159 research outputs found
Asymptotics of 4d spin foam models
We study the asymptotic properties of four-simplex amplitudes for various
four-dimensional spin foam models. We investigate the semi-classical limit of
the Ooguri, Euclidean and Lorentzian EPRL models using coherent states for the
boundary data. For some classes of geometrical boundary data, the asymptotic
formulae are given, in all three cases, by simple functions of the Regge action
for the four-simplex geometry.Comment: 10 pages, Proceedings for the 2nd Corfu summer school and workshop on
quantum gravity and quantum geometry, talk given by Winston J. Fairbair
Spin foams with timelike surfaces
Spin foams of 4d gravity were recently extended from complexes with purely
spacelike surfaces to complexes that also contain timelike surfaces. In this
article, we express the associated partition function in terms of vertex
amplitudes and integrals over coherent states. The coherent states are
characterized by unit 3--vectors which represent normals to surfaces and lie
either in the 2--sphere or the 2d hyperboloids. In the case of timelike
surfaces, a new type of coherent state is used and the associated completeness
relation is derived. It is also shown that the quantum simplicity constraints
can be deduced by three different methods: by weak imposition of the
constraints, by restriction of coherent state bases and by the master
constraint.Comment: 22 pages, no figures; v2: remarks on operator formalism added in
discussion; correction: the spin 1/2 irrep of the discrete series does not
appear in the Plancherel decompositio
(Broken) Gauge Symmetries and Constraints in Regge Calculus
We will examine the issue of diffeomorphism symmetry in simplicial models of
(quantum) gravity, in particular for Regge calculus. We find that for a
solution with curvature there do not exist exact gauge symmetries on the
discrete level. Furthermore we derive a canonical formulation that exactly
matches the dynamics and hence symmetries of the covariant picture. In this
canonical formulation broken symmetries lead to the replacements of constraints
by so--called pseudo constraints. These considerations should be taken into
account in attempts to connect spin foam models, based on the Regge action,
with canonical loop quantum gravity, which aims at implementing proper
constraints. We will argue that the long standing problem of finding a
consistent constraint algebra for discretized gravity theories is equivalent to
the problem of finding an action with exact diffeomorphism symmetries. Finally
we will analyze different limits in which the pseudo constraints might turn
into proper constraints. This could be helpful to infer alternative
discretization schemes in which the symmetries are not broken.Comment: 32 pages, 15 figure
A human cancer-predisposing polymorphism in Cdc25A is embryonic lethal in the mouse and promotes ASK-1 mediated apoptosis
<p>Abstract</p> <p>Background</p> <p>Failure to regulate the levels of Cdc25A phosphatase during the cell cycle or during a checkpoint response causes bypass of DNA damage and replication checkpoints resulting in genomic instability and cancer. During G1 and S and in cellular response to DNA damage, Cdc25A is targeted for degradation through the Skp1-cullin-β-TrCP (SCF<sup>β-TrCP</sup>) complex. This complex binds to the Cdc25A DSG motif which contains serine residues at positions 82 and 88. Phosphorylation of one or both residues is necessary for the binding and degradation to occur.</p> <p>Results</p> <p>We now show that mutation of serine 88 to phenylalanine, which is a cancer-predisposing polymorphic variant in humans, leads to early embryonic lethality in mice. The mutant protein retains its phosphatase activity both <it>in vitro </it>and in cultured cells. It fails to interact with the apoptosis signal-regulating kinase 1 (ASK1), however, and therefore does not suppress ASK1-mediated apoptosis.</p> <p>Conclusions</p> <p>These data suggest that the DSG motif, in addition to its function in Cdc25A-mediated degradation, plays a role in cell survival during early embyogenesis through suppression of ASK1-mediated apoptosis.</p
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Channelling optics for high quality imaging of sensory hair
A long distance microscope (LDM) is extended by a lens and aperture array. This newly formed channelling LDM is superior in high quality, high-speed imaging of large field of views (FOV). It allows imaging the same FOV like a conventional LDM, but at improved magnification. The optical design is evaluated by calculations with the ray tracing code ZEMAX. High-speed imaging of a 2 × 2 mm(2) FOV is realized at 3.000 frames per second and 1 μm per pixel image resolution. In combination with flow sensitive hair the optics forms a wall shear stress sensor. The optics images the direct vicinity of twenty-one flow sensitive hair distributed in a quadratic array. The hair consists of identical micro-pillars that are 20 μm in diameter, 390 μm in length and made from polydimethylsiloxane (PDMS). Sensor validation is conducted in the transition region of a wall jet in air. The wall shear stress is calculated from optically measured micro-pillar tip deflections. 2D wall shear stress distributions are obtained with currently highest spatiotemporal resolution. The footprint of coherent vortical structures far away from the wall is recovered in the Fourier spectrum of wall shear stress fluctuations. High energetic patterns of 2D wall shear stress distributions are identified by proper orthogonal decomposition (POD)
Topological Graph Polynomials in Colored Group Field Theory
In this paper we analyze the open Feynman graphs of the Colored Group Field
Theory introduced in [arXiv:0907.2582]. We define the boundary graph
\cG_{\partial} of an open graph \cG and prove it is a cellular complex.
