J. Ibanez

Role of electric charge and cosmological constant in structure scalars

The physical meaning of structure scalars is analyzed for charged dissipative spherical fluids and for neutral dust in the presence of cosmological constant. The role played by such factors in the structure scalars is clearly brought out and physical consequences are discussed. Particular attention needs to be paid to the changes introduced by the above mentioned factors in the inhomogeneity factor and the evolution of the expansion scalar and the shear tensor.

Poincaré-invariant gravitational field and equations of motion of two pointlike objects: The postlinear approximation of general relativity

Using a fast-motion approximation method we obtain the second-order gravitational field and equations of motion for two pointlike objects in algebraically closed form. A regularization procedure is used which is shown to guarantee the consistency of the approximation scheme. The equations of motion are then transformed within the framework of relativistic predictive mechanics into a system of ordinary differential equations.

TILTED LEMAITRE-TOLMAN-BONDI SPACETIMES: HYDRODYNAMIC AND THERMODYNAMIC PROPERTIES

(Dated: June 27, 2011)We consider Lemaitre–Tolman–Bondi spacetimes from the point of view of a tilted observer,i.e. one with respect to which the fluid is radially moving. The imperfect fluid and the congruencedescribed by its four–velocity, as seen by the tilted observer is studied in detail. It is shown that fromthe point of view of such tilted observer the fluid evolves non–reversibly (i.e. with non–vanishingrate of entropy production). The non–geodesic character of the tilted congruence is related to thenon–vanishing of the divergence of the 4–vector entropy flow. We determine the factor related to theexistence of energy–density inhomogeneities and describe its evolution, these results are comparedwith those obtained for the non–tilted observer. Finally, we exhibit a peculiar situation where thenon–tilted congruence might be unstable.

Organisms and their place in biology

In this paper we review the concept of organism analysing the main ideas related to it in the context of present biological theories. The discussion is focused and developed according to four key issues: individuality, organisation, autonomy and reproduction. Once these basic connections are established, a spectrum of possible entities that fall under the label ‘organism’ is looked over, with special emphasis on limit or controversial cases. The aim is to see whether they all share a set of common features and, if they do, why it is so difficult to reach a consensus on the definition of the term. Finally, we try to release somehow the tension between those hierarchical schemes proposed to account for life as a global phenomenon and those approaches that take organisms as the central target of (theoretical) biology, suggesting a possible middle-ground solution open for further research.

Dissipative collapse of axially symmetric, general relativistic, sources: A general framework and some applications

We carry out a general study on the collapse of axially (and reflection) symmetric sources in the context of general relativity. All basic equations and concepts required to perform such a general study are deployed. These equations are written down for a general anisotropic dissipative fluid. The proposed approach allows for analytical studies as well as for numerical applications. A causal transport equation derived from the Israel-Stewart theory is applied, to discuss some thermodynamic aspects of the problem. A set of scalar functions (the structure scalars) derived from the orthogonal splitting of the Riemann tensor are calculated and their role in the dynamics of the source is clearly exhibited. The characterization of the gravitational radiation emitted by the source is discussed.

Cognition and life : The autonomy of cognition

In this paper we propose a philosophical distinction between biological and cognitive domains based on two conditions that are postulated to obtain a useful characterization of cognition: biological grounding and explanatory sufficiency. According to this, we argue that the origin of cognition in natural systems (cognition as we know it) is the result of the appearance of an autonomous system embedded into another more generic one: the whole organism. This basic idea is complemented by another one: the formation and development of this system, in the course of evolution, cannot be understood but as the outcome of a continuous process of interaction between organisms and environment, between different organisms, and, specially, between the very cognitive organisms. Finally, we address the problem of the generalization of a theory of cognition (cognition as it could be) and conclude that this work would imply a grounding work on the problem of the origins developed in the frame of a confluence between both Artificial Life and an embodied Artificial Intelligence.

Self-Inspection Based Reproduction in Cellular Automata

In this work we analyze different existing cellular automata capable of reproducing, and conclude that reproduction in those models is attained as a fixed point of the more general operation of construction, instead of being founded as a higher level specialized mechanism. Thus reproduction becomes a very unstable property and this fact is an obstacle in the aim of studying it together with other Alife characteristics, mainly because of destructive interactions. We propose a cellular reproduction model based on self-inspection as an attempt to overcome at least partially these drawbacks. We argue that our model attains a better founded an more robust reproduction scheme, and briefly explore the possibilities of using it to model the emergence of reproducing structures, to study the interplay between autonomy and reproduction, and to use it as a basis for evolutionary experimental scenarios.

On the collision of gravitational plane waves: a class of soliton solutions

The inverse scattering method is used to obtain a class of solutions of the vacuum Einstein equations describing the space-time following the collision of two gravitational plane waves. The general features of these solutions are analyzed in terms of the behavior of the Weyl scalars, and some degenerate cases are discussed.

Axially symmetric static sources: A general framework and some analytical solutions

Departamento de Matema´tica Aplicada, Universidad de Salamanca, Salamanca 37007, Spain(Received 19 November 2012; published 8 January 2013)We provide all basic equations and concepts required to carry out a general study on axially symmetricstatic sources. The Einstein equations and the conservation equations are written down for a generalanisotropic static fluid endowed with axial symmetry. The structure scalars are calculated and theinhomogeneity factors are identified. Finally some exact analytical solutions were found. One of thesesolutions describes an incompressible spheroid with isotropic pressure and becomes the well-knowninterior Schwarzschild solution in the spherically symmetric limit; however, it cannot be matchedsmoothly to any Weyl exterior metric. Another family of solutions was found that corresponds to ananisotropic fluid distribution and can in principle be matched to a Weyl exterior.

Exponential-potential scalar field universes. I. Bianchi type I models.

We obtain a general exact solution of the Einstein field equations for the anisotropic Bianchi type I universes filled with an exponential-potential scalar field and study their dynamics. It is shown, in agreement with previous studies, that for a wide range of initial conditions the late-time behavior of the models is that of a power-law inflating Friedmann-Robertson-Walker (FRW) universe. This property does not hold, in contrast, when some degree of inhomogeneity is introduced, as discussed in our following paper.