Physical Review D (Particles, Fields, Gravitation, and Cosmology) -- 15 November 2009

Search for gravitational-wave bursts in the first year of the fifth LIGO science run

B. P. Abbott et al. (LIGO Scientific Collaboration)

Published 11 November 2009 (26 pages)

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We present the results obtained from an all-sky search for gravitational-wave (GW) bursts in the 64–2000 Hz frequency range in data collected by the LIGO detectors during the first year (November 2005—November 2006) of their fifth science run. The total analyzed live time was 268.6 days. Multiple hierarchical data analysis methods were invoked in this search. The overall sensitivity expressed in terms of the root-sum-square (rss) strain amplitude hrss for gravitational-wave bursts with various morphologies was in the range of 6×10^-22 Hz^-1/2 to a few×10^-21 Hz^-1/2. No GW signals were observed and a frequentist upper limit of 3.75 events per year on the rate of strong GW bursts was placed at the 90% confidence level. As in our previous searches, we also combined this rate limit with the detection efficiency for selected waveform morphologies to obtain event rate versus strength exclusion curves. In sensitivity, these exclusion curves are the most stringent to date.

Search for high frequency gravitational-wave bursts in the first calendar year of LIGO's fifth science run

B. P. Abbott et al. (LIGO Scientific Collaboration)

Published 11 November 2009 (14 pages)

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We present an all-sky search for gravitational waves in the frequency range 1 to 6 kHz during the first calendar year of LIGO's fifth science run. This is the first untriggered LIGO burst analysis to be conducted above 3 kHz. We discuss the unique properties of interferometric data in this regime. 161.3 days of triple-coincident data were analyzed. No gravitational events above threshold were observed and a frequentist upper limit of 5.4 year^-1 on the rate of strong gravitational-wave bursts was placed at a 90% confidence level. Implications for specific theoretical models of gravitational-wave emission are also discussed.

Performance of arm locking in LISA

Kirk McKenzie, Robert E. Spero, and Daniel A. Shaddock

Published 16 November 2009 (28 pages)

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For the Laser Interferometer Space Antenna (LISA) to reach its design sensitivity, the coupling of the free-running laser frequency noise to the signal readout must be reduced by more than 14 orders of magnitude. One technique employed to reduce the laser frequency noise will be arm locking, where the laser frequency is locked to the LISA arm length. In this paper we detail an implementation of arm locking. We investigate orbital effects (changing arm lengths and Doppler frequencies), the impact of errors in the Doppler knowledge that can cause pulling of the laser frequency, and the noise limit of arm locking. Laser frequency pulling is examined in two regimes: at lock acquisition and in steady state. The noise performance of arm locking is calculated with the inclusion of the dominant expected noise sources: ultrastable oscillator (clock) noise, spacecraft motion, and shot noise. We find that clock noise and spacecraft motion limit the performance of dual arm locking in the LISA science band. Studying these issues reveals that although dual arm locking [A. Sutton and D. A. Shaddock, Phys. Rev. D 78, 082001 (2008)] has advantages over single (or common) arm locking in terms of allowing high gain, it has disadvantages in both laser frequency pulling and noise performance. We address this by proposing a modification to the dual arm-locking sensor, a hybrid of common and dual arm-locking sensors. This modified dual arm-locking sensor has the laser frequency pulling characteristics and low-frequency noise coupling of common arm locking, but retains the control system advantages of dual arm locking. We present a detailed design of an arm-locking controller and perform an analysis of the expected performance when used with and without laser prestabilization. We observe that the sensor phase changes beneficially near unity-gain frequencies of the arm-locking controller, allowing a factor of 10 more gain than previously believed, without degrading stability. With a time-delay error of 3 ns (equivalent of 1 m interspacecraft ranging error), time-delay interferometry (TDI) is capable of suppressing 300 Hz/sqrt(Hz) of laser frequency noise to the required level. We show that if no interspacecraft laser links fail, arm locking alone surpasses this noise performance for the entire mission. If one interspacecraft laser link fails, arm locking alone will achieve this performance for all but approximately 1 h per year, when the arm length mismatch of the two remaining arms passes through zero. Therefore, the LISA sensitivity can be realized with arm locking and time-delay interferometry only, without any form of prestabilization.

Bayesian reconstruction of gravitational wave burst signals from simulations of rotating stellar core collapse and bounce

Christian Röver, Marie-Anne Bizouard, Nelson Christensen, Harald Dimmelmeier, Ik Siong Heng, and Renate Meyer

Published 17 November 2009 (17 pages)

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Presented in this paper is a technique that we propose for extracting the physical parameters of a rotating stellar core collapse from the observation of the associated gravitational wave signal from the collapse and core bounce. Data from interferometric gravitational wave detectors can be used to provide information on the mass of the progenitor model, precollapse rotation, and the nuclear equation of state. We use waveform libraries provided by the latest numerical simulations of rotating stellar core collapse models in general relativity, and from them create an orthogonal set of eigenvectors using principal component analysis. Bayesian inference techniques are then used to reconstruct the associated gravitational wave signal that is assumed to be detected by an interferometric detector. Posterior probability distribution functions are derived for the amplitudes of the principal component analysis eigenvectors, and the pulse arrival time. We show how the reconstructed signal and the principal component analysis eigenvector amplitude estimates may provide information on the physical parameters associated with the core collapse event.

Demagnified gravitational waves from cosmological double neutron stars and gravitational wave foreground cleaning around 1 Hz

Naoki Seto

Published 2 November 2009 (8 pages)

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Gravitational waves (GWs) from cosmological double neutron star binaries (NS+NS) can be significantly demagnified by the strong gravitational lensing effect, and the proposed future missions such as the Big Bang Observer or Deci-hertz Interferometer Gravitational Wave Observatory might miss some of the demagnified GW signals below a detection threshold. The undetectable binaries would form a GW foreground, which might hamper detection of a very weak primordial GW signal. We discuss the outlook of this potential problem, using a simple model based on the singular isothermal sphere lens profile. Fortunately, it is expected that, for a presumable merger rate of NS+NSs, the residual foreground would be below the detection limit OmegaGW,lim~10^-16 realized with the Big Bang Observer/Deci-hertz Interferometer Gravitational Wave Observatory by correlation analysis.

The Gödel universe: Exact geometrical optics and analytical investigations on motion

Frank Grave, Michael Buser, Thomas Müller, Günter Wunner, and Wolfgang P. Schleich

Published 5 November 2009 (19 pages)

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In this work we derive the analytical solution of the geodesic equations of Gödel's universe for both particles and light in a special set of coordinates, which reveals the physical properties of this spacetime in a very transparent way. We also recapitulate the equations of isometric transport for points and derive the solution for Gödel's universe. The equations of isometric transport for vectors are introduced and solved. We utilize these results to transform different classes of curves along Killing vector fields. In particular, we generate nontrivial closed timelike curves from circular closed timelike curves. The results can serve as a starting point for egocentric visualizations in the Gödel universe.

Reconstructing the neutron-star equation of state from astrophysical measurements

Feryal Özel and Dimitrios Psaltis

Published 5 November 2009 (7 pages)

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The properties of matter at ultrahigh densities, low temperatures, and with a significant asymmetry between protons and neutrons can be studied exclusively through astrophysical observations of neutron stars. We show that measurements of the masses and radii of neutron stars can lead to tight constraints on the pressure of matter at three fiducial densities, from 1.85 to 7.4 times the density of nuclear saturation, in a manner that is largely model independent and that captures the key characteristics of the equation of state. We demonstrate that observations with 10% uncertainties of at least three neutron stars can lead to measurements of the pressure at these fiducial densities with an accuracy of 0.11 dex or ~=30%. Observations of three neutron stars with 5% uncertainties are sufficient to distinguish at a better than 3sigma confidence level between currently proposed equations of state. In the electromagnetic spectrum, such accurate measurements will become possible for weakly magnetic neutron stars during thermonuclear flashes and in quiescence with future missions such as the International X-ray Observatory.

Present and future gamma-ray probes of the Cygnus OB2 environment

Luis A. Anchordoqui, Haim Goldberg, Russell D. Moore, Sergio Palomares-Ruiz, Diego F. Torres, and Thomas J. Weiler

Published 6 November 2009 (6 pages)

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The MAGIC Collaboration has provided new observational data pertaining to the TeV J2032+4130 gamma-ray source (within the Cygnus OB2 region), for energies Egamma>400 GeV. It is then appropriate to update the impact of these data on gamma-ray production mechanisms in stellar associations. We consider two mechanisms of gamma-ray emission, pion production and decay (PION) and photoexcitation of high-energy nuclei followed by prompt photoemission from the daughter nuclei (A^{[small star, filled]}). We find that while the data can be accommodated with either scenario, the A^{[small star, filled]} features a spectral bump, corresponding to the threshold for exciting the giant dipole resonance, which can serve to discriminate between them. We comment on neutrino emission and detection from the region if the PION and/or A^{[small star, filled]} processes are operative. We also touch on the implications for this analysis of future Fermi and Čerenkov Telescope array data.

