Maluf, R. V., & Olmo, G. J. (2023). Vacuum polarization and induced Maxwell and Kalb-Ramond effective action in very special relativity. Phys. Rev. D, 108(9), 095022–13pp.
Abstract: This work investigates the implications of very special relativity (VSR) on the calculation of vacuum polarization for fermions in the presence of Maxwell and Kalb-Ramond gauge fields in four-dimensional spacetime. We derive the SIM(2)-covariant gauge theory associated with an Abelian antisymmetric twotensor and its corresponding field strength. We demonstrate that the free VSR-Kalb-Ramond electrodynamics is equivalent to a massive scalar field with a single polarization. Furthermore, we determine an explicit expression for the effective action involving Maxwell and Kalb-Ramond fields due to fermionic vacuum polarization at one-loop order. The quantum corrections generate divergences free of nonlocal terms only in the VSR-Maxwell sector. At the same time, we observe UV/IR mixing divergences due to the entanglement of VSR-nonlocal effects with quantum higher-derivative terms for the Kalb-Ramond field. However, in the lower energy limit, the effective action can be renormalized like in the Lorentz invariant case.
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Martinez-Asencio, J., Olmo, G. J., & Rubiera-Garcia, D. (2012). Black hole formation from a null fluid in extended Palatini gravity. Phys. Rev. D, 86(10), 104010–8pp.
Abstract: We study the formation and perturbation of black holes by null fluxes of neutral matter in a quadratic extension of general relativity formulated a la Palatini. Working in a spherically symmetric space-time, we obtain an exact analytical solution for the metric that extends the usual Vaidya-type solution to this type of theory. We find that the resulting space-time is formally that of a Reissner-Nordstrom black hole but with an effective charge term carrying the wrong sign in front of it. This effective charge is directly related to the luminosity function of the radiation stream. When the ingoing flux vanishes, the charge term disappears and the space-time relaxes to that of a Schwarzschild black hole. We provide two examples that illustrate the formation of a black hole from Minkowski space and the perturbation by a finite pulse of radiation of an existing Schwarzschild black hole.
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Bambi, C., Olmo, G. J., & Rubiera-Garcia, D. (2015). Melvin universe in Born-Infeld gravity. Phys. Rev. D, 91(10), 104010–6pp.
Abstract: We consider a magnetic flux pointing in the z direction of an axially symmetric space-time (Melvin universe) in a Born-Infeld-type extension of general relativity (GR) formulated in the Palatini approach. Large magnetic fields could have been produced in the early Universe, and given rise to interesting phenomenology regarding wormholes and black hole remnants. We find a formal analytic solution to this problem that recovers the GR result in the appropriate limits. Our results set the basis for further extensions that could allow the embedding of pairs of black hole remnants in geometries with intense magnetic fields.
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Lobo, F. S. N., Olmo, G. J., Orazi, E., Rubiera-Garcia, D., & Rustam, A. (2020). Structure and stability of traversable thin-shell wormholes in Palatini f(R) gravity. Phys. Rev. D, 102(10), 104012–11pp.
Abstract: We study the structure and stability of traversable wormholes built as (spherically symmetric) thin shells in the context of Palatini f(R) gravity. Using a suitable junction formalism for these theories we find that the effective number of degrees of freedom on the shell is reduced to a single one, which fixes the equation of state to be that of massless stress-energy fields, contrary to the general relativistic and metric f(R) cases. Another major difference is that the surface energy density threading the thin shell, needed in order to sustain the wormhole, can take any sign and may even vanish, depending on the desired features of the corresponding solutions. We illustrate our results by constructing thin-shell wormholes by surgically grafting Schwarzschild space-times and show that these configurations are always linearly unstable. However, surgically joined Reissner-Nordstrom space-times allow for linearly stable, traversable thin-shell wormholes supported by a positive energy density provided that the (squared) mass-to-charge ratio, given by y = Q(2)/M-2, satisfies the constraint 1 < y < 9/8 (corresponding to overcharged Reissner-Nordstrom configurations having a photon sphere) and lies in a region bounded by specific curves defined in terms of the (dimensionless) radius of the shell x(0) = R/M.
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Menchon, C. C., Olmo, G. J., & Rubiera-Garcia, D. (2017). Nonsingular black holes, wormholes, and de Sitter cores from anisotropic fluids. Phys. Rev. D, 96(10), 104028–16pp.
Abstract: We study Born-Infeld gravity coupled to an anisotropic fluid in a static, spherically symmetric background. The free function characterizing the fluid is selected on the following grounds: i) recovery of the Reissner-Nordstrom solution of General Relativity at large distances, ii) fulfillment of classical energy conditions, and iii) inclusion of models of nonlinear electrodynamics as particular examples. Four branches of solutions are obtained, depending on the signs of two parameters on the gravity and matter sectors. On each branch, we discuss in detail the modifications on the innermost region of the corresponding solutions, which provides a plethora of configurations, including nonsingular black holes and naked objects, wormholes, and de Sitter cores. The regular character of these configurations is discussed according to the completeness of geodesics and the behavior of curvature scalars.
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