50,322 research outputs found

    Effective interactions from q-deformed inspired transformations

    Get PDF
    From the mass term for the transformed quark fields, we obtain effective contact interactions of the NJL type. The parameters of the model that maps a system of non-interacting transformed fields into quarks interacting via NJL contact terms are discussed

    Gravitational Wave Emission and Mass Extraction from a Perturbed Schwarzschild Black Hole (continue)

    Full text link
    A relativistic model for the emission of gravitational waves from an initially unperturbed Schwarzschild black hole, or spherical collapsing configuration, is completely integrated. The model consists basically of gravitational perturbations of the Robinson-Trautman type on the Schwarzschild spacetime. In our scheme of perturbation, gravitational waves may extract mass from the collapsing configuration. Robinson-Trautmann perturbations also include another mode of emission of mass, which we denote shell emission mode: in the equatorial plane of the configuration, a timelike (1+2)(1+2) shell of matter may be present, whose stress-energy tensor is modelled by neutrinos and strings emitted radially on the shell; no gravitational waves are present in this mode. The invariant characterization of gravitational wave perturbations and of the gravitational wave zone is made through the analysis of the structure of the curvature tensor and the use of the Peeling Theorem.Comment: 26 pages, LaTex, no figure

    Studying light propagation in a locally homogeneous universe through an extended Dyer-Roeder approach

    Full text link
    Light is affected by local inhomogeneities in its propagation, which may alter distances and so cosmological parameter estimation. In the era of precision cosmology, the presence of inhomogeneities may induce systematic errors if not properly accounted. In this vein, a new interpretation of the conventional Dyer-Roeder (DR) approach by allowing light received from distant sources to travel in regions denser than average is proposed. It is argued that the existence of a distribution of small and moderate cosmic voids (or "black regions") implies that its matter content was redistributed to the homogeneous and clustered matter components with the former becoming denser than the cosmic average in the absence of voids. Phenomenologically, this means that the DR smoothness parameter (denoted here by αE\alpha_E) can be greater than unity, and, therefore, all previous analyses constraining it should be rediscussed with a free upper limit. Accordingly, by performing a statistical analysis involving 557 type Ia supernovae (SNe Ia) from Union2 compilation data in a flat Λ\LambdaCDM model we obtain for the extended parameter, αE=1.26−0.54+0.68\alpha_E=1.26^{+0.68}_{-0.54} (1σ1\sigma). The effects of αE\alpha_E are also analyzed for generic Λ\LambdaCDM models and flat XCDM cosmologies. For both models, we find that a value of αE\alpha_E greater than unity is able to harmonize SNe Ia and cosmic microwave background observations thereby alleviating the well-known tension between low and high redshift data. Finally, a simple toy model based on the existence of cosmic voids is proposed in order to justify why αE\alpha_E can be greater than unity as required by supernovae data.Comment: 5 pages, 2 figures. Title modified, results unchanged. It matches version published as a Brief Report in Phys. Rev.

    Comment on "Constraining the smoothness parameter and dark energy using observational H(z) data"

    Full text link
    In this Comment we discuss a recent analysis by Yu et al. [RAA 11, 125 (2011)] about constraints on the smoothness α\alpha parameter and dark energy models using observational H(z)H(z) data. It is argued here that their procedure is conceptually inconsistent with the basic assumptions underlying the adopted Dyer-Roeder approach. In order to properly quantify the influence of the H(z)H(z) data on the smoothness α\alpha parameter, a χ2\chi^2-test involving a sample of SNe Ia and H(z)H(z) data in the context of a flat Λ\LambdaCDM model is reanalyzed. This result is confronted with an earlier approach discussed by Santos et al. (2008) without H(z)H(z) data. In the (Ωm,α\Omega_m, \alpha) plane, it is found that such parameters are now restricted on the intervals 0.66≤α≤1.00.66 \leq \alpha \leq 1.0 and 0.27≤Ωm≤0.370.27 \leq \Omega_m \leq 0.37 within 95.4% confidence level (2σ\sigma), and, therefore, fully compatible with the homogeneous case. The basic conclusion is that a joint analysis involving H(z)H(z) data can indirectly improve our knowledge about the influence of the inhomogeneities. However, this happens only because the H(z)H(z) data provide tighter constraints on the matter density parameter Ωm\Omega_m.Comment: 3 pages, 1 figure, submitted to Research in Astronomy and Astrophysic
    • …
    corecore