Determination of the neutrino flavor ratio at the astrophysical source

Kwang-Chang Lai, Guey-Lin Lin, and T. C. Liu

Published 11 November 2009 (11 pages)

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We discuss the reconstruction of neutrino flavor ratios at astrophysical sources through the future neutrino-telescope measurements. Taking the ranges of neutrino mixing parameters thetaij as those given by the current global fit, we demonstrate by a statistical method that the accuracies in the measurements of energy-independent ratios R[equivalent]phi(nuµ)/(phi(nue)+phi(nutau)) and S[equivalent]phi(nue)/phi(nutau) among integrated neutrino flux should both be better than 10% in order to distinguish between the pion source and the muon-damped source at the 3sigma level. The 10% accuracy needed for measuring R and S requires an improved understanding of the background atmospheric neutrino flux to a better than 10% level in the future. We discuss the applicability of our analysis to practical situations that the diffuse astrophysical neutrino flux arises from different types of sources and each point source has a neutrino flavor ratio varying with energies. We also discuss the effect of the leptonic CP phase on the flavor-ratio reconstruction.

Sensitivity of cosmic-ray experiments to ultrahigh-energy photons: Reconstruction of the spectrum and limits on the superheavy dark matter

O. E. Kalashev, G. I. Rubtsov, and S. V. Troitsky

Published 16 November 2009 (9 pages)

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We estimate the sensitivity of various experiments detecting ultrahigh-energy cosmic rays to primary photons with energies above 10¹⁹ eV. We demonstrate that the energy of a primary photon may be significantly (up to a factor of ~10) underestimated or overestimated for particular primary energies and arrival directions. We consider distortion of the reconstructed cosmic-ray spectrum for the photonic component. As an example, we use these results to constrain the parameter space of models of superheavy dark matter by means of both the observed spectra and available limits on the photon content. We find that a significant contribution of ultrahigh-energy particles (photons and protons) from decays of superheavy dark matter is allowed by all these constraints.

Effect of long-lived strongly interacting relic particles on big bang nucleosynthesis

Motohiko Kusakabe, Toshitaka Kajino, Takashi Yoshida, and Grant J. Mathews

Published 2 November 2009 (17 pages)

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It has been suggested that relic long-lived strongly interacting massive particles (SIMPs, or X particles) existed in the early universe. We study effects of such long-lived unstable SIMPs on big bang nucleosynthesis (BBN) assuming that such particles existed during the BBN epoch, but then decayed long before they could be detected. The interaction strength between an X particle and a nucleon is assumed to be similar to that between nucleons. We then calculate BBN in the presence of the unstable neutral charged X⁰ particles taking into account the capture of X⁰ particles by nuclei to form X nuclei. We also study the nuclear reactions and beta decays of X nuclei. We find that SIMPs form bound states with normal nuclei during a relatively early epoch of BBN. This leads to the production of heavy elements which remain attached to them. Constraints on the abundance of X⁰ particles during BBN are derived from observationally inferred limits on the primordial light element abundances. Particle models which predict long-lived colored particles with lifetimes longer than ~200 s are rejected based upon these constraints.

Nonthermal production of WIMPs, cosmic e^± excesses, and gamma rays from the Galactic Center

Xiao-Jun Bi, Robert Brandenberger, Paolo Gondolo, Tianjun Li, Qiang Yuan, and Xinmin Zhang

Published 4 November 2009 (10 pages)

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In this paper we propose a dark matter model and study aspects of its phenomenology. Our model is based on a new dark matter sector with a U(1)^[prime] gauge symmetry plus a discrete symmetry added to the standard model of particle physics. The new fields of the dark matter sector have no hadronic charges and couple only to leptons. Our model cannot only give rise to the observed neutrino mass hierarchy, but can also generate the baryon number asymmetry via nonthermal leptogenesis. The breaking of the new U(1)^[prime] symmetry produces cosmic strings. The dark matter particles are produced nonthermally from cosmic string loop decay which allows one to obtain sufficiently large annihilation cross sections to explain the observed cosmic ray positron and electron fluxes recently measured by the PAMELA, ATIC, PPB-BETS, Fermi-LAT, and HESS experiments while maintaining the required overall dark matter energy density. The high velocity of the dark matter particles from cosmic string loop decay leads to a low phase space density and thus to a dark matter profile with a constant density core in contrast to what happens in a scenario with thermally produced cold dark matter where the density keeps rising towards the center. As a result, the flux of gamma rays radiated from the final leptonic states of dark matter annihilation from the Galactic center is suppressed and satisfies the constraints from the HESS gamma-ray observations.

Archipelagian cosmology: Dynamics and observables in a universe with discretized matter content

Timothy Clifton and Pedro G. Ferreira

Published 5 November 2009 (26 pages)

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We consider a model of the Universe in which the matter content is in the form of discrete islands, rather than a continuous fluid. In the appropriate limits the resulting large-scale dynamics approach those of a Friedmann-Robertson-Walker (FRW) universe. The optical properties of such a space-time, however, do not. This illustrates the fact that the optical and “average” dynamical properties of a relativistic universe are not equivalent, and do not specify each other uniquely. We find the angular diameter distance, luminosity distance, and redshifts that would be measured by observers in these space-times, using both analytic approximations and numerical simulations. While different from their counterparts in FRW, the effects found do not look like promising candidates to explain the observations usually attributed to the existence of dark energy. This incongruity with standard FRW cosmology is not due to the existence of any unexpectedly large structures or voids in the Universe, but only to the fact that the matter content of the Universe is not a continuous fluid.

Inflation, baryogenesis, and gravitino dark matter at ultralow reheat temperatures

Kazunori Kohri, Anupam Mazumdar, and Narendra Sahu

Published 5 November 2009 (5 pages)

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It is quite possible that the reheat temperature of the Universe is extremely low close to the scale of big bang nucleosynthesis, i.e. TR~1–10 MeV. At such low reheat temperatures generating matter, antimatter asymmetry and synthesizing dark matter particles are challenging issues which need to be addressed within a framework of beyond the standard model physics. In this paper we point out that a successful cosmology can emerge naturally provided the R-parity violating interactions are responsible for the excess in baryons over antibaryons and at the same time they can explain the longevity of dark matter with the right abundance.

Exact cosmological solutions with nonminimal derivative coupling

Sergey V. Sushkov

Published 5 November 2009 (6 pages)

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We consider a gravitational theory of a scalar field phi with nonminimal derivative coupling to curvature. The coupling terms have the form kappa1Rphi,µphi^,µ and kappa2Rµnuphi^,µphi^,nu, where kappa1 and kappa2 are coupling parameters with dimensions of length squared. In general, field equations of the theory contain third derivatives of gµnu and phi. However, in the case -2kappa1=kappa2[equivalent]kappa, the derivative coupling term reads kappaGµnuphi^,µphi^,nu and the order of corresponding field equations is reduced up to second one. Assuming -2kappa1=kappa2, we study the spatially-flat Friedman-Robertson-Walker model with a scale factor a(t) and find new exact cosmological solutions. It is shown that properties of the model at early stages crucially depend on the sign of kappa. For negative kappa, the model has an initial cosmological singularity, i.e., a(t)~(t-ti)^2/3 in the limit t-->ti; and for positive kappa, the Universe at early stages has the quasi-de Sitter behavior, i.e., a(t)~e^Ht in the limit t-->-[infinity], where H=(3sqrt( kappa ))^-1. The corresponding scalar field phi is exponentially growing at t-->-[infinity], i.e., phi(t)~e^{-t/sqrt( kappa )}. At late stages, the Universe evolution does not depend on kappa at all; namely, for any kappa one has a(t)~t^1/3 at t-->[infinity]. Summarizing, we conclude that a cosmological model with nonminimal derivative coupling of the form kappaGµnuphi^,µphi^,nu is able to explain in a unique manner both a quasi-de Sitter phase and an exit from it without any fine-tuned potential.

Incompatibility of rotation curves with gravitational lensing for TeVeS theory

Ignacio Ferreras, Nick E. Mavromatos, Mairi Sakellariadou, and Muhammad Furqaan Yusaf

Published 6 November 2009 (10 pages)

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We constrain the one-parameter (alpha) class of TeVeS models by testing the theory against both rotation curve and strong gravitational lensing data on galactic scales, remaining fully relativistic in our formalism. The upshot of our analysis is that—at least in its simplest original form, which is the only one studied in the literature so far—TeVeS is ruled out, in the sense that the models cannot consistently fit simultaneously the two sets of data without including a significant dark matter component. It is also shown that the details of the underlying cosmological model are not relevant for our analysis, which pertains to galactic scales. The choice of the stellar initial mass function—which affects the estimates of baryonic mass—is found not to change our conclusions.

Gauge-invariant construction of quantum cosmology

Fumitoshi Amemiya and Tatsuhiko Koike

Published 6 November 2009 (9 pages)

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We present and analyze a gauge-invariant quantum theory of the Friedmann-Robertson-Walker universe with dust. We construct the reduced phase space spanned by gauge-invariant quantities by using the so-called relational formalism at the classical level. The reduced phase space thereby obtained can be quantized in the same manner as an ordinary mechanical system. We carry out the quantization and obtain the Schrödinger equation. This quantization procedure realizes a possible resolution to the problem of time and observables in canonical quantum gravity. We analyze the classical initial singularity of the theory by evolving a wave packet backward in time and evaluating the expectation value of the scale factor. It is shown that the initial singularity of the Universe is avoided by the quantum gravitational effects.

