401 research outputs found
Accelerated Cosmological Models in Ricci squared Gravity
Alternative gravitational theories described by Lagrangians depending on
general functions of the Ricci scalar have been proven to give coherent
theoretical models to describe the experimental evidence of the acceleration of
universe at present time. In this paper we proceed further in this analysis of
cosmological applications of alternative gravitational theories depending on
(other) curvature invariants. We introduce Ricci squared Lagrangians in minimal
interaction with matter (perfect fluid); we find modified Einstein equations
and consequently modified Friedmann equations in the Palatini formalism. It is
striking that both Ricci scalar and Ricci squared theories are described in the
same mathematical framework and both the generalized Einstein equations and
generalized Friedmann equations have the same structure. In the framework of
the cosmological principle, without the introduction of exotic forms of dark
energy, we thus obtain modified equations providing values of w_{eff}<-1 in
accordance with the experimental data. The spacetime bi-metric structure plays
a fundamental role in the physical interpretation of results and gives them a
clear and very rich geometrical interpretation.Comment: New version: 26 pages, 1 figure (now included), Revtex
Extended Loop Quantum Gravity
We discuss constraint structure of extended theories of gravitation (also
known as f(R) theories) in the vacuum selfdual formulation introduced in ref.
[1].Comment: 7 pages, few typos correcte
Scalar field propagation in the phi^4 kappa-Minkowski model
In this article we use the noncommutative (NC) kappa-Minkowski phi^4 model
based on the kappa-deformed star product, ({*}_h). The action is modified by
expanding up to linear order in the kappa-deformation parameter a, producing an
effective model on commutative spacetime. For the computation of the tadpole
diagram contributions to the scalar field propagation/self-energy, we
anticipate that statistics on the kappa-Minkowski is specifically
kappa-deformed. Thus our prescription in fact represents hybrid approach
between standard quantum field theory (QFT) and NCQFT on the kappa-deformed
Minkowski spacetime, resulting in a kappa-effective model. The propagation is
analyzed in the framework of the two-point Green's function for low,
intermediate, and for the Planckian propagation energies, respectively.
Semiclassical/hybrid behavior of the first order quantum correction do show up
due to the kappa-deformed momentum conservation law. For low energies, the
dependence of the tadpole contribution on the deformation parameter a drops out
completely, while for Planckian energies, it tends to a fixed finite value. The
mass term of the scalar field is shifted and these shifts are very different at
different propagation energies. At the Planckian energies we obtain the
direction dependent kappa-modified dispersion relations. Thus our
kappa-effective model for the massive scalar field shows a birefringence
effect.Comment: 23 pages, 2 figures; To be published in JHEP. Minor typos corrected.
Shorter version of the paper arXiv:1107.236
PCV97 Cost-Effectiveness Analysis of Ivabradine in Chronic Heart Failure in the Polish Setting
Differential structure on kappa-Minkowski space, and kappa-Poincare algebra
We construct realizations of the generators of the -Minkowski space
and -Poincar\'{e} algebra as formal power series in the -adic
extension of the Weyl algebra. The Hopf algebra structure of the
-Poincar\'{e} algebra related to different realizations is given. We
construct realizations of the exterior derivative and one-forms, and define a
differential calculus on -Minkowski space which is compatible with the
action of the Lorentz algebra. In contrast to the conventional bicovariant
calculus, the space of one-forms has the same dimension as the
-Minkowski space.Comment: 20 pages. Accepted for publication in International Journal of Modern
Physics
Unbraiding the braided tensor product
We show that the braided tensor product algebra
of two module algebras of a quasitriangular Hopf algebra is
equal to the ordinary tensor product algebra of with a subalgebra of
isomorphic to , provided there exists a
realization of within . In other words, under this assumption we
construct a transformation of generators which `decouples' (i.e.
makes them commuting). We apply the theorem to the braided tensor product
algebras of two or more quantum group covariant quantum spaces, deformed
Heisenberg algebras and q-deformed fuzzy spheres.Comment: LaTex file, 29 page
Knotted Solitons in a Charged Two-Condensate Bose System
By making use of the decomposition of U(1) gauge potential theory and the
\phi mapping method, we propose that a charged two-condensate Bose system
possesses vortex lines and two classes of knotted solitons. The topological
charges of the vortex lines are characterized by the Hopf indices and the
Brower degrees of \phi-mapping, and the knotted solitons are described by the
nontrivial Hopf invariant and the BF action, respectively.Comment: 12 pages,0 figure
Currents and Superpotentials in classical gauge theories: II. Global aspects and the example of Affine gravity
The conserved charges associated to gauge symmetries are defined at a
boundary component of space-time because the corresponding Noether current can
be rewritten on-shell as the divergence of a superpotential. However, the
latter is afflicted by ambiguities. Regge and Teitelboim found a procedure to
lift the arbitrariness in the Hamiltonian framework. An alternative covariant
formula was proposed by one of us for an arbitrary variation of the
superpotential, it depends only on the equations of motion and on the gauge
symmetry under consideration. Here we emphasize that in order to compute the
charges, it is enough to stay at a boundary of spacetime, without requiring any
hypothesis about the bulk or about other boundary components, so one may speak
of holographic charges. It is well known that the asymptotic symmetries that
lead to conserved charges are really defined at infinity, but the choice of
boundary conditions and surface terms in the action and in the charges is
usually determined through integration by parts whereas each component of the
boundary should be considered separately. We treat the example of gravity (for
any space-time dimension, with or without cosmological constant), formulated as
an Affine theory which is a natural generalization of the Palatini and
Cartan-Weyl (vielbein) first order formulations. We then show that the
superpotential associated to a Dirichlet boundary condition on the metric (the
one needed to treat asymptotically flat or AdS spacetimes) is the one proposed
by Katz, Bi\u{c}{\'a}k and Lynden-Bell and not that of Komar. We finally
discuss the KBL superpotential at null infinity.Comment: 16 pages, minor corrections and references added. Final version to
appear in CQ
New Cases of Universality Theorem for Gravitational Theories
The "Universality Theorem" for gravity shows that f(R) theories (in their
metric-affine formulation) in vacuum are dynamically equivalent to vacuum
Einstein equations with suitable cosmological constants. This holds true for a
generic (i.e. except sporadic degenerate cases) analytic function f(R) and
standard gravity without cosmological constant is reproduced if f is the
identity function (i.e. f(R)=R). The theorem is here extended introducing in
dimension 4 a 1-parameter family of invariants R' inspired by the
Barbero-Immirzi formulation of GR (which in the Euclidean sector includes also
selfdual formulation). It will be proven that f(R') theories so defined are
dynamically equivalent to the corresponding metric-affine f(R) theory. In
particular for the function f(R)=R the standard equivalence between GR and
Holst Lagrangian is obtained.Comment: 10 pages, few typos correcte
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