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Search Results: 1 - 10 of 4811 matches for " Wolfgang Kummer "
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Progress and problems in quantum gravity
Wolfgang Kummer
Physics , 2005,
Abstract: From the point of view of an uncompromising field theorist quantum gravity is beset with serious technical and, above all, conceptual problems with regard especially to the meaning of genuine "physical" observables. This situation is not really improved by the appearance of recent attempts to reformulate gravity within some novel framework.However, the original aim, a background-independent quantum theory of gravity, can be achieved in a particular area, namely 2d dilaton quantum gravity without any assumptions beyond standard quantum field theory. Some important by-products of the research of the "Vienna School" include the introduction of the concept of Poisson-Sigma models, a verification of the "virtual Black Hole" and the extensions to N = (1,1) and N = (2,2) 2d-supergravity, for which complete solutions of some old problems have been possible which are relevant for superstring theory
2d quantum gravity with torsion, dilaton theory and black hole formation
Wolfgang Kummer
Physics , 2000,
Abstract: Starting from the work of the author in 1990 with different collaborators, essential progress in 2d gravity theories has been made. Now all such theories (and not only certain special models) can be treated at the classical as well as at the quantum level. New physical insights have been obtained, as e.g. the ``virtual black hole''. The formalism developed in this context recently also finds increasing interest in mathematical physics.
P-Branes, Poisson-Sigma-Models and Embedding Approach to (p+1)- Dimensional Gravity
Igor Bandos,Wolfgang Kummer
Physics , 1997, DOI: 10.1142/S0217751X99002311
Abstract: A generalization of the embedding approach for d-dimensional gravity based upon p-brane theories is considered. We show that the D-dimensional p-brane coupled to an antisymmetric tensor field of rank (p+1) provides the dynamical basis for the description of d=(p+1) dimensional gravity in the isometric embedding formalism. ''Physical'' matter appears in such an approach as a manifestation of a D-dimensional antisymmetric tensor (generalized Kalb- Ramond) background. For the simplest case, the Lorentz harmonic formulation of the bosonic string in a Kalb-Ramond background and its relation to a first order Einstein-Cartan approach for d=2 dimensional gravity is analysed in some detail. A general Poisson-sigma-model structure emerges. For the minimal choice of D=3 an interesting ``dual'' formulation is found which has the structure of a Jackiw-Teitelboim theory, coupled minimally to a massive scalar field. Our approach is intended to serve as a preparation for the study of d- dimensional supergravity theory, either starting from the generalized action of free supersymmetric (d-1)-branes or $D_{(d-1)}$-branes, or from the corresponding geometric equations ('rheotropic' conditions).
A Polynomial First Order Action for the Dirichlet 3-brane
Igor Bandos,Wolfgang Kummer
Physics , 1997, DOI: 10.1016/S0370-2693(97)01128-3
Abstract: A new first order action for type IIB Dirichlet 3-brane is proposed. Its form is inspired by the superfield equations of motion obtained recently from the generalized action principle. The action involves auxiliary symmetric spin tensor fields. It seems promising for a reformulation of the generalized action in a structure most adequate for investigating the extrinsic geometry of the super-3- brane, but also for further studies of string dualities.
On Action Functionals for Interacting Brane Systems
Igor Bandos,Wolfgang Kummer
Physics , 2000,
Abstract: We present an action functional and derive equations of motion for a coupled system of a bosonic Dp--brane and an open string ending on the Dp-brane. With this example we address the key issues of the recently proposed method (hep-th/9905144, hep-th/9906041) for the construction of manifestly supersymmetric action functionals for interacting superbrane systems. We clarify, in particular, how the arbitrariness in sources localized on the intersection is related to the standard description of the flat D-branes as rigid planes where the string for endpoints 'live'.
