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Santiago Esteban Perez Bergliaffa

Publications and source records attributed to Santiago Esteban Perez Bergliaffa.

At least 19 recordsLinked to original sources
arXiv2026

Ultracompact anisotropic stars and gravastars in general relativity

We study static, spherically symmetric ultracompact objects in General Relativity with anisotropic stress. Using the covariant anisotropic equation of state and assuming homogeneous density, we construct a mostly analytical model admitting configurations beyond the Buchdahl limit and with compactness arbitrarily close to the black hole value. The solutions exhibit two regimes separated by a critical curve in the parameter space of compactness and anisotropy, associated with the divergence of the central pressure: regular anisotropic configurations with positive central pressure, and singular configurations with negative central pressure for arbitrary anisotropy. In the latter case, we show that introducing a thick shell removes the pressure divergence, yielding regular ultracompact gravastar configurations. We further construct both anisotropic and isotropic gravastar models using this thick-shell construction.

gr-qc↗
arXiv2026

An Analytical Two Incompressible Fluid Star with a Mixed Ordinary Dark Matter Core and an Ordinary Matter Envelope

We construct an analytical relativistic two fluid star characterized by a mixed core, where ordinary matter and dark matter coexist as two independently conserved incompressible perfect fluids, and an envelope composed exclusively of ordinary matter. The fluids exchange neither matter nor energy and interact only through the common spacetime geometry, with the ordinary component extending across the core envelope interface while the dark component is confined to the core. Despite the mixed core--single-fluid envelope structure and the internal interface, the system remains analytically tractable, allowing us to obtain explicit expressions for the pressures and metric functions and to follow directly the effects of the dark-matter fraction and relative core size. We determine the physically admissible parameter space and derive a Buchdahl like critical compactness associated with the divergence of the central pressure, whose value depends on the relative dark matter density and the size of the mixed core. The Schwarzschild constant density star and its standard critical value, $2M/R=8/9$, are recovered in the corresponding one fluid limit. The mass--radius analysis further shows that configurations with the same global compactness can correspond to distinct internal matter distributions. Beyond providing an analytically controlled description of a core-confined second component, the construction offers a useful benchmark for identifying qualitative trends that may subsequently be examined in more realistic dark matter admixed neutron star models, whose detailed treatment lies beyond the scope of the present work.

gr-qc↗
arXiv2026

Another Look at the Weak-Field Limit of Generalized Hybrid Metric-Palatini Gravity

We investigate the weak-field regime of generalized hybrid metric-Palatini theories, described by a generic function \(f(R,\mathcal{R})\), where \(R\) is the metric Ricci scalar and \(\mathcal{R}\) is constructed from an independent torsionless connection. Linearizing the field equations about Minkowski spacetime, we show, without using the scalar-tensor representation, that the theory propagates the usual massless spin-2 mode and two massive scalar modes, with an effective gravitational coupling. The absence of tachyonic and ghostlike instabilities at the linearized level, together with the nondegeneracy of the scalar sector, is shown to impose algebraic restrictions on the derivatives of \(f(R,\mathcal R)\) evaluated on the Minkowski background, which generalize previously obtained conditions. The Newtonian limit for an extended static source is derived, yielding a gravitational potential with two Yukawa corrections whose amplitudes are fixed by the scalar residues, while finite-size effects are encoded in source-dependent form factors. We determine the conditions under which the usual Newtonian limit is recovered and derive the effective post-Newtonian parameter \(γ_Σ\) governing light propagation. Finally, we compute the radial epicyclic frequency and the corresponding anomalous periapsis advance, and compare it with planetary precession data to constrain the parameters of a viable hierarchical scalar-mass regime.

gr-qc↗
arXiv2026

A connection between Gravitational Scalar-Tensor theories and Generalized Hybrid theories

We establish a correspondence between higher-derivative gravitational scalar-tensor theories of the form $Ψ(R,(\nabla R)^2,\Box R)$ and generalized hybrid metric-Palatini models $f(R,\mathcal{R})$. Restricting to the physically relevant case of linear dependence on $\Box R$, we make explicit that both frameworks can be reformulated in the Einstein frame as General Relativity minimally coupled to two interacting scalar fields, thereby opening the possibility of finding theories that are dynamically equivalent. This correspondence provides an explicit dictionary relating the functions that define the higher-derivative theory to the hybrid function $f(R,\mathcal{R})$, allowing for reconstruction in both directions. We illustrate the usefulness of the procedure with explicit examples.

gr-qc↗
arXiv2026

Scalar-tensor theories in the Lyra geometry: Invariance under local transformations of length units and the Jordan-Einstein frame conundrum

