https://www.um.edu.mt/library/oar/handle/123456789/403 2026-09-28T18:51:33Z 2026-09-28T18:51:33Z Sultana, Joseph https://www.um.edu.mt/library/oar/handle/123456789/149470 2026-09-28T11:07:03Z 2023-01-01T00:00:00Z

Title: Comment on “Gravitational lensing in Weyl gravity” Authors: Sultana, Joseph Abstract: In a recent paper [Phys. Rev. D 100, 024019 (2019)] the authors calculated the bending angle of light in Schwarzschild-de Sitter (SdS) spacetime and also in the static and spherically symmetric vacuum solution of Weyl’s conformal gravity, which is sometimes referred to as the Mannheim-Kazanas (MK) spacetime. To do this they used the standard Weinberg analysis which is normally used to calculate the bending angle of light in asymptotically flat spacetimes, but limited the integration to the position of the cosmological horizon in these spacetimes. In this paper we make some comments about the bending angle formulas obtained in these spacetimes. We point out that in the case of the MK spacetime this would still lead to an unphysical term in their formula for the deflection angle, which also occurred in previous light bending formulas for this spacetime based on similar analysis.

2023-01-01T00:00:00Z Sultana, Joseph https://www.um.edu.mt/library/oar/handle/123456789/149468 2026-09-28T11:00:30Z 2026-01-01T00:00:00Z

Title: Null Geodesics and Shadow Structure in Einstein–Weyl Gravity Authors: Sultana, Joseph Abstract: We investigate null geodesics, photon spheres and black hole shadows for the static spherically symmetric non-Schwarzschild black hole solution of Einstein-Weyl gravity, a higher-derivative extension of General Relativity containing a quadratic Weyl-curvature term. Such higher-curvature theories are motivated by attempts to formulate a quantum theory of gravity, where they improve the ultraviolet behaviour of the gravitational interaction, and also arise naturally as effective descriptions in approaches such as string theory. We employ the numerical black hole solution obtained by Lü et al. to compute the photon sphere, the shadow radius and the angular size of the shadow as observed by static observers. We show that, for black holes of equal mass, the photon sphere, shadow radius and angular size are consistently larger than those of the corresponding Schwarzschild black hole, with the deviations increasing monotonically with the higher-curvature coupling parameter a. Motivated by the Event Horizon Telescope observations of M87* and Sagittarius A*, we further compare the predicted shadow size with current observational uncertainties and derive phenomenological upper bounds on the dimensionless coupling $\alpha/m^2$. These results demonstrate that black hole shadow observations provide a promising avenue for testing Einstein-Weyl gravity and constraining quantum-motivated higher-curvature corrections to General Relativity.

2026-01-01T00:00:00Z Sciberras, Arnold Deidun, Alan Lo Valvo, Mario Sciberras, Jeffrey Borg Bonaci, Kurt Cortis, Bernard Faraone, Francesco Paolo https://www.um.edu.mt/library/oar/handle/123456789/149465 2026-09-28T10:33:16Z 2026-01-01T00:00:00Z

Title: The growing spread of the non-native brahminy blind snake, Indotyphlops braminus (Daudin, 1803), across the Maltese archipelago Authors: Sciberras, Arnold; Deidun, Alan; Lo Valvo, Mario; Sciberras, Jeffrey; Borg Bonaci, Kurt; Cortis, Bernard; Faraone, Francesco Paolo Abstract: The brahminy blindsnake, Indotyphlops braminus, native to the Indo-Malayan region, is a small fossorial snake with many populations worldwide introduced, whose presence in Malta was first reported in 2020 in a Marsa sport center. Its spread, as a "contaminant" of soil, has globally increased following the plant trade globalisation. Here, we update the distribution of I. braminus in the Maltese archipelago. Data were collected through targeted sampling and citizen science methods. Records with mutual distances greater than 100 m were treated as different sites. 47 records, spanning between 2017 and 2026, were collected, for a total of 65 I. braminus individuals. The species was found in 34 different sites in the islands of Malta (n=31), Gozo (n=2), and Comino (n=1). Most of the records was linked to highly anthropized environments: 38.3% from 12 different plant nurseries and garden centers, 17% inside or close to crops and greenhouses, 12.8% from six different public green areas, 10.6% from buildings and urbanized areas, 8.5% in or close to private gardens. In four cases the species was found outside of human settlement, such as eucalyptus reforestations and garrigue. Interestingly, in non-irrigated sites I. braminus has been sometimes found in relatively moist microhabitats, such as shady carob litter and close to artificial water-bodies. Our results highlight an unexpectedly wide distribution of I. braminus in the Maltese archipelago and confirm its dispersal pattern, clearly linked to plant trade. The species' occurrence is likely linked to multiple introductions rather than active spread. Observations in dry and seminatural areas, although apparently marginal, deserve further monitoring to assess whether they are accidental or due to the presence of stable populations.

2026-01-01T00:00:00Z Sultana, Joseph https://www.um.edu.mt/library/oar/handle/123456789/149421 2026-09-28T06:27:01Z 2026-01-01T00:00:00Z

Title: Black hole shadows in the Rh= ct universe Authors: Sultana, Joseph Abstract: The Lambda cold dark matter (CDM) model still enjoys the title of being the standard model in cosmology. Yet an alternative model that has received a lot of attention over the last decade is the universe. This model provides an age for the Universe similar to that of CDM and like CDM it is also based on a Friedmann–Robertson–Walker (FRW) cosmology. However, the total energy density and pressure p of the cosmic fluid for the model satisfy the simple equation of state (EOS), i.e. it has a vanishing total active gravitational mass. Unlike the CDM, the is a linearly expanding (coasting) model and therefore has a vanishing deceleration parameter, which apparently is not inconsistent with observations. In fact, various observational tests seem to favour the model over CDM. In this paper, we compute the shadow of a black hole embedded in the universe as observed by a comoving observer at an arbitrary distance from the black hole. To do this we use the method of matched asymptotic expansions introduced by Tsupko & Bisnovatyi-Kogan (IJMPD, 29 2050062 (2020)). We also compare these black hole shadows with those in other cosmological models including the CDM universe.

2026-01-01T00:00:00Z