Current Density Distributions and a Supersymmetric Action for Interacting Brane Systems
Igor Bandos,Wolfgang Kummer
Physics , 1999, DOI: 10.1016/S0370-2693(99)00928-4
Abstract: We propose a method to obtain a manifestly supersymmetric action functional for interacting brane systems. It is based on the induced map of the worldvolume of low-dimensional branes into the worldvolume of the space-time filling brane ((D-1)-brane), which may be either dynamical or auxiliary, and implies an identification of Grassmann coordinate fields of lower dimensional branes with an image of the Grassmann coordinate fields of that (D-1)-brane. With this identification the covariant current distribution forms with support on the superbrane worldvolumes become invariant under the target space supersymmetry and can be used to write the coupled superbrane action as an integral over the D-dimensional manifolds ((D-1)-brane worldvolume). We compare the equations derived from this new ('Goldstone fermion embedded') action with the ones produced by a more straightforward generalization of the free brane actions based on the incorporation of the boundary terms with Lagrange multipliers ('superspace embedded' action). We find that both procedures produce the same equations of motion and thus justify each other. Both actions are presented explicitly for the coupled system of a D=10 super-D3-brane and a fundamental superstring which ends on the super-D3-brane.
Superstring 'ending' on super-D9-brane: a supersymmetric action functional for the coupled brane system
Igor Bandos,Wolfgang Kummer
Physics , 1999, DOI: 10.1016/S0550-3213(99)00530-1
Abstract: A supersymmetric action functional describing the interaction of the fundamental superstring with the D=10, type IIB Dirichlet super-9-brane is presented. A set of supersymmetric equations for the coupled system is obtained from the action principle. It is found that the interaction of the string endpoints with the super-D9-brane gauge field requires some restrictions for the image of the gauge field strength. When those restrictions are not imposed, the equations imply the absence of the endpoints, and the equations coincide either with the ones of the free super-D9-brane or with the ones for the free closed type IIB superstring. Different phases of the coupled system are described. A generalization to an arbitrary system of intersecting branes is discussed.
Serotonin Increases Cilia-Driven Particle Transport via an Acetylcholine-Independent Pathway in the Mouse Trachea
Peter K?nig, Benjamin Krain, Gabriela Krasteva, Wolfgang Kummer
PLOS ONE , 2009, DOI: 10.1371/journal.pone.0004938
Abstract: Background Mucociliary clearance in the airways is driven by the coordinated beating of ciliated cells. Classical neuromediators such as noradrenalin and acetylcholine increase ciliary beat frequency and thus cilia-driven transport. Despite the fact that the neuromediator serotonin is ciliostimulatory in invertebrates and has been implied in releasing acetylcholine from the airway epithelium, its role in regulating cilia function in vertebrate airways is not established. Methodology/Principal Findings We examined the effects of serotonin on ciliary beat frequency and cilia-driven particle transport in the acutely excised submerged mouse trachea and determined the sources of serotonin in this tissue by immunohistochemistry. Serotonin (100 μM) increased cilary beat frequency (8.9±1.2 Hz to 17.0±2.7 Hz) and particle transport speed (38.9±4.6 μm/s to 83.4±8.3 μm/s) to an extent that was comparable to a supramaximal dose of ATP. The increase in particle transport speed was totally prevented by methysergide (100 μM). Blockade of muscarinic receptors by atropine (1 μM) did not reduce the effect of serotonin, although it was effective in preventing the increase in particle transport speed mediated by muscarine (100 μM). Immunohistochemistry demonstrated serotonin in mast cells pointing towards mast cells and platelets as possible endogenous sources of serotonin. Conclusions/Significance These results indicate that serotonin is a likely endogenous mediator that can increase cilia-driven transport independent from acetylcholine during activation of mast cells and platelets.
