118638108
Superluminal dispersion relations and the Unruh effect
In the context of quantum gravity phenomenology, we study the Unruh effect in the presence of superluminal dispersion relations. In particular, we estimate the response function and the probability rate for an accelerated detector coupled to a massless scalar field, whose dispersion relation becomes essentially quadratic beyond a threshold momentum ${k}_{p}$. By means of a perturbative analysis, we show that superluminal dispersion induces a correction to the Planckian spectrum, which tends to vanish as ${k}_{p}$ increases.
16113420
Self-T-dual brane cosmology and the cosmological constant problem
We consider a codimension-one brane embedded in a gravity-dilaton bulk action, whose symmetries are compatible with T-duality along the space-like directions parallel to the brane, and the bulk time-like direction. The equations of motions in the string frame allow for a smooth background obtained by the union of two symmetric patches of AdS space. The Poincare invariance of the solution appears to hold independently of the value of the brane vacuum energy, through a self-tuning property of the dilaton ground state. Moreover, the effective cosmology displays a bounce, at which the scale factor does not shrink to zero. Finally, by exploiting the T-duality symmetry, we show how to construct an ever-expanding Universe, along the lines of the Pre-Big-Bang scenario.
118790490
Observational signatures of pre-inflationary and lower-dimensional effective gravity
Several alternative ways to quantize gravitational interactions seem to indicate that gravity at short distances is effectively two-dimensional. If this were true, and the energy scale at which the dimensional transition occurs is of the order of the inflationary one, it is reasonable to investigate eventual signatures in the primordial perturbation spectra. In this paper we look at this possibility by assuming that the inflationary era was preceded by an effectively two-dimensional evolution of the Universe. We model the transition with a mode matching and we show that, in this case, no observational signatures are expected in the tensor and scalar power spectra nor in their ratio. We also show that assuming modified dispersion relations before the matching between two sets of four-dimensional modes leads to similar results.
18733372
Bubbles in anti-de Sitter space
We explore the bubble spacetimes which can be obtained from double analytic continuations of static and rotating black holes in anti-de Sitter space. In particular, we find that rotating black holes with elliptic horizon lead to bubble spacetimes only in dimension greater than five. For dimension greater than seven, the topology of the bubble can be non-spherical. However, a bubble spacetime is shown to arise from a rotating de Sitter black hole in four dimensions. In all cases, the evolution of the bubble is of de Sitter type. Double analytic continuations of hyperbolic black holes and branes are also discussed.
12251695
Self-T-Dual Brane Cosmology
We show how T-duality can be implemented with brane cosmology. As a result, we obtain a smooth bouncing cosmology with features similar to the ones of the pre-Big Bang scenario. Also, by allowing T-duality transformations along the time-like direction, we find a static solution that displays an interesting self tuning property.
119224226
Entropy of an acoustic black hole in Bose-Einstein condensates
We compute the entanglement entropy associated to the Hawking emission of a $(1+1)$-dimensional acoustic black hole in a Bose-Einstein condensate. We use the brick wall model proposed by 't Hooft, adapted to the momentum space, in order to tackle the case when high frequency dispersion is taken in account. As expected, we find that in the hydrodynamic limit the entropy only depends on the size of the box in the near-horizon region, as for gravitational $(1+1)$-dimensional black holes. When dispersion effects are considered, we find a correction that depends on the square of the size of the near-horizon region measured in units of healing length, very similar to the universal correction to the entropy found in the case of spin-1/2 Heisenberg XX chains.
1328077
Spinning Q-balls in Abelian Gauge Theories with positive potentials: existence and non existence
We study the existence of cylindrically symmetric electro-magneto-static solitary waves for a system of a nonlinear Klein–Gordon equation coupled with Maxwell’s equations in presence of a positive mass and of a nonnegative nonlinear potential. Nonexistence results are provided as well.
119281435
Neutron Transverse-Momentum Distributions and Polarized 3He within Light-Front Hamiltonian Dynamics
The possibility to extract the quark transverse-momentum distributions in the neutron from semi-inclusive deep inelastic electron scattering off polarized 3He is illustrated through an impulse approximation analysis in the Bjorken limit. The generalization of the analysis at finite momentum transfers in a Poincaré covariant framework is outlined. The definition of the light-front spin-dependent spectral function of a J = 1/2 system allows us to show that within the light-front dynamics only three of the six leading twist T-even transverse-momentum distributions are independent.
117142608
On the algebraic structure of link-diagrams on a 2-dimensional surface
Following and generalizing a paper by Turaev, we consider some algebraic structures on the set of (generalized) link-diagrams, meant simply as collections of immersed loops on a two dimensional surface, with the specification of an over/undercrossing prescription at each double point. This definition is general enough to be relevant not only to traditional knot-theory, but possibly to some statistical mechanical models. A coalgebra structure is introduced on some modules (over polynomial rings), generated by these diagrams. The compatibility of this coalgebra structure with the skein invariance and the invariance under Reidemeister moves is discussed. A Hopf algebra structure results only in some special cases, which are thoroughly examined. It is shown that a special choice of the ground (polynomial) ring over which the diagram module is defined, allows us to define link-invariants for links (in the ordinary sense) in Σ×[0,1],where Σ is a (closed or open) two-dimensional surface. These invariants generalize in a nontrivial way the Jones polynomials and the Homfly polynomials (at least when the last ones are computed for some special values of the variables). In a sequel paper the relation between the algebraic structures of link-diagrams, some special types of quantum groups and the quantum holonomy will be discussed.
898400
Supersymmetry and Fredholm modules over quantized spaces
The purpose of this paper is to apply the framework of non-commutative differential geometry to quantum deformations of a class of Kähler manifolds. For the examples of the Cartan domains of type I and flat space, we construct Fredholm modules over the quantized manifolds using the supercharges which arise in the quantization of supersymmetric generalizations of the manifolds. We compute an explicit formula for the Chern character on generators of the Toeplitz ℂ*-algebra.
209514446
OPERATIONAL EXPERIENCE WITH THE ELETTRA RF SYSTEM
The ELETTRA RF System has been in operation since October 1993 and has now exceeded 20000 running hours. With four 500 MHz cavities powered by four 60 kW transmitters, ELETTRA has successfully delivered synchrotron light to the users with 300 mA of stored current at 2 GeV in multibunch mode, which is 1.5 times the initial target value ( 200 mA at 2 GeV). The RF system has worked very satisfactorily. Apart from the usual problems of debugging a new system, the main problem encountered during the operation period came from the coaxial power transmission line. The global operational experience gained during these years of operation and the performances of the various elements making up the ELETTRA RF system will be reported.