arXiv subjects

João Luís Rosa

Publications and source records attributed to João Luís Rosa.

At least 37 records · Page 2Linked to original sources

arXiv2024

Dynamical reconstruction of the $Λ$CDM model in scalar-tensor $f(R,T)$ gravity

In this work, we use the dynamical system approach to explore the cosmological background evolution of the scalar-tensor representation of $f(R,T)$ gravity, where $R$ is the Ricci scalar and $T$ is the trace of the stress-energy tensor. The motivation for this work resides in finding dynamical cosmological behaviors comparable with the $Λ$CDM model without the necessity of recurring to a dark energy component. We introduce a set of dynamical variables that allow for a direct comparison with the cosmological standard model and the current experimental measurements, and develop a dynamical system framework to analyze the cosmological evolution of Friedmann-Lemaître-Robertson-Walker (FLRW) universes within this theory. In this framework, we obtain the critical points in the cosmological phase space and perform fully numerical integrations of the dynamical system to extract the cosmological behavior, subjected to initial conditions compatible with the measurements by the Planck satellite. The phase space of the theory is proven to feature fixed points associated with cosmological behaviors analogous to those of GR, whereas variations in the scalar field associated to the dependency in $T$ affect the phase space structure only quantitatively. Our results indicate that cosmological solutions featuring a radiation dominated epoch, followed by a transition into a matter dominated epoch, and finally a transition into an exponentially accelerated epoch, are allowed by the theory, while maintaining a present state compatible with the current measurements from the Planck satellite and solar system dynamics, and preserving the regularity of the scalar fields and their interaction potential.

gr-qc↗

arXiv2024

Observational imprints of gravastars from accretion disks and hot-spots

In this work, we analyze the observational properties of thin-shell gravastars under two astrophysical frameworks, namely surrounded by optically-thin accretion disks and orbited by hot-spots. We consider the thin-shell gravastar model with two free parameters, the gravastar radius and ratio of mass allocated at the thin-shell, and produce the corresponding observables via the use of numerical backwards ray-tracing codes. Regarding the observations of accretion disks, our results indicate that, due to the absence of a strong gravitational redshift effect, smooth gravastar configurations cannot reproduce shadow observations when internal emission is assumed. We thus expect such models to be excluded as candidates for supermassive objects in galactic cores. Nevertheless, thin-shell gravastars with a large portion of their total mass allocated at the surface can produce such an effect and are thus adequate candidates for black-hole mimickers. In the context of hot-spot orbits, the astrometrical observational properties of ultra-compact gravastars resemble closely those of other ultra-compact objects e.g. fluid stars and bosonic stars. However, for low-compacticity configurations, the time-integrated fluxes feature additional contributions in the form of a high-intensity plunge through image. These qualitative differences in the observational properties of gravastars in comparison with black-hole spacetimes could potentially be discriminated by the next generation of interferometric experiments in gravitational physics.

gr-qc↗

arXiv2024

Dynamical reconstruction of the $Λ$CDM model in hybrid metric-Palatini gravity

In this work, we apply the formalism of dynamical systems to analyze the viability of the $Λ$CDM model in a generalized form of the hybrid metric-Palatini gravity theory written in terms of its dynamically equivalent scalar-tensor representation. Adopting a matter distribution composed of two relativistic fluids described by the equations of state of radiation and pressureless dust, one verifies that the cosmological phase space features the usual curvature-dominated, radiation-dominated, matter-dominated, and exponentially accelerated fixed points, even in the absence of a dark energy component. A numerical integration of the dynamical equations describing the system, subjected to initial conditions consistent with the cosmographic observations from the Planck satellite and weak-field solar system dynamics, shows that cosmological solutions with the same behavior as the $Λ$CDM model in General Relativity (GR) are attainable in this theory, with the deviations from GR being exponentially suppressed at early-times and the scalar-field potential effectively playing the role of dark energy at late times.

gr-qc↗

arXiv2024

Accretion disks and relativistic line broadening in boson star spacetimes

In this work, we analyze the observational properties of static, spherically symmetric boson stars with fourth and sixth-order self-interactions, using the Julia-based general-relativistic radiative transfer code Skylight. We assume the boson stars are surrounded by an optically thick, geometrically thin accretion disk. We use the Novikov-Thorne model to compute the energy flux, introducing a physically based accretion model around these boson star configurations. Additionally, we calculate the relativistic broadening of emission lines, incorporating a lamppost corona model with full relativistic effects for the first time around a boson star. Our results show distinct observational features between quartic-potential boson stars and Schwarzschild black holes, owing to the presence of stable circular orbits at all radii around the former. On the other hand, compact solitonic boson stars, which possess an innermost stable circular orbit, have observational features closely similar to black holes. This similarity emphasizes their potential as black-hole mimickers. However, the compact boson stars, lacking an event horizon, have complex light-ring structures that produce potentially observable differences from black holes with future generations of experiments.