TRPM5, a taste-signaling transient receptor potential ion-channel, is a ubiquitous signaling component in chemosensory cells
Silke Kaske, Gabriele Krasteva, Peter K?nig, Wolfgang Kummer, Thomas Hofmann, Thomas Gudermann, Vladimir Chubanov
BMC Neuroscience , 2007, DOI: 10.1186/1471-2202-8-49
Abstract: Here, we systematically investigated the expression of TRPM5 in rat and mouse tissues. Apart from taste buds, where we found TRPM5 to be predominantly localized on the basolateral surface of taste receptor cells, TRPM5 immunoreactivity was seen in other chemosensory organs – the main olfactory epithelium and the vomeronasal organ. Most strikingly, we found solitary TRPM5-enriched epithelial cells in all parts of the respiratory and gastrointestinal tract. Based on their tissue distribution, the low cell density, morphological features and co-immunostaining with different epithelial markers, we identified these cells as brush cells (also known as tuft, fibrillovesicular, multivesicular or caveolated cells). In terms of morphological characteristics, brush cells resemble taste receptor cells, while their origin and biological role are still under intensive debate.We consider TRPM5 to be an intrinsic signaling component of mammalian chemosensory organs, and provide evidence for brush cells being an important cellular correlate in the periphery.Transient receptor potential (TRP) proteins form a large gene family of ion channels characterized by distinct activation mechanisms and biophysical properties. By sequence homology, members of the family fall into six subfamilies (TRPC, TRPV, TRPM, TRPML, TRPP, and TRPA). There is mounting evidence that TRP channels are involved in thermosensation, mechanosensation, smell and taste. A subset of TRP channels, called 'thermo-TRPs' (TRPV1-TRPV4, TRPA1 and TRPM8), have been found to be highly temperature dependent and are directly involved in heat and cold sensation in the peripheral nervous system [1]. Several TRP channels are mechanosensitive or activated by hypotonic challenge (TRPV4, TRPA1, TRPM3, PKD1 and TRPP2) [2]. TRPC2 is specifically expressed in the rodent sensory epithelium of the vomeronasal organ (VNO) where it plays a critical role in signaling processes triggered by pheromones [3,4]. More recently, evidence was obtai
Spinal afferent neurons projecting to the rat lung and pleura express acid sensitive channels
Michael Groth, Tanja Helbig, Veronika Grau, Wolfgang Kummer, Rainer V Haberberger
Respiratory Research , 2006, DOI: 10.1186/1465-9921-7-96
Abstract: The tracer DiI was either injected into the left lung or applied to the costal pleura. Retrogradely labelled dorsal root ganglion neurons were subjected to triple-labelling immunohistochemistry using antisera against TRPV1, ASIC3 and neurofilament 68 (marker for myelinated neurons), and their soma diameter was measured.Whereas 22% of pulmonary spinal afferents contained neither channel-immunoreactivity, at least one is expressed by 97% of pleural afferents. TRPV1+/ASIC3- neurons with probably slow conduction velocity (small soma, neurofilament 68-negative) were significantly more frequent among pleural (35%) than pulmonary afferents (20%). TRPV1+/ASIC3+ neurons amounted to 14 and 10% respectively. TRPV1-/ASIC3+ neurons made up between 44% (lung) and 48% (pleura) of neurons, and half of them presumably conducted in the A-fibre range (larger soma, neurofilament 68-positive).Rat pleural and pulmonary spinal afferents express at least two different acid-sensitive channels that make them suitable to monitor tissue acidification. Patterns of co-expression and structural markers define neuronal subgroups that can be inferred to subserve different functions and may initiate specific reflex responses. The higher prevalence of TRPV1+/ASIC3- neurons among pleural afferents probably reflects the high sensitivity of the parietal pleura to painful stimuli.The lower respiratory tract receives primary afferent fibres both from vagal sensory (nodose and jugular) and dorsal root ganglia (DRG) that transmit information to the brainstem and spinal cord, respectively [1-5]. To date, the vagal airway afferents have received particular interest. Functionally, they can be grouped in rapidly adapting mechanoreceptors (RARs, also called "irritant receptors"), slowly adapting mechanoreceptors (SARs), and C-fibre afferents [1,6]. At least in the guinea-pig, the functionally diverse classes are segregated between the two vagal sensory ganglia, with SARs specifically located in the upper, jugula
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