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

Papers by Francisco Lobo

Research paper thumbnail of Kinetic gravity braiding wormhole geometries

Physical Review D

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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...

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Research paper thumbnail of Warp drive basics

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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

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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

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Research paper thumbnail of General constraints on Horndeski wormhole throats

Physical Review D, 2020

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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.

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

Physical Review D, 2020

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

Physical Review D, 2020

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

Physical Review D, 2020

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

Physical Review D, 2021

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Research paper thumbnail of Wormhole Basics

Fundamental Theories of Physics, 2017

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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, [...

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

Physical Review D, 2021

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Research paper thumbnail of Thick brane solitons breaking Z2 symmetry

The Fourteenth Marcel Grossmann Meeting, Nov 16, 2017

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

The Fourteenth Marcel Grossmann Meeting, Nov 16, 2017

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

Physical Review D, 2019

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

Physical Review D, 2018

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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.

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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’.

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Research paper thumbnail of Kinetic gravity braiding wormhole geometries

Physical Review D

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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...

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Research paper thumbnail of Warp drive basics

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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

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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

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Research paper thumbnail of General constraints on Horndeski wormhole throats

Physical Review D, 2020

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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.

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

Physical Review D, 2020

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

Physical Review D, 2020

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

Physical Review D, 2020

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

Physical Review D, 2021

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Research paper thumbnail of Wormhole Basics

Fundamental Theories of Physics, 2017

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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, [...

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

Physical Review D, 2021

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Research paper thumbnail of Thick brane solitons breaking Z2 symmetry

The Fourteenth Marcel Grossmann Meeting, Nov 16, 2017

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

The Fourteenth Marcel Grossmann Meeting, Nov 16, 2017

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

Physical Review D, 2019

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

Physical Review D, 2018

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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.

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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’.

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