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Volume: 533

We investigate the spectrum of cosmological perturbations in a bounce cosmos modeled by a scalar field coupled to the string tachyon field (CSTB cosmos). By explicit computation of its primordial spectral index we show the power spectrum of curvature perturbations, generated during the tachyon matter dominated contraction phase, to be nearly scale invariant. We propose a unified space of parameters for a systematic study of inflationary/bouncing cosmologies. We find that CSTB cosmos is dual--in Wands's sense--to the slow-roll inflation model as can be easily seen from this unified parameter space. Guaranteed by the dynamical attractor behavior of CSTB Cosmos, this scale invariance is free of the fine-tuning problem, in contrast to the slow-roll inflation model., Comment: 19 pages, 2 figures

We investigate a new scenario of dark matter production in a bouncing universe, in which dark matter was produced completely out of equilibrium in the contracting as well as expanding phase. We explore possibilities of using dark matter as a probe of the bouncing universe, focusing on the relationship between a critical temperature of the bouncing universe and the present relic abundance of dark matter., Comment: 13 pages, 5 figures

When a D-brane wraps around a cycle of a curved manifold, the twisting of its normal bundle can induce chiral asymmetry in its worldvolume theory. We obtain the general form of the resulting anomalies for D-branes and their intersections. They are not cancelled among themselves, and the standard inflow mechanism does not apply at first sight because of their apparent lack of factorizability and the apparent vanishing of the corresponding inflow. We show however after taking into consideration the effects of the nontrivial topology of the normal bundles, the anomalies can be transformed into factorized forms and precisely cancelled by finite inflow from the Chern-Simons actions for the D-branes as long as the latter are well defined. We then consider examples in type II compactifications where the twisting of the normal bundles occurs and calculate the changes in the induced Ramond-Ramond charges on the D-branes., 25 pages, 0 figure; latex 2.09. Typos corrected. A reference and a comment added. Final version for publication in Nuclear Physics B

The M1 excitations in the nuclide 90Zr have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 J(π)=1+ states have been identified from observed ground-state transitions, revealing the fine structure of the giant M1 resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) μ(N)(2). The result for the total M1 strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.

The M1 excitations in the nuclide 90Zr have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 Ji^pi = 1+ states have been identified from observed ground-state transitions, revealing the fine structure of the giant M1 resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) mu_N^2 . The result for the total M1 strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.

Nuclear resonance fluorescence experiments were carried out at the High-Intensity �� -ray Source facility at Triangle Universities Nuclear Laboratory to characterize the low-energy dipole structure of 238 U using 100% linearly polarized photon beams from 2.0 to 6.2 MeV. 113 transitions corresponding to de-excitations to the ground state in 238 U were observed and the energy, spin, parity, integrated cross section, reduced width, and branching ratio were determined for each of these identified levels. The total E1 �� -ray interaction cross section was calculated and it was deduced that the observed concentration of low-lying E1 transitions were excited from the low-energy tail of the giant dipole resonance and were not a pygmy dipole resonance. Comparisons were made between quasiparticle random-phase approximation calculations and the experimentally observed strength. The observed and predicted M1 strength agreed well with each other. However, there was no similar agreement for the E1 strength.

The M1 excitations in the nuclide 90Zr have been studied in a photon-scattering experiment with monoenergetic and linearly-polarized beams from 7 to 11 MeV. More than 40 J^pi = 1+ states have been identified from observed ground-state transitions, revealing the fine structure of the giant M1 resonance with centroid energy of 9 MeV and sum strength of 3.4(3) µ_N^2 . The result for the total M1 strength and its fragmentation are discussed in the framework of three-phonon quasi-particle model.

The electromagnetic dipole strength of the nucleus 136Ba has been investigated at two facilities. At the bremsstrahlung facility at the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf, two measurements were performed with electron energies of 7.0 and 11.4 MeV. Photon scattering on the same nucleus has been investigated at the HI$gS facility of the Triangle Universities Nuclear Laboratory. The GEANT4 code has been used to determine detector response and non-nuclear scattered events. Thus it is possible to account for the dipole strength in the quasi-continuum of unresolvable transitions. A statistical code was used to simulate inelastic transitions and to determine the branching ratios of transitions to the ground-state. The resulting photo-absorption cross section is compared to QRPA and RQTBA calculations.

