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Magalhaes, R. B., Ribeiro, G. P., Lima, H. C. D. J., Olmo, G. J., & Crispino, L. C. B. (2024). Singular space-times with bounded algebraic curvature scalars. J. Cosmol. Astropart. Phys., 05(5), 114–34pp.
Abstract: We show that the absence of unbounded algebraic curvature invariants constructed from polynomials of the Riemann tensor cannot guarantee the absence of strong singularities. As a consequence, it is not sufficient to rely solely on the analysis of such scalars to assess the regularity of a given space-time. This conclusion follows from the analysis of incomplete geodesics within the internal region of asymmetric wormholes supported by scalar matter which arise in two distinct metric-affine gravity theories. These wormholes have bounded algebraic curvature scalars everywhere, which highlights that their finiteness does not prevent the emergence of pathologies (singularities) in the geodesic structure of space-time. By analyzing the tidal forces in the internal wormhole region, we find that the angular components are unbounded along incomplete radial time-like geodesics. The strength of the singularity is determined by the evolution of Jacobi fields along such geodesics, finding that it is of strong type, as volume elements are torn apart as the singularity is approached. Lastly, and for completeness, we consider the wormhole of the quadratic Palatini theory and present an analysis of the tidal forces in the entire space-time.
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Castillo-Felisola, O., Grez, B., Olmo, G. J., Orellana, O., & Perdiguero Garate, J. (2024). Cosmological solutions in polynomial affine gravity with torsion. Eur. Phys. J. C, 84(9), 900–12pp.
Abstract: The Polynomial Affine Gravity is an alternative gravitational model, where the interactions are mediated solely by the affine connection, instead of the metric tensor. In this paper, we explore the space of solutions to the field equations when the torsion fields are turned on, in a homogeneous and isotropic (cosmological) scenario. We explore various metric structures that emerge in the space of solutions.
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Magalhaes, R. B., Maso-Ferrando, A. S., Bombacigno, F., Olmo, G. J., & Crispino, L. C. B. (2024). Echoes from bounded universes. Phys. Rev. D, 110(4), 044058–21pp.
Abstract: We construct a general class of modified Ellis-Bronnikov wormholes, where one asymptotic Minkowski region is replaced by a bounded 2-sphere core, characterized by an asymptotic finite areal radius. We pursue an in-depth analysis of the resulting geometry, outlining that geodesic completeness is also guaranteed when the area function asymptotically shrinks to zero. Moreover, we perform an analysis of the circular orbits present in our model and conclude that stable circular orbits are allowed in the bounded region. As a consequence, a stable light ring may exist in the inner region and trapped orbits may appear within this bounded region. Such internal structure suggests that the bounded region can trap perturbations. Then, we study the evolution of scalar perturbations, bringing out how these geometric configurations can in principle affect the time-domain profiles of quasinormal modes, pointing out the distinctive features with respect to other black hole or wormhole geometries.
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Batool, A., Malik Sultan, A., Olmo, G. J., & Rubiera-Garcia, D. (2024). Stellar structure in f(R,T) gravity: Some exact solutions. Phys. Rev. D, 110(6), 064059–6pp.
Abstract: We find some exact solutions for constant-density and quark matter equations of state in stellar structure models framed within the f(R, T) = R + lambda(KT)-T-2 theory of gravity, where R is the curvature scalar, T the trace of the stress-energy tensor, and lambda some constant. These solutions correspond to specific values of the constant lambda and represent different compactness states of the corresponding stars, though only those made of quark matter can be regarded as physical. The latter modify the compactness (Buchdahl) limit of neutron stars upward for lambda > 0 until saturating the one of black holes. Our results show that it is possible to find useful insights on stellar structure in this class of theories, a fact that could be used for obtaining constraints on limiting masses such as the minimum hydrogen burning mass.
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Mora-Perez, G., Olmo, G. J., Rubiera-Garcia, D., & Saez-Chillon Gomez, D. (2024). Boundary terms and on-shell action in Ricci-based gravity theories: The Hamiltonian formulation. Phys. Rev. D, 110(8), 084051–11pp.
Abstract: Considering the so-called Ricci-based gravity theories, a family of extensions of general relativity whose action is given by a nonlinear function of contractions and products of the (symmetric part of the) Ricci tensor of an independent connection, the Hamiltonian formulation of the theory is obtained. To do so, the independent connection is decomposed in two parts, one compatible with a metric tensor and the other one given by a 3-rank tensor. Subsequently, the Riemann tensor is expressed in terms of its projected components onto a hypersurface, allowing one to construct the 3 & thorn; 1 decomposition of the theory and the corresponding Gauss-Codazzi relations, where the boundary terms naturally arise in the gravitational action. Finally, the Arnowitt-Deser-Misner (ADM) decomposition is followed in order to construct the corresponding Hamiltonian and the ADM energy for any Ricci-based gravity theory. The formalism is applied to the simple case of Schwarzschild spacetime.
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Almeida, C. A. S., Lima, F. C. E., Mishra, S. S., Olmo, G. J., & Sahoo, P. K. (2024). Thick brane in mimetic-like gravity. Nucl. Phys. B, 1009, 116747–9pp.
Abstract: We analyze a five-dimensional braneworld governed by a mimetic-like gravity, a plausible candidate for explaining dark matter. Within this scenario, we examine Friedmann-Lemaitre-Robertson-Walker (FLRW) branes and find that constant curvature and Minkowskian solutions are possible. We then show that the mimetic model leads to kink-like and lump-like thick brane solutions without the need for spontaneous symmetry breaking. Its stability against small perturbations is also verified.
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Andrade, I., Bazeia, D., Marques, M. A., Menezes, R., & Olmo, G. J. (2025). Analytical solutions for Maxwell-scalar system on radially symmetric spacetimes. Eur. Phys. J. C, 85(1), 27–15pp.
Abstract: We investigate Maxwell-scalar models on radially symmetric spacetimes in which the gauge and scalar fields are coupled via the electric permittivity. We find the conditions that allow for the presence of minimum energy configurations. In this formalism, the charge density must be written exclusively in terms of the components of the metric tensor and the scalar field is governed by first-order equations. We also find a manner to map the aforementioned equation into the corresponding one associated to kinks in (1, 1) spacetime dimensions, so we get analytical solutions for three specific spacetimes. We then calculate the energy density and show that the energy is finite. The stability of the solutions against contractions and dilations, following Derrick's argument, and around small fluctuations in the fields is also investigated. In this direction, we show that the solutions obeying the first-order framework are stable.
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