15703222
Newtonian limit of the singular f(R) gravity in the Palatini formalism
Recently D. Vollick [Phys. Rev. D68, 063510 (2003)] has shown that the inclusion of the 1/R curvature terms in the gravitational action and the use of the Palatini formalism offer an alternative explanation for cosmological acceleration. In this work we show not only that this model of Vollick does not have a good Newtonian limit, but also that any f(R) theory with a pole of order n in R=0 and its second derivative respect to R evaluated at Ro is not zero, where Ro is the scalar curvature of background, does not have a good Newtonian limit.
9451406
Superselection sectors in asymptotic quantization of gravity
Using the continuity of the scalar ψ2 (the mass aspect) at null infinity through 1° we show that the space of radiative solutions of general relativity can be thought of a fibered space where the value of ψ2 at 1° plays the role of the base space. We also show that the restriction of the available symplectic form to each “fiber” is degenerate along directions that represent supertranslations of the BMS group. By factoring out these gauge directions we obtain the reduced phase space for the radiation data. This reduced phase space posses a global structure, i.e., it does not distinguishes between future or past null infinity. Thus, it can be used as the space of quantum gravitons. Moreover, a (pre)Hilbert space can be constructed on each “fiber” if an appropriate definition of scalar product is provided. Since there is no natural correspondence between the reduced phase spaces of different leaves they define superselection sectors on the space of asymptotic quantum states. We briefly review the dynamics of...
22816939
The Collision of two slowly rotating, initially nonboosted, black holes in the close limit
We study the collision of two slowly rotating, initially non boosted, black holes in the close limit. A ``punctures'' modification of the Bowen - York method is used to construct conformally flat initial data appropriate to the problem. We keep only the lowest nontrivial orders capable of giving rise to radiation of both gravitational energy and angular momentum. We show that even with these simplifications an extension to higher orders of the linear Regge-Wheeler-Zerilli black hole perturbation theory, is required to deal with the evolution equations of the leading contributing multipoles. This extension is derived, together with appropriate extensions of the Regge-Wheeler and Zerilli equations. The data is numerically evolved using these equations, to obtain the asymptotic gravitational wave forms and amplitudes. Expressions for the radiated gravitational energy and angular momentum are derived and used together with the results of the numerical evolution to provide quantitative expressions for the relative contribution of different terms, and their significance is analyzed.
119520371
A Gravitational memory effect in 'boosted' black hole perturbation theory
Black hole perturbation theory, or more generally, perturbation theory on a Schwarzschild bockground, has been applied in several contexts, but usually under the simplifying assumption that the ADM momentum vanishes, namely, that the evolution is carried out and observed in the ``center of momentum frame''. In this paper we consider some consequences of the inclusion of a non vanishing ADM momentum in the initial data. We first provide a justification for the validity of the transformation of the initial data to the ``center of momentum frame'', and then analyze the effect of this transformation on the gravitational wave amplitude. The most significant result is the possibility of a type of gravitational memory effect that appears to have no simple relation with the well known Christodoulou effect.
42350703
Large quantum gravity effects and nonlocal variables
We reconsider here the model where large quantum gravity effects were first found, but now in its null surface formulation (NSF). We find that although the set of coherent states for Z, the basic variable of NSF, is as restricted as is the one for the metric, while some type of small deviations from these states may cause huge fluctuations on the metric, the corresponding fluctuations on Z remain small.
119068604
Radiating black hole solutions in Einstein-Gauss-Bonnet gravity
In this paper, we find some new exact solutions to the Einstein-Gauss-Bonnet equations. First, we prove a theorem which allows us to find a large family of solutions to the Einstein-Gauss-Bonnet gravity in n-dimensions. This family of solutions represents dynamic black holes and contains, as particular cases, not only the recently found Vaidya-Einstein-Gauss-Bonnet black hole, but also other physical solutions that we think are new, such as the Gauss-Bonnet versions of the Bonnor-Vaidya (de Sitter/anti-de Sitter) solution, a global monopole, and the Husain black holes. We also present a more general version of this theorem in which less restrictive conditions on the energy-momentum tensor are imposed. As an application of this theorem, we present the exact solution describing a black hole radiating a charged null fluid in a Born-Infeld nonlinear electrodynamics.
