Francisco Lobo | University of Lisbon (original) (raw)

Papers by Francisco Lobo

Research paper thumbnail of Kinetic gravity braiding wormhole geometries

Physical Review D

An interesting class of scalar-tensor models, denoted by kinetic gravity braiding (KGB), has rece... more An interesting class of scalar-tensor models, denoted by kinetic gravity braiding (KGB), has recently been proposed. These models contain interactions of the second derivatives of the scalar field that do not lead to additional degrees of freedom and exhibit peculiar features, such as an essential mixing of the scalar φ and tensor kinetic X terms. In this work, we consider the possibility that wormhole geometries are sustained by the KGB theory. More specifically, we present the full gravitational field equations in a static and spherically symmetric traversable wormhole background, and outline the general constraints at the wormhole throat, imposed by the flaring-out conditions. Furthermore, we present a plethora of analytical and numerical wormhole solutions by considering particular choices of the KGB factors. The analysis explicitly demonstrates that the KGB theory exhibits a rich structure of wormhole geometries, ranging from asymptotically flat solutions to asymptotically anti-de Sitter spacetimes.

Research paper thumbnail of Novel black-bounce geometries

We develop a number of novel "black-bounce" spacetimes. These are specific regular blac... more We develop a number of novel "black-bounce" spacetimes. These are specific regular black holes where the "area radius" always remains non-zero, thereby leading to a "throat" that is either timelike (corresponding to a traversable wormhole), spacelike (corresponding to a "bounce" into a future universe), or null (corresponding to a "one-way wormhole"). We shall first perform a general analysis of the regularity conditions for such a spacetime, and then consider a number of specific examples. The examples are constructed using a mass function similar to that of Fan--Wang, and fall into several particular cases, such as the original Simpson--Visser model, a Bardeen-type model, and other generalizations thereof. We shall analyse the regularity, the energy conditions, and the causal structure of these models. The main results are several new geometries, more complex than before, with two or more horizons, with the possibility of an extrem...

Research paper thumbnail of Warp drive basics

Research paper thumbnail of 0 50 70 63 v 1 1 4 Ju l 2 00 5 Stability analysis of dynamic thin shells

We analyze the stability of generic spherically symmetric thin shells to linearized perturbations... more We analyze the stability of generic spherically symmetric thin shells to linearized perturbations around static solutions. We include the momentum flux term in the conservation identity, deduced from the “ADM” constraint and the Lanczos equations. Following the Ishak-Lake analysis, we deduce a master equation which dictates the stable equilibrium configurations. Considering the transparency condition, we study the stability of thin shells around black holes, showing that our analysis is in agreement with previous results. Applying the analysis to traversable wormhole geometries, by considering specific choices for the form function, we deduce stability regions, and find that the latter may be significantly increased by considering appropriate choices for the redshift function. PACS numbers: 04.20.Cv, 04.20.Gz, 04.70.Bw † flobo@cosmo.fis.fc.ul.pt ‡ crawford@cosmo.fis.fc.ul.pt Stability analysis of dynamic thin shells 2

Research paper thumbnail of qc / 0 50 70 63 v 2 6 O ct 2 00 5 Stability analysis of dynamic thin shells

We analyze the stability of generic spherically symmetric thin shells to linearized perturbations... more We analyze the stability of generic spherically symmetric thin shells to linearized perturbations around static solutions. We include the momentum flux term in the conservation identity, deduced from the “ADM” constraint and the Lanczos equations. Following the Ishak-Lake analysis, we deduce a master equation which dictates the stable equilibrium configurations. Considering the transparency condition, we study the stability of thin shells around black holes, showing that our analysis is in agreement with previous results. Applying the analysis to traversable wormhole geometries, by considering specific choices for the form function, we deduce stability regions, and find that the latter may be significantly increased by considering appropriate choices for the redshift function. PACS numbers: 04.20.Cv, 04.20.Gz, 04.70.Bw † flobo@cosmo.fis.fc.ul.pt ‡ crawford@cosmo.fis.fc.ul.pt Stability analysis of dynamic thin shells 2

Research paper thumbnail of General constraints on Horndeski wormhole throats

Physical Review D, 2020

In this work, we consider the full Horndeski Lagrangian applied to wormhole geometries and presen... more In this work, we consider the full Horndeski Lagrangian applied to wormhole geometries and present the full gravitational field equations. We analyse the general constraints imposed by the flaring-out conditions at the wormhole throat and consider a plethora of specific subclasses of the Horndeski Lagrangian, namely, quintessence/phantom fields, k-essence, scalar-tensor theories, covariant galileons, nonminimal kinetic coupling, kinetic gravity braiding, and the scalar-tensor representation of Gauss-Bonnet couplings, amongst others. The generic constraints analysed in this work serve as a consistency check of the main solutions obtained in the literature and draws out new avenues of research in considering applications of specific subclasses of the Horndeski theory to wormhole physics.

