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We investigate the dynamical evolution of a homogeneous and isotropic flat universe filled with a barotropic fluid satisfying the linear equation of state in Rastall gravity. Using the dynamical system approach, we determined the fixed points of the system and studied their stability. We further explore the thermodynamic aspects at the apparent horizon by investigating the validity of the generalised second law of thermodynamics with an equilibrium description. Key words: Rastall gravity, dynamical system, cyclic universe, thermodynamics. Nous etudions un univers plat, homogene et isotrope rempli d'un fluide barotrope satisfaisant l'equation d'etat lineaire de la gravite de Rastall. Utilisant l'approche des systemes dynamiques, nous trouvons les points fixes et etudions leur stabilite. Nous explorons aussi les aspects thermodynamiques de l'horizon apparent en examinant la validite de la deuxieme loi de la thermodynamique avec description d'equilibre. Mots-cles : gravite de Rastall, systeme dynamique, univers cyclique, thermodynamique., 1. Introduction Cosmological modelling requires a theory of gravity and a description of the matter content in the universe. In Rastall's theory of gravity [1-3], the matter source is described [...]

We investigate the Friedmann-Lemaitre-Robertson-Walker (FLRW) type cosmology of the induced dark energy model and illustrate that the extra terms emerging from the fifth dimension can play the role of dark energy. The model predicts the expansion with deceleration at early time and acceleration in late time for an open universe. Key words: induced-matter theory, FLRW cosmology, dark energy, geometry of the universe. Nous etudions la cosmologie de type FLRW du modele de l'energie sombre induite et illustrons que les termes additionnels qui emergent de la cinquieme dimension peuvent jouer le role de l'energie sombre. Le modele predit une expansion avec deceleration dans un premier temps et acceleration plus tard pour un univers ouvert. [Traduit par la Redaction] Mots-cles : theorie de la matiere induite, cosmologie FLRW, energie sombre, geometrie de l'univers, acceleration (de l'expansion) de l'univers., 1. Introduction Observations of distant type-Ia supernovae indicate that the universe is currently accelerating in its expansion [1-4]. Therefore, in a school of thought, the universe should mainly be filled [...]

Relativistic jets are observed throughout the Universe and strongly affect their surrounding environments on a range of physical scales, from Galactic binary systems.sup.1 to galaxies and clusters of galaxies.sup.2. All types of accreting black hole and neutron star have been observed to launch jets.sup.3, with the exception of neutron stars with strong magnetic fields.sup.4,5 (higher than 10.sup.12 gauss), leading to the conclusion that their magnetic field strength inhibits jet formation.sup.6. However, radio emission recently detected from two such objects could have a jet origin, among other possible explanations.sup.7,8, indicating that this long-standing idea might need to be reconsidered. But definitive observational evidence of such jets is still lacking. Here we report observations of an evolving jet launched by a strongly magnetized neutron star accreting above the theoretical maximum rate given by the Eddington limit. The radio luminosity of the jet is two orders of magnitude fainter than those seen in other neutron stars with similar X-ray luminosities.sup.9, implying an important role for the properties of the neutron star in regulating jet power. Our result also shows that the strong magnetic fields of ultra-luminous X-ray pulsars do not prevent such sources from launching jets.Observations of a jet from a strongly magnetized neutron star reveal that strong magnetic fields do not prevent jet formation and suggest that stellar properties influence jet power., Author(s): J. van den Eijnden [sup.1] , N. Degenaar [sup.1] , T. D. Russell [sup.1] , R. Wijnands [sup.1] , J. C. A. Miller-Jones [sup.2] , G. R. Sivakoff [sup.3] [...]

This paper investigates the Godel's exact solution of the Einstein equations which describes a stationary homogeneous cosmological Universe inducing closed timelike curves CTCs). This model is generally dismissed because it exhibits a rotational symmetry and it requires a non zero cosmological constant in contradiction with the current astronomical observations. If the cosmological term is assumed to be slightly variable, we show that this metric can be compatible with the Hubble expansion, which makes the Godel model a viable representation of our Universe., Introduction In his original paper [1], Kurt Godel has derived an exact solution to Einstein's field equations in which the matter takes the form of a pressure-free perfect fluid (dust [...]

In this paper, as a toy model, we study the dynamics of a 5D bulk space-time with our universe as a 'test 3- brane' located in the bulk; an idealized model of a topological object whose physical properties are negligible in comparison with those of the bulk. We obtain the result that our universe experiences acceleration and phantom crossing as a result of the geometry of bulk space-time. PACS Nos: 04.50.-h, 11.25.-w, 95.36.+x Nous etudions ici la dynamique d'un espace-temps 5D (bulk) enrobant, oU notre univers est un 3-brane test correspondant a une surface de cet espace, ce qui nous donne un modele idealise d'un objet topologique dont les proprietes physiques sont negligeables comparees a celles de l'espace-temps enrobant. Notre univers subit une acceleration et le parametre de son equation d'etat croise la frontiere du fantome a cause de la geometrie de l'espace-temps. [Traduit par la Redaction], 1. Introduction Cosmological observations, such as Super-Nova Ia (SNIa), Wilkinson Microwave Anisotropy Probe (WMAP), Sloan Digital Sky Survey (SDSS), Chandra X-ray Observatory, etc., reveal some cross-checked information about our universe [...]