Unified model of k-inflation, dark matter, and dark energy

Nilok Bose and A. S. Majumdar

Published 10 November 2009 (7 pages)

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We present a k-essence model where a single scalar field is responsible for the early expansion of the Universe through the process of k inflation and at appropriate subsequent stages acts both as dark matter and dark energy. The Lagrangian contains a potential for the scalar field as well as a noncanonical kinetic term, and is of the form F(X)V(phi) which has been widely used as a k-essence Lagrangian. After the period of inflation is over the model can be approximated as purely kinetic k essence, generating dark matter and dark energy at late times. We show how observational results are used to put constraints on the parameters of this model.

Inflation and dark energy from three-forms

Tomi S. Koivisto and Nelson J. Nunes

Published 12 November 2009 (18 pages)

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Three-forms can give rise to viable cosmological scenarios of inflation and dark energy with potentially observable signatures distinct from standard single scalar field models. In this study, the background dynamics and linear perturbations of self-interacting three-form cosmology are investigated. The phase space of cosmological solutions possesses (super)-inflating attractors and saddle points, which can describe three-form driven inflation or dark energy. The quantum generation and the classical evolution of perturbations is considered. The scalar and tensor spectra from a three-form inflation and the impact from the presence of a three-form on matter perturbations are computed. Stability properties and equivalence of the model with alternative formulations are discussed.

Multimessenger constraints on the annihilating dark matter interpretation of the positron excess

Miguel Pato, Lidia Pieri, and Gianfranco Bertone

Published 13 November 2009 (15 pages)

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The rise in the energy spectrum of the positron ratio, observed by the PAMELA satellite above 10 GeV, and other cosmic-ray measurements, have been interpreted as a possible signature of dark matter annihilation in the Galaxy. However, the large number of free parameters, and the large astrophysical uncertainties, make it difficult to draw conclusive statements about the viability of this scenario. Here, we perform a multiwavelength, multimessenger analysis, that combines in a consistent way the constraints arising from different astrophysical observations. We show that if standard assumptions are made for the distribution of dark matter (we build models on the recent Via Lactea II and Aquarius simulations) and the propagation of cosmic rays, current dark matter models cannot explain the observed positron flux without exceeding the observed fluxes of antiprotons or gamma-ray and radio photons. To visualize the multimessenger constraints, we introduce “star plots,” a graphical method that shows in the same plot theoretical predictions and observational constraints for different messengers and wavelengths.

Sterile neutrinos produced near the electroweak scale: Mixing angles, MSW resonances, and production rates

Jun Wu, Chiu Man Ho, and Daniel Boyanovsky

Published 13 November 2009 (25 pages)

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We study the production of sterile neutrinos in the region T~MW in an extension beyond the standard model with the seesaw mass matrix originating in Yukawa couplings to Higgs-like scalars with masses and vev's of the order of the electroweak scale. Sterile neutrinos are produced by the decay of scalars and standard model vector bosons. We obtain the index of refraction, dispersion relations, mixing angles in the medium and production rates including those for right-handed sterile neutrinos, from the standard model and beyond the standard model self-energies. For 1<~MW/T<~3 we find narrow MSW resonances with k<~T for both left- and right-handed neutrinos even in absence of a lepton asymmetry in the (active) neutrino sector, as well as very low energy (k/T<<|xi|) narrow MSW resonances in the presence of a lepton asymmetry consistent with the bounds from Wilkinson Microwave Anisotropy Probe and Big Bang Nucleosynthesis. For small vacuum mixing angle, consistent with observational bounds, the absorptive part of the self-energies lead to a strong damping regime very near the resonances resulting in the exact degeneracy of the propagating modes with a concomitant breakdown of adiabaticity. We argue that cosmological expansion sweeps through the resonances, resonant and nonresonant sterile neutrino production results in a highly nonthermal distribution function enhanced at small momentum k<T, with potentially important consequences for their free-streaming length and transfer function at small scales.

Inflaton two-point correlation in the presence of a cosmic string

Chien-Yao Tseng and Mark B. Wise

Published 13 November 2009 (6 pages)

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Precise measurements of the microwave background anisotropy have confirmed the inflationary picture of approximately scale invariant, Gaussian primordial adiabatic density perturbations. However, there are some anomalies that suggest a small violation of rotational and/or translational invariance in the mechanism that generates the primordial density fluctuations. Motivated by this we study the two-point correlation of a massless scalar (the inflaton) when the stress tensor contains the energy density from an infinitely long straight cosmic string in addition to a cosmological constant.

Assisted dark energy

Junko Ohashi and Shinji Tsujikawa

Published 16 November 2009 (13 pages)

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Cosmological scaling solutions, which give rise to a scalar-field density proportional to a background fluid density during radiation and matter eras, are attractive to alleviate the energy scale problem of dark energy. In the presence of multiple scalar fields, the scaling solution can exit to the epoch of cosmic acceleration through the so-called assisted inflation mechanism. We study cosmological dynamics of a multifield system in details with a general Lagrangian density p=[summation]i = 1ⁿXig(Xie^lambdaiphii), where Xi=-([del]phii)²/2 is the kinetic energy of the ith field phii, lambdai is a constant, and g is an arbitrary function in terms of Yi=Xie^lambdaiphii. This covers most of the scalar-field models of dark energy proposed in literature that possess scaling solutions. Using the bound coming from big-bang nucleosynthesis and the condition under which each field cannot drive inflation as a single component of the universe, we find the following features: (i) a transient or eternal cosmic acceleration can be realized after the scaling matter era, (ii) a thawing property of assisting scalar fields is crucial to determine the evolution of the field equation of state wphi, and (iii) the field equation of state today can be consistent with the observational bound wphi<-0.8 in the presence of multiple scalar fields.

Constraint on coupled dark energy models from observations

Jun-Qing Xia

Published 16 November 2009 (9 pages)

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The coupled dark energy models, in which the quintessence scalar field nontrivially couples to the cold dark matter, have been proposed to explain the coincidence problem. In this paper we study the perturbations of coupled dark energy models and the effects of this interaction on the current observations. Here, we pay particular attention to its imprint on the late-time integrated Sachs-Wolfe effect. We perform a global analysis of the constraints on this interaction from the current observational data. Considering the typical exponential form as the interaction form, we obtain that the strength of interaction between dark sectors is constrained as beta<0.085 at 95% confidence level. Furthermore, we find that future measurements with smaller error bars could improve the constraint on the strength of coupling by a factor of 2, when compared to the present constraints.

Holes in the static Einstein universe and a model of the cosmological voids

Andrzej Odrzywo

ek

Published 17 November 2009 (10 pages)

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A spherically symmetric, static model of the cosmological voids is constructed in the framework of the Tolman-Oppenheimer-Volkov equation with the cosmological constant. Extension of the Tooper result (dimensionless form of the TOV equation) is provided for nonzero Lambda. Then, the equation is simplified in alpha-->0, lambda-->0, and lambda/alpha=const regime, suitable for largest structures in Lambda-dominated universe. Voids are treated as underdensity regions in the static Einstein universe. Both overdensity and underdensity (relative to static universe) solutions exist. They are identified with standard astrophysical spherical objects and voids, respectively. The model is tested against observed properties (the radius—the central density relation) and density profiles of voids. Analytical formulas for radial density contrast profile and radii of the voids are derived. Some consequences for cosmological N-body simulations are suggested. Hints on the dark matter/dark energy EOS filling the voids are provided.

CMB lensing constraints on dark energy and modified gravity scenarios

Erminia Calabrese, Asantha Cooray, Matteo Martinelli, Alessandro Melchiorri, Luca Pagano, Anže Slosar, and George F. Smoot

Published 19 November 2009 (10 pages)

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Weak gravitational lensing leaves a characteristic imprint on the cosmic microwave background temperature and polarization angular power spectra. Here, we investigate the possible constraints on the integrated lensing potential from future cosmic microwave background angular spectra measurements expected from Planck and EPIC. We find that Planck and EPIC will constrain the amplitude of the integrated projected potential responsible for lensing at 6% and 1% level, respectively, with very little sensitivity to the shape of the lensing potential. We discuss the implications of such a measurement in constraining dark energy and modified gravity scalar-tensor theories. We then discuss the impact of a wrong assumption on the weak lensing potential amplitude on cosmological parameter inference.

Impact of a strongly first-order phase transition on the abundance of thermal relics

Carroll Wainwright and Stefano Profumo

Published 19 November 2009 (11 pages)

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We study the impact of a strongly first-order electroweak phase transition on the thermal relic abundance of particle species that could constitute the dark matter and that decoupled before the phase transition occurred. We define a dilution factor induced by generic first-order phase transitions, and we explore the parameter space of the minimal supersymmetric extension to the standard model to determine which phase transition temperatures and dilution factors are relevant for the lightest neutralino as a dark matter candidate. We then focus on a specific toy-model setup that could give rise to a strongly first-order electroweak phase transition, and proceed to a detailed calculation of dilution factors and transition temperatures, comparing our findings to actual neutralino dark matter models. Typical models that would produce an excessive thermal relic density and that can be salvaged postulating a strongly first-order electroweak phase transition include massive (multi-TeV) wino or Higgsino-like neutralinos, as well as binolike neutralinos in a wider mass range, with masses as low as 400 GeV. If LHC data indicate an inferred thermal neutralino relic abundance larger than the cold dark matter density, the mismatch could thus potentially be explained by electroweak-scale physics that will also be thoroughly explored with collider experiments in the near future.