Research paper thumbnail of Dynamic wormhole geometries in hybrid metric-Palatini gravity

The European Physical Journal C, 2021

In this work, we analyse the evolution of time-dependent traversable wormhole geometries in a Fri... more In this work, we analyse the evolution of time-dependent traversable wormhole geometries in a Friedmann–Lemaître–Robertson–Walker background in the context of the scalar–tensor representation of hybrid metric-Palatini gravity. We deduce the energy–momentum profile of the matter threading the wormhole spacetime in terms of the background quantities, the scalar field, the scale factor and the shape function, and find specific wormhole solutions by considering a barotropic equation of state for the background matter. We find that particular cases satisfy the null and weak energy conditions for all times. In addition to the barotropic equation of state, we also explore a specific evolving wormhole spacetime, by imposing a traceless energy–momentum tensor for the matter threading the wormhole and find that this geometry also satisfies the null and weak energy conditions at all times.

Research paper thumbnail of Dynamic thin-shell black-bounce traversable wormholes

Physical Review D, 2020

Based on the recently introduced black-bounce spacetimes, we shall consider the construction of t... more Based on the recently introduced black-bounce spacetimes, we shall consider the construction of the related spherically symmetric thin-shell traversable wormholes within the context of standard general relativity. All of the really unusual physics is encoded in one simple parameter a which characterizes the scale of the bounce. Keeping the discussion as close as possible to standard general relativity is the theorist's version of only adjusting one feature of the model at a time. We shall modify the standard thin-shell traversable wormhole construction, each bulk region now being a black-bounce spacetime, and with the physics of the thin shell being (as much as possible) derivable from the Einstein equations. Furthermore, we shall apply a dynamical analysis to the throat by considering linearized radial perturbations around static solutions, and demonstrate that the stability of the wormhole is equivalent to choosing suitable properties for the exotic material residing on the wormhole throat. The construction is sufficiently novel to be interesting, and sufficiently straightforward to be tractable.

Research paper thumbnail of Thin-shell traversable wormhole crafted from a regular black hole with asymptotically Minkowski core

Physical Review D, 2020

Recently, a novel model for a regular black hole was advocated which possesses an asymptotically ... more Recently, a novel model for a regular black hole was advocated which possesses an asymptotically Minkowski core implemented via an exponential suppression (in the core region) of the Misner-Sharp quasi-local mass. Using this regular black hole as a template, we shall construct a spherically symmetric thin-shell traversable wormhole using the "cut-and-paste" technique, thereby constructing yet another black hole mimicker. The surface stress-energy at the wormhole throat is calculated, and the stability of the wormhole is analyzed. An important result is that, (as compared to their Schwarzschild thin-shell counterparts), increasing the exponential suppression of the Misner-Sharp quasi-local mass by increasing the suppression parameter a, also considerably increases the stability regions for these thin-shell wormholes, and furthermore minimizes the amount of energy condition violating exotic matter required to keep the wormhole throat open.

Research paper thumbnail of Structure and stability of traversable thin-shell wormholes in Palatini f(R) gravity

Research paper thumbnail of Novel black-bounce spacetimes: Wormholes, regularity, energy conditions, and causal structure

Physical Review D, 2021

We develop a number of novel "black-bounce" spacetimes. These are specific regular black holes wh... more We develop a number of novel "black-bounce" spacetimes. These are specific regular black holes where the "area radius" always remains non-zero, thereby leading to a "throat" that is either timelike (corresponding to a traversable wormhole), spacelike (corresponding to a "bounce" into a future universe), or null (corresponding to a "one-way wormhole"). We shall first perform a general analysis of the regularity conditions for such a spacetime, and then consider a number of specific examples. The examples are constructed using a mass function similar to that of Fan-Wang, and fall into several particular cases, such as the original Simpson-Visser model, a Bardeen-type model, and other generalizations thereof. We shall analyse the regularity, the energy conditions, and the causal structure of these models. The main results are several new geometries, more complex than before, with two or more horizons, with the possibility of an extremal case. We shall derive a general theorem regarding static space-time regularity, and another general theorem regarding (non)-satisfaction of the classical energy conditions.

