https://www.um.edu.mt/library/oar/handle/123456789/404 Sat, 03 Oct 2026 18:32:05 GMT 2026-10-03T18:32:05Z https://www.um.edu.mt/library/oar/handle/123456789/149626 Title: Comment on “Varying-G Cosmology with Type Ia Supernovae”[Am. J. Phys. 79, 57–62 (2011)] Authors: Sultana, Joseph Abstract: In a recent paper, Dungan and Prosper claim that the Type Ia supernovae data alone are not enough to distinguish between the standard KCDM model and other models with varying G. To substantiate this, they present two spatially flat variable G FRW models with K¼0 that fit well the Type Ia supernova data. In these models they assumed that the energy momentum tensor of the matter distribution is conserved. We show that this assumption is inconsistent with variable G cosmology when K is assumed to be constant, thus rendering the suggested models erroneous. Thu, 01 Jan 2015 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/149626 2015-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149625 Title: The statistics of BAT-to-XRT flux ratio in GRB : evidence for a characteristic value and its implications Authors: Kazanas, Demosthenes; Racusin, J.L.; Sultana, Joseph; Mastichiadis, A. Abstract: We present the statistics of the ratio, R, between the prompt and afterglow “plateau” fluxes of gamma-ray bursts (GRBs). We define this as the ratio of the mean prompt energy flux in Swift BAT and the Swift XRT one, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the “plateau”. Like the distribution of many other GRB observables, the histogram of R is log-normal with maximum at a value R m  2000, FWHM of about two decades, and with the entire distribution spanning about five decades in the value of R. We note that the peak of the distribution is close to the proton-to-electron mass ratio (Rm  mp me = 1836), as proposed to be the case in an earlier publication, on the basis of a specific model of the GRB dissipation process. It therefore appears that, in addition to the values of the energy of peak luminosity Epk ~ mec 2, GRBs present us with one more quantity with an apparent characteristic value. The fact that the values of both these quantities (Epk and R) are consistent with the same specific model invoked to account for the efficient conversion of their relativistic proton energies to electrons argues favorably for its underlying assumptions. Thu, 01 Jan 2015 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/149625 2015-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149624 Title: The Statistics of BAT-to-XRT Flux Ratio in GRB : evidence for a characteristic value and its implications Authors: Kazanas, Demosthenes; Racusin, J.L.; Sultana, Joseph; Mastichiadis, A. Abstract: We present the statistics of the ratio, R, between the prompt and afterglow “plateau” fluxes of GRB. This we define as the ratio between the mean prompt energy flux in Swift BAT and the Swift XRT one, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the “plateau”. Like the distribution of many other GRB observables, the histogram of R is log-normal with maximum at a value Rm ≃ 2, 000, FWHM of about 2 decades and with the entire distribution spanning about 5 decades in the value of R. We note that the peak of the distribution is close to the proton-to-electron mass ratio (Rm ≃ mp/me = 1836), as proposed to be the case in an earlier publication, on the basis of a specific model of the GRB dissipation process. It therefore appears that, in addition to the values of the energy of peak luminosity Epk ∼ mec 2, GRB present us with one more quantity with an apparent characteristic value. The fact that the values of both these quantities (Epk and R) are consistent with the same specific model invoked to account for the efficient conversion of their relativistic proton energies to electrons, argues favorably for its underlying assumptions. Fri, 01 Jan 2016 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/149624 2016-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149621 Title: Thermodynamic geodesics of a Reissner–Nordström black hole Authors: Farrugia, Christine R.; Sultana, Joseph Abstract: Starting from a Geometrothermodynamics metric for the space of thermodynamic equilibrium states in the mass representation, we use numerical techniques to analyse the thermodynamic geodesics of a supermassive Reissner Nordström black hole in isolation. Appropriate constraints are obtained by taking into account the processes of Hawking radiation and Schwinger pair-production. We model the black hole in line with the work of Hiscock and Weems (Phys Rev D 41:1142–1151, 1990). It can be deduced that the relation which the geodesics establish between the entropy S and electric charge Q of the black hole extremises changes in the black hole’s mass. Indeed, the expression for the entropy of an extremal black hole is an exact solution to the geodesic equation. We also find that in certain cases, the geodesics describe the evolution brought about by the constant emission of Hawking radiation and charged-particle pairs. Sun, 01 Jan 2017 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/149621 2017-01-01T00:00:00Z