Cosmology with unparticles

Bohdan Grzadkowski and José Wudka

Published 20 November 2009 (10 pages)

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We discuss cosmological consequences of the existence of physics beyond the standard model that exhibits Banks-Zaks and unparticle behavior in the UV and IR, respectively. We first derive the equation of state for unparticles and use it to obtain the temperature dependence of the corresponding energy and entropy densities. We then formulate the Boltzmann and Kubo equations for both the unparticles and the Banks-Zaks particles, and use these results to determine the equilibrium conditions between the standard model and the new physics. We conclude by obtaining the constraints on the effective number of degrees of freedom of unparticles imposed by big-bang nucleosynthesis.

Turning big bang into big bounce. I. Classical dynamics

kiewicz, and W

Published 4 November 2009 (8 pages)

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The big bounce (BB) transition within a flat Friedmann-Robertson-Walker model is analyzed in the setting of loop geometry underlying the loop cosmology. We solve the constraint of the theory at the classical level to identify physical phase space and find the Lie algebra of the Dirac observables. We express energy density of matter and geometrical functions in terms of the observables. It is the modification of classical theory by the loop geometry that is responsible for BB. The classical energy scale specific to BB depends on a parameter that should be fixed either by cosmological data or determined theoretically at quantum level, otherwise the energy scale stays unknown.

Equivalence principle implications of modified gravity models

Lam Hui, Alberto Nicolis, and Christopher W. Stubbs

Published 5 November 2009 (26 pages)

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Theories that attempt to explain the observed cosmic acceleration by modifying general relativity all introduce a new scalar degree of freedom that is active on large scales, but is screened on small scales to match experiments. We demonstrate that if such screening occurs via the chameleon mechanism, such as in f(R) theory, it is possible to have order unity violation of the equivalence principle, despite the absence of explicit violation in the microscopic action. Namely, extended objects such as galaxies or constituents thereof do not all fall at the same rate. The chameleon mechanism can screen the scalar charge for large objects but not for small ones (large/small is defined by the depth of the gravitational potential and is controlled by the scalar coupling). This leads to order one fluctuations in the ratio of the inertial mass to gravitational mass. We provide derivations in both Einstein and Jordan frames. In Jordan frame, it is no longer true that all objects move on geodesics; only unscreened ones, such as test particles, do. In contrast, if the scalar screening occurs via strong coupling, such as in the Dvali-Gabadadze-Porrati braneworld model, equivalence principle violation occurs at a much reduced level. We propose several observational tests of the chameleon mechanism: 1. small galaxies should accelerate faster than large galaxies, even in environments where dynamical friction is negligible; 2. voids defined by small galaxies would appear larger compared to standard expectations; 3. stars and diffuse gas in small galaxies should have different velocities, even if they are on the same orbits; 4. lensing and dynamical mass estimates should agree for large galaxies but disagree for small ones. We discuss possible pitfalls in some of these tests. The cleanest is the third one where the mass estimate from HI rotational velocity could exceed that from stars by 30% or more. To avoid blanket screening of all objects, the most promising place to look is in voids.

Asymptotically free scalar curvature-ghost coupling in quantum Einstein gravity

Astrid Eichhorn, Holger Gies, and Michael M. Scherer

Published 5 November 2009 (8 pages)

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We consider the asymptotic-safety scenario for quantum gravity which constructs a nonperturbatively renormalizable quantum gravity theory with the help of the functional renormalization group (RG). We verify the existence of a non-Gaussian fixed point and include a running curvature-ghost coupling as a first step towards the flow of the ghost sector of the theory. We find that the scalar curvature-ghost coupling is asymptotically free and RG relevant in the ultraviolet. Most importantly, the property of asymptotic safety discovered so far within the Einstein-Hilbert truncation and beyond remains stable under the inclusion of the ghost flow.

Smooth lattice construction of the Oppenheimer-Snyder spacetime

Leo Brewin and Jules Kajtar

Published 5 November 2009 (18 pages)

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+ -	Show Abstract We present test results for the smooth lattice method using an Oppenheimer-Snyder spacetime. The results are in excellent agreement with theory and numerical results from other authors.

Large-scale structure in brane-induced gravity. II. Numerical simulations

Kwan Chuen Chan and Román Scoccimarro

Published 6 November 2009 (19 pages)

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We use N-body simulations to study the nonlinear structure formation in brane-induced gravity, developing a new method that requires alternate use of Fast Fourier Transforms and relaxation. This enables us to compute the nonlinear matter power spectrum and bispectrum, the halo mass function, and the halo bias. From the simulation results, we confirm the expectations based on analytic arguments that the Vainshtein mechanism does operate as anticipated, with the density power spectrum approaching that of standard gravity within a modified background evolution in the nonlinear regime. The transition is very broad and there is no well defined Vainshtein scale, but roughly this corresponds to k*~=2h Mpc^-1 at redshift z=1 and k*~=1h Mpc^-1 at z=0. We checked that while extrinsic curvature fluctuations go nonlinear, and the dynamics of the brane-bending mode C receives important nonlinear corrections, this mode does get suppressed compared to density perturbations, effectively decoupling from the standard gravity sector. At the same time, there is no violation of the weak field limit for metric perturbations associated with C. We find good agreement between our measurements and the predictions for the nonlinear power spectrum presented in paper I, that rely on a renormalization of the linear spectrum due to nonlinearities in the modified gravity sector. A similar prediction for the mass function shows the right trends. Our simulations also confirm the induced change in the bispectrum configuration dependence predicted in paper I.

Large-scale structure in brane-induced gravity. I. Perturbation theory

Román Scoccimarro

Published 6 November 2009 (26 pages)

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We study the growth of subhorizon perturbations in brane-induced gravity using perturbation theory. We solve for the linear evolution of perturbations taking advantage of the symmetry under gauge transformations along the extra-dimension to decouple the bulk equations in the quasistatic approximation, which we argue may be a better approximation at large scales than thought before. We then study the nonlinearities in the bulk and brane equations, concentrating on the workings of the Vainshtein mechanism by which the theory becomes general relativity (GR) at small scales. We show that at the level of the power spectrum, to a good approximation, the effect of nonlinearities in the modified gravity sector may be absorbed into a renormalization of the gravitational constant. Since the relation between the lensing potential and density perturbations is entirely unaffected by the extra physics in these theories, the modified gravity can be described in this approximation by a single function, an effective gravitational constant for nonrelativistic motion that depends on space and time. We develop a resummation scheme to calculate it, and provide predictions for the nonlinear power spectrum. At the level of the large-scale bispectrum, the leading order corrections are obtained by standard perturbation theory techniques, and show that the suppression of the brane-bending mode leads to characteristic signatures in the non-Gaussianity generated by gravity, generic to models that become GR at small scales through second-derivative interactions. We compare the predictions in this work to numerical simulations in a companion paper.

Interaction of the Barbero-Immirzi field with matter and pseudoscalar perturbations

Simone Mercuri and Victor Taveras

Published 6 November 2009 (11 pages)

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In loop quantum gravity the classical point of departure is the Einstein-Hilbert action modified by the addition of the so-called Holst term. Classically, this term does not affect the equations of motion, but it induces a well-known quantization ambiguity in the quantum theory, parametrized by the Barbero-Immirzi parameter. Recently, it has been suggested to promote the Barbero-Immirzi parameter to a field. The resulting theory, obtainable starting from the usual Holst action, is general relativity coupled to a pseudoscalar field. However, this theory turns out to have an unconventional kinetic term for the Barbero-Immirzi field and a rather unnatural coupling with fermions. The main goal of this work is twofold: First, to propose a further generalization of the Holst action, which yields a theory of gravity and matter with a more natural coupling to the Barbero-Immirzi field; second, to study the possible implications for cosmology correlated to the existence of this new pseudoscalar field.

Thermodynamics of charged black holes with a nonlinear electrodynamics source

Hernán A. González, Mokhtar Hassaïne, and Cristián Martínez

Published 9 November 2009 (11 pages)

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We study the thermodynamical properties of electrically charged black hole solutions of a nonlinear electrodynamics theory defined by a power p of the Maxwell invariant, which is coupled to Einstein gravity in four and higher spacetime dimensions. Depending on the range of the parameter p, these solutions present different asymptotic behaviors. We compute the Euclidean action with the appropriate boundary term in the grand canonical ensemble. The thermodynamical quantities are identified and, in particular, the mass and the charge are shown to be finite for all classes of solutions. Interestingly, a generalized Smarr formula is derived and it is shown that this latter encodes perfectly the different asymptotic behaviors of the black hole solutions. The local stability is analyzed by computing the heat capacity and the electrical permittivity and we find that a set of small black holes is locally stable. In contrast to the standard Reissner-Nordström solution, there is a first-order phase transition between a class of these nonlinear charged black holes and the Minkowski spacetime.

