yousef bisabr - Academia.edu (original) (raw)

Papers by yousef bisabr

Physics Letters B, Nov 1, 2017

We have investigated a brane world model in which the gravitational field in the bulk is describe... more We have investigated a brane world model in which the gravitational field in the bulk is described both by a metric tensor and a minimally coupled scalar field. This scalar field is taken to be a chameleon with an appropriate potential function. The scalar field interacts with matter and there is an energy transfer between the two components. We find a late-time asymptotic solution which exhibits late-time accelerating expansion. We also show that the Universe recently crosses the phantom barrier without recourse to any exotic matter. We provide some thermodynamic arguments which constrain both the direction of energy transfer and dynamics of the extra dimension.

EPL (Europhysics Letters)

We have investigated some cosmological consequences of variation of the fine structure constant, ... more We have investigated some cosmological consequences of variation of the fine structure constant, α, in the context of the Bekenstein-Sandvik-Barrow-Magueijo (BSBM) model. In this context, such a variation is attributed to electric charge by letting the latter take on the value of a real scalar field, φ, which varies generally in space and time. We consider the dynamics of φ in the case that it is allowed to have an exponential potential. We have found solutions for evolutions of φ and α in the radiation and the matter dominated eras. By employing observational bounds on the temporal variation of α, we constrain the model parameters. We then investigate the impact of α-variation on redshift dependence of CMB temperature. It is shown that there is a deviation from the standard evolution of the temperature which contains two different contributions, one is important at high redshift eras and the other is mostly effective at low redshift eras.

There is a conformal equivalence between power law f (R) theories and scalar field theories in wh... more There is a conformal equivalence between power law f (R) theories and scalar field theories in which the scalar degree of freedom evolves under the action of an exponential potential function. In the scalar field representation there is a strong coupling of the scalar field with the matter sector due to the conformal transformation. We use chameleon mechanism to implement constraints on the potential function of the scalar field in order that the resulting model be in accord with Solar System experiments. Investigation of these constraints reveals that there may be no possibility to distinguish between a power law f (R) function and the usual Einstein-Hilbert Lagrangian density.

Recently, a model f (R) theory is proposed [1] which is cosmologically viable and distinguishable... more Recently, a model f (R) theory is proposed [1] which is cosmologically viable and distinguishable from ΛCDM. We use chameleon mechanism to investigate viability of the model in terms of Solar System experiments.

We consider a generalized Brans-Dicke model in which the scalar field has a potential function an... more We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple non-minimally with the matter sector. This anomalous gravitational coupling can in principle avoid the model to pass local gravity experiments. One then usually assumes that the scalar field has a chameleon behavior in the sense that it acquires a density-dependent effective mass. While it can take a small effective mass in cosmological (low-density environment) scale, it has a sufficiently heavy mass in Solar System (large-density environment) and then hides gravity tests. We will argue that such a chameleon behavior can not be generally realized and depends significantly on the forms attributed to the potential and the coupling functions.

We consider modified theories of gravity with a direct coupling between matter and geometry, deno... more We consider modified theories of gravity with a direct coupling between matter and geometry, denoted by an arbitrary function in terms of the Ricci scalar. Due to such a coupling, the matter stress tensor is no longer conserved and there is an energy transfer between the two components. By solving the conservation equation, we argue that the matter system should gain energy in this interaction, as demanded by the second law of thermodynamics. In a cosmological setting, we show that although this kind of interaction may account for cosmic acceleration, this latter together with direction of the energy transfer constrain the coupling function.

We consider a generalized Brans-Dicke model in which the scalar field has a potential function an... more We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple non-minimally with the matter sector. We assume a power law form for the potential and the coupling functions as the inputs of the model and show that acceleration of the universe can be realized for a constrained range of exponent of the potential function. We also argue that this accelerating phase is consistent with a large and positive Brans-Dicke parameter. In our analysis, the potential plays a more important role with respect to the coupling function in dynamics of the universe as the latter does not contribute to any of the relations characterizing evolution of scale factor of the universe and the scalar field. However, we will show that the coupling function is closely related to magnitude and direction of the energy transfer between matter and the scale field. We use this fact and some thermodynamic aspects of the model to put some constraints on the...

