Search results for " relativity"
showing 8 items of 1158 documents
Rotational effects on the oscillation frequencies of newly born proto-neutron stars
2003
In this paper we study the effects of rotation on the frequencies of the quasi-normal modes of a proto-neutron star (PNS) born in a gravitational collapse during the first minute of life. Our analysis starts a few tenths of seconds after the PNS formation, when the stellar evolution can be described by a sequence of equilibrium configurations. We use the evolutionary models developed by Pons et al. (1999; 2001) that describe how a non rotating star cools down and contracts while neutrino diffusion and thermalization processes dominate the stellar dynamics. For assigned values of the evolution time, we set the star into slow rotation and integrate the equations of stellar perturbations in th…
Akbarian Scepticism in Islam : Qūnawī's Sceptical Arguments from Relativity and Disagreement
2021
This study deals with the sceptical arguments by one of the most important figures in the philosophical Sufi tradition (the Akbarian school) and the foremost disciple of Ibn ʿArabī, Ṣadr al‐Dīn al‐Qūnawī. Though not a sceptic in the strict sense, Qūnawī employs sceptical arguments from relativity of rational knowledge and disagreement among philosophers to prove inefficacy of reason and rational procedures of knowledge in terms of achieving certain knowledge of metaphysical matters, namely of God and the ultimate principles of things. The paper questions Qūnawī's implicit assumption that, if there is disagreement on a proposition p, then p is relative and thus cannot provide certain knowled…
Quantum aspects originated by Gravitation: from cosmology to astrophysics
2018
El estudio de campos cuánticos que se propagan en espacio-tiempos clásicos, curvos y dinámicos, ofrece una primera manera de evaluar las consecuencias de la teoría cuántica cuando los fenómenos gravitatorios no son despreciables. Esta es una cuestión importante que debe ser abordada cuando un campo gravitatorio intenso juega un papel principal en la dinámica de un sistema físico, tal como ocurre durante el universo temprano (inflación cósmica) o en la formación de agujeros negros astrofísicos. Una de las características más sorprendentes de esta materia es quizás el hecho de que el cálculo de observables físicos, incluso para campos sin interacción, frecuentemente involucra operadores cuadr…
Pseudospectrum of Reissner-Nordström black holes: Quasinormal mode instability and universality
2021
Black hole spectroscopy is a powerful tool to probe the Kerr nature of astrophysical compact objects and their environment. The observation of multiple ringdown modes in gravitational waveforms could soon lead to high-precision gravitational spectroscopy, so it is critical to understand if the quasinormal mode spectrum is stable against perturbations. It was recently shown that the pseudospectrum can shed light on the spectral stability of black hole quasinormal modes. We study the pseudospectrum of Reissner-Nordstr\"om spacetimes and we find a spectral instability of scalar and gravitoelectric quasinormal modes in subextremal and extremal black holes, extending similar findings for the Sch…
Scattering of Co-Current Surface Waves on an Analogue Black Hole
2018
We report on what is to our knowledge the first scattering experiment of surface waves on an accelerating transcritical flow, which in the analogue gravity context is described by an effective spacetime with a black-hole horizon. This spacetime has been probed by an incident co-current wave, which partially scatters into an outgoing countercurrent wave on each side of the horizon. The measured scattering amplitudes are compatible with the predictions of the hydrodynamical theory, where the kinematical description in terms of the effective metric is exact.
Remembering an Era: Roger Penrose’s Paper on “Gravitational Collapse: The Role of General Relativity”
2018
Back in the 1960s, Einstein’s theory of general relativity re-emerged as a field of important research activity. Much of the impetus behind this resurgence came from powerful new mathematical ideas that Roger Penrose and Stephen Hawking applied to prove general singularity theorems for global space-time structures. Their results stirred the imaginations of astrophysicists and gave relativistic cosmology an entirely new research agenda. A decade later, black holes and the big hang model were on the tongues of nearly everyone who followed recent trends in science. As popular expositions dealing with quasars, pulsars, and the geometry of black holes began to appear in magazines and textbooks, …
Anomaly-free scale symmetry and gravity
2023
In this Letter, we address the question of whether the conformal invariance can be considered as a global symmetry of a theory of fundamental interactions. To describe Nature, this theory must contain a mechanism of spontaneous breaking of the scale symmetry. Besides that, the fundamental theory must include gravity, whereas all known extensions of the conformal invariance to the curved space-time suffer from the Weyl anomaly. We show that conformal symmetry can be made free from the quantum anomaly only in the flat space. The presence of gravity would reduce the global symmetry group of the fundamental theory to the scale invariance only. We discuss how the effective Lagrangian respecting …
A posteriori error estimates for time-dependent reaction-diffusion problems based on the Payne-Weinberger inequality
2015
We consider evolutionary reaction-diffusion problem with mixed Dirichlet--Robin boundary conditions. For this class of problems, we derive two-sided estimates of the distance between any function in the admissible energy space and exact solution of the problem. The estimates (majorants and minorants) are explicitly computable and do not contain unknown functions or constants. Moreover, it is proved that the estimates are equivalent to the energy norm of the deviation from the exact solution.