Ultrahigh precision cosmology from gravitational waves

Curt Cutler and Daniel E. Holz

Published 9 November 2009 (15 pages)

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We show that the Big Bang Observer (BBO), a proposed space-based gravitational-wave (GW) detector, would provide ultraprecise measurements of cosmological parameters. By detecting ~3×10⁵ compact-star binaries, and utilizing them as standard sirens, BBO would determine the Hubble constant to ~0.1%, and the dark-energy parameters w0 and wa to ~0.01 and ~0.1, respectively. BBO's dark-energy figure-of-merit would be approximately an order of magnitude better than all other proposed, dedicated dark-energy missions. To date, BBO has been designed with the primary goal of searching for gravitational waves from inflation, down to the level OmegaGW~10^-17; this requirement determines BBO's frequency band (deci-Hz) and its sensitivity requirement (strain measured to ~10^-24). To observe an inflationary GW background, BBO would first have to detect and subtract out ~3×10⁵ merging compact-star binaries, out to a redshift z~5. It is precisely this carefully measured foreground which would enable high-precision cosmology. BBO would determine the luminosity distance to each binary to ~ percent accuracy. In addition, BBO's angular resolution would be sufficient to uniquely identify the host galaxy for the majority of binaries; a coordinated optical/infrared observing campaign could obtain the redshifts. Combining the GW-derived distances and the electromagnetically-derived redshifts for such a large sample of objects, out to such high redshift, naturally leads to extraordinarily tight constraints on cosmological parameters. We emphasize that such “standard siren” measurements of cosmology avoid many of the systematic errors associated with other techniques: GWs offer a physics-based, absolute measurement of distance. In addition, we show that BBO would also serve as an exceptionally powerful gravitational-lensing mission, and we briefly discuss other astronomical uses of BBO, including providing an early warning system for all short/hard gamma-ray bursts.

Higher-dimensional solitons and black holes with a nonminimally coupled scalar field

Dominic Hosler and Elizabeth Winstanley

Published 9 November 2009 (16 pages)

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We study higher-dimensional soliton and hairy black hole solutions of the Einstein equations nonminimally coupled to a scalar field. The scalar field has no self-interaction potential but a cosmological constant is included. Nontrivial solutions exist only when the cosmological constant is negative and the constant governing the coupling of the scalar field to the Ricci scalar curvature is positive. At least some of these solutions are stable when this coupling constant is not too large.

No-go theorem for bimetric gravity with positive and negative mass

Manuel Hohmann and Mattias N. R. Wohlfarth

Published 10 November 2009 (10 pages)

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We argue that the most conservative geometric extension of Einstein gravity describing both positive and negative mass sources and observers is bimetric gravity and contains two copies of standard model matter which interact only gravitationally. Matter fields related to one of the metrics then appear dark from the point of view of an observer defined by the other metric, and so may provide a potential explanation for the dark universe. In this framework we consider the most general form of linearized field equations compatible with physically and mathematically well-motivated assumptions. Using gauge-invariant linear perturbation theory, we prove a no-go theorem ruling out all bimetric gravity theories that, in the Newtonian limit, lead to precisely opposite forces on positive and negative test masses.

Wormhole geometries in f(R) modified theories of gravity

Francisco S. N. Lobo and Miguel A. Oliveira

Published 11 November 2009 (9 pages)

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In this work, we construct traversable wormhole geometries in the context of f(R) modified theories of gravity. We impose that the matter threading the wormhole satisfies the energy conditions, so that it is the effective stress-energy tensor containing higher order curvature derivatives that is responsible for the null energy condition violation. Thus, the higher order curvature terms, interpreted as a gravitational fluid, sustain these nonstandard wormhole geometries, fundamentally different from their counterparts in general relativity. In particular, by considering specific shape functions and several equations of state, exact solutions for f(R) are found.

One-loop corrections to the photon propagator in curved-space QED

Bruno Gonçalves, Guilherme de Berredo-Peixoto, and Ilya L. Shapiro

Published 12 November 2009 (17 pages)

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We calculate and discuss the one-loop corrections to the photon sector of QED interacting to a background gravitational field. At high energies the fermion field can be taken as massless and the quantum terms can be obtained by integrating conformal anomaly. We present a covariant local expression for the corresponding effective action, similar to the one obtained earlier for the gravitational sector. At the moderate energies the quantum terms can be obtained through the heat-kernel method. In this way we derive the exact one-loop beta function for the electric charge in the momentum subtraction scheme and explore both massless and large-mass limits. The relation between the two approaches is shown and the difference discussed in view of the possible applications to cosmology and astrophysics.

Stochastic template placement algorithm for gravitational wave data analysis

I. W. Harry, B. Allen, and B. S. Sathyaprakash

Published 12 November 2009 (11 pages)

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This paper presents an algorithm for constructing matched-filter template banks in an arbitrary parameter space. The method places templates at random, then removes those which are “too close” together. The properties and optimality of stochastic template banks generated in this manner are investigated for some simple models. The effectiveness of these template banks for gravitational wave searches for binary inspiral waveforms is also examined. The properties of a stochastic template bank are then compared to the deterministically placed template banks that are currently used in gravitational wave data analysis.

Further improvements in the understanding of isotropic loop quantum cosmology

M. Martín-Benito, G. A. Mena Marugán, and J. Olmedo

Published 12 November 2009 (11 pages)

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The flat, homogeneous, and isotropic universe with a massless scalar field is a paradigmatic model in loop quantum cosmology. In spite of the prominent role that the model has played in the development of this branch of physics, there still remain some aspects of its quantization which deserve a more detailed discussion. These aspects include the kinematical resolution of the cosmological singularity, the precise relation between the solutions of the densitized and nondensitized versions of the quantum Hamiltonian constraint, the possibility of identifying superselection sectors which are as simple as possible, and a clear comprehension of the Wheeler-DeWitt (WDW) limit associated with the theory in those sectors. We propose an alternative operator to represent the Hamiltonian constraint which is specially suitable to deal with all these issues in a detailed and satisfactory way. In particular, with our constraint operator, the singularity decouples in the kinematical Hilbert space and can be removed already at this level. Thanks to this fact, we can densitize the quantum Hamiltonian constraint in a well-controlled manner. Additionally, together with the physical observables, this constraint superselects simple sectors for the universe volume, with a discrete support contained in a single semiaxis of the real line and for which the basic functions that encode the information about the geometry possess optimal physical properties. Namely, they provide a no-boundary description around the cosmological singularity and admit a well-defined WDW limit in terms of standing waves. Both properties explain the presence of a generic quantum bounce replacing the classical singularity at a fundamental level, in contrast with previous studies where the bounce was proved in concrete regimes—focusing on states with a marked semiclassical behavior—or for a simplified model.

Generalized Misner-Sharp energy in f(R) gravity

Rong-Gen Cai, Li-Ming Cao, Ya-Peng Hu, and Nobuyoshi Ohta

Published 13 November 2009 (9 pages)

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We study generalized Misner-Sharp energy in f(R) gravity in a spherically symmetric space-time. We find that unlike the cases of Einstein gravity and Gauss-Bonnet gravity, the existence of the generalized Misner-Sharp energy depends on a constraint condition in the f(R) gravity. When the constraint condition is satisfied, one can define a generalized Misner-Sharp energy, but it cannot always be written in an explicit quasilocal form. However, such a form can be obtained in a Friedmann-Robertson-Walker universe and for static spherically symmetric solutions with constant scalar curvature. In the Friedmann-Robertson-Walker universe, the generalized Misner-Sharp energy is nothing but the total matter energy inside a sphere with radius r, which acts as the boundary of a finite region under consideration. The case of scalar-tensor gravity is also briefly discussed.

Curvature corrections and topology change transition in brane-black hole systems: A perturbative approach

Viktor G. Czinner and Antonino Flachi

Published 13 November 2009 (19 pages)

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We consider curvature corrections to static, axisymmetric Dirac-Nambu-Goto membranes embedded into a spherically symmetric black hole spacetime with arbitrary number of dimensions. Since the next to leading order corrections in the effective brane action are quadratic in the brane thickness [script-l], we adopt a linear perturbation approach in [script-l]². The perturbations are general in the sense that they are not restricted to the Rindler zone nor to the near-critical solutions of the unperturbed system. As a result, an unexpected asymmetry in the perturbed system is found. In configurations, where the brane does not cross the black hole horizon, the perturbative approach does not lead to regular solutions if the number of the brane's spacetime dimensions D>3. This condition, however, does not hold for the horizon crossing solutions. Consequently we argue that the presented perturbative approach breaks down for subcritical type solutions near the axis of the system for D>3. Nevertheless, we can discuss topology-changing phase transitions in cases when D=2 or 3, i.e. when the brane is a one-dimensional string or a two-dimensional sheet, respectively. For the general case, a different, nonperturbative approach should be sought. Based on the energy properties of those branes that are quasistatically evolved from the equatorial configuration, we illustrate the results of the phase transition in the case of a D=3 brane. It is found that small thickness perturbations do not modify the order of the transition, i.e. it remains first order just as in the case of vanishing thickness.

