arXiv subjects

Raül Vera

Publications and source records attributed to Raül Vera.

At least 19 recordsLinked to original sources

arXiv2026

Rigidity and the interpretation of mass with a positive cosmological constant

We provide an explicit counterexample to the rigidity properties underlying the interpretation of mass in the presence of a positive cosmological constant $Λ$. Specifically, we construct a family of regular static stellar configurations satisfying the dominant and strong energy conditions, and containing no surface layers, for which the mass parameter of the outer Schwarzschild-de Sitter region can be positive, zero, or negative. The zero-mass configuration is precisely the one for which the outer vacuum region becomes exactly de Sitter, yielding a spacetime in which a de Sitter domain coexists with regular perfect-fluid matter. This contrasts sharply with the $Λ=0$ case, where a Minkowski domain cannot coexist with perfect-fluid regions satisfying the energy conditions. These results show that the static, spherically symmetric realization of the rigidity principle associated with the positive mass theorem for $Λ=0$ does not carry over to $Λ>0$.

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arXiv2026

Area bounds and gauge fixing: alternative canonical variables for loop gravity

We use a canonical parametrization of twisted geometries describing the classical phase space of loop quantum gravity on a fixed graph, and establish its explicit correspondence with the associated frame bases and spinorial descriptions. Applied to the two-vertex model, this framework yields analytical bounds on the evolution of the total area, proving the existence of a non-vanishing lower bound at finite times. These findings, previously observed only numerically, suggest a bounce-like behavior and highlight the usefulness of these variables for the study of more general configurations. As a second result, the canonical variables are shown to simplify the gauge-fixing procedure, generalizing previous results restricted to two-vertex models with four links.

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arXiv2026

Cosmological Spacetimes with Sign-Changing Spatial Curvature and Topological Transitions

Observational evidence, together with practical computations and modeling, supports a Euclidean spatial sector in the current cosmological model based on the FLRW metric. This, however, would imply that the total amount of matter and energy immediately after the Big Bang must have been infinite, an implication that could only be avoided through a transition from a closed to an open universe, a process forbidden in standard FLRW models. In this article, we investigate the spacetimes resulting from promoting the spatial curvature $k$ in FLRW spacetimes to a time-dependent function, $k \to k(t)$, allowing it to change sign and thereby allowing changes in the topology of the constant-$t$ slices. Although previously dismissed due to a classical theorem by Geroch, such transitions are shown to be consistent with global hyperbolicity when the comoving time is distinct from a Cauchy time, as recent work by one of the authors demonstrates. We construct three distinct geometries exhibiting this behavior using different representations of constant-curvature spaces. We analyze their global properties and identify mild conditions under which they remain globally hyperbolic. Furthermore, we characterize their Killing vectors, proving a general result for spherically symmetric spacetimes and compare them with known geometries in the literature.

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arXiv2025

Improving the inference of the stellar quantities using the extended $I$-Love-$Q$-$δM$ relations

In relativistic Astrophysics the $I$-Love-$Q$ relations refer to approximately EoS-independent relations involving the moment of inertia, Love number, and quadrupole moment through some quantities that are normalised by the mass $M_0$ of the background configuration of the perturbative scheme. Since $M_0$ is not an observable quantity, this normalisation hinders the direct applicability of the relations. A common remedy assumes that $M_0$ coincides with the actual mass of the star $M_S$; however, this approximation is only adequate for very slow rotation (when the dimensionless spin parameter is $χ_S<0.1$). The more accurate alternative approach, based on the $I$-Love-$Q$-$δM$ set of relations, circumvents this limitation by enabling the inference of $M_0$. Here we review both approaches and provide numerical comparisons.

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arXiv2025

Homothetic expansion of polyhedra in the two-vertex model: emergence of FLRW

The cosmological behavior associated to a U(N)-symmetry reduced sector of the loop-quantum-gravity truncation known as the two-vertex model is further explored in this work. We construct convenient frame bases that encode the whole classical phase space of the twisted geometry associated to the graph. We show that the polyhedra of the twisted geometry suffer under evolution an homothetic expansion, which strengthens the correspondence to the Robertson-Walker geometry.

