11,057 research outputs found
Probing Dynamical Dark Energy with Press-Schechter Mass Functions
Measurement of accelerated expansion in the Universe led to propose a new
cosmic fluid as its cause: dark energy. Its various incarnations offer a wealth
of models whose relevance it is important to discriminate via contacts with
observations. I will present my investigations on the influence of dynamical
dark energy models on the formation of non-linear dark matter structures. In
particular, I will focus on structures traced by the mass function of dark
matter haloes.Comment: submitted to the Proceedings of the 11th Marcel Grossmann Meeting,
MG11, Berlin, Germany, July 23-29, 2006 (2007) to be publishe
Separating expansion from contraction and generalizing TOV condition in spherically symmetric models with pressure
We investigate spherically symmetric solutions with pressure and discuss the
existence of a dividing shell separating expanding and collapsing regions. We
perform a 3+1 splitting and obtain gauge invariant conditions relating not only
the intrinsic spatial curvature of the shells to the ADM mass, but also a
function of the pressure which we introduce that generalises the
Tolman-Oppenheimer-Volkoff equilibrium condition. We consider the particular
case of a Lema\^itre-Tolman dust models with a cosmological constant (a
-CDM model) as an example of our results.Comment: 4pp, 1fig, AIP Conference: Proc. XXXI Spanish Relat. Meeting, ERE08,
Salamanca, Spain, 15-19 Sept., 2008 Septembre, Salamanca, Espagn
A unified solution to the small scale problems of the CDM model II: introducing parent-satellite interaction
We continue the study of the impact of baryon physics on the small scale
problems of the CDM model, based on a semi-analytical model (Del
Popolo, 2009). Withsuch model, we show how the cusp/core, missing satellite
(MSP), Too Big to Fail (TBTF) problems and the angular momentum catastrophe can
be reconciled with observations, adding parent-satellite interaction. Such
interaction between darkmatter (DM) and baryons through dynamical friction (DF)
can sufficiently flattenthe inner cusp of the density profiles to solve the
cusp/core problem. Combining, in our model, a Zolotov et al. (2012)-like
correction, similarly to Brooks et al. (2013), and effects of UV heating and
tidal stripping, the number of massive, luminous satellites, as seen in the Via
Lactea 2 (VL2) subhaloes,is in agreement with the numbers observed in the MW,
thus resolving the MSP and TBTF problems. The model also produces a
distribution of the angular spin parameter and angular momentum in agreement
with observations of the dwarfs studied by van den Bosch, Burkert, \\& Swaters
(2001).Comment: 24pp, 5figs. arXiv admin note: text overlap with arXiv:1404.367
Small scale problems of the CDM model: a short review
The CDM model, or concordance cosmology, as it is often called, is a
paradigm at its maturity. It is clearly able to describe the universe at large
scale, even if some issues remain open, such as the cosmological constant
problem , the small-scale problems in galaxy formation, or the unexplained
anomalies in the CMB. CDM clearly shows difficulty at small scales,
which could be related to our scant understanding, from the nature of dark
matter to that of gravity; or to the role of baryon physics, which is not well
understood and implemented in simulation codes or in semi-analytic models. At
this stage, it is of fundamental importance to understand whether the problems
encountered by the DCM model are a sign of its limits or a sign of our
failures in getting the finer details right. In the present paper, we will
review the small-scale problems of the CDM model, and we will discuss
the proposed solutions and to what extent they are able to give us a theory
accurately describing the phenomena in the complete range of scale of the
observed universe.Comment: 48pp 19 figs, invited review, accepted by Galaxie
A model comparison to predict heat transfer during spot GTA welding
The present work deals with the estimation of the time evolution of the weld fusion boundary. This moving boundary is the result of a spot GTA welding process on a 316L stainless steel disk. The estimation is based on the iterative regularization method. Indeed, the three problems: direct, in variation and adjoint, classically associated with this method, are solved by the finite element method in a two-dimensional axisymmetric domain. The originality of this work is to treat an experimental estimation of a front motion using a model with a geometry including only the solid phase. In this model, the evolution of this solid domain during the fusion is set with the ALE moving mesh method (Arbitrary Lagrangian Eulerian). The numerical developments are realized with the commercial code Comsol Multiphysics® coupled with the software Matlab®. The estimation method has been validated in a previous work using theoretical data ([1]). The experimental data, used here for this identification are, temperatures measured by thermocouples in the solid phase, the temporal evolution of the melt pool boundary observed at the surface by a fast camera and the maximal dimensions of the melted zone measured on macrographs. These experimental data are also compared with numerical results obtained from a heat and fluid flow model taking into account surface tension effects, Lorentz forces and the deformation of the melt pool surface under arc pressure
