112 research outputs found
A class of non-singular gravi-dilaton backgrounds
We present a class of static, spherically symmetric, non-singular solutions
of the tree-level string effective action, truncated to first order in
. In the string frame the solutions approach asymptotically (at and ) two different anti-de Sitter configurations, thus
interpolating between two maximally symmetric states of different constant
curvature. The radial-dependent dilaton defines a string coupling which is
everywhere finite, with a peak value that can be chosen arbitrarily small so as
to neglect quantum-loop corrections. This example stresses the possible
importance of finite-size corrections, typical of string theory, in
avoiding space-time singularities.Comment: 9 pages, LATEX, four figure included using EPSFIG. Essay written for
the 1997 Awards of the Gravity Research Foundation, and selected for
"Honorable Mention". To appear in Mod. Phys. Lett.
Symmetry breaking aspects of the effective Lagrangian for quantum black holes
The physical excitations entering the effective Lagrangian for quantum black
holes are related to a Goldstone boson which is present in the Rindler limit
and is due to the spontaneous breaking of the translation symmetry of the
underlying Minkowski space. This physical interpretation, which closely
parallels similar well-known results for the effective stringlike description
of flux tubes in QCD, gives a physical insight into the problem of describing
the quantum degrees of freedom of black holes. It also suggests that the
recently suggested concept of 'black hole complementarity' emerges at the
effective Lagrangian level rather than at the fundamental level.Comment: 11 pages, Latex,1 figur
A late-time transition in the equation of state versus Lambda-CDM
We study a model of the dark energy which exhibits a rapid change in its
equation of state w(z), such as occurs in vacuum metamorphosis. We compare the
model predictions with CMB, large scale structure and supernova data and show
that a late-time transition is marginally preferred over standard Lambda-CDM.Comment: 4 pages, 1 figure, to appear in the proceedings of XXXVIIth
Rencontres de Moriond, "The Cosmological Model", March 200
Thermal behavior of Quantum Cellular Automaton wires
We investigate the effect of a finite temperature on the behavior of logic
circuits based on the principle of Quantum Cellular Automata (QCA) and of
ground state computation. In particular, we focus on the error probability for
a wire of QCA cells that propagates a logic state. A numerical model and an
analytical, more approximate, model are presented for the evaluation of the
partition function of such a system and, consequently, of the desired
probabilities. We compare the results of the two models, assessing the limits
of validity of the analytical approach, and provide estimates for the maximum
operating temperature.Comment: 15 pages, 7 figures, uses revte
Non-local dilaton coupling to dark matter: cosmic acceleration and pressure backreaction
A model of non-local dilaton interactions, motivated by string duality
symmetries, is applied to a scenario of "coupled quintessence" in which the
dilaton dark energy is non-locally coupled to the dark-matter sources. It is
shown that the non-local effects tend to generate a backreaction which -- for
strong enough coupling -- can automatically compensate the acceleration due to
the negative pressure of the dilaton potential, thus asymptotically restoring
the standard (dust-dominated) decelerated regime. This result is illustrated by
analytical computations and numerical examples.Comment: 11 pages, 1 figure ep
A family of filters to search for frequency dependent gravitational wave stochastic backgrounds
We consider a three dimensional family of filters based on broken power law
spectra to search for gravitational wave stochastic backgrounds in the data
from Earth-based laser interferometers. We show that such templates produce the
necessary fitting factor for a wide class of cosmological backgrounds and
astrophysical foregrounds and that the total number of filters required to
search for those signals in the data from first generation laser
interferometers operating at the design sensitivity is fairly smallComment: 4 pages, 4 figures, uses iopart.cls, accepted for publications on
Classical and Quantum Gravity (Special Issue, Proceedings of Amaldi 2003
Recent developments in quantum string cosmology
In this talk we discuss the quantisation of a class of string cosmology
models characterised by scale factor duality invariance. The amplitudes for the
full set of classically allowed and forbidden transitions are computed by
applying the reduced phase space and path integral methods. In particular, the
path integral calculation clarifies the meaning of the instanton-like behaviour
of the transition amplitudes that has been first pointed out in previous
investigations.Comment: 4 pages, one eps figures, uses espcrc2.sty. Talk given at the Third
Conference on Constrained Dynamics and Quantum Gravit
Simulation of a non-invasive charge detector for quantum cellular automata
Information in a Quantum Cellular Automata architecture is encoded in the
polarizazion state of a cell, i.e., in the occupation numbers of the quantum
dots of which the cell is made up. Non-invasive charge detectors of single
electrons in a quantum dot are therefore needed, and recent experiments have
shown that a quantum constriction electrostatically coupled to the quantum dot
may be a viable solution. We have performed a numerical simulation of a system
made of a quantum dot and a nearby quantum point contact defined, by means of
depleting metal gates, in a two-dimensional electron gas at a GaAs/AlGaAs
heterointerface. We have computed the occupancy of each dot and the resistance
of the quantum wire as a function of the voltage applied to the plunger gate,
and have derived design criteria for achieving optimal sensitivity.Comment: 8 pages, RevTeX, epsf, 5 figure
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