119 research outputs found
L'immixtion et la trop grande expertise du propriétaire, cause d'exonération de responsabilité des architectes et des entrepreneurs sous l'article 1688 C.C.
In May 1977, the Supreme Court of Canada gave an authoritative judgment on the interpretation of article 1688 of the Civil Code dealing with the liability of the builder and the architect. In Davie Shipbuilding Ltd v. Cargill Grain Company Ltd and The Foundation Company of Canada Ltd, the court established a new method of rebutting the presumption of liability resulting from the article. Indeed, the Supreme Court found that where the owner intervened during the construction and was more qualified than the contractors in charge of the work to judge its quality, he could not invoke the presumption resulting from article 1688 C.C. and was responsible for any loss. The case is not an easy one — questions of law and facts are not simple in this dispute involving more than eleven million dollars
Shadows of black holes with dark matter halo
We investigate the shadow of an exact black hole solution of Einstein's
equations recently proposed by Cardoso et al., to describe a supermassive black
hole immersed in a dark matter halo. We analyze and discuss the light rings and
the gravitational lensing of this spacetime comparing them with an isolated
Schwarzschild black hole. Using backward ray-tracing techniques, we study the
shadows cast by such black hole when illuminated by a celestial sphere that
emits radiation isotropically. We find that when the dark matter distribution
concentrates near the event horizon of the black hole, multiple light rings
emerge. In this high compactness regime, the shadows and gravitational lensing
are significantly different from the Schwarzschild one. We also use the M87*
and SgrA* shadow data, obtained by the Event Horizon Telescope collaboration,
to constrain the parameters of the dark matter halo.Comment: 8 pages, 6 figure
On-axis tidal forces in Kerr spacetime
Tidal forces are an important feature of General Relativity, which are
related to the curvature tensor. We analyze the tidal tensor in Kerr spacetime,
with emphasis on the case along the symmetry axis of the Kerr black hole,
noting that tidal forces may vanish at a certain point, unlike in the
Schwarzschild spacetime, using Boyer-Lindquist coordinates. We study in detail
the effects of vanishing tidal forces in a body constituted of dust infalling
along the symmetry axis. We find the geodesic deviation equations and solve
them, numerically and analytically, for motion along the symmetry axis of the
Kerr geometry. We also point out that the intrinsic Gaussian curvature of the
event horizon at the symmetry axis is equal to the component along this axis of
the tidal tensor there.Comment: 13 pages, 15 figures, accepted for publication in The European
Physical Journal Plu
Singular space-times with bounded algebraic curvature scalars
We show that the absence of unbounded algebraic curvature invariants
constructed from polynomials of the Riemann tensor cannot guarantee the absence
of strong singularities. As a consequence, it is not sufficient to rely solely
on the analysis of such scalars to assess the regularity of a given space-time.
This conclusion follows from the analysis of incomplete geodesics within the
internal region of asymmetric wormholes supported by scalar matter which arise
in two distinct metric-affine gravity theories. These wormholes have bounded
algebraic curvature scalars everywhere, which highlights that their finiteness
does not prevent the emergence of pathologies (singularities) in the geodesic
structure of space-time. By analyzing the tidal forces in the internal wormhole
region, we find that the angular components are unbounded along incomplete
radial time-like geodesics. The strength of the singularity is determined by
the evolution of Jacobi fields along such geodesics, finding that it is of
strong type, as volume elements are torn apart as the singularity is
approached. Lastly, and for completeness, we consider the wormhole of the
quadratic Palatini theory and present an analysis of the tidal forces in the
entire space-time.Comment: 15 pages, 5 figure
Electrically charged regular black holes in nonlinear electrodynamics: light rings, shadows, and gravitational lensing
Within nonlinear electrodynamics (NED), photons follow null geodesics of an effective geometry, which
is different from the geometry of the spacetime itself. Over the last years, several works were dedicated to
investigate the motion of photons in the effective geometry of NED-based magnetically charged regular black
hole (RBH) solutions. However, there are few works considering electrically charged RBHs. We study the light
rings, shadows, and gravitational lensing of the electrically charged RBH solution proposed by Irina Dymnikova
(ID), which is a static and spherically symmetric spacetime with a NED source. We show that the shadow
associated to the effective geometry can be almost 10% bigger that the one associated to the standard geometry.
We also find that the ID solution may mimic the shadow properties of the Reissner-Nordstrom (RN) BH, for low- ¨
to-extreme values of the electric charge. Besides that, by using the backwards ray-tracing technique, we obtain
that ID and RN BH solutions can have a very similar gravitational lensing, for some values of the correspondent
electric charges. We also show that the motion of photons in the effective geometry can be interpreted as a
non-geodesic curve submitted to a 4-force term, from the perspective of an observer in the standard geometry.publishe
Tachigali rodolfo-rocioii, una nueva especie arbórea de Leguminosae de los bosques Andino Amazónicos de Perú
Como resultado de los esfuerzos de la sinergia entre la taxonomÃa y ecologÃa en los estudios del género Tachigali Aubl. (Caesalpinioideae, Leguminosae), se describe e ilustra a la nueva especie Tachigali rodolfo-rocioii, proveniente de los bosques húmedos Amazónicos del sur y centro de Perú. Se compara y discute con las especies morfológicamente más próximas T. amarumayu y T. prancei
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