815 research outputs found

    Towards Dynamic Control of Wettability by Using Functionalized Altitudinal Molecular Motors on Solid Surfaces

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    We report the synthesis of altitudinal molecular motors that contain functional groups in their rotor part. In an approach to achieve dynamic control over the properties of solid surfaces, a hydrophobic perfluorobutyl chain and a relatively hydrophilic cyano group were introduced to the rotor part of the motors. Molecular motors were attached to quartz surfa-ces by using interfacial 1,3-dipolar cycloadditions. To test the effect of the functional groups on the rotary motion, photochemical and thermal isomerization studies of the motors were per-formed both in solution and when attached to the surface. We found that the substituents have no significant effect on the thermal and photochemical processes, and the functionalized motors preserved their rotary function both in solution and on a quartz surface. Preliminary results on the influence of the functional groups on surface wettability are also described

    Codimension Two Compactifications and the Cosmological Constant Problem

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    We consider solutions of six dimensional Einstein equations with two compact dimensions. It is shown that one can introduce 3-branes in this background in such a way that the effective four dimensional cosmological constant is completely independent of the brane tensions. These tensions are completely arbitrary, without requiring any fine tuning. We must, however, fine tune bulk parameters in order to obtain a sufficiently small value for the observable cosmological constant. We comment in the effective four dimensional description of this effect at energies below the compactification scale.Comment: 4 pages, rextex

    No Time Machine Construction in Open 2+1 Gravity with Timelike Total Energy Momentum

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    It is shown that in 2+1 dimensional gravity an open spacetime with timelike sources and total energy momentum cannot have a stable compactly generated Cauchy horizon. This constitutes a proof of a version of Kabat's conjecture and shows, in particular, that not only a Gott pair cannot be formed from processes such as the decay of a single cosmic string as has been shown by Carroll et al., but that, in a precise sense, a time machine cannot be constructed at all.Comment: 7 pages. Several changes and 3 figures added. To appear in Phys. Rev.

    Exact Black Holes and Gravitational Shockwaves on Codimension-2 Branes

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    We derive exact gravitational fields of a black hole and a relativistic particle stuck on a codimension-2 brane in DD dimensions when gravity is ruled by the bulk DD-dimensional Einstein-Hilbert action. The black hole is locally the higher-dimensional Schwarzschild solution, which is threaded by a tensional brane yielding a deficit angle and includes the first explicit example of a `small' black hole on a tensional 3-brane. The shockwaves allow us to study the large distance limits of gravity on codimension-2 branes. In an infinite locally flat bulk, they extinguish as 1/rD41/r^{D-4}, i.e. as 1/r21/r^2 on a 3-brane in 6D6D, manifestly displaying the full dimensionality of spacetime. We check that when we compactify the bulk, this special case correctly reduces to the 4D Aichelburg-Sexl solution at large distances. Our examples show that gravity does not really obstruct having general matter stress-energy on codimension-2 branes, although its mathematical description may be more involved.Comment: 18 pages, LaTeX; v2: added references, version to appear in JHE

    Relating the Cosmological Constant and Supersymmetry Breaking in Warped Compactifications of IIB String Theory

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    It has been suggested that the observed value of the cosmological constant is related to the supersymmetry breaking scale M_{susy} through the formula Lambda \sim M_p^4 (M_{susy}/M_p)^8. We point out that a similar relation naturally arises in the codimension two solutions of warped space-time varying compactifications of string theory in which non-isotropic stringy moduli induce a small but positive cosmological constant.Comment: 7 pages, LaTeX, references added and minor changes made, (v3) map between deSitter and global cosmic brane solutions clarified, supersymmetry breaking discussion improved and references adde

