103 research outputs found
Space-time deformations as extended conformal transformations
A definition of space-time metric deformations on an -dimensional manifold
is given. We show that such deformations can be regarded as extended conformal
transformations. In particular, their features can be related to the
perturbation theory giving a natural picture by which gravitational waves are
described by small deformations of the metric. As further result, deformations
can be related to approximate Killing vectors (approximate symmetries) by which
it is possible to parameterize the deformed region of a given manifold. The
perspectives and some possible physical applications of such an approach are
discussed.Comment: 9 page
A CFT description of the BTZ black hole: topology versus geometry (or thermodynamics versus statistical mechanics
In this paper we review the properties of the black hole entropy in the light
of a general conformal field theory treatment. We find that the properties of
horizons of the BTZ black holes in ADS_{3}, can be described in terms of an
effective unitary CFT_{2} with central charge c=1 realized in terms of the
Fubini-Veneziano vertex operators.
It is found a relationship between the topological properties of the black
hole solution and the infinite algebra extension of the conformal group in 2D,
SU(2,2), i.e. the Virasoro Algebra, and its subgroup SL(2,Z) which generates
the modular symmetry. Such a symmetry induces a duality for the black hole
solution with angular momentum J\neq 0. On the light of such a global symmetry
we reanalyze the Cardy formula for CFT_{2} and its possible generalization to
D>2 proposed by E. Verlinde.Comment: 21 page
Universal Tunnelling Time in photonic Barriers
Tunnelling transit time for a frustrated total internal reflection in a
double-prism experiment was measured using microwave radiation. We have found
that the transit time is of the same order of magnitude as the corresponding
transit time measured either in an undersized waveguide (evanescent modes) or
in a photonic lattice. Moreover we have established that in all such
experiments the tunnelling transit time is approximately equal to the
reciprocal 1/f of the corresponding frequency of radiation.Comment: 8 pages, 4 figures, 1 tabl
f(R) cosmology with torsion
f(R)-gravity with geometric torsion (not related to any spin fluid) is
considered in a cosmological context. We derive the field equations in vacuum
and in presence of perfect-fluid matter and discuss the related cosmological
models. Torsion vanishes in vacuum for almost all arbitrary functions f(R)
leading to standard General Relativity. Only for f(R)=R^{2}, torsion gives
contribution in the vacuum leading to an accelerated behavior . When material
sources are considered, we find that the torsion tensor is different from zero
even with spinless material sources. This tensor is related to the logarithmic
derivative of f'(R), which can be expressed also as a nonlinear function of the
trace of the matter energy-momentum tensor. We show that the resulting
equations for the metric can always be arranged to yield effective Einstein
equations. When the homogeneous and isotropic cosmological models are
considered, terms originated by torsion can lead to accelerated expansion. This
means that, in f(R) gravity, torsion can be a geometric source for
acceleration.Comment: 13 page
The Cardy-Verlinde equation in a spherical symmetric gravitational collapse
The Cardy-Verlinde formula is analyzed in the contest of the gravitational
collapse. Starting from the holographic principle, we show how the equations
for a homogeneous and isotropic gravitational collapse describe the formation
of the black hole entropy. Some comments on the role of the entangled entropy
and the connection with the c-theorem are made
Inflaton Potential Reconstruction and Generalized Equations of State
We extend a previous analysis concerning cosmological fluids with generalized
equations of state in order to study inflationary scenarios. In the framework
of the slow-roll approximation we find the expressions for the perturbation
parameters , and the density perturbation spectra in terms of
the adiabatic index \ga as a function of the universe scale factor. This
connection allows to find straightforwardly \ga corresponding, for example,
to the simplest {\it chaotic} model and to the Harrison-Zeldovich potential and
shows its capability to be applied to more complicate situations. Finally, we
use this description to develop a new approach to the early universe dynamics,
based on a expansion, where is the e-fold number. To this aim, we
introduce a set of suitable dimensionless variables and show that at the
zero-th order in , an improved slow-roll approximation is obtained.Comment: 15 pages, plain Latex, to appear in Mod. Phys. Lett.
Tomographic entropy and cosmology
The probability representation of quantum mechanics including propagators and
tomograms of quantum states of the universe and its application to quantum
gravity and cosmology are reviewed. The minisuperspaces modeled by oscillator,
free pointlike particle and repulsive oscillator are considered. The notion of
tomographic entropy and its properties are used to find some inequalities for
the tomographic probability determining the quantum state of the universe. The
sense of the inequality as a lower bound for the entropy is clarified.Comment: 19 page
- …