326 research outputs found
How (Not) to Palatini
We revisit the problem of defining non-minimal gravity in the first order
formalism. Specializing to scalar-tensor theories, which may be disguised as
`higher-derivative' models with the gravitational Lagrangians that depend only
on the Ricci scalar, we show how to recast these theories as Palatini-like
gravities. The correct formulation utilizes the Lagrange multiplier method,
which preserves the canonical structure of the theory, and yields the
conventional metric scalar-tensor gravity. We explain the discrepancies between
the na\"ive Palatini and the Lagrange multiplier approach, showing that the
na\"ive Palatini approach really swaps the theory for another. The differences
disappear only in the limit of ordinary General Relativity, where an accidental
redundancy ensures that the na\"ive Palatini works there. We outline the
correct decoupling limits and the strong coupling regimes. As a corollary we
find that the so-called `Modified Source Gravity' models suffer from strong
coupling problems at very low scales, and hence cannot be a realistic
approximation of our universe. We also comment on a method to decouple the
extra scalar using the chameleon mechanism.Comment: 18 pages, LaTeX; added references and minor improvements in sec
A Lorentz-Poincar\'e type interpretation of the Weak Equivalence Principle
The validity of the Weak Equivalence Principle relative to a local inertial
frame is detailed in a scalar-vector gravitation model with Lorentz-Poincar\'e
type interpretation. Given the previously established first Post-Newtonian
concordance of dynamics with General Relativity, the principle is to this order
compatible with GRT. The gravitationally modified Lorentz transformations, on
which the observations in physical coordinates depend, are shown to provide a
physical interpretation of \emph{parallel transport}. A development of
``geodesic'' deviation in terms of the present model is given as well.Comment: v1: 9 pages, 2 figures, v2: version to appear in International
Journal of Theoretical Physic
Two-atom dark states in electromagnetic cavities
The center-of-mass motion of two two-level atoms coupled to a single damped
mode of an electromagnetic resonator is investigated. For the case of one atom
being initially excited and the cavity mode in the vacuum state it is shown
that the atomic time evolution is dominated by the appearance of dark states.
These states, in which the initial excitation is stored in the internal atomic
degrees of freedom and the atoms become quantum mechanically entangled, are
almost immune against photon loss from the cavity. Various properties of the
dark states within and beyond the Raman-Nath approximation of atom optics are
worked out.Comment: 8 pages, 4 figure
Coherent Electron-Phonon Coupling in Tailored Quantum Systems
The coupling between a two-level system and its environment leads to
decoherence. Within the context of coherent manipulation of electronic or
quasiparticle states in nanostructures, it is crucial to understand the sources
of decoherence. Here, we study the effect of electron-phonon coupling in a
graphene and an InAs nanowire double quantum dot. Our measurements reveal
oscillations of the double quantum dot current periodic in energy detuning
between the two levels. These periodic peaks are more pronounced in the
nanowire than in graphene, and disappear when the temperature is increased. We
attribute the oscillations to an interference effect between two alternative
inelastic decay paths involving acoustic phonons present in these materials.
