4,408 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.
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
Phi-Psi model for Electrodynamics in dielectric media: exact quantisation in the Heisenberg representation
We investigate the quantization in the Heisenberg representation of a model
which represents a simplification of the Hopfield model for dielectric media,
where the electromagnetic field is replaced by a scalar field and the
role of the polarization field is played by a further scalar field . The
model, which is quadratic in the fields, is still characterized by a nontrivial
physical content, as the physical particles correspond to the polaritons of the
standard Hopfield model of condensed matter physics. Causality is also taken
into account and a discussion of the standard interaction representation is
also considered.Comment: 9 page
Point-particle method to compute diffusion-limited cellular uptake
We present an efficient point-particle approach to simulate
reaction-diffusion processes of spherical absorbing particles in the
diffusion-limited regime, as simple models of cellular uptake. The exact
solution for a single absorber is used to calibrate the method, linking the
numerical parameters to the physical particle radius and uptake rate. We study
configurations of multiple absorbers of increasing complexity to examine the
performance of the method, by comparing our simulations with available exact
analytical or numerical results. We demonstrate the potentiality of the method
in resolving the complex diffusive interactions, here quantified by the
Sherwood number, measuring the uptake rate in terms of that of isolated
absorbers. We implement the method in a pseudo-spectral solver that can be
generalized to include fluid motion and fluid-particle interactions. As a test
case of the presence of a flow, we consider the uptake rate by a particle in a
linear shear flow. Overall, our method represents a powerful and flexible
computational tool that can be employed to investigate many complex situations
in biology, chemistry and related sciences.Comment: 13 pages, 13 figure
The IR-Completion of Gravity: What happens at Hubble Scales?
We have recently proposed an "Ultra-Strong" version of the Equivalence
Principle (EP) that is not satisfied by standard semiclassical gravity. In the
theory that we are conjecturing, the vacuum expectation value of the (bare)
energy momentum tensor is exactly the same as in flat space: quartically
divergent with the cut-off and with no spacetime dependent (subleading) ter ms.
The presence of such terms seems in fact related to some known difficulties,
such as the black hole information loss and the cosmological constant problem.
Since the terms that we want to get rid of are subleading in the high-momentum
expansion, we attempt to explore the conjectured theory by "IR-completing" GR.
We consider a scalar field in a flat FRW Universe and isolate the first
IR-correction to its Fourier modes operators that kills the quadratic (next to
leading) time dependent divergence of the stress energy tensor VEV. Analogously
to other modifications of field operators that have been proposed in the
literature (typically in the UV), the present approach seems to suggest a
breakdown (here, in the IR, at large distances) of the metric manifold
description. We show that corrections to GR are in fact very tiny, become
effective at distances comparable to the inverse curvature and do not contain
any adjustable parameter. Finally, we derive some cosmological implications. By
studying the consistency of the canonical commutation relations, we infer a
correction to the distance between two comoving observers, which grows as the
scale factor only when small compared to the Hubble length, but gets relevant
corrections otherwise. The corrections to cosmological distance measures are
also calculable and, for a spatially flat matter dominated Universe, go in the
direction of an effective positive acceleration.Comment: 27 pages, 2 figures. Final version, references adde
Instability of the superfluid flow as black-hole lasing effect
We show that the instability leading to the decay of the one-dimensional
superfluid flow through a penetrable barrier are due to the black-hole lasing
effect. This dynamical instability is triggered by modes resonating in an
effective cavity formed by two horizons enclosing the barrier. The location of
the horizons is set by , with being the local fluid
velocity and sound speed, respectively. We compute the critical velocity
analytically and show that it is univocally determined by the horizons
configuration. In the limit of broad barriers, the continuous spectrum at the
origin of the Hawking-like radiation and of the Landau energetic instability is
recovered.Comment: 18 pages, 3 figure
Energy transfer in nonlinear network models of proteins
We investigate how nonlinearity and topological disorder affect the energy
relaxation of local kicks in coarse-grained network models of proteins. We find
that nonlinearity promotes long-range, coherent transfer of substantial energy
to specific, functional sites, while depressing transfer to generic locations.
Remarkably, transfer can be mediated by the self-localization of discrete
breathers at distant locations from the kick, acting as efficient
energy-accumulating centers.Comment: 4 pages, 3 figure
Modelling a Particle Detector in Field Theory
Particle detector models allow to give an operational definition to the
particle content of a given quantum state of a field theory. The commonly
adopted Unruh-DeWitt type of detector is known to undergo temporary transitions
to excited states even when at rest and in the Minkowski vacuum. We argue that
real detectors do not feature this property, as the configuration "detector in
its ground state + vacuum of the field" is generally a stable bound state of
the underlying fundamental theory (e.g. the ground state-hydrogen atom in a
suitable QED with electrons and protons) in the non-accelerated case. As a
concrete example, we study a local relativistic field theory where a stable
particle can capture a light quantum and form a quasi-stable state. As
expected, to such a stable particle correspond energy eigenstates of the full
theory, as is shown explicitly by using a dressed particle formalism at first
order in perturbation theory. We derive an effective model of detector (at
rest) where the stable particle and the quasi-stable configurations correspond
to the two internal levels, "ground" and "excited", of the detector.Comment: 13 pages, references added, final versio
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