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.
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.
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.
966351
The Fractal Distribution of Galaxies and the Transition to Homogeneity
There is an ongoing debate in cosmology about the value of the length scale at which "homogeneity" in the matter distribution is reached or even if such a scale exists. In the wake of this debate, we intend in this Letter to clarify the meaning of the statement transition to homogeneity and of the concept of correlation length. We show that there are two scales, each associated with the fractal and with the homogeneous regimes of the matter distribution, respectively. The distinction between both scales has deep consequences, for there can be a regime that, despite having small fluctuations around the average density, exhibits large clusters of galaxies.
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.
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.
14722905
On the reliability of initial conditions for dissipationless cosmological simulations
We present the study of ten random realisations of a density field characterised by a cosmological power spectrum P(k) at redshift z = 50. The reliability of such initial conditions for N-body simulations is tested with respect to their correlation properties. The power spectrum P(k) and the mass variance σM(r) do not show detectable deviations from the desired behaviour in the intermediate range of scales between the mean interparticle distance and the simulation volume. The estimator for ξ(r) is too noisy to detect any reliable signal at the initial redshift z = 50. The particle distributions are then evolved forward until z = 0. This allows us to explore the cosmic variance stemming from the random nature of the initial conditions. With cosmic variance we mean the fact that a simulation represents a single realisation of the stochastic initial conditions whereas the real Universe contains many realisations of regions of the size of the box; this problem affects most importantly the scales at about the fundamental mode. We study morphological descriptors of the matter distribution such as the genus, as well as the internal properties of the largest object(s) forming in the box. We find that the scatter is at least comparable to the scatter in the fundamental mode.
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.