Physics Letters B, 2017

We have investigated a brane world model in which the gravitational field in the bulk is describe... more We have investigated a brane world model in which the gravitational field in the bulk is described both by a metric tensor and a minimally coupled scalar field. This scalar field is taken to be a chameleon with an appropriate potential function. The scalar field interacts with matter and there is an energy transfer between the two components. We find a late-time asymptotic solution which exhibits late-time accelerating expansion. We also show that the Universe recently crosses the phantom barrier without recourse to any exotic matter. We provide some thermodynamic arguments which constrain both the direction of energy transfer and dynamics of the extra dimension.

International Journal of Modern Physics D, 2018

We deal with a dynamical mechanism in which a large cosmological constant, as suggested by inflat... more We deal with a dynamical mechanism in which a large cosmological constant, as suggested by inflationary scenarios, decays due to expansion of the universe. This mechanism has its origin in the gravitational coupling of the vacuum density. We assume that the vacuum couples anomalously to gravity that is the metric tensor that appears the gravitational part is not the same as that appears the matter part as suggested by weak equivalence principle. Instead, the two metric tensors are taken to be conformally related. We show that this provides a dynamical mechanism which works during expansion of the universe. We also consider some observational consequences of such a gravitational model.

Gravitation and Cosmology, 2018

In a homogenous and isotropic cosmology, we introduce general exact solutions for some modified g... more In a homogenous and isotropic cosmology, we introduce general exact solutions for some modified gravity models. In particular, we introduce exact solutions for powerlaw f (R) gravity and Brans-Dicke theory in Einstein and Jordan conformal frames. In the Brans-Dicke case, the solutions are presented for both single and double exponential potentials in Einstein frame which correspond to power-law potentials in Jordan frame. Our analysis for extracting general exact solutions can also be generalized to those scalartensor theories in which the scalar field has an exponential coupling to Ricci scalar.

International Journal of Modern Physics D, 2019

We consider a generalized Brans–Dicke model in which the scalar field has a self-interacting pote... more We consider a generalized Brans–Dicke model in which the scalar field has a self-interacting potential function. The scalar field is also allowed to couple nonminimally with the matter part. We assume that it has a chameleon behavior in the sense that it acquires a density-dependent effective mass. We consider two different types of matter systems which couple with the chameleon, dust and vacuum. In the first case, we find a set of exact solutions when the potential has an exponential form. In the second case, we find a power-law exact solution for the scale factor. In this case, we will show that the vacuum density decays during expansion due to coupling with the chameleon.

Physics Letters B, 2010

It is well-known that f (R) theories in Einstein frame is conformally equivalent to quintessence ... more It is well-known that f (R) theories in Einstein frame is conformally equivalent to quintessence models in which the scalar field minimally couples with gravity. If there exists a matter system in Jordan frame, then it interacts with the scalar field in Einstein frame due to the conformal transformations. This interaction, in general, may lead to changes of fundamental constants. Here we will consider possible time variation of fine structure constant in a general f (R) theory. We will use observational bounds on these variations and argue that it provides a criterion for constraining f (R) models.

Physics Letters B, 2010

Recently, a model f (R) theory is proposed [1] which is cosmologically viable and distinguishable... more Recently, a model f (R) theory is proposed [1] which is cosmologically viable and distinguishable from ΛCDM. We use chameleon mechanism to investigate viability of the model in terms of Solar System experiments.

Gravitation and Cosmology, 2012

The f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravit... more The f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. The scalar field couples with the matter sector and the coupling term is given by the conformal factor. We apply the holographic principle to such an interacting model in spatially flat and curved universes. We show that the model leads to a constant ratio of energy densities of dark matter to dark energy in a spatially flat universe. In a spatially curved universe, the ratio is not a constant and the evolution seems to be model-dependent. However, we argue that any cosmologically viable f (R) model can lead to a nearly constant ratio of energy densities and therefore alleviate the coincidence problem.

Gravitation and Cosmology, 2010

There is a conformal equivalence between power law f (R) theories and scalar field theories in wh... more There is a conformal equivalence between power law f (R) theories and scalar field theories in which the scalar degree of freedom evolves under the action of an exponential potential function. In the scalar field representation there is a strong coupling of the scalar field with the matter sector due to the conformal transformation. We use chameleon mechanism to implement constraints on the potential function of the scalar field in order that the resulting model be in accord with Solar System experiments. Investigation of these constraints reveals that there may be no possibility to distinguish between a power law f (R) function and the usual Einstein-Hilbert Lagrangian density.