This interpretation predicts the oscillations to wash out when temperature is
increased, as observed experimentally.Comment: 11 pages, 4 figure
Multidisciplinary Approach to Unravelling the Relative Contribution of Different Oxylipins in Indirect Defense of Arabidopsis thaliana
The oxylipin pathway is commonly involved in induced plant defenses, and is the main signal-transduction pathway induced by insect folivory. Herbivory induces the production of several oxylipins, and consequently alters the so-called ‘oxylipin signature’ in the plant. Jasmonic acid (JA), as well as pathway intermediates are known to induce plant defenses. Indirect defense against herbivorous insects comprises the production of herbivore-induced plant volatiles (HIPVs). To unravel the precise oxylipin signal-transduction underlying the production of HIPVs in Arabidopsis thaliana and the resulting attraction of parasitoid wasps, we used a multidisciplinary approach that includes molecular genetics, metabolite analysis, and behavioral analysis. Mutant plants affected in the jasmonate pathway (18:0 and/or 16:0 -oxylipin routes; mutants dde2-2, fad5, opr3) were studied to assess the effects of JA and its oxylipin intermediates 12-oxo-phytodienoate (OPDA) and dinor-OPDA (dnOPDA) on HIPV emission and parasitoid (Diadegma semiclausum) attraction. Interference with the production of the oxylipins JA and OPDA altered the emission of HIPVs, in particular terpenoids and the phenylpropanoid methyl salicylate, which affected parasitoid attraction. Our data show that the herbivore-induced attraction of parasitoid wasps to Arabidopsis plants depends on HIPVs that are induced through the 18:0 oxylipin-derivative JA. Furthermore, our study shows that the 16:0-oxylipin route towards dnOPDA does not play a role in HIPV induction, and that the role of 18:0 derived oxylipin-intermediates, such as OPDA, is either absent or limited
On the problem of interactions in quantum theory
The structure of representations describing systems of free particles in the
theory with the invariance group SO(1,4) is investigated. The property of the
particles to be free means as usual that the representation describing a
many-particle system is the tensor product of the corresponding single-particle
representations (i.e. no interaction is introduced). It is shown that the mass
operator contains only continuous spectrum in the interval
and such representations are unitarily equivalent to ones describing
interactions (gravitational, electromagnetic etc.). This means that there are
no bound states in the theory and the Hilbert space of the many-particle system
contains a subspace of states with the following property: the action of free
representation operators on these states is manifested in the form of different
interactions. Possible consequences of the results are discussed.Comment: 35 pages, Late
Cosmic Microwave Background Temperature and Polarization Anisotropy in Brans-Dicke Cosmology
We develop a formalism for calculating cosmic microwave background (CMB)
temperature and polarization anisotropies in cosmological models with
Brans-Dicke gravity. We then modify publicly available Boltzmann codes to
calculate numerically the temperature and polarization power spectra. Results
are illustrated with a few representative models. Comparing with the
general-relativistic model with the same cosmological parameters, both the
amplitude and the width of the acoustic peaks are different in the Brans-Dicke
models. We use a covariance-matrix calculation to investigate whether the
effects of Brans-Dicke gravity are degenerate with those of variation in other
cosmological parameters and to simultaneously determine whether forthcoming CMB
maps might be able to distinguish Brans-Dicke and general-relativistic
cosmology. Although the predicted power spectra for plausible Brans-Dicke
models differ from those in general relativity only slightly, we find that MAP
and/or the Planck Surveyor may in principle provide a test of Brans-Dicke
theory that is competitive to solar-system tests. For example, if all other
parameters except for the CMB normalization are fixed, a value of the
Brans-Dicke parameter omega as large as 500 could be identified with MAP, and
for Planck, values as large as omega \simeq3000 could be identified; these
sensitivities are decreased roughly by a factor of 3 if we marginalize over the
baryon density, Hubble constant, spectral index, and reionization optical
depth. In more general scalar-tensor theories, omega may evolve with time, and
in this case, the CMB probe would be complementary to that from solar-system
tests.Comment: 27 pages, 10 figures, typeset using RevTe
Attractor Solution in Coupled Yang-Mills Field Dark Energy Models
We investigate the attractor solution in the coupled Yang-Mills field dark
energy models with the general interaction term, and obtain the constraint
equations for the interaction if the attractor solution exists. The research
also shows that, if the attractor solution exists, the equation-of-state of the
dark energy must evolve from to , which is slightly suggested
by the observation. At the same time, the total equation-of-state in the
attractor solution is , the universe is a de Sitter expansion, and
the cosmic big rip is naturally avoided. These features are all independent of
the interacting forms.Comment: 6 pages, no figure. version for publication. IJMPD accepte
Spontaneous emission of an atom in front of a mirror
Motivated by a recent experiment [J. Eschner {\it et al.}, Nature {\bf 413},
495 (2001)], we now present a theoretical study on the fluorescence of an atom
in front of a mirror. On the assumption that the presence of the distant mirror
and a lens imposes boundary conditions on the electric field in a plane close
to the atom, we derive the intensities of the emitted light as a function of an
effective atom-mirror distance. The results obtained are in good agreement with
the experimental findings.Comment: 8 pages, 6 figures, revised version, references adde
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