17481438
Influence of the cosmological expansion on small systems
The effect of the large-scale cosmological expansion on small systems is studied in the light of modern cosmological models of large-scale structure. We identify certain assumptions of earlier works which render them unrealistic regarding these cosmological models. The question is reanalyzed by dropping these assumptions to conclude that a given small system can experience either an expansion or a contraction of cosmological origin.
122277485
Beam test results from the SPAKEBAB electromagnetic calorimeter
Abstract We present the results of beam tests with high energy (6–150 GeV) electrons and muons of a new type of electromagnetic calorimeter, consisting of very thin interleaved lead and scintillator plates and read out by means of a large number of wavelength shifting fibers. A crucial feature of this detector is the absence of physical boundaries between the readout towers. We have measured the effects of crosstalk between cells that results from this feature and more common calorimeter properties such as the energy and position resolutions, signal linearity, signal uniformity and light yield. The results are among the very best ever obtained for sampling calorimeters.
18437289
Hydrodynamic approach to the evolution of cosmic structures – II. Study of N‐body simulations at z= 0
We present a series of cosmological N-body simulations which make use of the hydrodynamic approach to the evolution of structures. This approach addresses explicitly the existence of a finite spatial resolution and the dynamical effect of subresolution degrees of freedom. We adapt this method to cosmological simulations of the standard Lambda cold dark matter structure formation scenario and study the effects induced at redshift z= 0 by this novel approach on the large-scale clustering patterns as well as (individual) dark matter haloes. 
 
 
 
Comparing these simulations to usual N-body simulations, we find that (i) the new (hydrodynamic) model entails a proliferation of low-mass haloes and (ii) dark matter haloes have a higher degree of rotational support. These results agree with the theoretical expectation about the qualitative behaviour of the ‘correction terms’ introduced by the hydrodynamic approach: these terms act as a drain of inflow kinetic energy and a source of vorticity by the small-scale tidal torques and shear stresses.
14856382
Study of corrections to the dust model via perturbation theory
This work reports on the application of the Eulerian perturbation theory to a recently proposed model of cosmological structure formation by gravitational instability. Its physical meaning is discussed in detail and put in perspective of previous works. The model incorporates in a systematic fashion corrections to the popular dust model owing to multistreaming and, more generally, the small-scale, virialized degrees of freedom. It features a time-dependent length-scale L(t) estimated to be L/r 0 ∼ 10 - 1 [r 0 (t) is the non-linear scale, at which = 1]. The model provides a new angle on the dust model and allows us to overcome some of its limitations. Thus, the scale L(t) works as a physically meaningful short-distance cut-off for the divergences appearing in the perturbation expansion of the dust model when there is too much initial power on small scales. The model also incorporates the generation of vorticity by tidal forces; according to the perturbational result, the filtered vorticity for standard cold dark matter initial conditions should be significant today only at scales below ∼ 1 h - 1 Mpc.
2374089
Numerical study of the cosmological velocity field as a function of density
We report on a new study of the velocity distribution in N-body simulations. We investigate the center-of-mass and internal kinetic energies of coarsening cells as a function of time, cell size and cell mass. By using self-similar cosmological models, we are able to derive theoretical predictions for comparison and to assess the influence of finite-size and resolution effects. The most interesting result is the discovery of a polytropic-like relationship between the average velocity dispersion (internal kinetic energy) and the mass density in an intermediate range of densities, κint ∝ � 2−η . The exponent η measures the deviations from the virial prediction, ηvirial = 0. For self-similar models, η depends only on the spectral index of the initial power spectrum. We also study CDM models and confirm a previous result that the same polytropic-like dependence exists, with a time and coarsening length dependent η. The dependence κint(� ) is an important input for a recently proposed theoretical model of cosmological structure formation which improves over the standard dust model (pressureless fluid) by regularizing the density singularities.