8,231 research outputs found
Quintessence as a run-away dilaton
We consider a late-time cosmological model based on a recent proposal that
the infinite-bare-coupling limit of superstring/M-theory exists and has good
phenomenological properties, including a vanishing cosmological constant, and a
massless, decoupled dilaton. As it runs away to , the dilaton can
play the role of the quintessence field recently advocated to drive the
late-time accelerated expansion of the Universe. If, as suggested by some
string theory examples, appreciable deviations from General Relativity persist
even today in the dark matter sector, the Universe may smoothly evolve from an
initial "focusing" stage, lasting untill radiation--matter equality, to a
"dragging" regime, which eventually gives rise to an accelerated expansion with
frozen .Comment: 31 pages, latex, 5 figures included using epsfig. New references
added and misprints corrected. To appear in Phys. Rev.
Scalar-tensor theories, trace anomalies and the QCD-frame
We consider the quantum effects of matter fields in scalar-tensor theories
and clarify the role of trace anomaly when switching between conformally
related `frames'. We exploit the property that the couplings between the scalar
and the gauge fields are not frame-invariant in order to define a `QCD-frame',
where the scalar is not coupled to the gluons. We show that this frame is a
natural generalization of the `Jordan frame' in the case of non-metric theories
and that it is particularly convenient for gravitational phenomenology: test
bodies have trajectories that are as close as possible to geodesics with
respect to such a metric and equivalence principle violations are directly
proportional to the scalar coupling parameters written in this frame. We show
how RG flow and decoupling work in metric and non-metric theories. RG-running
commutes with the operation of switching between frames at different scales.
When only matter loops are considered, our analysis confirms that metricity is
stable under radiative corrections and shows that approximate metricity is
natural in a technical sense.Comment: 10 pages. Minor changes to the main text, appendix added. To appear
on PR
Heat wave propagation in a nonlinear chain
We investigate the propagation of temperature perturbations in an array of
coupled nonlinear oscillators at finite temperature. We evaluate the response
function at equilibrium and show how the memory effects affect the diffusion
properties. A comparison with nonequilibrium simulations reveals that the
telegraph equation provides a reliable interpretative paradigm for describing
quantitatively the propagation of a heat pulse at the macroscopic level. The
results could be of help in understanding and modeling energy transport in
individual nanotubes.Comment: Revised version, 1 fig. adde
Behaviour of traditional Portuguese timber roof structures
The aim of this paper is to present the results of a structural analysis of common trusses traditionally used in roof construction in Portugal. The study includes the results of a preliminary survey intending to assess the geometry, materials and on site pathologies, as well as a twodimensional linear elastic static and dynamic analysis. The trusses behaviour under symmetric and non-symmetric loads, the king post/tie-beam connection, the stiffness of the joints and the incorrect positioning of the purlins, were some of the structural aspects that have been investigated
Experimental analysis of original and strengthened traditional timber connections
Tests on full-scale unstrengthened connections were performed under monotonic and cyclic loading. Attention has been principally focused on the birdsmouth joint, because of its common use in practice. Different strengthening solutions with metal elements have been evaluated
Modelling of timber joints in traditional structures
Original unstrengthened timber connections and the effects of different strengthening techniques
have been evaluated experimentally with tests on full-scale birdsmouth joints. Experimental results
show that structural response of traditional timber connections under cyclic loading cannot be
represented by common constraint models, like perfect hinges or rigid joints, but should be using
semi-rigid and friction based models. A research program has investigated the behaviour of old
timber joints and examined strengthening criteria. The main parameters affecting the mechanical
behaviour of the connection have been singled out. A synthetic model of cyclic behaviour has been
adapted on the basis of experimental results
Self-Ordered stationary states of driven quantum degenerate gases in optical resonators
We study the role of quantum statistics in the self-ordering of ultracold
bosons and fermions moving inside an optical resonator with transverse coherent
pumping. For few particles we numerically compute the nonequilibrium dynamics
of the density matrix towards the self-ordered stationary state of the coupled
atom-cavity system. We include quantum fluctuations of the particles and the
cavity field. These fluctuations in conjunction with cavity cooling determine
the stationary distribution of the particles, which exhibits a transition from
a homogeneous to a spatially ordered phase with the appearance of a
superradiant scattering peak in the cavity output spectrum. At the same time
the cavity field -function changes from a single to a double peaked
distribution. While the ordering threshold is generally lower for bosons, we
confirm the recently predicted zero pump strength threshold for superradiant
scattering for fermions when the cavity photon momentum coincides with twice
the Fermi momentum.Comment: 8 pages, 6 figures (v2: added one reference
A relativistic non-relativistic Goldstone theorem: gapped Goldstones at finite charge density
We adapt the Goldstone theorem to study spontaneous symmetry breaking in
relativistic theo- ries at finite charge density. It is customary to treat
systems at finite density via non-relativistic Hamiltonians. Here we highlight
the importance of the underlying relativistic dynamics. This leads to seemingly
new results whenever the charge in question is spontaneously broken and does
not commute with other broken charges. We find that that the latter interpolate
gapped excitations. In contrast, all existing versions of the Goldstone theorem
predict the existence of gapless modes. We derive exact non-perturbative
expressions for their gaps, in terms of the chemical potential and of the
symmetry algebra.Comment: 5 pages. v2: minor modifications, matches the PRL versio
Path integral quantization of the relativistic Hopfield model
The path integral quantization method is applied to a relativistically
covariant version of the Hopfield model, which represents a very interesting
mesoscopic framework for the description of the interaction between quantum
light and dielectric quantum matter, with particular reference to the context
of analogue gravity. In order to take into account the constraints occurring in
the model, we adopt the Faddeev-Jackiw approach to constrained quantization in
the path integral formalism. In particular we demonstrate that the propagator
obtained with the Faddeev-Jackiw approach is equivalent to the one which, in
the framework of Dirac canonical quantization for constrained systems, can be
directly computed as the vacuum expectation value of the time ordered product
of the fields. Our analysis also provides an explicit example of quantization
of the electromagnetic field in a covariant gauge and coupled with the
polarization field, which is a novel contribution to the literature on the
Faddeev-Jackiw procedure.Comment: 16 page
Exact quantisation of the relativistic Hopfield model
We investigate the quantisation in the Heisenberg representation of a
relativistically covariant version of the Hopfield model for dielectric media,
which entails the interaction of the quantum electromagnetic field with the
matter dipole fields. The matter fields are represented by a mesoscopic
polarization field. A full quantisation of the model is provided in a covariant
gauge, with the aim of maintaining explicit relativistic covariance. Breaking
of the Lorentz invariance due to the intrinsic presence in the model of a
preferred reference frame is also taken into account. Relativistic covariance
forces us to deal with the unphysical (scalar and longitudinal) components of
the fields, furthermore it introduces, in a more tricky form, the well-known
dipole ghost of standard QED in a covariant gauge. In order to correctly
dispose of this contribution, we implement a generalized Lautrup trick.
Furthermore, causality and the relation of the model with the Wightman axioms
are also discussed.Comment: 24 page
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