gr-qc↗

arXiv2024

Gravitationally induced matter creation in scalar-tensor $f(R,T_{μν}T^{μν})$ gravity

In this work, we analyze the possibility of gravitationally induced matter creation in the so-called Energy-Momentum-Squared gravity (EMSG), i.e. $f(R,T_{μν}T^{μν})$ gravity, in its dynamically equivalent scalar-tensor representation. Given the explicit nonminimal coupling between matter and geometry in this theory, the energy-momentum tensor is not generally covariantly conserved, which motivates the study of cosmological scenarios by resorting to the formalism of irreversible thermodynamics of open systems. We start by deriving the universe matter creation rates and subsequent thermodynamical properties, such as, the creation pressure, temperature evolution, and entropy evolution, in the framework of $f(R,T_{μν}T^{μν})$ gravity. These quantities are then analyzed for a Friedmann-Lemaître-Robertson-Walker (FLRW) background with a scale factor described by the de Sitter solution, under different assumptions for the mater distribution, namely a vacuum universe, a constant density universe, and a time-varying density universe. Finally, we explore cosmological solutions with varying Hubble parameters and provide a comparison with the standard cosmological model. Our results indicate that the cosmological evolution in the framework of EMSG are in close agreement with the observational cosmological data for low redshift.

gr-qc↗

arXiv2024

Spherically symmetric vacuum solutions in 1-Parameter New General Relativity and their phenomenology

In this work, we study spherically symmetric vacuum solutions in 1-parameter New General Relativity (NGR), a specific theory in teleparallel gravity which is constructed from the three possible quadratic scalars obtained from torsion with arbitrary coefficients satisfying the requirements for the absence of ghosts. In this class of modified theories of gravity, the observable effects of gravity result from the torsion rather than the curvature of the spacetime. Unlike in GR, where the fundamental quantity is the metric from which the Levi-Civita connection is derived, in teleparallel theories of gravity the fundamental variable is the tetrad, from which one constructs the metric and the teleparallel connection. We consider the most general tetrad for spherical symmetry and we derive the corresponding field equations. Under adequate assumptions, we find three different branches of vacuum solutions and discuss their associated phenomenology. In particular, we analyze the photon sphere, the classical tests of GR such as the light deflection, the Shapiro delay, and the perihelion shift, and also the Komar mass, while providing a detailed comparison with their Schwarzschild spacetime counterparts. Finally, we analyze how the observational imprints from accretion disks and shadows are affected in comparison with their GR counterparts, and conclude that the free parameters of the model might induce additional attractive or repulsive effects to the propagation of photons, depending on their values.

gr-qc↗

arXiv2024

Observational properties of hot-spots orbiting relativistic fluid spheres

In this work we analyze the observational properties of relativistic fluid spheres when orbited by isotropically emitting sources, known as hot spots. We consider fluid star configurations in four different regimes of compacticity, from the Buchdahl limit to non-ultra compact solutions, thus obtaining fluid stars with qualitatively different geodesic structures and observational properties. We show that the observational properties for fluid stars at the Buchdahl limit are qualitatively similar to the ones for the Schwarzschild black hole, whereas for more dilute configurations one can find observational properties similar to other fundamentally different models e.g. bosonic star configurations with and without self interactions. For solutions with a radius in the range $2.25M<R<3M$, where $M$ is the total mass of the fluid star, the presence of a light-ring (LR) pair, which becomes degenerate at $R=3M$, leads to the appearance of additional observational signatures e.g. secondary images and LR contributions, which allow one to distinguish these models from their black-hole counterparts. Fluid star configurations supported by thin shells are also analyzed and it is proven that the stability of the inner LR is increased for these solutions, resulting in a non-differentiable extremum in the effective potential of the photons. Our results suggest that compact fluid star configurations provide a suitable and physically relevant alternative to the black-hole scenario in accordance with the current generation of observational experiments.

