952 research outputs found
Dualism between Physical Frames and Time in Quantum Gravity
In this work we present a discussion of the existing links between the
procedures of endowing the quantum gravity with a real time and of including in
the theory a physical reference frame. More precisely, as first step, we
develop the canonical quantum dynamics, starting from the Einstein equations in
presence of a dust fluid and arrive to a Schroedinger evolution. Then, by
fixing the lapse function in the path integral of gravity, we get a
Schroedinger quantum dynamics, of which eigenvalues problem provides the
appearance of a dust fluid in the classical limit. The main issue of our
analysis is to claim that a theory, in which the time displacement invariance,
on a quantum level, is broken, is indistinguishable from a theory for which
this symmetry holds, but a real reference fluid is included.Comment: 9 pages, submitted to Mod. Phys. Lett. A, major replacements in
section 3 and
Revised Canonical Quantum Gravity via the Frame Fixing
We present a new reformulation of the canonical quantum geometrodynamics,
which allows to overcome the fundamental problem of the frozen formalism and,
therefore, to construct an appropriate Hilbert space associate to the solution
of the restated dynamics. More precisely, to remove the ambiguity contained in
the Wheeler-DeWitt approach, with respect to the possibility of a (3 +
1)-splitting when the space-time is in a quantum regime, we fix the reference
frame (i.e. the lapse function and the shift vector) by introducing the
so-called kinematical action; as a consequence the new super-Hamiltonian
constraint becomes a parabolic one and we arrive to a Schroedinger-like
approach for the quantum dynamics. In the semiclassical limit our theory
provides General Relativity in the presence of an additional energy-momentum
density contribution coming from no longer zero eigenvalues of the Hamiltonian
constraints; the interpretation of these new contributions comes out in natural
way as soon as it is recognized that the kinematical action can be recasted in
such a way it describes a pressureless, but, in general, non geodesic perfect
fluid.Comment: 24 pages, 0 figures, to appear on Int. Jour. Mod. Phys.
The Immirzi Parameter as an Instanton Angle
The Barbero-Immirzi parameter is a one parameter quantization ambiguity
underpinning the loop approach to quantum gravity that bears tantalizing
similarities to the theta parameter of gauge theories such as Yang-Mills and
QCD. Despite the apparent semblance, the Barbero-Immirzi field has resisted a
direct topological interpretation along the same lines as the theta-parameter.
Here we offer such an interpretation. Our approach begins from the perspective
of Einstein-Cartan gravity as the symmetry broken phase of a de Sitter gauge
theory. From this angle, just as in ordinary gauge theories, a theta-term
emerges from the requirement that the vacuum is stable against quantum
mechanical tunneling. The Immirzi parameter is then identified as a combination
of Newton's constant, the cosmological constant, and the theta-parameter.Comment: 24 page
LAND USE FROM SEASONAL ARCHAEOLOGICAL SITES: THE ARCHAEOBOTANICAL EVIDENCE OF SMALL ROMAN FARMHOUSES IN CINIGIANO, SOUTH-EASTERN TUSCANY - CENTRAL ITALY
this paper focuses on the archaeobotanical study of two Roman small farmhouses, san Martino and poggio dell’amore, located near cinigiano, in the province of grosseto-tuscany. the sites were probably occupied during seasonal agricultural works, a peculiar typology of site that has not been usually identified in Roman sites of central italy. the integrated analyses of pollen, non pollen palynomorphs, charcoal particles and seeds/fruits help to obtain interesting details on the site function, land use and palaeoenvironment of these archaeological contexts. the archaeobotanical reconstruction shows that the landscape was fairly treeless. pastures surrounded the small buildings while cereal fields were probably less extended or further away than legume fields cultivated to forage. shrubs and some fruit trees might mark boundaries of fields, while the woods, including oak woods, were distributed far from the sites. anthropogenic pollen indicators, spores of coprophilous fungi and parasite eggs point to the presence of excrements in the sites suggesting that the small buildings were used as small barns for domestic animals, or a temporary shed
Gravity from a fermionic condensate of a gauge theory
The most prominent realization of gravity as a gauge theory similar to the
gauge theories of the standard model comes from enlarging the gauge group from
the Lorentz group to the de Sitter group. To regain ordinary Einstein-Cartan
gravity the symmetry must be broken, which can be accomplished by known
quasi-dynamic mechanisms. Motivated by symmetry breaking models in particle
physics and condensed matter systems, we propose that the symmetry can
naturally be broken by a homogenous and isotropic fermionic condensate of
ordinary spinors. We demonstrate that the condensate is compatible with the
Einstein-Cartan equations and can be imposed in a fully de Sitter invariant
manner. This lends support, and provides a physically realistic mechanism for
understanding gravity as a gauge theory with a spontaneously broken local de
Sitter symmetry.Comment: 16 page
Generalized Chern-Simons Modified Gravity in First-Order Formalism
We propose a generalization of Chern-Simons (CS) modified gravity in
first-order formalism. CS modified gravity action has a term that comes from
the chiral anomaly which is Pontryagin invariant. First-order CS modified
gravity is a torsional theory and in a space-time with torsion the chiral
anomaly includes a torsional topological term called Nieh-Yan invariant. We
generalize the CS modified gravity by adding the Nieh-Yan term to the action
and find the effective theory. We compare the generalized theory with the