We discuss a cosmological model in which the string gauge field coupled universally to matter gives rise to an extra centripetal force and will have observable signatures on cosmological and astronomical observations. Several tests are performed using data including galaxy rotation curves of 22 spiral galaxies of varied luminosities and sizes, and perihelion precessions of planets in the solar system. The rotation curves of the same group of galaxies are independently fit using a dark matter model with the generalized Navarro--Frenk--White (NFW) profile and the string model. A Remarkable fit of galaxy rotation curves is achieved using the one-parameter string model as compared to the three-parameter dark matter model with the NFW profile. The average $\chi^2$ value of the NFW fit is 9\% better than that of the string model at a price of two more free parameters. Furthermore, from the string model, we can give a dynamical explanation for the phenomenological Tully-Fisher relation. We are able to derive a relation between field strength, galaxy size and luminosity, which can be verified with data from the 22 galaxies. To further test the hypothesis of the universal existence of the string gauge field, we apply our string model to the solar system. Constraint on the magnitude of the string field in the solar system is deduced from the current ranges for any anomalous perihelion precession of planets allowed by the latest observations. The field distribution resembles a dipole field originating from the Sun. The string field strength deduced from the solar system observations is of a similar magnitude as the field strength needed to sustain the rotational speed of the Sun inside the Milky Way. This hypothesis can be tested further by future observations with higher precision., Comment: 36 pages, 33 figures, published version

We introduce a string-inspired model for a bouncing/cyclic universe, utilizing the scalar-tachyon coupling as well as contribution from curvature in a closed universe. The universe undergoes the locked inflation, tachyon matter dominated rolling expansion, turnaround and contraction, as well as the subsequent deflation and "bounce" in each cycle of the cosmological evolution. We perform extensive analytic and numerical studies of the above evolution process. The minimum size of the universe is nonzero for generic initial values. The smooth bounce are made possible because of the negative contribution to effective energy density by the curvature term. No ghosts are ever generated at any point in the entire evolution of the universe, with the Null, Weak, and Dominant Energy Conditions preserved even at the bounce points, contrary to many bounce models previously proposed. And the Strong Energy Condition is satisfied in periods with tachyon matter domination., Comment: Extensive revision with comprehensive analytic and numerical studies of the locked inflation, the tachyon matter dominated process--rolling expansion, turnaround and contraction--as well as subsequent deflation and bounce process carefully presented. 31 pages and 21 figures

Background: Survey data over the last several decades suggests that emergency department (ED) access to diagnostic ultrasound performed by the radiology department is unreliable, particularly outside of regular business hours. Objective: To evaluate the association between the time of day of patient presentation and the use of diagnostic ultrasound services in United States (U.S.) EDs. Methods: This was a cross-sectional study of ED patient visits using the National Hospital Ambulatory Medical Care Survey for the years 2003 to 2005. Our main outcome measure was the use of diagnostic ultrasound during the ED patient visit as abstracted from the medical record. We performed multivariate analyses to identify any association between ultrasound use and time of presentation for all patients, as well as for two subgroups who are more likely to need ultrasound as part of their routine workup: patients at risk of deep venous thrombosis, and patients at risk for ectopic pregnancy. Results: During the three-year period, we analyzed 110,447 patient encounters, representing 39 million national visits. Of all ED visits, 2.6% received diagnostic ultrasound. Presenting to the ED “off hours” (defined as Monday through Friday 7pm to 7am and weekends) was associated with a lower rate of ultrasound use independent of potential confounders (odds ratio [OR] 0.73, 95% confidence interval [CI]: 0.65 - 0.82). Patients at increased risk of deep venous thrombosis who presented to the ED during “off hours” were also less likely to undergo diagnostic ultrasound (OR 0.34, 95% CI: 0.15 - 0.79). Similarly, patients at increased risk of ectopic pregnancy received fewer diagnostic ultrasounds during “off hours” (OR 0.56, 95% CI 0.35 - 0.91). Conclusion: In U.S. EDs, ultrasound use was lower during “off hours,” even among patient populations where its use would be strongly indicated. [West J Emerg Med. 2011;12(1):90-95.]

High-sensitivity studies of E1 and M1 transitions observed in the reaction 138Ba(gamma,gamma{'}) at energies below the one-neutron separation energy have been performed using the nearly monoenergetic and 100% linearly polarized photon beams of the HIgammaS facility. The electric dipole character of the so-called "pygmy" dipole resonance was experimentally verified for excitations from 4.0 to 8.6 MeV. The fine structure of the M1 "spin-flip" mode was observed for the first time in N=82 nuclei.