gr-qc↗

arXiv2024

Junction conditions in gravity theories with extra scalar degrees of freedom

In this work we present a general method to obtain the junction conditions of modified theories of gravity whose action can be written in the form $f\left(X_1,...,X_n\right)$, where $X_1$ to $X_n$ are any combination of scalar dependencies, e.g. the Ricci scalar $R$, the trace of the stress-energy tensor $T$, the Gauss-Bonnet invariant $\mathcal G$, among others. We discriminate the junction conditions into three sub-groups: the immediate conditions, arising from the imposition of regularity of the relevant quantities in the distribution formalism; the differential conditions, arising from the differential terms in the field equations; and the coupling conditions, arising from the interaction between different scalars $X_i$. Writing the modified field equations in terms of a linear combination of the different contributions of the scalars $X_i$ allows one to analyze the direct and differential junction conditions independently for each of these scalars, whereas the coupling junction conditions can be analyzed separately afterwards. We show that the coupling junction conditions induced on a scalar $X_i$ due to a coupling with a scalar $X_j$ are of the same form as the differential junction conditions of the scalar $X_j$, but applied to the analogous quantity in the framework of the scalar $X_i$. We provide a complete analysis of three different types of spacetime matching, namely smooth matching, matching with a thin-shell, and matching with double gravitational layers, and we also describe under which conditions the full sets of junction conditions might be simplified. Our results are applicable to several well-known theories of gravity e.g. $f\left(R\right)$, $f\left(T\right)$, and $f\left(\mathcal G\right)$, and can be straightforwardly extrapolated to other theories with scalar dependencies and more complicated scenarios where several dependencies are present simultaneously.

gr-qc↗

arXiv2024

Optical appearance of black holes surrounded by a dark matter halo

Black holes in General Relativity are described by space-time metrics that are simpler in comparison to non-vacuum compact objects. However, given the universality of the gravitational pull, it is expected that dark matter accumulates around astrophysical black holes, which can have an impact in the overall gravitational field, especially at galactic centers, and induce non-negligible effects in their observational imprints. In this work we study the optical appearance of a spherically symmetric black hole both when orbited by isotropically emitting light sources and when surrounded by a (geometrically and optically thin) accretion disk, while immersed in a dark matter halo. The black hole geometry plus the dark matter halo come as a solution of Einstein's field equations coupled to an anisotropic fluid whose density component follows a Hermquist-type distribution. Even in situations in which the geodesic description differs profoundly from the isolated black hole case, we find minor modifications to the primary and secondary tracks of the isotropic orbiting sources, and to the width, location, and relative luminosity of the corresponding photon rings as compared to the Schwarzschild black hole at equal black hole mass and emission models. This fact troubles distinguishing between both geometries using present observations of very-long baseline interferometry.

gr-qc↗

arXiv2023

Non-exotic traversable wormholes in $f\left(R,T_{ab}T^{ab}\right)$ gravity

In this work we analyze traversable wormhole spacetimes in the framework of a covariant generalization of Einstein's General Relativity known as energy-momentum squared gravity, or $f\left(R,\mathcal T\right)$ gravity, where $R$ is the Ricci scalar, $\mathcal T=T_{ab}T^{ab}$, and $T_{ab}$ is the energy-momentum tensor. Considering a linear $f\left(R,\mathcal T\right)=R+γ\mathcal T$ form, we show that a wide variety of wormhole solutions for which the matter fields satisfy all the energy conditions, namely the null, weak, strong and dominant energy conditions, exist in this framework, without the necessity for a fine-tuning of the free parameters that describe the model. Due to the complexity of the field equations these solutions are obtained through an analytical recursive algorithm. A drawback of the solutions obtained is that they are not naturally localized, and thus a matching with an external vacuum is required. For that purpose, we derive the junction conditions for the theory, and we prove that a matching between two spacetimes must always be smooth, i.e., no thin-shells are allowed at the boundary. Finally, we use these junction conditions to match the interior wormhole spacetime to an exterior vacuum described by the Schwarzschild solution, thus obtaining traversable localized static and spherically symmetric wormhole solutions satisfying all energy conditions for the whole spacetime range. We also prove that the methods outlined in this work can be straightforwardly generalized to more complicated dependencies of the action in $\mathcal T$, as long as crossed terms between $R$ and $\mathcal T$ are absent.

gr-qc↗

arXiv2023

Shadows and photon rings of regular black holes and geonic horizonless compact objects