The Lyra geometry provides an interesting approach to develop purely geometrical scalar-tensor theories. Here we present a theory on Lyra manifolds which contains generalizations of both Brans-Dicke gravity and Einstein-Gauss-Bonnet scalar-tensor theory. It is shown that the symmetry group of gravitational theories on the Lyra geometry comprises not only coordinate transformations but also local transformations of length units, so that the Lyra function is a conformal factor which locally fixes the unit of length. The Lyra geometry is thus a generalization of Riemannian geometry which includes spacetime-dependent length units. By performing a Lyra transformation to a frame in which the unit of length is globally fixed, it is shown that General Relativity (GR) is obtained from the Lyra Scalar-Tensor Theory (LyST). Through the same procedure, even in the presence of matter fields, it is found that Brans-Dicke gravity and the Einstein-Gauss-Bonnet scalar-tensor theory are obtained from their Lyra counterparts. It is argued that this approach is consistent with the Mach-Dicke principle, since the strength of gravity in Brans-Dicke-Lyra is controlled by the scale function. It might be possible that any known scalar-tensor theory can be naturally geometrized by considering a particular Lyra frame, for which the scalar field is the function which locally controls the unit of length. The Jordan-Einstein frame conundrum is also assessed from the perspective of Lyra transformations, it is shown that the Lyra geometry makes explicit that the two frames are only different representations of the same theory, so that in the Einstein frame the unit of length varies locally. The Lyra formalism is then shown to be better suited for exploring scalar-tensor gravity, since in its well-defined structure the conservation of the energy-momentum tensor and geodesic motion are assured in the Einstein frame.

gr-qc↗
arXiv2021

Non-linear electrodynamics non-minimally coupled to gravity:symmetric-hyperbolicity and causal structure

It is shown here that symmetric hyperbolicity, which guarantees well-posedness, leads to a set of two inequalities for matrices whose elements are determined by a given theory. As a part of the calculation, carried out in a mostly-covariant formalism, the general form for the symmetrizer, valid for a general Lagrangian theory, was obtained. When applied to nonlinear electromagnetism linearly coupled to curvature, the inequalities lead to strong constraints on the relevant quantities, which were illustrated with applications to particular cases. The examples show that non-linearity leads to constraints on the field intensities, and non-minimal coupling imposes restrictions on quantities associated to curvature.

gr-qc↗
arXiv2021

Hairy Black Holes from Horndeski Theory

We present an exact static black hole solution of Einstein field equations in the framework of Horndeski Theory by imposing spherical symmetry and choosing the coupling constants in the Lagrangian so that the only singularity in the solution is at $r=0$. The analytical extension is built in two particular domains of the parametric space. In the first domain we obtain a solution exhibiting an event horizon analogous to that of the Schwarzschild geometry. For the second domain, we show that the metric displays an exterior event horizon and a Cauchy horizon which encloses a singularity. For both branches we obtain the corresponding Hawking temperature which, when compared to that of the Schwarzschild black hole, acquires a correction proportional to a combination of the coupling constants. Such a correction also modifies the definition of the entropy of the black hole.

gr-qc↗
arXiv2021

Particle Production in Accelerated Thin Bubbles

We investigate the creation of scalar particles inside a region delimited by a bubble which is expanding with non-zero acceleration. The bubble is modelled as a thin shell and plays the role of a moving boundary, thus influencing the fluctuations of the test scalar field inside it. Bubbles expanding in Minkowski spacetime as well as those dividing two de Sitter spacetimes are explored in a unified way. Our results for the Bogoliubov coefficient $β_k$ in the adiabatic approximation show that in all cases the creation of scalar particles decreases with the mass, and is much more significant in the case of nonzero curvature. They also show that the dynamics of the bubble and its size are relevant for particle creation, but in the dS-dS case the combination of both effects leads to a behaviour different from that of Minkowski space-time, due to the presence of a length scale (the Hubble radius of the internal geometry).

gr-qc↗
arXiv2020

A novel exact magnetic black hole solution in four-dimensional extended scalar-tensor-Gauss-Bonnet theory

In this work the first exact asymptotically flat static and spherically symmetric black hole solution for $(3+1)$-dimensional ESTGB is presented, with a model of nonlinear electrodynamics -- that reduces to Maxwell's theory in the weak field limit and satisfies the weak energy condition -- as a source. The solution has a nonzero magnetic charge, and scalar hair, which turns out to be dependent of the magnetic charge. It is characterized by the ADM mass $m$ and the magnetic charge $q$. Depending on the range of these parameters, the solution describes black holes with different structure. In the case $m\geq0$ and $q\geq0$, it shares many of the characteristics of the Schwarzschild solution. For $m>0$ and $q<0$, it is akin to the Reissner-Nordström metric. In the case $m=0$, it represents a purely magnetic black hole.

