Magalhaes, R. B., Crispino, L. C. B., & Olmo, G. J. (2022). Compact objects in quadratic Palatini gravity generated by a free scalar field. Phys. Rev. D, 105(6), 064007–15pp.
Abstract: We study the correspondence that connects the space of solutions of general relativity (GR) with that of Ricci-based gravity theories (RBGs) of the f(R, Q) type in the metric-affinc formulation, where Q = R(mu nu)R(mu nu). We focus on the case of scalar matter and show that when one considers a free massless scalar in the GR frame, important simplifications arise that allow one to establish the correspondence for arbitrary f (R, Q) Lagrangian. We particularize the analysis to a quadratic f (R, Q) theory and use the spherically symmetric, static solution of Jannis-Newman-Winicour as seed to generate new compact objects in our target theory. We find that two different types of solutions emerge, one representing naked singularities and another corresponding to asymmetric wormholes with bounded curvature scalars everywhere. The latter solutions, nonetheless, are geodesically incomplete.
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Olmo, G. J., Rubiera-Garcia, D., & Sanchez-Puente, A. (2018). Accelerated observers and the notion of singular spacetime. Class. Quantum Gravity, 35(5), 055010–18pp.
Abstract: Geodesic completeness is typically regarded as a basic criterion to determine whether a given spacetime is regular or singular. However, the principle of general covariance does not privilege any family of observers over the others and, therefore, observers with arbitrary motions should be able to provide a complete physical description of the world. This suggests that in a regular spacetime, all physically acceptable observers should have complete paths. In this work we explore this idea by studying the motion of accelerated observers in spherically symmetric spacetimes and illustrate it by considering two geodesically complete black hole spacetimes recently described in the literature. We show that for bound and locally unbound accelerations, the paths of accelerated test particles are complete, providing further support to the regularity of such spacetimes.
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Bazeia, D., Losano, L., Olmo, G. J., & Rubiera-Garcia, D. (2017). Geodesically complete BTZ-type solutions of 2+1 Born-Infeld gravity. Class. Quantum Gravity, 34(4), 045006–21pp.
Abstract: We study Born-Infeld gravity coupled to a static, non-rotating electric field in 2 + 1 dimensions and find exact analytical solutions. Two families of such solutions represent geodesically complete, and hence nonsingular, spacetimes. Another family represents a point-like charge with a singularity at the center. Despite the absence of rotation, these solutions resemble the charged, rotating BTZ solution of general relativity but with a richer structure in terms of horizons. The nonsingular character of the first two families turn out to be attached to the emergence of a wormhole structure on their innermost region. This seems to be a generic prediction of extensions of general relativity formulated in metric-affine (or Palatini) spaces, where metric and connection are regarded as independent degrees of freedom.
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Guerrero, M., Olmo, G. J., Rubiera-Garcia, D., & Saez-Chillon Gomez, D. (2022). Multiring images of thin accretion disk of a regular naked compact object. Phys. Rev. D, 106(4), 044070–13pp.
Abstract: We discuss the importance of multiring images in the optical appearance of a horizonless spherically symmetric compact object, when illuminated by an optically thin accretion disk. Such an object corresponds to a subcase of an analytically tractable extension of the Kerr solution dubbed as the “eye of the storm” by Simpson and Visser in [J. Cosmol. Astropart. Phys. 03 (2022) 011], which merits in removing curvature singularities via an asymptotically Minkowski core, while harboring both a critical curve and an infinite potential barrier at the center for null geodesics. This multiring structure is induced by light rays winding several times around the object, and whose luminosity is significantly boosted as compared to the Schwarzschild solution by the modified shape of the potential. Using three toy profiles for the emission of an infinitely thin disk, truncated at its inner edge (taking its maximum value there) and having different decays with the distance, we discuss the image created by up to eight rings superimposed on top of the direct emission of the disk as its edge is moved closer to the center of the object. Our results point to the existence of multiring images with a non-negligible luminosity in shadow observations when one allows for the existence of other compact objects in the cosmic zoo beyond the Schwarzschild solution. Such multiring images could be detectable within the future projects on very long baseline interferometry.
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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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Olmo, G. J., Rubiera-Garcia, D., & Sanchez-Puente, A. (2015). Geodesic completeness in a wormhole spacetime with horizons. Phys. Rev. D, 92(4), 044047–16pp.