The optical appearance of a body compact enough to feature an unstable bound orbit, when surrounded by an accretion disk, is expected to be dominated by a luminous ring of radiation enclosing a central brightness depression typically known as the shadow. Despite observational limitations, the rough details of this picture have been now confirmed by the results of the EHT Collaboration on the imaging of the M87 and Milky Way supermassive central objects. However, the precise characterization of both features - ring and shadow - depends on the interaction between the background geometry and the accretion disk, thus being a fertile playground to test our theories on the nature of compact objects and the gravitational field itself in the strong-field regime. In this work we use both features in order to test a continuous family of solutions interpolating between regular black holes and horizonless compact objects, which arise within the Eddington-inspired Born-Infeld theory of gravity, a viable extension of Einstein's General Relativity (GR). To this end we consider seven distinctive classes of such configurations (five black holes and two traversable wormholes) and study their optical appearances under illumination by a geometrically and optically thin accretion disk, emitting monochromatically with three analytic intensity profiles previously suggested in the literature. We build such images and consider the sub-ring structure created by light rays crossing the disk more than once and existing on top of the main ring of radiation. We discuss in detail the modifications as compared to their GR counterparts, the Lyapunov exponents of unstable nearly-bound orbits, as well as the differences between black hole and traversable wormholes for the three intensity profiles.

gr-qc↗

arXiv2023

Imaging compact boson stars with hot-spots and thin accretion disks

In this work we consider the observational properties of compact boson stars with self-interactions orbited by isotropically emitting (hot-spot) sources and optically thin accretion disks. We consider two families of boson stars supported by quartic and sixth-order self-interaction potentials, and choose three samples of each of them in growing compactness; only those with large enough compactness are capable to hold light-rings, namely, null bound orbits. For the hot-spots, using inclination angles $θ=\{20^\circ, 50^\circ, 80^\circ \}$ we find a secondary track plunge-through image of photons crossing the interior of the boson star, which can be further decomposed into additional images if the star is compact enough. For accretion disks we find that the latter class of stars actually shows a sequence of additional secondary images in agreement with the hot-spot analysis, a feature absent in typical black hole space-times. Furthermore, we also find a shadow-like central brightness depression for some of these stars in both axial observations and at the inclination angles above. We discuss our findings in relation to the capability of boson stars to effectively act as black hole mimickers in their optical appearances as well as potential observational discriminators.

gr-qc↗

arXiv2023

Scalar-tensor representation to $f(R,T)-$brane with Gauss-Bonnet gravity

In this paper, we generalize the analysis of the $f(R,T)-$brane via the inclusion of a term proportional to the Gauss-Bonnet invariant. We consider an action of the form $F(R,G,T)=f(R,T)+αG$, where $T$ is the trace of the stress-energy tensor, $R$ is the Ricci scalar, and $α$ is a real parameter that controls the contribution of the Gauss-Bonnet invariant $G$. We introduce the first-order formalism to obtain solutions for the source field of the brane in the special case where $f(R,T)=R+βT$ and illustrate its procedure with an application to the sine-Gordon model. We also investigate the general case of the $f(R,T)-$brane via the use of the scalar-tensor formalism, where we also use the first-order formalism to obtain solutions. Finally, we investigate the linear stability of the brane under tensor perturbations of the the modified Einstein's field equations. Our results indicate that the Gauss-Bonnet term may induce qualitatively different behaviors of the quantities on the brane, provided that its contribution is large enough.

gr-qc↗

arXiv2023

Observational properties of relativistic fluid spheres with thin accretion disks

In this work we analyze the observational properties of incompressible relativistic fluid spheres with and without thin-shells, when surrounded by thin accretion disks. We consider a set of six configurations with different combinations of the star radius $R$ and the thin-shell radius $r_Σ$ to produce solutions with neither thin-shells nor light-rings, with either of those features, and with both. Furthermore, we consider three different models for the intensity profile of the accretion disk, based on the Gralla-Lupsasca-Marrone (GLM) disk model, for which the peaks of intensity occur at the Innermost Stable Circular Orbit (ISCO), the Light-Ring (LR), and the center of the star. The observed images and intensity profiles for an asymptotic observer are produced using a Mathematica-based ray-tracing code. Our results indicate that, in the absence of a light-ring, the presence of a thin-shell produces a negligible effect in the observational properties of the stars. However, when the spacetime features a light-ring, the portion of the mass of the star that is stored in the thin-shell has a strong effect on its observational properties, particularly in the magnitude of the central gravitational redshift effect responsible for causing a central shadow-like dimming in the observed images. A comparison with the Schwarzschild spacetime is also provided and the most compact configurations are shown to produce observational imprints similar to those of black-hole solutions, with subtle qualitative differences, most notably extra secondary image components that decrease the radius of the shadow and are potentially observable.