Using this structure we generalize the topological (Bollobas-Riordan) Tutte
polynomials associated to (ribbon) graphs to topological polynomials adapted to
Colored Group Field Theory graphs in arbitrary dimension
Encoding simplicial quantum geometry in group field theories
We show that a new symmetry requirement on the GFT field, in the context of
an extended GFT formalism, involving both Lie algebra and group elements,
leads, in 3d, to Feynman amplitudes with a simplicial path integral form based
on the Regge action, to a proper relation between the discrete connection and
the triad vectors appearing in it, and to a much more satisfactory and
transparent encoding of simplicial geometry already at the level of the GFT
action.Comment: 15 pages, 2 figures, RevTeX, references adde
Operator Spin Foam Models
The goal of this paper is to introduce a systematic approach to spin foams.
We define operator spin foams, that is foams labelled by group representations
and operators, as the main tool. An equivalence relation we impose in the set
of the operator spin foams allows to split the faces and the edges of the
foams. The consistency with that relation requires introduction of the
(familiar for the BF theory) face amplitude. The operator spin foam models are
defined quite generally. Imposing a maximal symmetry leads to a family we call
natural operator spin foam models. This symmetry, combined with demanding
consistency with splitting the edges, determines a complete characterization of
a general natural model. It can be obtained by applying arbitrary (quantum)
constraints on an arbitrary BF spin foam model. In particular, imposing
suitable constraints on Spin(4) BF spin foam model is exactly the way we tend
to view 4d quantum gravity, starting with the BC model and continuing with the
EPRL or FK models. That makes our framework directly applicable to those
models. Specifically, our operator spin foam framework can be translated into
the language of spin foams and partition functions. We discuss the examples: BF
spin foam model, the BC model, and the model obtained by application of our
framework to the EPRL intertwiners.Comment: 19 pages, 11 figures, RevTex4.
Coupling gauge theory to spinfoam 3d quantum gravity
We construct a spinfoam model for Yang-Mills theory coupled to quantum
gravity in three dimensional riemannian spacetime. We define the partition
function of the coupled system as a power series in g_0^2 G that can be
evaluated order by order using grasping rules and the recoupling theory. With
respect to previous attempts in the literature, this model assigns the
dynamical variables of gravity and Yang-Mills theory to the same simplices of
the spinfoam, and it thus provides transition amplitudes for the spin network
states of the canonical theory. For SU(2) Yang-Mills theory we show explicitly
that the partition function has a semiclassical limit given by the Regge
discretization of the classical Yang-Mills action.Comment: 18 page
Spinning Loop Black Holes
In this paper we construct four Kerr-like spacetimes starting from the loop
black hole Schwarzschild solutions (LBH) and applying the Newman-Janis
transformation. In previous papers the Schwarzschild LBH was obtained replacing
the Ashtekar connection with holonomies on a particular graph in a
minisuperspace approximation which describes the black hole interior. Starting
from this solution, we use a Newman-Janis transformation and we specialize to
two different and natural complexifications inspired from the complexifications
of the Schwarzschild and Reissner-Nordstrom metrics. We show explicitly that
the space-times obtained in this way are singularity free and thus there are no
naked singularities. We show that the transformation move, if any, the
causality violating regions of the Kerr metric far from r=0. We study the
space-time structure with particular attention to the horizons shape. We
conclude the paper with a discussion on a regular Reissner-Nordstrom black hole
derived from the Schwarzschild LBH and then applying again the Newmann-Janis
transformation.Comment: 18 pages, 18 figure
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