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

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

Naoki Seto

Published 2 November 2009 (8 pages)

103001 Full Text: PDF (204 kB) | Buy Article

+

Show Abstract

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)

103002 Full Text: PDF (900 kB) | Buy Article

+

Show Abstract

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)

103003 Full Text: PDF (440 kB) | Buy Article

+

Show Abstract

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)

103004 Full Text: PDF (194 kB) | Buy Article

+

Show Abstract

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.

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)

103501 Full Text: PDF (425 kB) | Buy Article

+

Show Abstract

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)

103502 Full Text: PDF (226 kB) | Buy Article

+

Show Abstract

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)

103503 Full Text: PDF (655 kB) | Buy Article

+

Show Abstract

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)

103504 Full Text: PDF (89 kB) | Buy Article

+

Show Abstract

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)

103505 Full Text: PDF (151 kB) | Buy Article

+

Show Abstract

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)

103506 Full Text: PDF (347 kB) | Buy Article

+

Show Abstract

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)

103507 Full Text: PDF (1903 kB) | Buy Article

+

Show Abstract

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.

Turning big bang into big bounce. I. Classical dynamics

kiewicz, and W

Published 4 November 2009 (8 pages)

104001 Full Text: PDF (156 kB) | Buy Article

+

Show Abstract

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)

104002 Full Text: PDF (453 kB) | Buy Article

+

Show Abstract

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)

104003 Full Text: PDF (208 kB) | Buy Article

+

Show Abstract

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)

104004 Full Text: PDF (1396 kB) | Buy Article

+ -	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)

104005 Full Text: PDF (936 kB) | Buy Article

+

Show Abstract

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)

104006 Full Text: PDF (691 kB) | Buy Article

+

Show Abstract

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)

104007 Full Text: PDF (171 kB) | Buy Article

+

Show Abstract

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.

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

Zhong Chao Wu

Published 3 November 2009 (5 pages)

105001 Full Text: PDF (89 kB) | Buy Article

+

Show Abstract

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)

105002 Full Text: PDF (499 kB) | Buy Article

+

Show Abstract

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)

105003 Full Text: PDF (346 kB) | Buy Article

+

Show Abstract

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)

105004 Full Text: PDF (956 kB) | Buy Article

+

Show Abstract

[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.

Article Lookup:

A new free weekly publication from APS

SELECTED ARTICLES (0)

SELECTED ARTICLES (0)

Physical Review D

November 2009

Volume 80, Number 10 , partial issue

ARTICLES

ERRATA

	Erratum: Infrared singularities of QCD amplitudes with massive partons [Phys. Rev. D 79, 125004 (2009)] Thomas Becher and Matthias Neubert Published 2 November 2009 (1 page) 109901(E) Full Text: PDF (26 kB)
	No abstract available.