2,704 research outputs found
From the elasticity theory to cosmology and vice versa
The paper shows how a generalization of the elasticity theory to four
dimensions and to space-time allows for a consistent description of the
homogeneous and isotropic universe, including the accelerated expansion. The
analogy is manifested by the inclusion in the traditional Lagrangian of general
relativity of an additional term accounting for the strain induced in the
manifold (i.e. in space-time) by the curvature, be it induced by the presence
of a texture defect or by a matter/energy distribution. The additional term is
sufficient to account for various observed features of the universe and to give
a simple interpretation for the so called dark energy. Then, we show how the
same approach can be adopted back in three dimensions to obtain the equilibrium
configuration of a given solid subject to strain induced by defects or applied
forces. Finally, it is shown how concepts coming from the familiar elasticity
theory can inspire new approaches to cosmology and in return how methods
appropriated to General Relativity can be applied back to classical problems of
elastic deformations in three dimensions.Comment: 11 pages, 3 figure
Angular momentum effects in weak gravitational fields
It is shown that, contrary to what is normally expected, it is possible to
have angular momentum effects on the geometry of space time at the laboratory
scale, much bigger than the purely Newtonian effects. This is due to the fact
that the ratio between the angular momentum of a body and its mass, expressed
as a length, is easily greater than the mass itself, again expressed as a
length.Comment: LATEX, 8 page
Einstein-Cartan theory as a theory of defects in space-time
The Einstein-Cartan theory of gravitation and the classical theory of defects
in an elastic medium are presented and compared. The former is an extension of
general relativity and refers to four-dimensional space-time, while we
introduce the latter as a description of the equilibrium state of a
three-dimensional continuum. Despite these important differences, an analogy is
built on their common geometrical foundations, and it is shown that a
space-time with curvature and torsion can be considered as a state of a
four-dimensional continuum containing defects. This formal analogy is useful
for illustrating the geometrical concept of torsion by applying it to concrete
physical problems. Moreover, the presentation of these theories using a common
geometrical basis allows a deeper understanding of their foundations.Comment: 18 pages, 7 EPS figures, RevTeX4, to appear in the American Journal
of Physics, revised version with typos correcte
Relativistic contraction and related effects in noninertial frames
Although there is no relative motion among different points on a rotating
disc, each point belongs to a different noninertial frame. This fact, not
recognized in previous approaches to the Ehrenfest paradox and related
problems, is exploited to give a correct treatment of a rotating ring and a
rotating disc. Tensile stresses are recovered, but, contrary to the prediction
of the standard approach, it is found that an observer on the rim of the disc
will see equal lengths of other differently moving objects as an inertial
observer whose instantaneous position and velocity are equal to that of the
observer on the rim. The rate of clocks at various positions, as seen by
various observers, is also discussed. Some results are generalized for
observers arbitrarily moving in a flat or a curved spacetime. The generally
accepted formula for the space line element in a non-time-orthogonal frame is
found inappropriate in some cases. Use of Fermi coordinates leads to the result
that for any observer the velocity of light is isotropic and is equal to ,
providing that it is measured by propagating a light beam in a small
neighborhood of the observer.Comment: 15 pages, significantly revised version, title changed, to appear in
Phys. Rev.
Interacting supernovae and supernova impostors. LSQ13zm: an outburst heralds the death of a massive star
We report photometric and spectroscopic observations of the optical transient LSQ13zm. Historical data reveal the presence of an eruptive episode (that we label as â2013aâ) followed by a much brighter outburst (â2013bâ) three weeks later, that we argue to be the genuine supernova explosion. This sequence of events closely resemble those observed for SN 2010mc and (in 2012) SN 2009ip. The absolute magnitude reached by LSQ13zm during 2013a (M_R=â14.87±0.25mag) is comparable with those of supernova impostors, while that of the 2013b event (M_R=â18.46±0.21mag) is consistent with those of interacting supernovae. Our spectra reveal the presence of a dense and structured circumstellar medium, probably produced through numerous pre-supernova mass-loss events. In addition, we find evidence for high-velocity ejecta, with a fraction of gas expelled at more than 20000 km s^(â1). The spectra of LSQ13zm show remarkable similarity with those of well-studied core-collapse supernovae. From the analysis of the available photometric and spectroscopic data, we conclude that we first observed the last event of an eruptive sequence from a massive star, likely a Luminous Blue Variable, which a short time later exploded as a core-collapse supernova. The detailed analysis of archival images suggest that the host galaxy is a star-forming Blue Dwarf Compact Galaxy
Emission vs Fermi coordinates: applications to relativistic positioning systems
A 4-dimensional relativistic positioning system for a general spacetime is
constructed by using the so called "emission coordinates". The results apply in
a small region around the world line of an accelerated observer carrying a
Fermi triad, as described by the Fermi metric. In the case of a Schwarzschild
spacetime modeling the gravitational field around the Earth and an observer at
rest at a fixed spacetime point, these coordinates realize a relativistic
positioning system alternative to the current GPS system. The latter is indeed
essentially conceived as Newtonian, so that it necessarily needs taking into
account at least the most important relativistic effects through Post-Newtonian
corrections to work properly. Previous results concerning emission coordinates
in flat spacetime are thus extended to this more general situation.
Furthermore, the mapping between spacetime coordinates and emission coordinates
is completely determined by means of the world function, which in the case of a
Fermi metric can be explicitly obtained.Comment: 12 pages iop style, 2 eps figures, to appear on Classical and Quantum
Gravity, 200
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