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arXiv2024

New characterization of Robertson-Walker geometries involving a single timelike curve

Our aim in this paper is two-fold. We establish a novel geometric characterization of the Roberson-Walker (RW) spacetime and, along the process, we find a canonical form of the RW metric associated to an arbitrary timelike curve and an arbitrary space frame. A known characterization establishes that a spacetime foliated by constant curvature leaves whose orthogonal flow (the cosmological flow) is geodesic, shear-free, and with constant expansion on each leaf, is RW. We generalize this characterization by relaxing the condition on the expansion. We show it suffices to demand that the spatial gradient and Laplacian of the expansion on a single arbitrary timelike curve vanish. In General Relativity these local conditions are equivalent to demanding that the energy flux measured by the cosmological flow, as well as its divergence, are zero on a single arbitrary timelike curve. The proof allows us to construct canonically adapted coordinates to the arbitrary curve, thus well-fitted to an observer with an arbitrary motion with respect to the cosmological flow.

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arXiv2024

$I$-Love-$Q$, and $δM$ too: The role of the mass in universal relations of compact stars

In the study of rotating neutron stars the $I$-Love-$Q$ relations refer to the existence of various approximate, equation of state-independent relations involving the moment of inertia, the Love number and the quadrupole moment. These relations are relevant for observational astrophysics, since they allow (in theory) the inference of any two quantities within the $I$-Love-$Q$ triad out of the third one alone. However, the quantities involved in the relations are, in fact, normalized by a parameter $M_0$ that arises in the usual perturbative analytical approach as the mass of the background configuration. Since $M_0$ is not the mass of the rotating star $M_S$, it is not an observational quantity, which may affect the application of the relations to actual observations. This situation is usually ignored in most studies by taking $M_0$ to be the mass of the star, an approximation that can, in some cases, be inconsistent. In this paper we extract the value of $M_0$ using an $\textit{extended}$ version of the universal relations that involve a fourth parameter, $δM$, proportional to the difference $M_S-M_0$. We analyze to which degree this extended set of relations yields a more precise inference of compact star properties and equation of state parameters.

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arXiv2024

Review on the matching conditions for the tidal problem: towards the application to more general contexts

The tidal problem is used to obtain the tidal deformability (or Love number) of stars. The semi-analytical study is usually treated in perturbation theory as a first order perturbation problem over a spherically symmetric background configuration consisting of a stellar interior region matched across a boundary to a vacuum exterior region that models the tidal field. The field equations for the metric and matter perturbations at the interior and exterior regions are complemented with corresponding boundary conditions. The data of the two problems at the common boundary are related by the so called matching conditions. These conditions for the tidal problem are known in the contexts of perfect fluid stars and superfluid stars modelled by a two-fluid. Here we review the obtaining of the matching conditions for the tidal problem starting from a purely geometrical setting, and present them so that they can be readily applied to more general contexts, such as other types of matter fields, different multiple layers or phase transitions. As a guide on how to use the matching conditions, we recover the known results for perfect fluid and superfluid neutron stars.

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arXiv2023

Revisiting the $I$-Love-$Q$ relations for superfluid neutron stars

We study the tidal problem and the resulting $I$-Love-$Q$ approximate universal relations for rotating superfluid neutron stars in the Hartle-Thorne formalism. Superfluid stars are described in this work by means of a two-fluid model consisting of superfluid neutrons and all other charged constituents. We employ a stationary and axisymmetric perturbation scheme to second order around a static and spherically symmetric background. Recently, we used this scheme to study isolated rotating superfluid stars. In this paper it is applied to analyze the axially symmetric sector of the tidal problem in a binary system. We show that a consistent use of perturbative matching theory amends the original two-fluid formalism for the tidal problem to account for the possible non-zero value of the energy density at the boundary of the star. This is exemplified by building numerically different stellar models spanning three equations of state. Significant departures from universality are found when the correct matching relations are not taken into account. We also present an augmented set of universal relations for superfluid neutron stars which includes the contribution to the total mass of the star at second order, $δM$. Therefore, our results complete the set of universal relations for rotating superfluid stars, generalizing our previous findings in the perfect fluid case.

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arXiv2023

An effective model for the quantum Schwarzschild black hole

We present an effective theory to describe the quantization of spherically symmetric vacuum in loop quantum gravity. We include anomaly-free holonomy corrections through a canonical transformation of the Hamiltonian of general relativity, such that the modified constraint algebra closes. The system is then provided with a fully covariant and unambigous geometric description, independent of the gauge choice on the phase space. The resulting spacetime corresponds to a singularity-free (black-hole/white-hole) interior and two asymptotically flat exterior regions of equal mass. The interior region contains a minimal smooth spacelike surface that replaces the Schwarzschild singularity. We find the global causal structure and the maximal analytical extension. Both Minkowski and Schwarzschild spacetimes are directly recovered as particular limits of the model.

