1,908 research outputs found
Impulsive cylindrical gravitational wave: one possible radiative form emitted from cosmic strings and corresponding electromagnetic response
The cosmic strings (CSs) may be one type of important source of gravitational waves (GWs), and it has been intensively studied due to its special properties such as the cylindrical symmetry. The CSs would generate not only usual continuous GWs, but also impulsive GWs that bring about a more concentrated energy and consist of different GW components, broadly covering low-, intermediate- and high-frequency bands simultaneously. These features might underlie interesting electromagnetic (EM) responses to these GWs generated by the CSs. In this paper, with novel results and effects, we firstly calculate the analytical solutions of perturbed EM fields caused by the interaction between impulsive cylindrical GWs (which would be one of possible forms emitted from CSs) and background celestial high magnetic fields or widespread cosmological background magnetic fields, by using the exact form of the Einstein–Rosen metric rather than the planar approximation usually applied. The results show that perturbed EM fields are also in the impulsive form in accordant to the GW pulse, and the asymptotic behaviors of the perturbed EM fields are fully consistent with the asymptotic behaviors of the energy density, energy flux density, and Riemann curvature tensor of the corresponding impulsive cylindrical GWs. The analytical solutions naturally give rise to the accumulation effect (due to the synchro-propagation of perturbed EM fields and the GW pulse, because of their identical propagating velocities, i.e., the speed of light), which is proportional to the distance . Based on this accumulation effect, in consideration of very widely existing background galactic–extragalactic magnetic fields in all galaxies and galaxy clusters, we for the first time predict potentially observable effects in the region of the Earth caused by the EM response to GWs from the CSs
Determination of f+K(0) and extraction of |Vcs| from semileptonic D decays
By globally analyzing all existing measured branching fractions and partial rates in different four momentum transfer-squared q2 bins of D→Ke+νe decays, we obtain the product of the form factor and magnitude of Cabibbo–Kobayashi–Maskawa matrix element Vcs to be f+K(0)|Vcs|=0.717±0.004 . With this product, we determine the D→K semileptonic form factor f+K(0)=0.737±0.004±0.000 in conjunction with the value of |Vcs| determined from the standard model global fit. Alternately, with the product together with the input of the form factor f+K(0) calculated in lattice quantum chromodynamics (LQCD) recently, we extract |Vcs|D→Ke+νe=0.962±0.005±0.014 , where the error is still dominated by the uncertainty of the form factor calculated in LQCD. Combining the |Vcs|Ds+→ℓ+νℓ=1.012±0.015±0.009 extracted from all existing measurements of Ds+→ℓ+νℓ decays and |Vcs|D→Ke+νe=0.962±0.005±0.014 together, we find the most precisely determined |Vcs| to be |Vcs|=0.983±0.011 , which improves the accuracy of the PDG’2014 value |Vcs|PDG′2014=0.986±0.016 by 45%
Gauss–Bonnet coupling constant as a free thermodynamical variable and the associated criticality
The thermodynamic phase space of Gauss–Bonnet (GB) AdS black holes is extended, taking the inverse of the GB coupling constant as a new thermodynamic pressure PGB . We studied the critical behavior associated with PGB in the extended thermodynamic phase space at fixed cosmological constant and electric charge. The result shows that when the black holes are neutral, the associated critical points can only exist in five dimensional GB-AdS black holes with spherical topology, and the corresponding critical exponents are identical to those for the Van der Waals system. For charged GB-AdS black holes, it is shown that there can be only one critical point in five dimensions (for black holes with either spherical or hyperbolic topologies), which also requires the electric charge to be bounded within some appropriate range; while in d>5 dimensions, there can be up to two different critical points at the same electric charge, and the phase transition can occur only at temperatures which are not in between the two critical values
The structure of f ( R )-brane model
Recently, a family of interesting analytical brane solutions were found in f ( R ) gravity with f(R)=R+αR2 in Bazeia et al. (Phys Lett B 729:127 2014 ). In these solutions, the inner brane structure can be turned on by tuning the value of the parameter α . In this paper, we investigate how the parameter α affects the localization and the quasilocalization of the tensorial gravitons around these solutions. It is found that, in a range of α , despite the brane having an inner structure, there is no graviton resonance. However, in some other regions of the parameter space, although the brane has no internal structure, the effective potential for the graviton Kaluza–Klein (KK) modes has a singular structure, and there exist a series of graviton resonant modes. The contribution of the massive graviton KK modes to Newton’s law of gravity is discussed briefly
Cosmic constraint on the unified model of dark sectors with or without a cosmic string fluid in the varying gravitational constant theory