Research paper thumbnail of Wormhole Basics

Fundamental Theories of Physics, 2017

Research paper thumbnail of Beyond Einstein’s General Relativity: Hybrid metric-Palatini gravity and curvature-matter couplings

International Journal of Modern Physics D, 2020

Einstein’s General Relativity (GR) is possibly one of the greatest intellectual achievements ever... more Einstein’s General Relativity (GR) is possibly one of the greatest intellectual achievements ever conceived by the human mind. In fact, over the last century, GR has proven to be an extremely successful theory, with a well established experimental footing, at least for weak gravitational fields. Its predictions range from the existence of black holes and gravitational radiation (now confirmed) to the cosmological models. Indeed, a central theme in modern Cosmology is the perplexing fact that the Universe is undergoing an accelerating expansion, which represents a new imbalance in the governing gravitational equations. The cause of the late-time cosmic acceleration remains an open and tantalizing question, and has forced theorists and experimentalists to question whether GR is the correct relativistic theory of gravitation. This has spurred much research in modified theories of gravity, where extensions of the Hilbert–Einstein action describe the gravitational field, in particular, [...

Research paper thumbnail of Wormhole geometries induced by action-dependent Lagrangian theories

Physical Review D, 2021

In this work, we explore wormhole geometries in a recently proposed modified gravity theory arisi... more In this work, we explore wormhole geometries in a recently proposed modified gravity theory arising from a non-conservative gravitational theory, tentatively denoted action-dependent Lagrangian theories. The generalized gravitational field equation essentially depends on a background fourvector λ µ , that plays the role of a coupling parameter associated with the dependence of the gravitational Lagrangian upon the action, and may generically depend on the spacetime coordinates. Considering wormhole configurations, by using "Buchdahl coordinates", we find that the four-vector is given by λµ = (0, 0, λ θ , 0), and that the spacetime geometry is severely restricted by the condition gttguu = −1, where u is the radial coordinate. We find a plethora of specific asymptotically flat, symmetric and asymmetric, solutions with power law choices for the function λ, by generalizing the Ellis-Bronnikov solutions and the recently proposed black bounce geometries, amongst others. We show that these compact objects possess a far richer geometrical structure than their general relativistic counterparts.

Research paper thumbnail of Thick brane solitons breaking Z2 symmetry

The Fourteenth Marcel Grossmann Meeting, Nov 16, 2017

New soliton solutions for thick branes in 4 + 1 dimensions are considered in this article. In par... more New soliton solutions for thick branes in 4 + 1 dimensions are considered in this article. In particular, brane models based on the sine-Gordon (SG), ϕ 4 and ϕ 6 scalar fields are investigated; in some cases Z 2 symmetry is broken. Besides, these soliton solutions are responsible for supporting and stabilizing the thick branes. In these models, the origin of the symmetry breaking resides in the fact that the modified scalar field potential may have non-degenerate vacuua and these non-degenerate vacuua determine the cosmological constant on both sides of the brane. At last, in order to explore the particle motion in the neighborhood of the brane, the geodesic equations along the fifth dimension are studied.

Research paper thumbnail of Irreversible matter creation processes through a nonminimal curvature-matter coupling

The Fourteenth Marcel Grossmann Meeting, Nov 16, 2017

An interesting cosmological history was proposed by Prigogine et al. who considered the Universe ... more An interesting cosmological history was proposed by Prigogine et al. who considered the Universe as a thermodynamically open system. This scenario is characterized by a process of matter creation, which corresponds to an irreversible energy flow from the gravitational field to the pressureless matter fluid. Here, we show that the gravitationally induced particle production may arise from a nonminimal curvature-matter coupling. By considering the equivalent scalar-tensor theory, the cosmological implications of the model are discussed. As all known natural systems tend to a state of thermodynamic equilibrium, and assuming the universe is not different in this respect, we also discuss the conditions to attain the equilibrium state.

