8,699 research outputs found
Four-dimensional Quantum Gravity with a Cosmological Constant from Three-dimensional Holomorphic Blocks
Prominent approaches to quantum gravity struggle when it comes to
incorporating a positive cosmological constant in their models. Using
quantization of a complex Chern-Simons theory we
include a cosmological constant, of either sign, into a model of quantum
gravity.Comment: 5 pages and 2 figure
SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry
We study the expectation value of a nonplanar Wilson graph operator in
SL(2,C) Chern-Simons theory on . In particular we analyze its asymptotic
behaviour in the double-scaling limit in which both the representation labels
and the Chern-Simons coupling are taken to be large, but with fixed ratio. When
the Wilson graph operator has a specific form, motivated by loop quantum
gravity, the critical point equations obtained in this double-scaling limit
describe a very specific class of flat connection on the graph complement
manifold. We find that flat connections in this class are in correspondence
with the geometries of constant curvature 4-simplices. The result is fully
non-perturbative from the perspective of the reconstructed geometry. We also
show that the asymptotic behavior of the amplitude contains at the leading
order an oscillatory part proportional to the Regge action for the single
4-simplex in the presence of a cosmological constant. In particular, the
cosmological term contains the full-fledged curved volume of the 4-simplex.
Interestingly, the volume term stems from the asymptotics of the Chern-Simons
action. This can be understood as arising from the relation between
Chern-Simons theory on the boundary of a region, and a theory defined by an
action in the bulk. Another peculiarity of our approach is that the sign
of the curvature of the reconstructed geometry, and hence of the cosmological
constant in the Regge action, is not fixed a priori, but rather emerges
semiclassically and dynamically from the solution of the equations of motion.
In other words, this work suggests a relation between 4-dimensional loop
quantum gravity with a cosmological constant and SL(2,C) Chern-Simons theory in
3-dimensions with knotted graph defects.Comment: 54+11 pages, 9 figure
Spacetime thermodynamics without hidden degrees of freedom
A celebrated result by Jacobson is the derivation of Einstein's equations
from Unruh's temperature, the Bekenstein-Hawking entropy and the Clausius
relation. This has been repeatedly taken as evidence for an interpretation of
Einstein's equations as equations of state for unknown degrees of freedom
underlying the metric. We show that a different interpretation of Jacobson
result is possible, which does not imply the existence of additional degrees of
freedom, and follows only from the quantum properties of gravity. We introduce
the notion of quantum gravitational Hadamard states, which give rise to the
full local thermodynamics of gravity.Comment: 12 pages, 1 figur
Evolution and Modern Approaches for Thermal Analysis of Electrical Machines
In this paper, the authors present an extended survey on the evolution and the modern approaches in the thermal analysis of electrical machines. The improvements and the new techniques proposed in the last decade are analyzed in depth and compared in order to highlight the qualities and defects of each. In particular, thermal analysis based on lumped-parameter thermal network, finite-element analysis, and computational fluid dynamics are considered in this paper. In addition, an overview of the problems linked to the thermal parameter determination and computation is proposed and discussed. Taking into account the aims of this paper, a detailed list of books and papers is reported in the references to help researchers interested in these topics
On the Generalized Kramers Problem with Oscillatory Memory Friction
The time-dependent transmission coefficient for the Kramers problem exhibits
different behaviors in different parameter regimes. In the high friction regime
it decays monotonically ("non-adiabatic"), and in the low friction regime it
decays in an oscillatory fashion ("energy-diffusion-limited"). The generalized
Kramers problem with an exponential memory friction exhibits an additional
oscillatory behavior in the high friction regime ("caging"). In this paper we
consider an oscillatory memory kernel, which can be associated with a model in
which the reaction coordinate is linearly coupled to a nonreactive coordinate,
which is in turn coupled to a heat bath. We recover the non-adiabatic and
energy-diffusion-limited behaviors of the transmission coefficient in
appropriate parameter regimes, and find that caging is not observed with an
oscillatory memory kernel. Most interestingly, we identify a new regime in
which the time-dependent transmission coefficient decays via a series of rather
sharp steps followed by plateaus ("stair-like"). We explain this regime and its
dependence on the various parameters of the system
A Structural Comparison of Ordered and Non-Ordered Ion Doped Silicate Bioactive Glasses
One of the key benefits of sol-gel-derived glasses is the presence of a mesoporous structure
and the resulting increase in surface area. This enhancement in textural properties has a significant
e ect on the physicochemical properties of the materials. In this context the aim of this study was to
investigate how sol-gel synthesis parameters can influence the textural and structural properties of
mesoporous silicate glasses. We report the synthesis and characterization of metal ion doped sol-gel
derived glasses with di erent dopants in the presence or absence of a surfactant (Pluronic P123)
used as structure-directing templating agent. Characterization was done by several methods. Using
a structure directing agent led to larger surface areas and highly ordered mesoporous structures.
The chemical structure of the non-ordered glasses was modified to a larger extent than the one
of the ordered glasses due to increased incorporation of dopant ions into the glass network. The
results will help to further understand how the properties of sol-gel glasses can be controlled by
incorporation of metal dopants, in conjunction with control over the textural properties, and will be
important to optimize the properties of sol-gel glasses for specific applications, e.g., drug delivery,
bone regeneration, wound healing, and antibacterial materials.European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 643050, project “HyMedPoly
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