gr-qc↗

arXiv2022

Shadows of boson and Proca stars with thin accretion disks

In this work, we obtain the shadow images of spherically symmetric scalar boson and Proca stars using analytical fittings of numerical solutions, when illuminated by a geometrically thin accretion disk. We chose a sample of four boson and four Proca stars with radii ranging from more compact configurations with $R\sim 9M$ to more dilute configurations with $R\sim 20M$, where $M$ is the total mass of the bosonic star. In these configurations, the absence of the photon sphere (the locus of unstable bound geodesics) makes the optical appearance of these stars to be dominated by a single luminous ring enclosing a central brightness depression, and no further light rings are available. We show that if one considers face-on observations and a disk model whose emission is truncated at some finite radius at which the luminosity attains its maximum value, both the size of the shadow, as well as the luminosity and depth of the bright region, are heavily influenced by the emission profile, with the choice of the type and parameters of the bosonic stars in our samples having a sub-dominant influence. These differences are nonetheless significantly magnified when one allows the accretion disk to extend close enough to the center of the star. Our results point out that even though bosonic stars are horizonless and do not have a photon sphere, some of them may be able to produce conventional black hole shadow-like images provided that their compactness is large enough, thus being potentially consistent with current and future observations.

gr-qc↗

arXiv2022

Non-exotic traversable wormhole solutions in linear $f\left(R,T\right)$ gravity

In this work we analyze traversable wormhole solutions in the linear form of $f\left(R,T\right)=R+λT$ gravity satisfying the Null, Weak, Strong, and Dominant Energy Conditions (NEC, WEC, SEC, and DEC respectively) for the entire spacetime. These solutions are obtained via a fully analytical parameter space analysis of the free parameters of the wormhole model, namely the exponents controlling the degree of the redshift and shape functions, the radius of the wormhole throat $r_0$, the value of the redshift function at the throat $ζ_0$, and the coupling parameter $λ$. Bounds on these free parameters for which the energy conditions are satisfied for the entire spacetime are deduced and two explicit solutions are provided. Even if some of these bounds are violated, leading to the violation of the NEC at some critical radius $r_c>r_0$, it is still possible to find physically relevant wormhole solutions via a matching with an exterior vacuum spacetime in the region where the energy conditions are still satisfied. For this purpose, we deduce the set of junction conditions for the form of $f\left(R,T\right)$ considered and provide an explicit example. These results seem to indicate that a wide variety of non-exotic wormhole solutions are attainable in the $f\left(R,T\right)$ theory without the requirement of fine-tuning.

gr-qc↗

arXiv2022

Junction conditions of Palatini $f\left(\mathcal R,T\right)$ gravity

We work out the junction conditions for the Palatini $f(\mathcal{R},T)$ extension of General Relativity, where $f$ is an arbitrary function of the curvature scalar $\mathcal{R}$ of an independent connection, and of the trace $T$ of the stress-energy tensor of the matter fields. We find such conditions on the allowed discontinuities of several geometrical and matter quantities, some of which depart from their metric counterparts, and in turn extend their Palatini $f(\mathcal{R})$ versions via some new $T$-dependent terms. Moreover, we also identify some "exceptional cases" of $f(\mathcal{R},T)$ Lagrangians such that some of these conditions can be discarded, thus allowing for further discontinuities in $\mathcal{R}$ and $T$ and, in contrast with other theories of gravity, they are shown to not give rise to extra components in the matter sector e.g. momentum fluxes and double gravitational layers. We discuss how these junction conditions, together with the non-conservation of the stress-energy tensor ascribed to these theories, may induce non-trivial changes in the shape of specific applications such as traversable thin-shell wormholes.

gr-qc↗

arXiv2022

Multi-kink braneworld configurations in the scalar-tensor representation of $f(R,T)$ gravity

In this work we investigate the $f(R,T)$ brane in the scalar-tensor representation, where the solutions of the equations of motions for the source field engender topological defects with two-kink profiles. We use the first-order formalism to obtain analytical solutions for the source field of the brane and analyze how these solutions modify the structure of the auxiliary fields arising from the scalar-tensor representation of the theory. We found that when the model engenders two-kink solutions, the auxiliary fields are modified in order to allow for the appearance of an internal structure. In addition, the stability potential and zero mode also have their internal structure modified by two-kink solution.

gr-qc↗