Research paper thumbnail of Observational constraints of f(Q) gravity

Research paper thumbnail of Wormhole geometries supported by three-form fields

Physical Review D, 2018

In this work, we find novel static and spherically symmetric wormhole geometries using a threefor... more In this work, we find novel static and spherically symmetric wormhole geometries using a threeform field. By solving the gravitational field equations, we find a variety of analytical and numerical solutions and show that it is possible for the matter fields threading the wormhole to satisfy the null and weak energy conditions throughout the spacetime, when the three-form field is present. In these cases, the form field is responsible for supporting the wormhole and all the exoticity is confined to it. Thus, the three-form curvature terms, which may be interpreted as a gravitational fluid, sustain these wormhole geometries. We also show that in the case of a vanishing redshift function the field can display a cosmological constant behavior.

Research paper thumbnail of Palatini wormholes and energy conditions from the prism of general relativity

The European physical journal. C, Particles and fields, 2017

Wormholes are hypothetical shortcuts in spacetime that in general relativity unavoidably violate ... more Wormholes are hypothetical shortcuts in spacetime that in general relativity unavoidably violate all of the pointwise energy conditions. In this paper, we consider several wormhole spacetimes that, as opposed to the standard procedure frequently employed in the literature, arise directly from gravitational actions including additional terms resulting from contractions of the Ricci tensor with the metric, and which are formulated assuming independence between metric and connection (Palatini approach). We reinterpret such wormhole solutions under the prism of General Relativity and study the matter sources that thread them. We discuss the size of violation of the energy conditions in different cases and how this is related to the same spacetimes when viewed from the modified gravity side.

Research paper thumbnail of From the Flamm–Einstein–Rosen bridge to the modern renaissance of traversable wormholes

International Journal of Modern Physics D, 2016

We consider the possibility of multiply-connected spacetimes, ranging from the Flamm–Einstein–Ros... more We consider the possibility of multiply-connected spacetimes, ranging from the Flamm–Einstein–Rosen bridge, geons, and the modern renaissance of traversable wormholes. A fundamental property in wormhole physics is the flaring-out condition of the throat, which through the Einstein field equation entails the violation of the null energy condition (NEC). In the context of modified theories of gravity, it has also been shown that the normal matter can be imposed to satisfy the energy conditions, and it is the higher order curvature terms, interpreted as a gravitational fluid, that sustain these nonstandard wormhole geometries, fundamentally different from their counterparts in general relativity (GR). We explore interesting features of these geometries, in particular, the physical properties and characteristics of these ‘exotic spacetimes’.

Research paper thumbnail of Kinetic gravity braiding wormhole geometries

Physical Review D

An interesting class of scalar-tensor models, denoted by kinetic gravity braiding (KGB), has rece... more An interesting class of scalar-tensor models, denoted by kinetic gravity braiding (KGB), has recently been proposed. These models contain interactions of the second derivatives of the scalar field that do not lead to additional degrees of freedom and exhibit peculiar features, such as an essential mixing of the scalar φ and tensor kinetic X terms. In this work, we consider the possibility that wormhole geometries are sustained by the KGB theory. More specifically, we present the full gravitational field equations in a static and spherically symmetric traversable wormhole background, and outline the general constraints at the wormhole throat, imposed by the flaring-out conditions. Furthermore, we present a plethora of analytical and numerical wormhole solutions by considering particular choices of the KGB factors. The analysis explicitly demonstrates that the KGB theory exhibits a rich structure of wormhole geometries, ranging from asymptotically flat solutions to asymptotically anti-de Sitter spacetimes.

Research paper thumbnail of Novel black-bounce geometries

We develop a number of novel "black-bounce" spacetimes. These are specific regular blac... more We develop a number of novel "black-bounce" spacetimes. These are specific regular black holes where the "area radius" always remains non-zero, thereby leading to a "throat" that is either timelike (corresponding to a traversable wormhole), spacelike (corresponding to a "bounce" into a future universe), or null (corresponding to a "one-way wormhole"). We shall first perform a general analysis of the regularity conditions for such a spacetime, and then consider a number of specific examples. The examples are constructed using a mass function similar to that of Fan--Wang, and fall into several particular cases, such as the original Simpson--Visser model, a Bardeen-type model, and other generalizations thereof. We shall analyse the regularity, the energy conditions, and the causal structure of these models. The main results are several new geometries, more complex than before, with two or more horizons, with the possibility of an extrem...

