358 research outputs found
Non-Trivial Vacua in Higher-Derivative Gravitation
A discussion of an extended class of higher-derivative classical theories of
gravity is presented. A procedure is given for exhibiting the new propagating
degrees of freedom, at the full non-linear level, by transforming the
higher-derivative action to a canonical second-order form. For general
fourth-order theories, described by actions which are general functions of the
scalar curvature, the Ricci tensor and the full Riemann tensor, it is shown
that the higher-derivative theories may have multiple stable vacua. The vacua
are shown to be, in general, non-trivial, corresponding to deSitter or
anti-deSitter solutions of the original theory. It is also shown that around
any vacuum the elementary excitations remain the massless graviton, a massive
scalar field and a massive ghost-like spin-two field. The analysis is extended
to actions which are arbitrary functions of terms of the form ,
and it is shown that such theories also have a non-trivial vacuum structure.Comment: 25 pages, LaTeX2e with AMS-LaTeX 1.2, 7 eps figure
Dark Energy Dominance and Cosmic Acceleration in First Order Formalism
The current accelerated universe could be produced by modified gravitational
dynamics as it can be seen in particular in its Palatini formulation. We
analyze here a specific non-linear gravity-scalar system in the first order
Palatini formalism which leads to a FRW cosmology different from the purely
metric one. It is shown that the emerging FRW cosmology may lead either to an
effective quintessence phase (cosmic speed-up) or to an effective phantom
phase. Moreover, the already known gravity assisted dark energy dominance
occurs also in the first order formalism. Finally, it is shown that a dynamical
theory able to resolve the cosmological constant problem exists also in this
formalism, in close parallel with the standard metric formulation.Comment: 21 pages, LaTeX file, no figures. Replaced version to be published on
Phys. Rev.
Ostrogradski Formalism for Higher-Derivative Scalar Field Theories
We carry out the extension of the Ostrogradski method to relativistic field
theories. Higher-derivative Lagrangians reduce to second differential-order
with one explicit independent field for each degree of freedom. We consider a
higher-derivative relativistic theory of a scalar field and validate a powerful
order-reducing covariant procedure by a rigorous phase-space analysis. The
physical and ghost fields appear explicitly. Our results strongly support the
formal covariant methods used in higher-derivative gravity.Comment: 22 page
Equivalence of black hole thermodynamics between a generalized theory of gravity and the Einstein theory
We analyze black hole thermodynamics in a generalized theory of gravity whose
Lagrangian is an arbitrary function of the metric, the Ricci tensor and a
scalar field. We can convert the theory into the Einstein frame via a
"Legendre" transformation or a conformal transformation. We calculate
thermodynamical variables both in the original frame and in the Einstein frame,
following the Iyer--Wald definition which satisfies the first law of
thermodynamics. We show that all thermodynamical variables defined in the
original frame are the same as those in the Einstein frame, if the spacetimes
in both frames are asymptotically flat, regular and possess event horizons with
non-zero temperatures. This result may be useful to study whether the second
law is still valid in the generalized theory of gravity.Comment: 14 pages, no figure
Gauge Fixing in Higher Derivative Gravity
Linearized four-derivative gravity with a general gauge fixing term is
considered. By a Legendre transform and a suitable diagonalization procedure it
is cast into a second-order equivalent form where the nature of the physical
degrees of freedom, the gauge ghosts, the Weyl ghosts, and the intriguing
"third ghosts", characteristic to higher-derivative theories, is made explicit.
The symmetries of the theory and the structure of the compensating
Faddeev-Popov ghost sector exhibit non-trivial peculiarities.Comment: 21 pages, LaTe
The dynamical equivalence of modified gravity revisited
We revisit the dynamical equivalence between different representations of
vacuum modified gravity models in view of Legendre transformations. The
equivalence is discussed for both bulk and boundary space, by including in our
analysis the relevant Gibbons-Hawking terms. In the f(R) case, the Legendre
transformed action coincides with the usual Einstein frame one. We then
re-express the R+f(G) action, where G is the Gauss-Bonnet term, as a second
order theory with a new set of field variables, four tensor fields and one
scalar and study its dynamics. For completeness, we also calculate the
conformal transformation of the full Jordan frame R+f(G) action. All the
appropriate Gibbons-Hawking terms are calculated explicitly.Comment: 17 pages; v3: Revised version. New comments added in Sections 3 & 5.
New results added in Section 6. Version to appear in Class. Quantum Gravit
The high school competencies scale (H-comp scale): A first validation study
Researchers widely explored non-intellective study factors because they play a central role in academic performance and are potentially more modifiable than intellective ones. The scientific literature suggests that the non-intellective factors can be classified into three main areas: self-concept, which refers to self-esteem and efficacy, motivation and emotional reactions; the area of study, related to study dedication and operative skills; and the area of relationships, comprising those with family, fellow students and teachers. Basing on these findings, the C-Comp Scale has been developed and tested in the past, addressed to college students. This study aimed to adapt and test a new version of this questionnaire on high school students. Methods. A pilot study was conducted on 364 Italian high school students to adapt and test the new version of the questionnaire, called the H-Comp Scale. The following study, conducted on 792 Italian high school students, provided further evidence of its reliability, structural validity, and concurrent validity with general self-efficacy, academic self-efficacy, social self-efficacy, and academic performance. Results. The H-Comp Scale showed to possess excellent reliability and structural and concurrent validity. The final version is composed of twelve subscales, aggregated in three areas, with just 48 items: Study (Intrinsic Motivation, Extrinsic Motivation, Time Management, Study Dedication), Self (Learning Assessment, General Self-Esteem, Self-Efficacy, Reaction to Failures, Emotional Control), and Relationships (Family Relationships, Fellow Student Relationships, Teacher Relationships). Conclusions. The H-Comp Scale would be a useful and easy-to-use instrument to support school counselors, tutors, teachers, and researchers in exploring different types of non-intellective variables, to better project educational intervention aimed to improve high school students’ academic performance and satisfaction
Evidence for Strong Itinerant Spin Fluctuations in the Normal State of CeFeAsO(0.89)F(0.11) Iron-Oxypnictides
The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide
superconductors has been investigated with x-ray absorption and photoemission
spectroscopy. All the data exhibit signatures of Fe d-electron itinerancy.
Exchange multiplets appearing in the Fe 3s core level indicate the presence of
itinerant spin fluctuations. These findings suggest that the underlying physics
and the origin of superconductivity in these materials are likely to be quite
different from those of the cuprate high-temperature superconductors. These
materials provide opportunities for elucidating the role of magnetic
fluctuations in high-temperature superconductivity.Comment: Shorter version. Accepted in Phys. Rev. Let
Higher-Derivative Boson Field Theories and Constrained Second-Order Theories
As an alternative to the covariant Ostrogradski method, we show that
higher-derivative relativistic Lagrangian field theories can be reduced to
second differential-order by writing them directly as covariant two-derivative
theories involving Lagrange multipliers and new fields. Despite the intrinsic
non-covariance of the Dirac's procedure used to deal with the constraints, the
explicit Lorentz invariance is recovered at the end. We develop this new
setting on the grounds of a simple scalar model and then its applications to
generalized electrodynamics and higher-derivative gravity are worked out. For a
wide class of field theories this method is better suited than Ostrogradski's
for a generalization to 2n-derivative theoriesComment: 31 pages, Plain Te
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