Classical and Quantum Strings in compactified pp-waves and Godel type Universes

We consider Neveu-Schwarz pp-waves with spacetime supersymmetry. Upon compactification of a spacelike direction, these backgrounds develop Closed Null Curves (CNCs) and Closed Timelike Curves (CTCs), and are U-dual to supersymmetric Godel type universes. We study classical and quantum strings in this background, with emphasis on the strings winding around the compact direction. We consider two types of strings: long strings stabilized by NS flux and rotating strings which are stabilized against collapse by angular momentum. Some of the latter strings wrap around CNCs and CTCs, and are thus a potential source of pathology. We analyze the partition function, and in particular discuss the effects of these string states. Although our results are not conclusive, the partition function seems to be dramatically altered due to the presence of CNCs and CTCs. We discuss some interpretations of our results, including a possible sign of unitary violation.Comment: 42 pages, LaTeX, 2 figure

Logarithmic Corrections to Rotating Extremal Black Hole Entropy in Four and Five Dimensions

We compute logarithmic corrections to the entropy of rotating extremal black holes using quantum entropy function i.e. Euclidean quantum gravity approach. Our analysis includes five dimensional supersymmetric BMPV black holes in type IIB string theory on T^5 and K3 x S^1 as well as in the five dimensional CHL models, and also non-supersymmetric extremal Kerr black hole and slowly rotating extremal Kerr-Newmann black holes in four dimensions. For BMPV black holes our results are in perfect agreement with the microscopic results derived from string theory. In particular we reproduce correctly the dependence of the logarithmic corrections on the number of U(1) gauge fields in the theory, and on the angular momentum carried by the black hole in different scaling limits. We also explain the shortcomings of the Cardy limit in explaining the logarithmic corrections in the limit in which the (super)gravity description of these black holes becomes a valid approximation. For non-supersymmetric extremal black holes, e.g. for the extremal Kerr black hole in four dimensions, our result provides a stringent testing ground for any microscopic explanation of the black hole entropy, e.g. Kerr/CFT correspondence.Comment: LaTeX file, 50 pages; v2: added extensive discussion on the relation between boundary condition and choice of ensemble, modified analysis for slowly rotating black holes, all results remain unchanged, typos corrected; v3: minor additions and correction

Structure and Function of the Hair Cell Ribbon Synapse

Faithful information transfer at the hair cell afferent synapse requires synaptic transmission to be both reliable and temporally precise. The release of neurotransmitter must exhibit both rapid on and off kinetics to accurately follow acoustic stimuli with a periodicity of 1 ms or less. To ensure such remarkable temporal fidelity, the cochlear hair cell afferent synapse undoubtedly relies on unique cellular and molecular specializations. While the electron microscopy hallmark of the hair cell afferent synapse — the electron-dense synaptic ribbon or synaptic body — has been recognized for decades, dissection of the synapse’s molecular make-up has only just begun. Recent cell physiology studies have added important insights into the synaptic mechanisms underlying fidelity and reliability of sound coding. The presence of the synaptic ribbon links afferent synapses of cochlear and vestibular hair cells to photoreceptors and bipolar neurons of the retina. This review focuses on major advances in understanding the hair cell afferent synapse molecular anatomy and function that have been achieved during the past years