Gravitation and Cosmology, 2012

We study capability of f (R) gravity models to allow crossing the phantom boundary in both Jordan... more We study capability of f (R) gravity models to allow crossing the phantom boundary in both Jordan and Einstein conformal frames. In Einstein frame, these models are equivalent to Einstein gravity together with a scalar field minimally coupled to gravity. This scalar degree of freedom appears as a quintessence field with a coupling with the matter sector. We investigate evolution of the equation of sate parameter for some cosmologically viable f (R) gravity models in both conformal frames. This investigation (beyond mere theoretical arguments) acts as an operational tool to distinguish physical status of the two conformal frames. It shows that the two conformal frames have not the same physical status.

Physics Letters B, 2010

The f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravit... more The f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. We shall explore phantom behavior of f (R) models in this frame and compare the results with those of the usual notion of phantom scalar field.

Physical Review D, 2012

We consider modified theories of gravity with a direct coupling between matter and geometry, deno... more We consider modified theories of gravity with a direct coupling between matter and geometry, denoted by an arbitrary function in terms of the Ricci scalar. Due to such a coupling, the matter stress tensor is no longer conserved and there is an energy transfer between the two components. By solving the conservation equation, we argue that the matter system should gain energy in this interaction, as demanded by the second law of thermodynamics. In a cosmological setting, we show that although this kind of interaction may account for cosmic acceleration, this latter together with direction of the energy transfer constrain the coupling function.

Physica Scripta, 2011

It is well-known that f (R) gravity models formulated in Einstein conformal frame are equivalent ... more It is well-known that f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. In this case, the scalar field couples with the matter sector and the coupling term is given by the conformal factor. We apply the holographic principle to such interacting models. In a spatially flat universe, we show that the Einstein frame representation of f (R) models leads to a constant ratio of energy densities of dark matter to dark energy.

Physica Scripta, 2009

We propose a scalar-tensor representation of f (R) theories with use of conformal transformations... more We propose a scalar-tensor representation of f (R) theories with use of conformal transformations. In this representation, the model takes the form of the Brans-Dicke model with a potential function and a non-zero kinetic term for the scalar field. In this case, the scalar field may interact with matter systems and the corresponding matter stress tensor may be non-conserved.

Physics Letters B, Nov 1, 2017

We have investigated a brane world model in which the gravitational field in the bulk is describe... more We have investigated a brane world model in which the gravitational field in the bulk is described both by a metric tensor and a minimally coupled scalar field. This scalar field is taken to be a chameleon with an appropriate potential function. The scalar field interacts with matter and there is an energy transfer between the two components. We find a late-time asymptotic solution which exhibits late-time accelerating expansion. We also show that the Universe recently crosses the phantom barrier without recourse to any exotic matter. We provide some thermodynamic arguments which constrain both the direction of energy transfer and dynamics of the extra dimension.

EPL (Europhysics Letters)

We have investigated some cosmological consequences of variation of the fine structure constant, ... more We have investigated some cosmological consequences of variation of the fine structure constant, α, in the context of the Bekenstein-Sandvik-Barrow-Magueijo (BSBM) model. In this context, such a variation is attributed to electric charge by letting the latter take on the value of a real scalar field, φ, which varies generally in space and time. We consider the dynamics of φ in the case that it is allowed to have an exponential potential. We have found solutions for evolutions of φ and α in the radiation and the matter dominated eras. By employing observational bounds on the temporal variation of α, we constrain the model parameters. We then investigate the impact of α-variation on redshift dependence of CMB temperature. It is shown that there is a deviation from the standard evolution of the temperature which contains two different contributions, one is important at high redshift eras and the other is mostly effective at low redshift eras.

There is a conformal equivalence between power law f (R) theories and scalar field theories in wh... more There is a conformal equivalence between power law f (R) theories and scalar field theories in which the scalar degree of freedom evolves under the action of an exponential potential function. In the scalar field representation there is a strong coupling of the scalar field with the matter sector due to the conformal transformation. We use chameleon mechanism to implement constraints on the potential function of the scalar field in order that the resulting model be in accord with Solar System experiments. Investigation of these constraints reveals that there may be no possibility to distinguish between a power law f (R) function and the usual Einstein-Hilbert Lagrangian density.

Recently, a model f (R) theory is proposed [1] which is cosmologically viable and distinguishable... more Recently, a model f (R) theory is proposed [1] which is cosmologically viable and distinguishable from ΛCDM. We use chameleon mechanism to investigate viability of the model in terms of Solar System experiments.