Observations indicate that most of the universal matter is invisible and the gravitational constant G ( t ) maybe depends on time. A theory of the variational G (VG) is explored in this paper, naturally producing the useful dark components in the universe. We utilize the following observational data: lookback time data, model-independent gamma ray bursts, growth function of matter linear perturbations, type Ia supernovae data with systematic errors, CMB, and BAO, to restrict the unified model (UM) of dark components in VG theory. Using the best-fit values of the parameters with the covariance matrix, constraints on the variation of G are GG0z=3.5≃1.0015-0.0075+0.0071 and G˙Gtoday≃-0.7252-2.3645+2.3645×10-13year-1 , with small uncertainties around the constants. The limit on the equation of state of dark matter is w0dm=0.0072-0.0170+0.0170 , assuming w0de=-1 in the unified model, and the dark energy is w0de=-0.9986-0.0011+0.0011 , assuming w0dm=0 a priori. The restrictions on the UM parameters are Bs=0.7442-0.0132-0.0292+0.0137+0.0262 and α=0.0002-0.0209-0.0422+0.0206+0.0441 with 1σ and 2σ confidence level. In addition, the effects of a cosmic string fluid on the unified model in VG theory are investigated. In this case it is found that the Λ CDM ( Ωs=0 , β=0 , and α=0 ) is included in this VG-UM model at 1σ confidence level, and larger errors are given: Ωs=-0.0106-0.0305-0.0509+0.0312+0.0582 (dimensionless energy density of cosmic string), GG0z=3.5≃1.0008-0.0584+0.0620 , and G˙Gtoday≃-0.3496-26.3135+26.3135×10-13year-1
Warped brane worlds in critical gravity
We investigate the brane models in arbitrary dimensional critical gravity presented in Lu and Pope (Phys Rev Lett 106:181302, 2011 ). For the models of the thin branes with codimension one, the Gibbons–Hawking surface term and the junction conditions are derived, with which the analytical solutions for the flat, AdS, and dS branes are obtained at the critical point of the critical gravity. It is found that all these branes are embedded in an AdS n spacetime, but, in general, the effective cosmological constant Λ of the AdS n spacetime is not equal to the naked one Λ0 in the critical gravity, which can be positive, zero, and negative. Another interesting result is that the brane tension can also be positive, zero, or negative, depending on the symmetry of the thin brane and the values of the parameters of the theory, which is very different from the case in general relativity. It is shown that the mass hierarchy problem can be solved in the braneworld model in the higher-derivative critical gravity. We also study the thick brane model and find analytical and numerical solutions of the flat, AdS, and dS branes. It is found that some branes will have inner structure when some parameters of the theory are larger than their critical values, which may result in resonant KK modes for some bulk matter fields. The flat branes with positive energy density and AdS branes with negative energy density are embedded in an n -dimensional AdS spacetime, while the dS branes with positive energy density are embedded in an n -dimensional Minkowski one
Black hole solution and strong gravitational lensing in Eddington-inspired Born–Infeld gravity
A new theory of gravity called Eddington-inspired Born–Infeld (EiBI) gravity was recently proposed by Bañados and Ferreira. This theory leads to some exciting new features, such as free of cosmological singularities. In this paper, we first obtain a charged EiBI black hole solution with a nonvanishing cosmological constant when the electromagnetic field is included in. Then based on it, we study the strong gravitational lensing by the asymptotic flat charged EiBI black hole. The strong deflection limit coefficients and observables are shown to closely depend on the additional coupling parameter κ in the EiBI gravity. It is found that, compared with the corresponding charged black hole in general relativity, the positive coupling parameter κ will shrink the black hole horizon and photon sphere. Moreover, the coupling parameter will decrease the angular position and relative magnitudes of the relativistic images, while increase the angular separation, which may shine new light on testing such gravity theory in near future by the astronomical instruments
Brane worlds in gravity with auxiliary fields
Recently, Pani et al. explored a new theory of gravity by adding nondynamical fields, i.e., gravity with auxiliary fields (Phys Rev D 88:121502, 2013 ). In this gravity theory, higher-order derivatives of matter fields generically appear in the field equations. In this paper we extend this theory to any dimensions and discuss the thick braneworld model in five dimensions. Domain wall solutions are obtained numerically. The stability of the brane system under tensor perturbations is analyzed. We find that the system is stable under tensor perturbations and the gravity zero mode is localized on the brane. Therefore, the four-dimensional Newtonian potential can be realized on the brane
Spherical top-hat collapse of a viscous unified dark fluid
In this paper, we test the spherical collapse of a viscous unified dark fluid (VUDF) which has constant adiabatic sound speed and show the nonlinear collapse for VUDF, baryons, and dark matter, which are important in forming the large-scale structure of our Universe. By varying the values of the model parameters α and ζ0 , we discuss their effects on the nonlinear collapse of the VUDF model, and we compare its result to the Λ CDM model. The results of the analysis show that, within the spherical top-hat collapse framework, larger values of α and smaller values of ζ0 make the structure formation earlier and faster, and the other collapse curves are almost distinguished with the curve of Λ CDM model if the bulk viscosity coefficient ζ0 is less than 10-3
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