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.
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.
43375390
Hydrodynamic approach to the evolution of cosmological structures
A hydrodynamic formulation of the evolution of large-scale structure in the Universe is presented. It relies on the spatially coarse-grained description of the dynamical evolution of a many-body gravitating system. Because of the assumed irrelevance of short-range (``collisional'') interactions, the way to tackle the hydrodynamic equations is essentially different from the usual case. The main assumption is that the influence of the small scales over the large-scale evolution is weak: this idea is implemented in the form of a large-scale expansion for the coarse-grained equations. This expansion builds a framework in which to derive in a controlled manner the popular ``dust'' model (as the lowest-order term) and the ``adhesion'' model (as the first-order correction). It provides a clear physical interpretation of the assumptions involved in these models and also the possibility to improve over them.
14575890
Morphology of the large-scale structure
The Minkowski functionals are a mathematical tool to quantify morphological features of patterns. Some applications to the matter distribution in galaxy catalogues and N-body simulations are reviewed, with an emphasis on the effects of cosmic variance. The conclusions are that (i) the observed large-scale morphology is sensitive to cosmic variance on scales much larger than the nonlinear length (approx. 8 Mpc/h), and (ii) the large-scale morphology predicted by simulations is thus affected by finite-size effects, but nonetheless a Lambda-CDM model is favored.
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.
13112109
Velocity dispersion in N-body simulations of CDM models
This work reports on a study of the spatially coarse-grained velocity dispersion in cosmological N-body simulations (OCDM and ΛCDM models) as a function of time (redshifts z = 0–4) and of the coarsening length (0.6–20 h−1 Mpc). The main result is the discovery of a polytropic relationship ℐ1 ∝ ϱ2–η between the velocity-dispersion kinetic energy density of the coarsening cells, ℐ1, and their mass density, ϱ. The exponent η, dependent on time and coarsening scale, is a compact measure of the deviations from the naive virial prediction ηvirial = 0. This relationship supports the “polytropic assumption” which has been employed in theoretical models for the growth of cosmological structure by gravitational instability. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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.
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.
109580082
The CMS pixel detector
The Compact Muon Solenoid Experiment (CMS) will start taking data at the Large Hadron Collider (LHC) in 2007 with the largest silicon tracking detector ever built. The central components of this tracker consist of two forward/backward pixel disks on each side of the interaction region and three barrel pixel layers. The silicon pixel detector will be crucial to pattern recognition and track reconstruction in the hadronic collisions of CMS and will play a key role in the physics program of the LHC. During the 2007 pilot physics run of the LHC, CMS will run with a subset of the final detector to be installed in 2008. In this report, the final design and results from test beam runs and expected performance of the detector are given. The expected radiation tolerance and projected lifetime of the pixel detector will be discussed as well its impact on the physics program of CMS.