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arXiv2023

Singularity resolution by holonomy corrections: Spherical charged black holes in cosmological backgrounds

We study spherical charged black holes in the presence of a cosmological constant with corrections motivated by the theory of loop quantum gravity. The effective theory is constructed at the Hamiltonian level by introducing certain correction terms under the condition that the modified constraints form a closed algebra. The corresponding metric tensor is then carefully constructed ensuring that the covariance of the theory is respected, that is, in such a way that different gauge choices on phase space simply correspond to different charts of the same spacetime solution. The resulting geometry is characterized by four parameters: the three usual ones that appear in the general relativistic limit (describing the mass, the charge, and the cosmological constant), as well as a polymerization parameter, which encodes the quantum-gravity corrections. Contrary to general relativity, where this family of solutions is generically singular, in this effective model the presence of the singularity depends on the values of the parameters. The specific ranges of values that define the family of singularity-free spacetimes are explicitly found, and their global structure is analyzed. In particular, the mass and the cosmological constant need to be nonnegative to provide a nonsingular geometry, while there can only be a bounded, relatively small, amount of charge. These conditions are suited for any known spherical astrophysical black hole in the de Sitter cosmological background, and thus this model provides a globally regular description for them.

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arXiv2023

A revised formalism for slowly-rotating superfluid neutron stars in general relativity

We discuss slowly-rotating, general relativistic, superfluid neutron stars in the Hartle-Thorne formulation. The composition of the stars is described by a simple two-fluid model which accounts for superfluid neutrons and all other constituents. We apply a perturbed matching framework to derive a new formalism for slowly-rotating superfluid neutron stars, valid up to second-order perturbation theory, building on the original formulation reported by Andersson and Comer in 2001. The present study constitutes an extension of previous work in the single-fluid case where it was shown that the Hartle-Thorne formalism needs to be amended since it does not provide the correct results when the energy density does not vanish at the surface of the star. We discuss in detail the corrections that need to be applied to the original two-fluid formalism in order to account for non vanishing energy densities at the boundary. In the process, we also find a correction needed in the computation of the deformation of the stellar surface in the original two-fluid model in all cases (irrespective of the value of the energy density at the surface). The discrepancies found between the two formalisms are illustrated by building numerical stellar models, focusing on the comparison in the calculation of the stellar mass, the deformation of the star, and in the Kepler limit of rotation. In particular, using a toy-model equation of state for which the energy density does not vanish at the boundary of the star we demonstrate that the corrections to the formalism we find impact the structure of slowly-rotating superfluid neutron stars in a significant way.

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arXiv2022

Nonsingular spherically symmetric black-hole model with holonomy corrections

We present a covariant model of a spherically symmetric black hole with corrections motivated by loop quantum gravity. The effective modifications, parametrized by a positive constant $λ$, are implemented through a canonical transformation and a linear combination of the constraints of general relativity, in such a way that the theory remains free of anomalies and general relativity is recovered for $λ=0$. In addition, the corresponding metric is constructed in a fully covariant way to ensure that gauge transformations on phase space correspond to coordinate changes. The solution for each gauge choice provides a chart and corresponding line element of a spacetime solution whose geometry is unambiguously determined in terms of the parameter $λ$ and a constant of motion $m$. For positive values of $m$, the solution is asymptotically flat and contains a globally hyperbolic black-hole/white-hole region with a minimal spacelike hypersurface that replaces the Schwarzschild singularity. The corresponding exterior regions are isometric and, in particular, allow the computation of the ADM mass. The procedure to obtain the global causal structure of the solution yields also its maximal analytic extension.

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arXiv2020

Gauge fixing and regularity of axially symmetric and axistationary second order perturbations around spherical backgrounds

Perturbation theory in geometric theories of gravitation is a gauge theory of symmetric tensors defined on a Lorentzian manifold (the background spacetime). The gauge freedom makes uniqueness problems in perturbation theory particularly hard as one needs to understand in depth the process of gauge fixing before attempting any uniqueness proof. This is the first paper of a series of two aimed at deriving an existence and uniqueness result for rigidly rotating stars to second order in perturbation theory in General Relativity. A necessary step is to show the existence of a suitable choice of gauge and to understand the differentiability and regularity properties of the resulting gauge tensors in some "canonical form", particularly at the centre of the star. With a wider range of applications in mind, in this paper we analyse the fixing and regularity problem in a more general setting. In particular we tackle the problem of the Hodge-type decomposition into scalar, vector and tensor components on spheres of symmetric and axially symmetric tensors with finite differentiability down to the origin, exploiting a strategy in which the loss of differentiability is as low as possible. Our primary interest, and main result, is to show that stationary and axially symmetric second order perturbations around static and spherically symmetric background configurations can indeed be rendered in the usual "canonical form" used in the literature while loosing only one degree of differentiability and keeping all relevant quantities bounded near the origin.