Assuming that the matter in the background geometry is a free gas and that no phase transitions were occurring in the early Universe, we discuss the thermodynamics of this closed system using classical approaches. We find that essential cosmological quantities, such as the Hubble parameter H, the scaling factor a, and the curvature parameter k, can be derived from this simple model, which on one hand fulfills and entirely obeys the laws of thermodynamics, and on the other hand, its results are compatible with the Friedmann-Robertson-WaLker model and the Einstein field equations. Including a finite bulk viscosity coefficient leads to important changes in all these cosmological quantities. Accordingly, our picture about the evolution of the Universe and its astrophysical consequences seems to undergoing a radical revision. We find that k strongly depends on the thermodynamics of background matter. The time scale at which negative curvature might take place depends on the relation between the matter content and the total energy. Using quantum and statistical approaches, we introduce expressions for H and the bulk viscosity coefficient ξ. PACS Nos: 98.80.Es, 98.80.Cq, 12.38.Mh, 25.75.-q Supposant que la matiere dans la geometrie de fond de l'univers est un gaz libre et qu'aucune transition de phase ne s'est produite dans l'univers primordial, nous etudions la thermodynamique de ce systeme ferme en utilisant une approche classique. Nous trouvons que les quantites cosmologiques essentielles, comme le parametre de Hubble H, le facteur d'echelle a et le parametre de courbure k peuvent (etre obtenu de ce simple modele qui satisfait et obeit completement aux lois de la thermodynamique. De plus, les resultats sont compatibles avec le modele de Robertson-Walker et avec les equations de champ d'Einstein. L'introduction d'un coefficient fini de viscosite volumique (bulk) amene d'importants changements dans toutes ces quantites cosmologiques. Par consequent, notre vision de revolution de l'Univers et ses consequences astrophysiques semblent devoir etre modifiees de facjon radicale. Nous trouvons que k depend fortement de la thermodynamique de la matiere du fond. L'echelle de temps, a laquelle une courbure negative peut s'etablir, depend de la relation entre le contenu en matiere et l'energie totale. Utilisant des approches quantiques et statistiques, nous introduisons des expressions pour H et le coefficient de viscosite volumique ξ. [Traduit par la Redaction], 1. Introduction We assume that the cosmological background geometry filled with matter or radiation can be characterized by well-known physical laws, such as equations of state and thermodynamics. It leads [...]

A question of mirror symmetry of the universe is considered in terms of the relationship between observer and the observed system. By including observer into the system, the handedness becomes a purely relative property; thereby the system represents both left and right spatial orders. Topologically interpreted, this logical conclusion suggests that universe represents a multiply connected space of the properties related to Mobius strip. Keywords: universe, Alice matter, handedness, Mobius strip, Poincare sphere, One of the most intriguing cases of human experience is connected with looking at the mirror. This, otherwise trivial activity, gives us a possibility of reflection that goes far beyond [...]

We apply the new dynamics of 3-space to cosmology by deriving a Hubble expansion solution. This dynamics involves two constants; G and α--the fine structure constant. This solution gives an [...]

A fascinating and deep question about nature is what one would see if one could probe space and time at smaller and smaller distances. Already the 19th century founders of modern geometry contemplated the possibility that a piece of empty space that looks completely smooth and structureless to the naked eye might have an intricate microstructure at a much smaller scale. Our vastly increased understanding of the physical world acquired during the 20th century has made this a certainty. The laws of quantum theory tell us that looking at spacetime at ever smaller scales requires ever larger energies and, according to Einstein's theory of general relativity, this will alter spacetime itself: it will acquire structure in the form of curvature. What we still lack is a definitive theory of quantum gravity to give us a detailed and quantitative description of the highly curved and quantum-fluctuating geometry of spacetime at this so-called Planck scale. This article outlines a particular approach to constructing such a theory, that of Causal Dynamical Triangulations, and its achievements so far in deriving from first principles why spacetime is what it is, from the tiniest realms of the quantum to the large-scale structure of the universe.

Take a length of rope, loop and weave it around itself and connect its ends. The result, of course, is a knot. Creating a knot seems simple, yet knot theory […]

Cosmic strings are linear concentrations of energy that may be formed at phase transitions in the very early universe. At one time they were thought to provide a possible origin for the density inhomogeneities from which galaxies eventually develop, though this idea has been ruled out, primarily by observations of the cosmic microwave background (CMB). Fundamental strings are the supposed building blocks of all matter in superstring theory or its modern version, M-theory. These two concepts were originally very far apart, but recent developments have brought them closer. The 'brane-world' scenario in particular suggests the existence of macroscopic fundamental strings that could well play a role very similar to that of cosmic strings. In this paper, we outline these new developments, and also analyse recent observational evidence, and prospects for the future.

Abstract: In this paper I try to make as much sense aspossible of, first, the extensive philosophicalliterature concerned with the status of `Wateris [H.sub.2]O' and, second, the implications ofPutnam's invention of Twin Earth, anotherpossible world stipulated to be just like Earth, except that water is XYZ, not[H.sub.2]O. Article History: Registration Date: 19/10/2004