Accretion of nonminimally coupled scalar fields into black holes

Manuela G. Rodrigues and Alberto Saa

Published 13 November 2009 (6 pages)

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By using a quasistationary approach, we consider the mass evolution of Schwarzschild black holes in the presence of a nonminimally coupled cosmological scalar field. The mass evolution equation is analytically solved for generic coupling, revealing a qualitatively distinct behavior from the minimal coupling case. In particular, for black hole masses smaller than a certain critical value, the accretion of the scalar field can lead to mass decreasing even if no phantom energy is involved. The physical validity of the adopted quasistationary approach and some implications of our result for the evolution of primordial and astrophysical black holes are discussed. More precisely, we argue that black hole observational data could be used to place constraints on the nonminimally coupled energy content of the Universe.

Shear viscosity and instability from third order Lovelock gravity

Xian-Hui Ge, Sang-Jin Sin, Shao-Feng Wu, and Guo-Hong Yang

Published 16 November 2009 (8 pages)

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We calculate the ratio of shear viscosity to entropy density for charged black branes in third order Lovelock theory. For chargeless black branes, the result turns out to be consistent with the prediction made by Brustein and Medved [Phys. Rev. D 79, 021901 (2009)].. We find that the third order Lovelock gravity term does not contribute to causality violation unlike the Gauss-Bonnet term. The stability of the black brane again requires the value of the Lovelock coupling constant to be bounded by 1/4 in the infinite dimensionality limit.

Towards a supersymmetric generalization of the Schwarzschild black hole

J. C. López-Domínguez, O. Obregón, and S. Zacarías

Published 16 November 2009 (8 pages)

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The Wheeler-DeWitt (WDW) equation for the Kantowski-Sachs model can also be understood as the WDW equation corresponding to the Schwarzschild black hole due to the well known diffeomorphism between these two metrics. The WDW equation and its solutions are ignorant of the coordinate patch one is using, only by imposing coordinate conditions we can differentiate between cosmological and black hole models. At that point, the foliation parameter t or r will appear in the solution of interest. In this work we supersymmetrize this WDW equation obtaining an extra term in the potential with two possible signs. The WKB method is then applied, giving rise to two classical equations. It is shown that the event horizon can never be reached because very near to it, the extra term in the potential, for each one of the equations, is more relevant than the one that corresponds to Schwarzschild. One can then study the asymptotic cases in which one of the two terms in the Hamiltonian dominates the behavior. One of them corresponds to the usual Schwarzschild black hole. We will study here the other two asymptotic regions; they provide three solutions. All of them have a singularity in r=0 and depending on an integration constant C they can also present a singularity in r=C². Neither of these solutions have a Newtonian limit. The black hole solution we study is analyzed between the singularity r=C² and a maximum radius rm. We find an associated mass, considering the related cosmological solution inside r=C², and based on the holographic principle an entropy can be assigned to this asymptotic solution.

Instability of small Lovelock black holes in even dimensions

Tomohiro Takahashi and Jiro Soda

Published 16 November 2009 (6 pages)

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We study the stability of static black holes in Lovelock theory, which is a natural higher dimensional generalization of Einstein theory. We derive a master equation for tensor perturbations in general Lovelock theory. It turns out that the resultant equation is characterized by one functional which determines the background black hole solutions. Thus, the stability issue of static black holes under tensor perturbations in general dimensions is reduced to an algebraic problem. We show that small Lovelock black holes in even-dimensions are unstable.

Einstein-Maxwell system in 3+1 form and initial data for multiple charged black holes

Miguel Alcubierre, Juan Carlos Degollado, and Marcelo Salgado

Published 16 November 2009 (20 pages)

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We consider the Einstein-Maxwell system as a Cauchy initial value problem taking the electric and magnetic fields as independent variables. Maxwell's equations in curved spacetimes are derived in detail using a 3+1 formalism and their hyperbolic properties are analyzed, showing that the resulting system is symmetric hyperbolic. We also focus on the problem of finding initial data for multiple charged black holes assuming time-symmetric initial data and using a puncturelike method to solve the Hamiltonian and the Gauss constraints. We study the behavior of the resulting initial data families, and show that previous results in this direction can be obtained as particular cases of our approach.

Accretion process onto super-spinning objects

Cosimo Bambi, Katherine Freese, Tomohiro Harada, Rohta Takahashi, and Naoki Yoshida

Published 17 November 2009 (10 pages)

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The accretion process onto spinning objects in Kerr spacetimes is studied with numerical simulations. Our results show that accretion onto compact objects with Kerr parameter (characterizing the spin) |a|<M and |a|>M is very different. In the superspinning case, for |a| moderately larger than M, the accretion onto the central object is extremely suppressed due to a repulsive force at short distance. The accreting matter cannot reach the central object, but instead is accumulated around it, forming a high density cloud that continues to grow. The radiation emitted in the accretion process will be harder and more intense than the one coming from standard black holes; e.g. gamma-rays could be produced as seen in some observations. Gravitational collapse of this cloud might even give rise to violent bursts. As |a| increases, a larger amount of accreting matter reaches the central object and the growth of the cloud becomes less efficient. Our simulations find that a quasisteady state of the accretion process exists for |a|/M>~1.4, independently of the mass accretion rate at large radii. For such high values of the Kerr parameter, the accreting matter forms a thin disk at very small radii. We provide some analytical arguments to strengthen the numerical results; in particular, we estimate the radius where the gravitational force changes from attractive to repulsive and the critical value |a|/M[approximate]1.4 separating the two qualitatively different regimes of accretion. We briefly discuss the observational signatures which could be used to look for such exotic objects in the Galaxy and/or in the Universe.

Emergence of scalar matter from spin foam model

Peng Xu and Yongge Ma

Published 17 November 2009 (5 pages)

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A spin foam model of three dimensional gravity nonminimally coupled with a scalar field is studied. By discretization of the scalar field, the model is worked out precisely in a purely combinational way. It is shown that the quantum physics of the scalar matter is totally encoded into the modified dynamics of SU(2) spin-network states which describe the quantum geometry of space. It turns out that the physics of the scalar matter coupled with gravity manifested in the low energy scale can be viewed as the phenomena that emerged from this microscopic construction. This gives rise to a radical observation on the issue of the unification of geometry and matter.

Hamiltonian of a spinning test particle in curved spacetime

Enrico Barausse, Etienne Racine, and Alessandra Buonanno

Published 17 November 2009 (17 pages)

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Using a Legendre transformation, we compute the unconstrained Hamiltonian of a spinning test particle in a curved spacetime at linear order in the particle spin. The equations of motion of this unconstrained Hamiltonian coincide with the Mathisson-Papapetrou-Pirani equations. We then use the formalism of Dirac brackets to derive the constrained Hamiltonian and the corresponding phase space algebra in the Newton-Wigner spin supplementary condition, suitably generalized to curved spacetime, and find that the phase space algebra (q,p,S) is canonical at linear order in the particle spin. We provide explicit expressions for this Hamiltonian in a spherically symmetric spacetime, both in isotropic and spherical coordinates, and in the Kerr spacetime in Boyer-Lindquist coordinates. Furthermore, we find that our Hamiltonian, when expanded in post-Newtonian (PN) orders, agrees with the Arnowitt-Deser-Misner canonical Hamiltonian computed in PN theory in the test particle limit. Notably, we recover the known spin-orbit couplings through 2.5PN order and the spin-spin couplings of type SKerrS (and SKerr²) through 3PN order, SKerr being the spin of the Kerr spacetime. Our method allows one to compute the PN Hamiltonian at any order, in the test particle limit and at linear order in the particle spin. As an application we compute it at 3.5PN order.

Electromagnetic absorption cross section of Reissner-Nordström black holes revisited

Luís C. B. Crispino, Atsushi Higuchi, and Ednilton S. Oliveira

Published 17 November 2009 (8 pages)

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The absorption cross section of Reissner-Nordström black holes for the electromagnetic field is computed numerically for arbitrary frequencies, taking into account the coupling of the electromagnetic and gravitational perturbations. We also compute the conversion coefficients of electromagnetic to gravitational waves by scattering from a Reissner-Nordström black hole.

Quantization of higher spin fields

J. W. Wagenaar and T. A. Rijken

Published 18 November 2009 (18 pages)

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In this article we quantize (massive) higher spin (1<=j<=2) fields by means of Dirac's constrained Hamilton procedure both in the situation were they are totally free and were they are coupled to (an) auxiliary field(s). A full constraint analysis and quantization is presented by determining and discussing all constraints and Lagrange multipliers and by giving all equal times (anti)commutation relations. Also we construct the relevant propagators. In the free case we obtain the well-known propagators and show that they are not covariant, which is also well known. In the coupled case we do obtain covariant propagators (in the spin-3/2 case this requires b=0) and show that they have a smooth massless limit connecting perfectly to the massless case (with auxiliary fields). We notice that in our system of the spin-3/2 and spin-2 case the massive propagators coupled to conserved currents only have a smooth limit to the pure massless spin-propagator, when there are ghosts in the massive case.

