrodinger International Boltzmanngasse, Greg Huey, Greg Huey, Reza Tavakol, Reza Tavakol
We study the robustness of the quintessence tracking scenario in the context of more general cosmological models that derive from high-energy physics. We consider the effects of inclusion of multiple...
Robustness of the Quintessence Scenario in Particle Cosmologies (2001)
We study the robustness of the quintessence tracking scenario in the context of more general cosmological models that derive from high-energy physics. We consider the effects of inclusion of multiple...
Cosmic no-hair: non-linear asymptotic stability of de Sitter universe (2001)
Bruni, Marco, Mena, Filipe C., Tavakol, Reza
We study the asymptotic stability of de Sitter spacetime with respect to non-linear perturbations, by considering second order perturbations of a flat Robertson-Walker universe with dust and a...
Aspects of the dynamics underlying solar and stellar dynamo models (2001)
Observations of the Sun and solar-type stars continue to reveal phenomena whose understanding is very likely to require a nonlinear framework. Here we shall concentrate on two such phenomena, namely...
Spatiotemporal fragmentation and the uncertainties in the solar rotation law (2001)
Covas, Eurico, Tavakol, Reza, Vorontsov, Sergei, Moss, David
Analyses of recent helioseismic data indicate that the dynamical regimes at the base of the convection zone can be different from those observed at the top, having either significantly shorter...
In--out intermittency in PDE and ODE models (2001)
Covas, Eurico, Tavakol, Reza, Ashwin, Peter, Tworkowski, Andrew, Brooke, John
We find concrete evidence for a recently discovered form of intermittency, referred to as in--out intermittency, in both PDE and ODE models of mean field dynamos. This type of intermittency...
The influence of noise on scalings for in-out intermittency (2001)
Ashwin, Peter, Covas, Eurico, Tavakol, Reza
We study the effects of noise on a recently discovered form of intermittency, referred to as in-out intermittency. This type of intermittency, which reduces to on-off in systems with a skew product...
Dynamical variations of the differential rotation in the solar convection zone (2000)
Covas, Eurico, Tavakol, Reza, Moss, David
Recent analyses of helioseismological observations seem to suggest the presence of two new phenomena connected with the dynamics of the solar convective zone. Firstly, there are present torsional...
Covas, Eurico, Tavakol, Reza, Moss, David
Recent analyses of the helioseismic observations indicate that the previously observed surface torsional oscillations with periods of about 11 years extend significantly downwards into the solar...
Torsional oscillations in the solar convection zone (2000)
Covas, Eurico, Tavakol, Reza, Moss, David, Tworkowski, Andrew
Recent analysis of the helioseismic observations indicate that the previously observed surface torsional oscillations extend significantly downwards into the solar convection zone. In an attempt to...
Absence of supersensitivity to small input signals in generalized on--off systems (2000)
It has recently been shown that nonlinear skew product dynamical systems with invariant subspaces which are capable of displaying on-off intermittency can show supersensitivity to small input...
Initial data and spherical dust collapse (2000)
Mena, Filipe C., Tavakol, Reza, Joshi, Pankaj S.
By considering families of radial null geodesics, we study the subsets of initial data that lead to naked singularities and black holes in inhomogeneous spherical dust collapse. We introduce the...
On Holography and Cosmology (1999)
We consider a recent generalisation by Bousso of an earlier holography proposal by Fischler and Susskind. We demonstrate that in general inhomogeneous universes such a proposal would involve...
On Applications of Campbell's Embedding Theorem (1999)
Lidsey, James E., Romero, Carlos, Tavakol, Reza, Rippl, Steve
A little known theorem due to Campbell is employed to establish the local embedding of a wide class of 4-dimensional spacetimes in 5-dimensional Ricci-flat spaces. An embedding for the class of...
Multiple forms of intermittency in PDE dynamo models (1999)
We find concrete evidence for the presence of crisis-induced and Pomeau-Manneville Type-I intermittencies in an axisymmetric PDE mean-field dynamo model. These findings are of potential importance...
Intermittency as a possible underlying mechanism for solar and stellar variability (1999)
We briefly discuss the status of the intermittency hypothesis, according to which the grand minima type variability in solar-type stars may be understood in terms of dynamical intermittency. We...
Tilted String Cosmologies (1999)
Clancy, Dominic, Feinstein, Alexander, Lidsey, James E., Tavakol, Reza
Global symmetries of the string effective action are employed to generate tilted, homogeneous Bianchi type VI_h string cosmologies from a previously known stiff perfect fluid solution to Einstein...
Inhomogeneous Einstein-Rosen String Cosmology (1999)
Clancy, Dominic, Feinstein, Alexander, Lidsey, James E., Tavakol, Reza
Families of anisotropic and inhomogeneous string cosmologies containing non-trivial dilaton and axion fields are derived by applying the global symmetries of the string effective action to a...
Evolution of the density contrast in inhomogeneous dust models (1998)
Mena, Filipe C., Tavakol, Reza
With the help of families of density contrast indicators, we study the tendency of gravitational systems to become increasingly lumpy with time. Depending upon their domain of definition, these...
Initial Conditions in String Cosmology (1998)
Clancy, Dominic, Lidsey, James E., Tavakol, Reza
We take a critical look at a recent conjecture concerning the past attractor in the pre-big-bang scenario. We argue that the Milne universe is unlikely to be a general past attractor for such models...