We prove a no-ghost theorem for a bosonic string propagating in Nappi-Witten spacetime. This is achieved in two steps. We first demonstrate unitarity for a class of NW/U(1) modules: the norm of any state which is primary with respect to a chosen timelike U(1) is non-negative. We then show that physical states - states satisfying the Virasoro constraints - in a class of modules of an affinisation of the Nappi-Witten algebra are contained in the NW/U(1) modules. Similar to the case of strings on $AdS_3$, in order to saturate the spectrum obtained in light-cone quantization we are led to include modules with energy not bounded from below, which are related to modules with energy bounded from below by spectral flow automorphisms., Comment: 24 pages, 1 figure

Wess-Zumino-Witten (WZW) models are abstract conformal field theories with an infinite-dimensional symmetry which accounts for their integrability, and at the same time they have a sigma-model description of closed-string propagation on group manifolds which, in turn, endows the models with an intuitive geometric meaning. We exploit this dual algebraic and geometric property of WZW models to construct an explicit example of a field-dependent reflection matrix for open strings in the Nappi-Witten model. Demanding the momentum outflow at the boundary to be zero determines a certain combination of the left and right chiral currents at the boundary. This same reflection matrix is obtained algebraically from an inner automorphism, giving rise to a space-filling D-brane. Half of the infinite-dimensional affine Kac-Moody symmetry present in the closed-string theory is preserved by this unique combination of the left and the right chiral currents. The operator-product expansions of these boundary currents are computed explicitly and they are shown to obey the same current algebra as those of the closed-string chiral currents. Different choices of the inner automorphisms correspond to different background gauge field configurations. Only those B-field configurations, and the corresponding D-branes, that preserve the diagonal part of the infinite-dimensional chiral algebras are allowed. In this way the existence of the D-branes in curved spaces is further constrained by the underlying symmetry of the ambient spacetime.

WZW models are abstract conformal field theories with an infinite dimensional symmetry which accounts for their integrability, and at the same time they have a sigma model description of closed string propagation on group manifolds which, in turn, endows the models with an intuitive geometric meaning. We exploit this dual algebraic and geometric property of WZW models to construct an explicit example of a field-dependent reflection matrix for open-strings in Nappi-Witten model. Demanding the momentum outflow at the boundary to be zero determines a certain combination of the left and right chiral currents at the boundary. This same reflection matrix is obtained algebraically from an inner automorphism, giving rise to a space-filling D-brane. Half of the infinite dimensional affine Kac-Moody symmetry present in the closed-string theory is preserved by this unique combination of the left and the right chiral currents. The OPEs of these boundary currents are computed explicitly and they are showed to obey the same current algebra as those of the closed-string chiral currents. Different choices of the inner automorphisms correspond to different background gauge field configurations. Only those B-field configurations, and the corresponding D-branes, that preserve the diagonal part of the infinite dimensional chiral algebras are allowed. In this way the existence of the D-branes in curved spaces is further constrained by the underlying symmetry of the ambient spacetime., Comment: title and abstract changed; improved presentation; references added; version to appear in PRD

Remarkable fit of galaxy rotation curves is achieved using a simple model from string theory. The rotation curves of the same group of galaxies are also fit using dark matter model with the generalized Navarro-Frenk-White profile for comparison. String model utilizes three free parameters vs five in the dark matter model. The average chi-squared of the string model fit is 1.649 while that of the dark matter model is 1.513. The generalized NFW profile fits marginally better at a price of two more free parameters.

This is a speculative attempt to connect string theory with cosmological observation. Inspired by an exactly solvable model in string theory, and based on the assumption that all matter is made of strings, individual stars will couple universally to this string gauge field and execute Landau orbits, much like electrons in an external magnetic field. This three-parameter phenomenological model can adequately fit the galaxy rotation curves. The extra centripetal acceleration provided by the background field can hence account for the "missing mass" needed to sustain the high rotation speed beyond the bulk of the stellar mass. The rotation speed of the stars on the outskirts of a galaxy is predicted to be linearly rising with distance., 15 pages, 1 table, 3 figures,1 appendix, LaTex in JHEP style