34 research outputs found
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 ā the electron-dense synaptic ribbon or synaptic body ā has been recognized for decades, dissection of the synapseās 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