Spin-spin interaction in the spin-precession equations

János Majár

Published 19 November 2009 (7 pages)

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One of the most active field of research in general relativity is the description of the spin properties of astrophysical objects. The main tool in the description of the gravitational waves emitted by compact binary systems in the inspiral era is the post-Newtonian (PN) approximation, where spin effects, namely, spin-orbit and spin-spin interactions, become important in higher orders. These interactions are described with coupled differential equations in general, but the PN approximation scheme gives the opportunity to solve them order by order. In the present work the effects of the spin-orbit and spin-spin interactions are described in the spin-precession equations with the use of the PN approximation up to 1.5 PN order. The decoupled angular equations describing the evolution of the direction of the spin vectors are given, and they are solved both in the eccentric and circular orbit cases. Since the spin-precession equations do not have a Newtonian contribution it is nontrivial to determine the relative order of the different variables. This analysis is also included up to the order of the spin-spin interaction.

Lifshitz black hole in three dimensions

Eloy Ayón-Beato, Alan Garbarz, Gaston Giribet, and Mokhtar Hassaïne

Published 20 November 2009 (4 pages)

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We show that three-dimensional massive gravity admits Lifshitz metrics with generic values of the dynamical exponent z as exact solutions. At the point z=3, exact black hole solutions that are asymptotically Lifshitz arise. These spacetimes are three-dimensional analogues of those that were recently proposed as gravity duals for anisotropic scale invariant fixed points.

Noether symmetry approach in phantom quintessence cosmology

S. Capozziello, E. Piedipalumbo, C. Rubano, and P. Scudellaro

Published 20 November 2009 (10 pages)

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In the framework of phantom quintessence cosmology, we use the Noether Symmetry Approach to obtain general exact solutions for the cosmological equations. This result is achieved by the quintessential (phantom) potential determined by the existence of the symmetry itself. A comparison between the theoretical model and observations is worked out. In particular, we use type Ia supernovae and large-scale structure parameters determined from the 2-degree Field Galaxy Redshift Survey and from the Wide part of the VIMOS-VLT Deep Survey ). It turns out that the model is compatible with the presently available observational data. Moreover we extend the approach to include radiation. We show that it is compatible with data derived from recombination and it seems that quintessence do not affect nucleosynthesis results.

Commutativity of substitution and variation in the actions of quantum field theory

Zhong Chao Wu

Published 3 November 2009 (5 pages)

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There exists a paradox in quantum field theory: substituting a field configuration, which solves a subset of the field equations into the action, and varying it is not necessarily equivalent to substituting that configuration into the remaining field equations. We take the S⁴ and Freund-Rubin–like instantons as two examples to clarify the paradox. One must match the specialized configuration field variables with the corresponding boundary conditions by adding appropriate Legendre terms to the action. Some comments are made regarding exceptional degenerate cases.

Amelioration of the little hierarchy problem in SU(4)_c×SU(2)_L×SU(2)_R

Ilia Gogoladze, Mansoor Ur Rehman, and Qaisar Shafi

Published 5 November 2009 (8 pages)

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The little hierarchy problem encountered in the constrained minimal supersymmetric model can be ameliorated in supersymmetric models based on the gauge symmetry G422[equivalent]SU(4)c×SU(2)L×SU(2)R. The standard assumption in the constrained minimal supersymmetric model [and in SU(5) and SO(10)] of universal gaugino masses can be relaxed in SU(4)c×SU(2)L×SU(2)R, and this leads to a significant improvement in the degree of fine-tuning required to implement radiative electroweak breaking in the presence of a characteristic supersymmetry breaking scale of around a TeV. Examples of Higgs and sparticle mass spectra realized with 10% fine-tuning are presented.

Fermionic Casimir effect for parallel plates in the presence of compact dimensions with applications to nanotubes

S. Bellucci and A. A. Saharian

Published 6 November 2009 (13 pages)

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We evaluate the Casimir energy and force for a massive fermionic field in the geometry of two parallel plates on background of Minkowski spacetime with an arbitrary number of toroidally compactified spatial dimensions. The bag boundary conditions are imposed on the plates and periodicity conditions with arbitrary phases are considered along the compact dimensions. The Casimir energy is decomposed into purely topological, single plate and interaction parts. With independence of the lengths of the compact dimensions and the phases in the periodicity conditions, the interaction part of the Casimir energy is always negative. In order to obtain the resulting force, the contributions from both sides of the plates must be taken into account. Then, the forces coming from the topological parts of the vacuum energy cancel out and only the interaction term contributes to the Casimir force. Applications of the general formulae to Kaluza-Klein-type models and carbon nanotubes are given. In particular, we show that for finite-length metallic nanotubes, the Casimir forces acting on the tube edges are always attractive, whereas for semiconducting-type ones, they are attractive for small lengths of the nanotube and repulsive for large lengths.

Fundamental length in quantum theories with [script P][script T]-symmetric Hamiltonians. II. The case of quantum graphs

Miloslav Znojil

Published 6 November 2009 (20 pages)

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[script P][script T]-symmetrization of quantum graphs is proposed as an innovation where an adjustable, tunable nonlocality is admitted. The proposal generalizes the [script P][script T]-symmetric square-well models of Ref. [M. Znojil, Phys. Rev. D 80, 045022 (2009).] (with real spectrum and with a variable fundamental length theta) which are reclassified as the most elementary quantum q-pointed-star graphs with minimal q=2. Their equilateral q=3,4,… generalizations are considered, with interactions attached to the vertices. Runge-Kutta discretization of coordinates simplifies the quantitative analysis by reducing our graphs to star-shaped lattices of N=qK+1 points. The resulting bound-state spectra are found real in an N-independent interval of couplings lambda[is-an-element-of](-1,1). Inside this interval the set of closed-form metrics Thetaj^(N)(lambda) is constructed, defining independent eligible local (at j=0) or increasingly nonlocal (at j=1,2,…) inner products in the respective physical Hilbert spaces of states [script H]j^(N)(lambda). In this way each graph is assigned a menu of nonequivalent, optional probabilistic quantum interpretations.

Information field theory for cosmological perturbation reconstruction and nonlinear signal analysis

Torsten A. Enßlin, Mona Frommert, and Francisco S. Kitaura

Published 9 November 2009 (35 pages)

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We develop information field theory (IFT) as a means of Bayesian inference on spatially distributed signals, the information fields. A didactical approach is attempted. Starting from general considerations on the nature of measurements, signals, noise, and their relation to a physical reality, we derive the information Hamiltonian, the source field, propagator, and interaction terms. Free IFT reproduces the well-known Wiener-filter theory. Interacting IFT can be diagrammatically expanded, for which we provide the Feynman rules in position-, Fourier-, and spherical-harmonics space, and the Boltzmann-Shannon information measure. The theory should be applicable in many fields. However, here, two cosmological signal recovery problems are discussed in their IFT formulation. (1) Reconstruction of the cosmic large-scale structure matter distribution from discrete galaxy counts in incomplete galaxy surveys within a simple model of galaxy formation. We show that a Gaussian signal, which should resemble the initial density perturbations of the Universe, observed with a strongly nonlinear, incomplete and Poissonian-noise affected response, as the processes of structure and galaxy formation and observations provide, can be reconstructed thanks to the virtue of a response-renormalization flow equation. (2) We design a filter to detect local nonlinearities in the cosmic microwave background, which are predicted from some early-Universe inflationary scenarios, and expected due to measurement imperfections. This filter is the optimal Bayes' estimator up to linear order in the nonlinearity parameter and can be used even to construct sky maps of nonlinearities in the data.

New self-dual solutions of SU(2) Yang-Mills theory in Euclidean Schwarzschild space

Ricardo A. Mosna and Gustavo Marques Tavares

Published 9 November 2009 (7 pages)

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We present a systematic study of spherically symmetric self-dual solutions of SU(2) Yang-Mills theory on Euclidean Schwarzschild space. All the previously known solutions are recovered and a new one-parameter family of instantons is obtained. The newly found solutions have continuous actions and interpolate between the classic Charap and Duff instantons. We examine the physical properties of this family and show that it consists of dyons of unit (magnetic and electric) charge.

Ground states of holographic superconductors

Steven S. Gubser and Abhinav Nellore

Published 9 November 2009 (16 pages)

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We investigate the ground states of the Abelian Higgs model in AdS4 with various choices of parameters, and with no deformations in the ultraviolet other than a chemical potential for the electric charge under the Abelian gauge field. For W-shaped potentials with symmetry-breaking minima, an analysis of infrared asymptotics suggests that the ground state has emergent conformal symmetry in the infrared when the charge of the complex scalar is large enough. But when this charge is too small, the likeliest ground state has Lifshitz-like scaling in the infrared. For positive mass quadratic potentials, Lifshitz-like scaling is the only possible infrared behavior for constant nonzero values of the scalar. The approach to Lifshitz-like scaling is shown in many cases to be oscillatory.