We consider a generalized Brans-Dicke model in which the scalar field has a potential function an... more We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple non-minimally with the matter sector. This anomalous gravitational coupling can in principle avoid the model to pass local gravity experiments. One then usually assumes that the scalar field has a chameleon behavior in the sense that it acquires a density-dependent effective mass. While it can take a small effective mass in cosmological (low-density environment) scale, it has a sufficiently heavy mass in Solar System (large-density environment) and then hides gravity tests. We will argue that such a chameleon behavior can not be generally realized and depends significantly on the forms attributed to the potential and the coupling functions.

We consider modified theories of gravity with a direct coupling between matter and geometry, deno... more We consider modified theories of gravity with a direct coupling between matter and geometry, denoted by an arbitrary function in terms of the Ricci scalar. Due to such a coupling, the matter stress tensor is no longer conserved and there is an energy transfer between the two components. By solving the conservation equation, we argue that the matter system should gain energy in this interaction, as demanded by the second law of thermodynamics. In a cosmological setting, we show that although this kind of interaction may account for cosmic acceleration, this latter together with direction of the energy transfer constrain the coupling function.

We consider a generalized Brans-Dicke model in which the scalar field has a potential function an... more We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple non-minimally with the matter sector. We assume a power law form for the potential and the coupling functions as the inputs of the model and show that acceleration of the universe can be realized for a constrained range of exponent of the potential function. We also argue that this accelerating phase is consistent with a large and positive Brans-Dicke parameter. In our analysis, the potential plays a more important role with respect to the coupling function in dynamics of the universe as the latter does not contribute to any of the relations characterizing evolution of scale factor of the universe and the scalar field. However, we will show that the coupling function is closely related to magnitude and direction of the energy transfer between matter and the scale field. We use this fact and some thermodynamic aspects of the model to put some constraints on the...

Physics Letters B, 2017

We have investigated a brane world model in which the gravitational field in the bulk is describe... more We have investigated a brane world model in which the gravitational field in the bulk is described both by a metric tensor and a minimally coupled scalar field. This scalar field is taken to be a chameleon with an appropriate potential function. The scalar field interacts with matter and there is an energy transfer between the two components. We find a late-time asymptotic solution which exhibits late-time accelerating expansion. We also show that the Universe recently crosses the phantom barrier without recourse to any exotic matter. We provide some thermodynamic arguments which constrain both the direction of energy transfer and dynamics of the extra dimension.

International Journal of Modern Physics D, 2018

We deal with a dynamical mechanism in which a large cosmological constant, as suggested by inflat... more We deal with a dynamical mechanism in which a large cosmological constant, as suggested by inflationary scenarios, decays due to expansion of the universe. This mechanism has its origin in the gravitational coupling of the vacuum density. We assume that the vacuum couples anomalously to gravity that is the metric tensor that appears the gravitational part is not the same as that appears the matter part as suggested by weak equivalence principle. Instead, the two metric tensors are taken to be conformally related. We show that this provides a dynamical mechanism which works during expansion of the universe. We also consider some observational consequences of such a gravitational model.

Gravitation and Cosmology, 2018

In a homogenous and isotropic cosmology, we introduce general exact solutions for some modified g... more In a homogenous and isotropic cosmology, we introduce general exact solutions for some modified gravity models. In particular, we introduce exact solutions for powerlaw f (R) gravity and Brans-Dicke theory in Einstein and Jordan conformal frames. In the Brans-Dicke case, the solutions are presented for both single and double exponential potentials in Einstein frame which correspond to power-law potentials in Jordan frame. Our analysis for extracting general exact solutions can also be generalized to those scalartensor theories in which the scalar field has an exponential coupling to Ricci scalar.

International Journal of Modern Physics D, 2019

We consider a generalized Brans–Dicke model in which the scalar field has a self-interacting pote... more We consider a generalized Brans–Dicke model in which the scalar field has a self-interacting potential function. The scalar field is also allowed to couple nonminimally with the matter part. We assume that it has a chameleon behavior in the sense that it acquires a density-dependent effective mass. We consider two different types of matter systems which couple with the chameleon, dust and vacuum. In the first case, we find a set of exact solutions when the potential has an exponential form. In the second case, we find a power-law exact solution for the scale factor. In this case, we will show that the vacuum density decays during expansion due to coupling with the chameleon.