Research paper thumbnail of Warp drive basics

Research paper thumbnail of 0 50 70 63 v 1 1 4 Ju l 2 00 5 Stability analysis of dynamic thin shells

We analyze the stability of generic spherically symmetric thin shells to linearized perturbations... more We analyze the stability of generic spherically symmetric thin shells to linearized perturbations around static solutions. We include the momentum flux term in the conservation identity, deduced from the “ADM” constraint and the Lanczos equations. Following the Ishak-Lake analysis, we deduce a master equation which dictates the stable equilibrium configurations. Considering the transparency condition, we study the stability of thin shells around black holes, showing that our analysis is in agreement with previous results. Applying the analysis to traversable wormhole geometries, by considering specific choices for the form function, we deduce stability regions, and find that the latter may be significantly increased by considering appropriate choices for the redshift function. PACS numbers: 04.20.Cv, 04.20.Gz, 04.70.Bw † flobo@cosmo.fis.fc.ul.pt ‡ crawford@cosmo.fis.fc.ul.pt Stability analysis of dynamic thin shells 2

Research paper thumbnail of qc / 0 50 70 63 v 2 6 O ct 2 00 5 Stability analysis of dynamic thin shells

We analyze the stability of generic spherically symmetric thin shells to linearized perturbations... more We analyze the stability of generic spherically symmetric thin shells to linearized perturbations around static solutions. We include the momentum flux term in the conservation identity, deduced from the “ADM” constraint and the Lanczos equations. Following the Ishak-Lake analysis, we deduce a master equation which dictates the stable equilibrium configurations. Considering the transparency condition, we study the stability of thin shells around black holes, showing that our analysis is in agreement with previous results. Applying the analysis to traversable wormhole geometries, by considering specific choices for the form function, we deduce stability regions, and find that the latter may be significantly increased by considering appropriate choices for the redshift function. PACS numbers: 04.20.Cv, 04.20.Gz, 04.70.Bw † flobo@cosmo.fis.fc.ul.pt ‡ crawford@cosmo.fis.fc.ul.pt Stability analysis of dynamic thin shells 2

Research paper thumbnail of General constraints on Horndeski wormhole throats

Physical Review D, 2020

In this work, we consider the full Horndeski Lagrangian applied to wormhole geometries and presen... more In this work, we consider the full Horndeski Lagrangian applied to wormhole geometries and present the full gravitational field equations. We analyse the general constraints imposed by the flaring-out conditions at the wormhole throat and consider a plethora of specific subclasses of the Horndeski Lagrangian, namely, quintessence/phantom fields, k-essence, scalar-tensor theories, covariant galileons, nonminimal kinetic coupling, kinetic gravity braiding, and the scalar-tensor representation of Gauss-Bonnet couplings, amongst others. The generic constraints analysed in this work serve as a consistency check of the main solutions obtained in the literature and draws out new avenues of research in considering applications of specific subclasses of the Horndeski theory to wormhole physics.

Research paper thumbnail of Dynamic wormhole geometries in hybrid metric-Palatini gravity

The European Physical Journal C, 2021

In this work, we analyse the evolution of time-dependent traversable wormhole geometries in a Fri... more In this work, we analyse the evolution of time-dependent traversable wormhole geometries in a Friedmann–Lemaître–Robertson–Walker background in the context of the scalar–tensor representation of hybrid metric-Palatini gravity. We deduce the energy–momentum profile of the matter threading the wormhole spacetime in terms of the background quantities, the scalar field, the scale factor and the shape function, and find specific wormhole solutions by considering a barotropic equation of state for the background matter. We find that particular cases satisfy the null and weak energy conditions for all times. In addition to the barotropic equation of state, we also explore a specific evolving wormhole spacetime, by imposing a traceless energy–momentum tensor for the matter threading the wormhole and find that this geometry also satisfies the null and weak energy conditions at all times.

Research paper thumbnail of Dynamic thin-shell black-bounce traversable wormholes

Physical Review D, 2020

Based on the recently introduced black-bounce spacetimes, we shall consider the construction of t... more Based on the recently introduced black-bounce spacetimes, we shall consider the construction of the related spherically symmetric thin-shell traversable wormholes within the context of standard general relativity. All of the really unusual physics is encoded in one simple parameter a which characterizes the scale of the bounce. Keeping the discussion as close as possible to standard general relativity is the theorist's version of only adjusting one feature of the model at a time. We shall modify the standard thin-shell traversable wormhole construction, each bulk region now being a black-bounce spacetime, and with the physics of the thin shell being (as much as possible) derivable from the Einstein equations. Furthermore, we shall apply a dynamical analysis to the throat by considering linearized radial perturbations around static solutions, and demonstrate that the stability of the wormhole is equivalent to choosing suitable properties for the exotic material residing on the wormhole throat. The construction is sufficiently novel to be interesting, and sufficiently straightforward to be tractable.