Abstract: The geometry of a spacetime containing a wormhole generated by a spherically symmetric electric field is investigated in detail. These solutions arise in high-energy extensions of general relativity formulated within the Palatini approach and coupled to Maxwell electrodynamics. Even though curvature divergences generically arise at the wormhole throat, we find that these spacetimes are geodesically complete. This provides an explicit example where curvature divergences do not imply spacetime singularities.
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Maso-Ferrando, A., Sanchis-Gual, N., Font, J. A., & Olmo, G. J. (2024). Numerical evolutions of boson stars in Palatini f(R) gravity. Phys. Rev. D, 109(4), 044042–14pp.
Abstract: We investigate the time evolution of spherically symmetric boson stars in Palatini f(R) gravity through numerical relativity computations. Employing a novel approach that establishes a correspondence between modified gravity with scalar matter and general relativity with modified scalar matter, we are able to use the techniques of numerical relativity to simulate these systems. Specifically, we focus on the quadratic theory f(R) = R + xi R2 and compare the obtained solutions with those in general relativity, exploring both positive and negative values of the coupling parameter xi. Our findings reveal that boson stars in Palatini f(R) gravity exhibit both stable and unstable evolutions. The latter give rise to three distinct scenarios: migration toward a stable configuration, complete dispersion, and gravitational collapse leading to the formation of a baby universe structure.
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Afonso, V. I., Olmo, G. J., Orazi, E., & Rubiera-Garcia, D. (2019). Correspondence between modified gravity and general relativity with scalar fields. Phys. Rev. D, 99(4), 044040–15pp.
Abstract: We describe a novel procedure to map the field equations of nonlinear Ricci-based metric-affine theories of gravity, coupled to scalar matter described by a given Lagrangian, into the field equations of general relativity coupled to a different scalar field Lagrangian. Our analysis considers examples with a single and N real scalar fields, described either by canonical Lagrangians or by generalized functions of the kinetic and potential terms. In particular, we consider several explicit examples involving foRthorn theories and the Eddington-inspired Born-Infeld gravity model, coupled to different scalar field Lagrangians. We show how the nonlinearities of the gravitational sector of these theories can be traded to nonlinearities in the matter fields and how the procedure allows to find new solutions on both sides of the correspondence. The potential of this procedure for applications of scalar field models in astrophysical and cosmological scenarios is highlighted.
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Olmo, G. J., Rubiera-Garcia, D., & Wojnar, A. (2019). Minimum main sequence mass in quadratic Palatini f(R) gravity. Phys. Rev. D, 100(4), 044020–9pp.
Abstract: General relativity yields an analytical prediction of a minimum required mass of roughly similar to 0.08-0.09 M-circle dot for a star to stably burn sufficient hydrogen to fully compensate photospheric losses and, therefore, to belong to the main sequence. Those objects below this threshold ( brown dwarfs) eventually cool down without any chance to stabilize their internal temperature. In this work we consider quadratic Palatini f(R) gravity and show that the corresponding Newtonian hydrostatic equilibrium equation contains a new term whose effect is to introduce a weakening/strengthening of the gravitational interaction inside astrophysical bodies. This fact modifies the general relativity prediction for this minimum main sequence mass. Through a crude analytical modeling we use this result in order to constraint a combination of the quadratic f(R) gravity parameter and the central density according to astrophysical observations.
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Bazeia, D., Losano, L., Olmo, G. J., Rubiera-Garcia, D., & Sanchez-Puente, A. (2015). Classical resolution of black hole singularities in arbitrary dimension. Phys. Rev. D, 92(4), 044018–15pp.
Abstract: A metric-affine approach is employed to study higher-dimensional modified gravity theories involving different powers and contractions of the Ricci tensor. It is shown that the field equations are always second-order, as opposed to the standard metric approach, where this is only achieved for Lagrangians of the Lovelock type. We point out that this property might have relevant implications for the AdS/CFT correspondence in black hole scenarios. We illustrate these aspects by considering the case of Born-Infeld gravity in d dimensions, where we work out exact solutions for electrovacuum configurations. Our results put forward that black hole singularities in arbitrary dimensions can be cured in a purely classical geometric scenario governed by second-order field equations.
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