Physics Letters B, 2010

It is well-known that f (R) theories in Einstein frame is conformally equivalent to quintessence ... more It is well-known that f (R) theories in Einstein frame is conformally equivalent to quintessence models in which the scalar field minimally couples with gravity. If there exists a matter system in Jordan frame, then it interacts with the scalar field in Einstein frame due to the conformal transformations. This interaction, in general, may lead to changes of fundamental constants. Here we will consider possible time variation of fine structure constant in a general f (R) theory. We will use observational bounds on these variations and argue that it provides a criterion for constraining f (R) models.

Physics Letters B, 2010

Recently, a model f (R) theory is proposed [1] which is cosmologically viable and distinguishable... more Recently, a model f (R) theory is proposed [1] which is cosmologically viable and distinguishable from ΛCDM. We use chameleon mechanism to investigate viability of the model in terms of Solar System experiments.

Gravitation and Cosmology, 2012

The f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravit... more The f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. The scalar field couples with the matter sector and the coupling term is given by the conformal factor. We apply the holographic principle to such an interacting model in spatially flat and curved universes. We show that the model leads to a constant ratio of energy densities of dark matter to dark energy in a spatially flat universe. In a spatially curved universe, the ratio is not a constant and the evolution seems to be model-dependent. However, we argue that any cosmologically viable f (R) model can lead to a nearly constant ratio of energy densities and therefore alleviate the coincidence problem.

Gravitation and Cosmology, 2010

There is a conformal equivalence between power law f (R) theories and scalar field theories in wh... more There is a conformal equivalence between power law f (R) theories and scalar field theories in which the scalar degree of freedom evolves under the action of an exponential potential function. In the scalar field representation there is a strong coupling of the scalar field with the matter sector due to the conformal transformation. We use chameleon mechanism to implement constraints on the potential function of the scalar field in order that the resulting model be in accord with Solar System experiments. Investigation of these constraints reveals that there may be no possibility to distinguish between a power law f (R) function and the usual Einstein-Hilbert Lagrangian density.

Gravitation and Cosmology, 2012

We study capability of f (R) gravity models to allow crossing the phantom boundary in both Jordan... more We study capability of f (R) gravity models to allow crossing the phantom boundary in both Jordan and Einstein conformal frames. In Einstein frame, these models are equivalent to Einstein gravity together with a scalar field minimally coupled to gravity. This scalar degree of freedom appears as a quintessence field with a coupling with the matter sector. We investigate evolution of the equation of sate parameter for some cosmologically viable f (R) gravity models in both conformal frames. This investigation (beyond mere theoretical arguments) acts as an operational tool to distinguish physical status of the two conformal frames. It shows that the two conformal frames have not the same physical status.

Physics Letters B, 2010

The f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravit... more The f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. We shall explore phantom behavior of f (R) models in this frame and compare the results with those of the usual notion of phantom scalar field.

Physical Review D, 2012

We consider modified theories of gravity with a direct coupling between matter and geometry, deno... more We consider modified theories of gravity with a direct coupling between matter and geometry, denoted by an arbitrary function in terms of the Ricci scalar. Due to such a coupling, the matter stress tensor is no longer conserved and there is an energy transfer between the two components. By solving the conservation equation, we argue that the matter system should gain energy in this interaction, as demanded by the second law of thermodynamics. In a cosmological setting, we show that although this kind of interaction may account for cosmic acceleration, this latter together with direction of the energy transfer constrain the coupling function.

Physica Scripta, 2011

It is well-known that f (R) gravity models formulated in Einstein conformal frame are equivalent ... more It is well-known that f (R) gravity models formulated in Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. In this case, the scalar field couples with the matter sector and the coupling term is given by the conformal factor. We apply the holographic principle to such interacting models. In a spatially flat universe, we show that the Einstein frame representation of f (R) models leads to a constant ratio of energy densities of dark matter to dark energy.

Physica Scripta, 2009

We propose a scalar-tensor representation of f (R) theories with use of conformal transformations... more We propose a scalar-tensor representation of f (R) theories with use of conformal transformations. In this representation, the model takes the form of the Brans-Dicke model with a potential function and a non-zero kinetic term for the scalar field. In this case, the scalar field may interact with matter systems and the corresponding matter stress tensor may be non-conserved.