Research paper thumbnail of Thin-shell traversable wormhole crafted from a regular black hole with asymptotically Minkowski core

Physical Review D, 2020

Recently, a novel model for a regular black hole was advocated which possesses an asymptotically ... more Recently, a novel model for a regular black hole was advocated which possesses an asymptotically Minkowski core implemented via an exponential suppression (in the core region) of the Misner-Sharp quasi-local mass. Using this regular black hole as a template, we shall construct a spherically symmetric thin-shell traversable wormhole using the "cut-and-paste" technique, thereby constructing yet another black hole mimicker. The surface stress-energy at the wormhole throat is calculated, and the stability of the wormhole is analyzed. An important result is that, (as compared to their Schwarzschild thin-shell counterparts), increasing the exponential suppression of the Misner-Sharp quasi-local mass by increasing the suppression parameter a, also considerably increases the stability regions for these thin-shell wormholes, and furthermore minimizes the amount of energy condition violating exotic matter required to keep the wormhole throat open.

Research paper thumbnail of Structure and stability of traversable thin-shell wormholes in Palatini f(R) gravity

Research paper thumbnail of Novel black-bounce spacetimes: Wormholes, regularity, energy conditions, and causal structure

Physical Review D, 2021

We develop a number of novel "black-bounce" spacetimes. These are specific regular black holes wh... more We develop a number of novel "black-bounce" spacetimes. These are specific regular black holes where the "area radius" always remains non-zero, thereby leading to a "throat" that is either timelike (corresponding to a traversable wormhole), spacelike (corresponding to a "bounce" into a future universe), or null (corresponding to a "one-way wormhole"). We shall first perform a general analysis of the regularity conditions for such a spacetime, and then consider a number of specific examples. The examples are constructed using a mass function similar to that of Fan-Wang, and fall into several particular cases, such as the original Simpson-Visser model, a Bardeen-type model, and other generalizations thereof. We shall analyse the regularity, the energy conditions, and the causal structure of these models. The main results are several new geometries, more complex than before, with two or more horizons, with the possibility of an extremal case. We shall derive a general theorem regarding static space-time regularity, and another general theorem regarding (non)-satisfaction of the classical energy conditions.

Research paper thumbnail of Wormhole Basics

Fundamental Theories of Physics, 2017

Research paper thumbnail of Beyond Einstein’s General Relativity: Hybrid metric-Palatini gravity and curvature-matter couplings

International Journal of Modern Physics D, 2020

Einstein’s General Relativity (GR) is possibly one of the greatest intellectual achievements ever... more Einstein’s General Relativity (GR) is possibly one of the greatest intellectual achievements ever conceived by the human mind. In fact, over the last century, GR has proven to be an extremely successful theory, with a well established experimental footing, at least for weak gravitational fields. Its predictions range from the existence of black holes and gravitational radiation (now confirmed) to the cosmological models. Indeed, a central theme in modern Cosmology is the perplexing fact that the Universe is undergoing an accelerating expansion, which represents a new imbalance in the governing gravitational equations. The cause of the late-time cosmic acceleration remains an open and tantalizing question, and has forced theorists and experimentalists to question whether GR is the correct relativistic theory of gravitation. This has spurred much research in modified theories of gravity, where extensions of the Hilbert–Einstein action describe the gravitational field, in particular, [...

Research paper thumbnail of Wormhole geometries induced by action-dependent Lagrangian theories

Physical Review D, 2021

In this work, we explore wormhole geometries in a recently proposed modified gravity theory arisi... more In this work, we explore wormhole geometries in a recently proposed modified gravity theory arising from a non-conservative gravitational theory, tentatively denoted action-dependent Lagrangian theories. The generalized gravitational field equation essentially depends on a background fourvector λ µ , that plays the role of a coupling parameter associated with the dependence of the gravitational Lagrangian upon the action, and may generically depend on the spacetime coordinates. Considering wormhole configurations, by using "Buchdahl coordinates", we find that the four-vector is given by λµ = (0, 0, λ θ , 0), and that the spacetime geometry is severely restricted by the condition gttguu = −1, where u is the radial coordinate. We find a plethora of specific asymptotically flat, symmetric and asymmetric, solutions with power law choices for the function λ, by generalizing the Ellis-Bronnikov solutions and the recently proposed black bounce geometries, amongst others. We show that these compact objects possess a far richer geometrical structure than their general relativistic counterparts.