    Cosmology of codimension-two braneworlds

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    We present a comprehensive study of the cosmological solutions of 6D braneworld models with azimuthal symmetry in the extra dimensions, moduli stabilization by flux or a bulk scalar field, and which contain at least one 3-brane that could be identified with our world. We emphasize an unusual property of these models: their expansion rate depends on the 3-brane tension either not at all, or in a nonstandard way, at odds with the naive expected dimensional reduction of these systems to 4D general relativity at low energies. Unlike other braneworld attempts to find a self-tuning solution to the cosmological constant problem, the apparent failure of decoupling in these models is not associated with the presence of unstabilized moduli; rather it is due to automatic cancellation of the brane tension by the curvature induced by the brane. This provides some corroboration for the hope that these models provide a distinctive step toward understanding the smallness of the observed cosmological constant. However, we point out some challenges for obtaining realistic cosmology within this framework.Comment: 30 pages, 4 figures; generalized result for nonconventional Friedmann equation, added referenc

    Disappearing Dark Matter in Brane World Cosmology: New Limits on Noncompact Extra Dimensions

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    We explore cosmological implications of dark matter as massive particles trapped on a brane embedded in a Randall-Sundrum noncompact higher dimension AdS5AdS_5 space. It is an unavoidable consequence of this cosmology that massive particles are metastable and can disappear into the bulk dimension. Here, we show that a massive dark matter particle (e.g. the lightest supersymmetric particle) is likely to have the shortest lifetime for disappearing into the bulk. We examine cosmological constraints on this new paradigm and show that disappearing dark matter is consistent (at the 95% confidence level) with all cosmological constraints, i.e. present observations of Type Ia supernovae at the highest redshift, trends in the mass-to-light ratios of galaxy clusters with redshift, the fraction of X-ray emitting gas in rich clusters, and the spectrum of power fluctuations in the cosmic microwave background. A best 2σ2 \sigma concordance region is identified corresponding to a mean lifetime for dark matter disappearance of 15Γ18015 \le \Gamma^{-1} \le 80 Gyr. The implication of these results for brane-world physics is discussed.Comment: 7 pages, 7 figures, new cosmological constraints added, accepted for publication in PR

    BPS Domain Walls in Large N Supersymmetric QCD

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    We explicitly construct BPS domain walls interpolating between neighboring chirally asymmetric vacua in a model for large N pure supersymmetric QCD. The BPS equations for the corresponding ZN{\bf Z}_N symmetric order parameter effective Lagrangian reduce to those in the ANA_N Landau-Ginsburg model assuming that the higher derivative terms in the K{\"a}hler potential are suppressed in the large N limit. These BPS domain walls, which have vanishing width in the large N limit, can be viewed as supermembranes embedded in a (3+1)-dimensional supersymmetric QCD background. The supermembrane couples to a three-form supermultiplet whose components we identify with the composite fields of supersymmetric QCD. We also discuss certain aspects of chromoelectric flux tubes (open strings) ending on these walls which appear to support their interpretation as D-branes.Comment: 24 REVTEX pages. Journal version. Comments added, typos correcte

    Non stationary Einstein-Maxwell fields interacting with a superconducting cosmic string

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    Non stationary cylindrically symmetric exact solutions of the Einstein-Maxwell equations are derived as single soliton perturbations of a Levi-Civita metric, by an application of Alekseev inverse scattering method. We show that the metric derived by L. Witten, interpreted as describing the electrogravitational field of a straight, stationary, conducting wire may be recovered in the limit of a `wide' soliton. This leads to the possibility of interpreting the solitonic solutions as representing a non stationary electrogravitational field exterior to, and interacting with, a thin, straight, superconducting cosmic string. We give a detailed discussion of the restrictions that arise when appropiate energy and regularity conditions are imposed on the matter and fields comprising the string, considered as `source', the most important being that this `source' must necessarily have a non- vanishing minimum radius. We show that as a consequence, it is not possible, except in the stationary case, to assign uniquely a current to the source from a knowledge of the electrogravitational fields outside the source. A discussion of the asymptotic properties of the metrics, the physical meaning of their curvature singularities, as well as that of some of the metric parameters, is also included.Comment: 14 pages, no figures (RevTex
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