A dual null formalism for the collapse of fluids in a cosmological background
In this work we revisit the definition of Matter Trapping Surfaces (MTS)
introduced in previous investigations and show how it can be expressed in the
so-called dual null formalism developed for Trapping Horizons (TH). With the
aim of unifying both approaches, we construct a 2+2 threading from the 1+3
flow, and thus isolate one prefered spatial direction, that allows
straightforward translation into a dual nul subbasis, and to deduce the
geometric apparatus that follows. We remain as general as possible, reverting
to spherical symmetry only when needed, and express the MTS conditions in terms
of 2-expansion of the flow, then in purely geometric form of the dual null
expansions. The Raychadhuri equations that describe both MTS and TH are written
and interpreted using the previously defined gTOV (generalized
Tolman-Oppenheimer-Volkov) functional introduced in previous work. Further
using the Misner-Sharp mass and its previous perfect fluid definition, we
relate the spatial 2-expansion to the fluid pressure, density and acceleration.
The Raychaudhuri equations also allows us to define the MTS dynamic condition
with first order differentials so the MTS conditions are now shown to be all
first order differentials. This unified formalism allows one to realise that
the MTS can only exist in normal regions, and so it can exist only between
black hole horizons and cosmological horizons. Finally we obtain a relation
yielding the sign, on a TH, of the non-vanishing null expansion which
determines the nature of the TH from fluid content, and flow characteristics.
The 2+2 unified formalism here investigated thus proves a powerful tool to
reveal, in the future extensions, more of the very rich and subtle relations
between MTS and TH.Comment: 10pp 1 fig. corrected for equation labels, cross listing correcte
Dark Energy-Dark Matter Interaction from the Abell Cluster A586
We find that deviation from the virial equilibrium of the Abell Cluster A586
yields evidence of the interaction between dark matter and dark energy. We
argue that this interaction might imply a violation of the Equivalence
Principle. Our analysis show that evidence is found in the context of two
different models of dark energy-dark matter interaction.Comment: Talk presented by O.B. at Encuentros Relativistas Espanoles 2007,
Puerto de la Cruz, Tenerife, Spain, 10-14 September 2007. 6 pages, 3 figures
and style fil
Image Watermaking With Biometric Data For Copyright Protection
In this paper, we deal with the proof of ownership or legitimate usage of a
digital content, such as an image, in order to tackle the illegitimate copy.
The proposed scheme based on the combination of the watermark-ing and
cancelable biometrics does not require a trusted third party, all the exchanges
are between the provider and the customer. The use of cancelable biometrics
permits to provide a privacy compliant proof of identity. We illustrate the
robustness of this method against intentional and unintentional attacks of the
watermarked content
Spherically symmetric models: separating expansion from contraction in models with anisotropic pressures
We investigate spherically symmetric spacetimes with an anisotropic fluid and
discuss the existence and stability of a dividing shell separating expanding
and collapsing regions. We find that the dividing shell is defined by a
relation between the pressure gradients, both isotropic and anisotropic, and
the strength of the fields induced by the Misner-Sharpe mass inside the
separating shell and by the pressure fluxes. This balance is a generalization
of the Tolman-Oppenheimer- Volkoff equilibrium condition which defines a local
equilibrium condition, but conveys also a non- local character given the
definition of the Misner-Sharpe mass. We present a particular solution with
dust and radiation that provides an illustration of our results.Comment: 4pp Towards New Paradigms: Proceeding Of The Spanish Relativity
Meeting 2011. AIP Conference Proceedings, Volume 1458, pp. 487-490 (2012).
Published in AIP Conf.Proc. 1458 (2011) 487-49
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