Research paper thumbnail of Thick brane solitons breaking Z2 symmetry

The Fourteenth Marcel Grossmann Meeting, Nov 16, 2017

New soliton solutions for thick branes in 4 + 1 dimensions are considered in this article. In par... more New soliton solutions for thick branes in 4 + 1 dimensions are considered in this article. In particular, brane models based on the sine-Gordon (SG), ϕ 4 and ϕ 6 scalar fields are investigated; in some cases Z 2 symmetry is broken. Besides, these soliton solutions are responsible for supporting and stabilizing the thick branes. In these models, the origin of the symmetry breaking resides in the fact that the modified scalar field potential may have non-degenerate vacuua and these non-degenerate vacuua determine the cosmological constant on both sides of the brane. At last, in order to explore the particle motion in the neighborhood of the brane, the geodesic equations along the fifth dimension are studied.

Research paper thumbnail of Irreversible matter creation processes through a nonminimal curvature-matter coupling

The Fourteenth Marcel Grossmann Meeting, Nov 16, 2017

An interesting cosmological history was proposed by Prigogine et al. who considered the Universe ... more An interesting cosmological history was proposed by Prigogine et al. who considered the Universe as a thermodynamically open system. This scenario is characterized by a process of matter creation, which corresponds to an irreversible energy flow from the gravitational field to the pressureless matter fluid. Here, we show that the gravitationally induced particle production may arise from a nonminimal curvature-matter coupling. By considering the equivalent scalar-tensor theory, the cosmological implications of the model are discussed. As all known natural systems tend to a state of thermodynamic equilibrium, and assuming the universe is not different in this respect, we also discuss the conditions to attain the equilibrium state.

Research paper thumbnail of Observational constraints of f(Q) gravity

Research paper thumbnail of Wormhole geometries supported by three-form fields

Physical Review D, 2018

In this work, we find novel static and spherically symmetric wormhole geometries using a threefor... more In this work, we find novel static and spherically symmetric wormhole geometries using a threeform field. By solving the gravitational field equations, we find a variety of analytical and numerical solutions and show that it is possible for the matter fields threading the wormhole to satisfy the null and weak energy conditions throughout the spacetime, when the three-form field is present. In these cases, the form field is responsible for supporting the wormhole and all the exoticity is confined to it. Thus, the three-form curvature terms, which may be interpreted as a gravitational fluid, sustain these wormhole geometries. We also show that in the case of a vanishing redshift function the field can display a cosmological constant behavior.

Research paper thumbnail of Palatini wormholes and energy conditions from the prism of general relativity

The European physical journal. C, Particles and fields, 2017

Wormholes are hypothetical shortcuts in spacetime that in general relativity unavoidably violate ... more Wormholes are hypothetical shortcuts in spacetime that in general relativity unavoidably violate all of the pointwise energy conditions. In this paper, we consider several wormhole spacetimes that, as opposed to the standard procedure frequently employed in the literature, arise directly from gravitational actions including additional terms resulting from contractions of the Ricci tensor with the metric, and which are formulated assuming independence between metric and connection (Palatini approach). We reinterpret such wormhole solutions under the prism of General Relativity and study the matter sources that thread them. We discuss the size of violation of the energy conditions in different cases and how this is related to the same spacetimes when viewed from the modified gravity side.

Research paper thumbnail of From the Flamm–Einstein–Rosen bridge to the modern renaissance of traversable wormholes

International Journal of Modern Physics D, 2016

We consider the possibility of multiply-connected spacetimes, ranging from the Flamm–Einstein–Ros... more We consider the possibility of multiply-connected spacetimes, ranging from the Flamm–Einstein–Rosen bridge, geons, and the modern renaissance of traversable wormholes. A fundamental property in wormhole physics is the flaring-out condition of the throat, which through the Einstein field equation entails the violation of the null energy condition (NEC). In the context of modified theories of gravity, it has also been shown that the normal matter can be imposed to satisfy the energy conditions, and it is the higher order curvature terms, interpreted as a gravitational fluid, that sustain these nonstandard wormhole geometries, fundamentally different from their counterparts in general relativity (GR). We explore interesting features of these geometries, in particular, the physical properties and characteristics of these ‘exotic spacetimes’.