https://www.um.edu.mt/library/oar/handle/123456789/404 Thu, 01 Oct 2026 06:30:33 GMT 2026-10-01T06:30:33Z https://www.um.edu.mt/library/oar/handle/123456789/149569 Title: Ellis wormhole without a phantom scalar field Authors: Cañate, Pedro; Sultana, Joseph; Kazanas, Demosthenes Abstract: In this paper, we present an exact solution for (3 þ 1)-dimensional Einstein-scalar-Gauss-Bonnet theory (EsGB) in electrovacuum. The solution is characterized by only one parameter, Q, which in general can be associated with the electromagnetic field and the scalar field. We show that the solution corresponds to a charged wormhole with a throat at the region r ¼ jQj and is also supported by a real scalar field having a positive kinetic term. We show that the solution belongs to the most general class of solutions known as Ellis wormholes but without the need for “exotic matter” or a phantom scalar field. Tue, 01 Jan 2019 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/149569 2019-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149568 Title: Testing viable f(R) models with the angular-diameter distance to compact quasar cores Authors: Sultana, Joseph; Melia, Fulvio; Kazanas, Demosthenes Abstract: We consider here some popular fðRÞ models generally viewed as possible alternatives to the existence of dark energy in General Relativity. For each of these, we compute the redshift zmax at which the angular diameter distance dAðzÞ is expected to reach its maximum value. This turning point in dAðzÞ was recently measured in a model-independent way using compact quasar cores and was found to occur at zmax ¼ 1.70 0.20. We compare the predictions of zmax for the fðRÞ models with this observed value to test their viability at a deeper level than has been attempted thus far, thereby quantifying an important observational difference between such modified gravity scenarios and standard Lambda Cold Dark Matter (ΛCDM) cosmology. Our results show that, while the most popular fðRÞ models today are consistent with this measurement to within 1σ, the turning point zmax will allow us to prioritize these alternative gravity theories as the measurement precision continues to improve, particularly with regard to how well they mitigate the tension between the predictions of ΛCDM and the observations. For example, while the Hu-Sawicki version of fðRÞ increases this tension, the Starobinky model reduces it. Tue, 01 Jan 2019 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/149568 2019-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149567 Title: Obtaining analytical approximations to black hole solutions in higher-derivative gravity using the homotopy analysis method Authors: Sultana, Joseph Abstract: The Homotopy Analysis Method (HAM) is considered as a very useful method for obtaining analytical approximate solutions to various nonlinear differential equations arising in many different areas of science and engineering. Despite this, it is seldom used to obtain solutions in General Relativity and particularly higher order theories of gravity, where due to the complexity and nonlinearity of the field equations, most of the known solutions are numerical. We consider the case of a non-Schwarzschild static and spherically symmetric black hole solution in higher derivative gravity that has been studied recently. We obtain an analytical approximation using HAM and compare it with the numerical solution. Tue, 01 Jan 2019 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/149567 2019-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149565 Title: Hairy black holes in Einstein–Weyl gravity Authors: Sultana, Joseph Abstract: Higher derivative extensions of Einstein’s general relativity are commonly viewed as alternative and effective theories of gravity, not just due to the fact that these arise naturally in the string theory approach to gravity, but mainly for their use in cosmology to generate geometric dark energy models. In a recent paper [Phys. Rev. Lett. 114, 171601 (2015)] Lü et al. used numerical methods to obtain static, spherically symmetric and asymptotically flat black hole solutions in vacuum Einstein-Weyl gravity and showed that these are different than the Schwarzschild black hole. Inspired by the absence of any no-hair theorem for Einstein-Weyl gravity, in this paper we derive numerical examples of black hole solutions with massive scalar hair, and study the effect of the scalar field on the black hole structure. We limit ourselves to static and spherically symmetric solutions which are asymptotically flat such that the scalar field is regular on the horizon and vanishes at infinity. Wed, 01 Jan 2020 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/149565 2020-01-01T00:00:00Z