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arXiv2020

Existence and uniqueness of compact rotating configurations in GR in second order perturbation theory

Existence and uniqueness of rotating fluid bodies in equilibrium is still poorly understood in General Relativity (GR). Apart from the limiting case of infinitely thin disks, the only known global results in the stationary rotating case (Heilig [14] and Makino [21] [arXiv:1705.07392]) show existence in GR nearby a Newtonian configuration (under suitable additional restrictions). In this work we prove existence and uniqueness of rigidly (slowly) rotating fluid bodies in equilibrium to second order in perturbation theory in GR. The most widely used perturbation framework to describe slowly rigidly rotating stars in the strong field regime is the Hartle-Thorne model. The model involves a number of hypotheses, some explicit, like equatorial symmetry or that the perturbation parameter is proportional to the rotation, but some implicit, particularly on the structure and regularity of the perturbation tensors and the conditions of their matching at the surface. In this work, with basis on the gauge results obtained in [25], the Hartle-Thorne model is fully derived from first principles and only assuming that the perturbations describe a rigidly rotating finite perfect fluid ball (with no layer at the surface) with the same barotropic equation of state as the static ball. Rigidly rotating fluid balls are analyzed consistently in second order perturbation theory by imposing only basic differentiability requirements and boundedness. Our results prove in particular that, at this level of approximation, the spacetime must be indeed equatorially symmetric and is fully determined by two parameters, namely the central pressure and the uniform angular velocity of the fluid.

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arXiv2017

Completion of the universal I-Love-Q relations in compact stars including the mass

In a recent paper we applied a rigorous perturbed matching framework to show the amendment of the mass of rotating stars in Hartle's model. Here, we apply this framework to the tidal problem in binary systems. Our approach fully accounts for the correction to the Love numbers needed to obtain the universal $I$-Love-$Q$ relations. We compute the corrected mass vs radius configurations of rotating quark stars, revisiting a classical paper on the subject. These corrections allow us to find a universal relation involving the second-order contribution to the mass $δM$. We thus complete the set of universal relations for the tidal problem in binary systems, involving four perturbation parameters, namely $I$, Love, $Q$, and $δM$. These relations can be used to obtain the perturbation parameters directly from observational data.

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arXiv2015

Junction conditions in quadratic gravity: thin shells and double layers

The junction conditions for the most general gravitational theory with a Lagrangian containing terms quadratic in the curvature are derived. We include the cases with a possible concentration of matter on the joining hypersurface -termed as thin shells, domain walls or braneworlds in the literature- as well as the proper matching conditions where only finite jumps of the energy-momentum tensor are allowed. In the latter case we prove that the matching conditions are more demanding than in General Relativity. In the former case, we show that generically the shells/domain walls are of a new kind because they possess, in addition to the standard energy-momentum tensor, a double layer energy-momentum contribution which actually induces an external energy flux vector and an external scalar pressure/tension on the shell. We prove that all these contributions are necessary to make the entire energy-momentum tensor divergence-free, and we present the field equations satisfied by these energy-momentum quantities. The consequences of all these results are briefly analyzed.

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arXiv2015

Revisiting Hartle's model using perturbed matching theory to second order: amending the change in mass

Hartle's model describes the equilibrium configuration of a rotating isolated compact body in perturbation theory up to second order in General Relativity. The interior of the body is a perfect fluid with a barotropic equation of state, no convective motions and rigid rotation. That interior is matched across its surface to an asymptotically flat vacuum exterior. Perturbations are taken to second order around a static and spherically symmetric background configuration. Apart from the explicit assumptions, the perturbed configuration is constructed upon some implicit premises, in particular the continuity of the functions describing the perturbation in terms of some background radial coordinate. In this work we revisit the model within a modern general and consistent theory of perturbative matchings to second order, which is independent of the coordinates and gauges used to describe the two regions to be joined. We explore the matching conditions up to second order in full. The main particular result we present is that the radial function $m_0$ (in the setting of the original work) of the second order perturbation tensor, contrary to the original assumption, presents a jump at the surface of the star, which is proportional to the value of the energy density of the background configuration there. As a consequence, the change in mass needed by the perturbed configuration to keep the value of the central energy density unchanged must be amended. We also discuss some subtleties that arise when studying the deformation of the star.

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