In-out intermittency in PDE and ODE models of axisymmetric mean-field dynamos (1998)
Covas, Eurico, Tavakol, Reza, Ashwin, Peter, Tworkowski, Andrew, Brooke, John M.
Employing some recent results in dynamics of systems with invariant subspaces we find evidence in both truncated and full axisymmetric mean-field dynamo models of a recently discovered type of...
Scale Factor Dualities in Anisotropic Cosmologies (1998)
Clancy, Dominic, Lidsey, James E., Tavakol, Reza
The concept of scale factor duality is considered within the context of the spatially homogeneous, vacuum Brans-Dicke cosmologies. In the Bianchi class A, it is found that duality symmetries exist...
Transverse instability for non-normal parameters (1998)
Ashwin, Peter, Covas, Eurico, Tavakol, Reza
We consider the behaviour of attractors near invariant subspaces on varying a parameter that does not preserve the dynamics in the invariant subspace but is otherwise generic, in a smooth dynamical...
Mean Field Dynamos With Algebraic and Dynamic Alpha-Quenchings (1998)
Andrew Tworkowski, Eurico Covas, Reza Tavakol, Axel Brandenburg
Calculations for mean field dynamo models (in both spherical and spherical shells), with both algebraic and dynamic ff--quenchings. show qualitative as well as quantitative differences and...
Transverse Instability for Non-Normal Parameters (1998)
Peter Ashwin, Eurico Covas, Reza Tavakol
. Suppose a smooth dynamical system has an invariant subspace and a parameter that leaves the dynamics in the invariant subspace invariant while changing the normal dynamics. Then we say the...
Non-normal parameter blowout bifurcation: An example in a truncated mean-field dynamo model (1997)
Covas, Eurico, Ashwin, Peter, Tavakol, Reza
We examine global dynamics and bifurcations occurring in a truncated model of a stellar mean-field dynamo. This model has symmetry-forced invariant subspaces for the dynamics and we find examples of...
Axisymmetric mean field dynamos with dynamic and algebraic $\alpha$--quenchings (1997)
Covas, Eurico, Tavakol, Reza, Tworkowski, Andrew, Brandenburg, Axel
We study axisymmetric mean field spherical and spherical shell dynamo models, with both dynamic and algebraic $\alpha$--quenchings. Our results show that there are qualitative as well as quantitative...
Non-normal parameter blowout bifurcation: an example in a truncated mean field dynamo model (1997)
Covas, Eurico, Ashwin, Peter, Tavakol, Reza
We examine global dynamics and bifurcations occurring in a truncated model of a stellar mean field dynamo. This model has symmetry-forced invariant subspaces for the dynamics and we find examples of...
Crisis-induced intermittency in truncated mean field dynamos (1997)
We investigate the detailed dynamics of a truncated $\alpha\omega$ dynamo model with a dynamic $\alpha$ effect. We find the presence of multiple attractors, including two chaotic attractors with a...
Dynamos with different formulations of a dynamic alpha-effect (1997)
Covas, Eurico, Tworkowski, Andrew, Brandenburg, Axel, Tavakol, Reza
We investigate the behaviour of $\alpha\Omega$ dynamos with a dynamic $\alpha$, whose evolution is governed by the imbalance between a driving and a damping term. We focus on truncated versions of...
Robustness of truncated alpha-Omega dynamos with a dynamic alpha (1997)
Covas, Eurico, Tworkowski, Andrew, Tavakol, Reza, Brandenburg, Axel
In a recent work (Covas et al. 1997), the behaviour and the robustness of truncated $\alpha\Omega$ dynamos with a dynamic $\alpha$ were studied with respect to a number of changes in the driving term...
On the Domain of Applicability of General Relativity (1997)
Tavakol, Reza, Zalaletdinov, Roustam
We consider the domain of applicability of general relativity (GR), as a classical theory of gravity, by considering its applications to a variety of settings of physical interest as well as its...
$D$-Dimensional Gravity from $(D+1)$ Dimensions (1995)
Rippl, Steve, Romero, Carlos, Tavakol, Reza
We generalise Wesson's procedure, whereby vacuum $(4+1)-$dimensional field equations give rise to $(3+1)-$dimensional equations with sources, to arbitrary dimensions. We then employ this...
Kinematics and Dynamics of $f(R)$ Theories of Gravity (1995)
Rippl, Steve, Van Elst, Henk, Tavakol, Reza, Taylor, David
We generalise the equations governing relativistic fluid dynamics given by Ehlers and Ellis for general relativity, and by Maartens and Taylor for quadratic theories, to generalised $f(R)$ theories...
Constraints on Inflationary Solutions in the Presence of Shear and Bulk Viscosity (1994)
Van Elst, Henk, Dunsby, Peter, Tavakol, Reza
Inflationary models and their claim to solve many of the outstanding problems in cosmology have been the subject of a great deal of debate over the last few years. A major sticking point has been the...
Quantum Cosmology and Higher-Order Lagrangian Theories (1994)
Van Elst, Henk, Lidsey, James E, Tavakol, Reza
In this paper the quantum cosmological consequences of introducing a term cubic in the Ricci curvature scalar $R$ into the Einstein--Hilbert action are investigated. It is argued that this term...