Testing symmetries in effective models of higher derivative field theories

C. Marat Reyes

Published 11 November 2009 (13 pages)

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Higher derivative field theories with interactions raise serious doubts about their validity due to severe energy instabilities. In many cases the implementation of a direct perturbation treatment to excise the dangerous negative-energies from a higher derivative field theory may lead to violations of Lorentz and other symmetries. In this work we study a perturbative formulation for higher derivative field theories that allows the construction of a low-energy effective field theory being a genuine perturbations over the ordinary-derivative theory and having a positive-defined Hamiltonian. We show that some discrete symmetries are recovered in the low-energy effective theory when the perturbative method to reduce the negative-energy degrees of freedom from the higher derivative theory is applied. In particular, we focus on the higher derivative Maxwell-Chern-Simons model which is a Lorentz invariant and parity-odd theory in 2+1 dimensions. The parity violation arises in the effective action of QED3 as a quantum correction from the massive fermionic sector. We obtain the effective field theory which remains Lorentz invariant, but parity invariant to the order considered in the perturbative expansion.

Constraining nonstandard neutrino-quark interactions with solar, reactor, and accelerator data

F. J. Escrihuela, O. G. Miranda, M. A. Tórtola, and J. W. F. Valle

Published 12 November 2009 (9 pages)

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We present a reanalysis of nonstandard neutrino-down-quark interactions of electron and tau neutrinos using solar, reactor, and accelerator data. In addition updating the analysis by including new solar data from SNO phase III and Borexino, as well as new KamLAND data and solar fluxes, a key role is played in our analysis by the combination of these results with the CHARM data. The latter allows us to better constrain the axial and axial-vector electron and tau-neutrino nonstandard interaction parameters characterizing the deviations from the standard model predictions.

Quantum complex scalar fields and noncommutativity

Ricardo Amorim and Everton M. C. Abreu

Published 12 November 2009 (6 pages)

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In this work we analyze complex scalar fields using a new framework where the object of noncommutativity theta^µnu represents independent degrees of freedom. In a first quantized formalism, theta^µnu and its canonical momentum piµnu are seen as operators living in some Hilbert space. This structure is compatible with the minimal canonical extension of the Doplicher-Fredenhagen-Roberts algebra and is invariant under an extended Poincaré group of symmetry. In a second quantized formalism perspective, we present an explicit form for the extended Poincaré generators and the same algebra is generated via generalized Heisenberg relations. We also introduce a source term and construct the general solution for the complex scalar fields using the Green's function technique.

Rotating optical cavity experiment testing Lorentz invariance at the 10^-17 level

S. Herrmann, A. Senger, K. Möhle, M. Nagel, E. V. Kovalchuk, and A. Peters

Published 12 November 2009 (8 pages)

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We present an improved laboratory test of Lorentz invariance in electrodynamics by testing the isotropy of the speed of light. Our measurement compares the resonance frequencies of two orthogonal optical resonators that are implemented in a single block of fused silica and are rotated continuously on a precision air bearing turntable. An analysis of data recorded over the course of one year sets a limit on an anisotropy of the speed of light of Deltac/c~1×10^-17. This constitutes the most accurate laboratory test of the isotropy of c to date and allows to constrain parameters of a Lorentz violating extension of the standard model of particle physics down to a level of 10^-17.

Taylor-Lagrange renormalization scheme: Application to light-front dynamics

P. Grangé, J.-F. Mathiot, B. Mutet, and E. Werner

Published 13 November 2009 (14 pages)

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The recently proposed renormalization scheme based on the definition of field operators as operator valued distributions acting on specific test functions is shown to be very convenient in explicit calculations of physical observables within the framework of light-front dynamics. We first recall the main properties of this procedure based on identities relating the test functions to their Taylor remainder of any order expressed in terms of Lagrange's formulas, hence the name given to this scheme. We thus show how it naturally applies to the calculation of state vectors of physical systems in the covariant formulation of light-front dynamics. As an example, we consider the case of the Yukawa model in the simple two-body Fock state truncation.

Compact baby Skyrmions

C. Adam, P. Klimas, J. Sánchez-Guillén, and A. Wereszczyński

Published 13 November 2009 (14 pages)

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For the baby Skyrme model with a specific potential, compacton solutions, i.e., configurations with a compact support and parabolic approach to the vacuum, are derived. Specifically, in the nontopological sector, we find spinning Q-balls and Q-shells, as well as peakons. Moreover, we obtain compact baby skyrmions with nontrivial topological charge. All these solutions may form stable multisoliton configurations provided they are sufficiently separated.

Quantum noncanonical field theory: Symmetries and interaction

J. M. Carmona, J. L. Cortés, J. Induráin, and D. Mazón

Published 16 November 2009 (9 pages)

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The symmetry properties of a proposal to go beyond relativistic quantum field theory based on a modification of the commutation relations of fields are identified. Poincaré invariance in an auxiliary spacetime is found in the Lagrangian version of the path integral formulation. This invariance is contrasted with the idea of doubly (or deformed) special relativity. This analysis is then used to go from the free theory of a complex field to an interacting field theory.

Crossing symmetry and modular invariance in conformal field theory and S duality in gauge theory

Dimitri V. Nanopoulos and Dan Xie

Published 16 November 2009 (5 pages)

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In this paper, we explore the relation between crossing symmetry and modular invariance in conformal field theory and S duality in gauge theory. It is shown that partition functions of different S dual theories of N=2 SU(2) gauge theory with four fundamentals can be derived from the crossing symmetry of the Liouville four-point function. We also show that the partition function of N=4 SU(2) gauge theory can be derived from the Liouville partition function on torus.

Effect of the gluon condensate on the holographic heavy quark potential

Youngman Kim, Bum-Hoon Lee, Chanyong Park, and Sang-Jin Sin

Published 16 November 2009 (7 pages)

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The gluon condensate is very sensitive to the QCD deconfinement transition since its value changes drastically with the deconfinement transition. We calculate the gluon condensate dependence of the heavy quark potential in AdS/CFT to study how the property of the heavy quarkonium is affected by a relic of the deconfinement transition. We observe that the heavy quark potential becomes deeper as the value of the gluon condensate decreases. We interpret this as a dropping of the heavy quarkonium mass just above the deconfinement transition. We finally argue that dropping of the gluon condensate and the pure thermal effect are competing with each other in the physics of heavy quarkonium at high temperature.

Anomalies, unparticles, and Seiberg duality

Jamison Galloway, John McRaven, and John Terning

Published 16 November 2009 (7 pages)

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We calculate triangle anomalies for fermions with noncanonical scaling dimensions. The most well-known example of such fermions (aka unfermions) occurs in Seiberg duality where the matching of anomalies (including mesinos with scaling dimensions between 3/2 and 5/2) is a crucial test of duality. By weakly gauging the nonlocal action for an unfermion, we calculate the one-loop three-current amplitude. Despite the fact that there are more graphs with more complicated propagators and vertices, we find that the calculation can be completed in a way that nearly parallels the usual case. We show that the anomaly factor for fermionic unparticles is independent of the scaling dimension and identical to that for ordinary fermions. This can be viewed as a confirmation that unparticle actions correctly capture the physics of conformal fixed point theories like Banks-Zaks or supersymmetric QCD.

Asymptotically free four-Fermi theory in 4 dimensions at the z=3 Lifshitz-like fixed point

Avinash Dhar, Gautam Mandal, and Spenta R. Wadia

Published 17 November 2009 (10 pages)

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We show that a Nambu-Jona-Lasinio type four-fermion coupling at the z=3 Lifshitz-like fixed point in 3+1 dimensions is asymptotically free and generates a mass scale dynamically. This result is nonperturbative in the limit of a large number of fermion species. The theory is ultraviolet complete and at low energies exhibits Lorentz invariance as an emergent spacetime symmetry. Many of our results generalize to z=d in odd d spatial dimensions; z=d=1 corresponds to the Gross-Neveu model. The above mechanism of mass generation has potential applications to the fermion mass problem and to dynamical electroweak symmetry breaking. We present a scenario in which a composite Higgs field arises from a condensate of these fermions, which then couples to quarks and leptons of the standard model. Such a scenario could eliminate the need for the Higgs potential and the associated hierarchy problem. We also show that the axial anomaly formula at z=3 coincides with the usual one in the relativistic domain.

Quantum corrections to the Larmor radiation formula in scalar electrodynamics

A. Higuchi and P. J. Walker

Published 17 November 2009 (12 pages)

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We use the semiclassical approximation in perturbative scalar quantum electrodynamics to calculate the quantum correction to the Larmor radiation formula to first order in Planck's constant in the nonrelativistic approximation, choosing the initial state of the charged particle to be a momentum eigenstate. We calculate this correction in two cases: in the first case the charged particle is accelerated by a time-dependent but space-independent vector potential whereas in the second case it is accelerated by a time-independent vector potential which is a function of one spatial coordinate. We find that the corrections in these two cases are different even for a charged particle with the same classical motion. The correction in each case turns out to be nonlocal in time in contrast to the classical approximation.

Supersymmetry preserving mass deformation of [script N]=1 super Yang-Mills theory in D=2+1 dimensions

Abhishek Agarwal

Published 20 November 2009 (11 pages)

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Physical Review D

November 2009

Volume 80, Number 10 , partial issue

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