first-order CS modified gravity and comment on the similarities and
differences.Comment: 8 pages, an author added, new paragraphs, comments and references
added, published in Gen. Relativ. Gravi
Corrigendum to “Pollen-based paleoenvironmental and paleoclimatic change at Lake Ohrid (south-eastern Europe) during the past 500 ka” published in Biogeosciences, 13, 1423–1437, 2016
In this corrigendum we report an updated pollen record from the Lake Ohrid DEEP site spanning the past 500 ka whereby we have reprocessed and re-analyzed 104 samples affected by chemical procedure problems that occurred in one palynological laboratory. Firstly, these samples were affected by the use of wrong containers, causing in- adequate settling of particles at the set centrifuging speed. Secondly, HCl and HF treatments were combined without the prescribed intermediate centrifuging and decanting steps. The inaccuracy in the protocol resulted in the loss of smaller pollen grains and in the overrepresentation of bisaccate ones in most of the re-analyzed samples. We therefore provide an updated set of figures with the new data and have revised the description of the results, discussion and conclusions re- ported in Sadori et al. (2016) where necessary. We stress that the majority of the original results and conclusions remain valid, while the records’ reliability and resolution have improved as 12 samples that had been omitted in the original study because of low count sums are now included in the revised dataset (Sadori et al., 2018)
Selective laser sintering of hydroxyapatite reinforced polyethylene composites for bioactive implants and tissue scaffold development
Selective laser sintering (SLS) has been investigated for the production of bioactive implants and tissue scaffolds using composites of high-density polyethylene (HDPE) reinforced with hydroxyapatite (HA) with the aim of achieving the rapid manufacturing of customized implants. Single-layer and multilayer block specimens made of HA-HDPE composites with 30 and 40 vol % HA were sintered successfully using a CO2 laser sintering system. Laser power and scanning speed had a significant effect on the sintering behaviour. The degree of particle fusion and porosity were influenced by the laser processing parameters, hence control can be attained by varying these parameters. Moreover, the SLS processing allowed exposure of HA particles on the surface of the composites and thereby should provide bioactive products. Pores existed in the SLS-fabricated composite parts and at certain processing parameters a significant fraction of the pores were within the optimal sizes for tissue regeneration. The results indicate that the SLS technique has the potential not only to fabricate HA-HDPE composite products but also to produce appropriate features for their application as bioactive implants and tissue scaffolds
Generating functional for the gravitational field: implementation of an evolutionary quantum dynamics
We provide a generating functional for the gravitational field, associated to
the relaxation of the primary constraints as extended to the quantum sector.
This requirement of the theory, relies on the assumption that a suitable time
variable exist, when taking the T-products of the dynamical variables. More
precisely, we start from the gravitational field equations written in the
Hamiltonian formalism and expressed via Misner-like variables; hence we
construct the equation to which the T-products of the dynamical variables obey
and transform this paradigm in terms of the generating functional, as taken on
the theory phase-space. We show how the relaxation of the primary constraints
(which correspond to break down the invariance of the quantum theory under the
4-diffeomorphisms) is summarized by a free functional taken on the Lagrangian
multipliers, accounting for such constraints in the classical theory. The issue
of our analysis is equivalent to a Gupta-Bleuler approach on the quantum
implementation of all the gravitational constraints; in fact, in the limit of
small , the quantum dynamics is described by a Schr\"odinger equation,
as soon as the mean values of the momenta, associated to the lapse function and
the shift vector, are not vanishing. Finally we show how, in the classical
limit, the evolutionary quantum gravity reduces to General Relativity in the
presence of an Eckart fluid, which corresponds to the classical counterpart of
the physical clock, introduced in the quantum theory.Comment: 23 pages, no figures, to appear on International Journal of Modern
Physics
Surface terms, Asymptotics and Thermodynamics of the Holst Action
We consider a first order formalism for general relativity derived from the
Holst action. This action is obtained from the standard Palatini-Hilbert form
by adding a topological-like term and can be taken as the starting point for
loop quantum gravity and spin foam models. The equations of motion derived from
the Holst action are, nevertheless, the same as in the Palatini formulation.
Here we study the form of the surface terms of the action for general
boundaries as well as the symplectic current in the covariant formulation of
the theory. Furthermore, we analyze the behavior of the surface terms in
asymptotically flat space-times. We show that the contribution to the
symplectic structure from the Holst term vanishes and one obtains the same
asymptotic expressions as in the Palatini action. It then follows that the
asymptotic Poincare symmetries and conserved quantities such as energy, linear
momentum and relativistic angular momentum found here are equivalent to those
obtained from the standard Arnowitt, Deser and Misner formalism. Finally, we
consider the Euclidean approach to black hole thermodynamics and show that the
on-shell Holst action, when evaluated on some static solutions containing
horizons, yields the standard thermodynamical relations.Comment: 16 page
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