gr-qc↗
arXiv2020

Thermodynamical and dynamical equilibrium of a self-gravitating uncharged thin shell

The dynamical stability of massive thin shells with a given equation of state (EOS) is here compared (both for the barotropic and non-barotropic case) with the results coming from thermodynamical stability. Our results show that the restrictions in the parameter space of equilibrium configurations of the shell following from thermodynamical stability are much more stringent that those obtained from dynamical stability. As a byproduct, we furnish evidence that the link between the maximum mass along a sequence of equilibrium configurations and the onset of dynamical stability is valid for EOS of the type $P=P(σ, R)$.

gr-qc↗
arXiv2019

Entropy bounds and nonlinear electrodynamics

Bekenstein's inequality sets a bound on the entropy of a charged macroscopic body. Such a bound is understood as a universal relation between physical quantities and fundamental constants of nature that should be valid for any physical system. We reanalyze the steps that lead to this entropy bound considering a charged object in conformity to Born-Infeld electrodynamics and show that the bound depends of the underlying theory used to describe the physical system. Our result shows that the nonlinear contribution to the electrostatic self-energy causes a raise in the entropy bound. As an intermediate step to obtain this result, we exhibit a general way to calculate the form of the electric field for a given nonlinear electrodynamics in Schwarzschild spacetime.

gr-qc↗
arXiv2018

An introductory study to white dwarfs

White dwarfs are compact objects that stand against gravitational collapse by their internal pressure of degenerate matter. In this work we aimed to perform an introductory study on these stars, using two equations of state (EOS): (I) an ideal Fermi gas and (II) the one by Baym Pethick and Sutherland (BPS). In addition, we analyzed these two equations of state in two scenarios, the Newtonian and the one from General Relativity, which allowed us to analyze the effects of curved space-time. We also studied a rotating white dwarf, finding a metric for this case. With these two cases, with rotation (J = cte) and static (J = 0) well determined, we introduced the concepts of instability by the turning point criterion and dynamic instability criterion. Finally, with the RNS program, we performed the numerical resolution with BPS equation of state in the rotating case in order to analyze the behavior of the turning point criterion in these two cases.

astro-ph.SR↗
arXiv2017

On the emergence of the ${\bfΛ}$CDM model from self-interacting Brans-Dicke theory in ${\bf d= 5}$

We investigate whether a self-interacting Brans-Dicke theory in $d=5$ without matter and with a time-dependent metric can describe, after dimensional reduction to $d=4$, the FLRW model with accelerated expansion and non-relativistic matter. By rewriting the effective 4-dimensional theory as an autonomous three-dimensional dynamical system and studying its critical points, we show that the $Λ$CDM cosmology cannot emerge from such a model. This result suggests that a richer structure in $d=5$ may be needed to obtain the accelerated expansion as well as the matter content of the 4-dimensional universe.

gr-qc↗
arXiv2014

Stability of the accretion of a ghost condensate onto the Schwarzschild black hole

We study the linear stability of a nongravitating, steady-state, spherically symmetric ghost condensate accreting onto a Schwarzschild black hole using two methods. The first one is based on the conservation of the energy-momentum tensor of the perturbations (whose propagation is determined by the effective metric), and involves the determination of the sign of the time derivative of the energy of the perturbations. The second method employs the positivity of the effective potential. Both methods yield the result that the system is stable, but the second one is less practical, since it involves lengthier calculations and requires the explicit form of the background solution for the scalar field.

gr-qc↗
arXiv2014

On the stability of black holes with nonlinear electromagnetic fields

The stability of three static and spherically symmetric black hole solutions with nonlinear electromagnetism as a source is investigated in three different ways. We show that the specific heat of all the solutions displays an infinite discontinuity with a change of sign, but the turning point method indicates that the solutions are thermodynamically stable (much in the same way as in the case of the Reissner-Nordstrom geometry). We also show that the black holes analyzed here are dynamically stable, thus suggesting that there may be a relation between thermodynamical and dynamical stability for nonvacuum black holes.

gr-qc↗
arXiv2011

An overview of f(R) theories

A brief introduction to theories of the gravitational field with a Lagrangian that is a function of the scalar curvature is given. The emphasis will be placed in formal developments, while comparison to observation will be discussed in the chapter by S. Jorás in this volume.

gr-qc↗
arXiv2011

Effective metric in nonlinear scalar field theories

We discuss several features of the propagation of perturbations in nonlinear scalar field theories using the effective metric. It is shown that the effective metric can be classified according to whether the gradient of the scalar field is timelike, null, or spacelike, and this classification is illustrated with two examples. We shall also show that different signatures for the effective metric are allowed.

gr-qc↗