Acoustic oscillations and viscosity

Using a simple thermo-hydrodynamic model that respects relativistic causality, we revisit the analysis of qualitative features of acoustic oscillations in the photon-baryon fluid. The growing photon mean free path introduces transient effects that can be modelled by the causal generalization of relativistic Navier-Stokes-Fourier theory. Causal thermodynamics provides a more satisfactory hydrodynamic approximation to kinetic theory than the quasi-stationary (and non-causal) approximations arising from standard thermodynamics or from expanding the photon distribution to first order in the Thomson scattering time. The causal approach introduces small corrections to the dispersion relation obtained in quasi-stationary treatments. A dissipative contribution to the speed of sound slightly increases the frequency of the oscillations. The diffusion damping scale is slightly increased by the causal corrections. Thus quasi-stationary approximations tend to over-estimate the spacing and under-estimate the damping of acoustic peaks. In our simple model, the fractional corrections at decoupling are $\gtrsim 10^{-3}$.Comment: Improved version with new quantitative estimates and some corrections. We show how quasi-stationary approximations based on expanding the photon distribution to first order in the Thomson time tend to under-estimate the frequency and damping of acoustic oscillation

Qualitative Analysis of Causal Cosmological Models

The Einstein's field equations of Friedmann-Robertson-Walker universes filled with a dissipative fluid described by both the {\em truncated} and {\em non-truncated} causal transport equations are analyzed using techniques from dynamical systems theory. The equations of state, as well as the phase space, are different from those used in the recent literature. In the de Sitter expansion both the hydrodynamic approximation and the non-thermalizing condition can be fulfilled simultaneously. For $\Lambda=0$ these expansions turn out to be stable provided a certain parameter of the fluid is lower than 1/2. The more general case $\Lambda>0$ is studied in detail as well.Comment: RevTeX file, 23 pages. Accepted for publication in J. Math. Phy

Collisional equilibrium, particle production and the inflationary universe

Particle production processes in the expanding universe are described within a simple kinetic model. The equilibrium conditions for a Maxwell-Boltzmann gas with variable particle number are investigated. We find that radiation and nonrelativistic matter may be in equilibrium at the same temperature provided the matter particles are created at a rate that is half the expansion rate. Using the fact that the creation of particles is dynamically equivalent to a nonvanishing bulk pressure we calculate the backreaction of this process on the cosmological dynamics. It turns out that the `adiabatic' creation of massive particles with an equilibrium distribution for the latter necessarily implies power-law inflation. Exponential inflation in this context is shown to become inconsistent with the second law of thermodynamics after a time interval of the order of the Hubble time.Comment: 19 pages, latex, no figures, to appear in Phys.Rev.

Stress effects in structure formation

Residual velocity dispersion in cold dark matter induces stresses which lead to effects that are absent in the idealized dust model. A previous Newtonian analysis showed how this approach can provide a theoretical foundation for the phenomenological adhesion model. We develop a relativistic kinetic theory generalization which also incorporates the anisotropic velocity dispersion that will typically be present. In addition to density perturbations, we consider the rotational and shape distortion properties of clustering. These quantities together characterize the linear development of density inhomogeneity, and we find exact solutions for their evolution. As expected, the corrections are small and arise only in the decaying modes, but their effect is interesting. One of the modes for density perturbations decays less rapidly than the standard decaying mode. The new rotational mode generates precession of the axis of rotation. The new shape modes produce additional distortion that remains frozen in during the subsequent (linear) evolution, despite the rapid decay of the terms that caused it.Comment: significantly improved discussion of kinetic theory of CDM velocity dispersion; to appear Phys. Rev.

El problema de la matèria fosca de l'univers

La Cosmologia com a intent d’entendre el Cosmos com a realitat global en la qual poder emmarcar la existència de l’ésser humà, és tan antiga com els mites que han acompanyat al naixement de les diverses civilitzacions que ens han precedit. Tanmateix, l’assoliment d’una aproximació científica i observacional a l’origen, evolució i estructura de l’Univers és força recent. Dels diverses i important avenços científics del segle XX, hem de destacar la teoria General de la Relativitat d’Einstein (1916) com a marc teòric en el qual fonamentar els estudis cosmològics. Conforma l’entramat que dóna sentit i coherència a les cada vegada més copioses observacions i dades que recollim. Malgrat que els intents en superar-la i reemplaçar-la no han escassejat, aquests no han reeixit a data d’avui. Sens dubte, altres fites a destacar, aquest cop observacionals, són: (a) el descobriment de l’expansió de l’univers per E. Hubble (1929), és a dir, que les galàxies s’allunyen entre sí amb velocitats proporcionals a les distàncies entre elles, (b) la radiació de fons de microones (1965) que impregna tot l’Univers. Des de ja fa algunes dècades s’han anat recollint evidències, cada cop més contundents, que indiquen la presència de quantitats ingents de matèria no detectable de forma directa (fosca) i de naturalesa desconeguda. Els últims anys, però, han estat testimoni de progressos observacionals notables que han posat de relleu que el ritme al qual l’univers s’expandeix, està accelerant-se a un ritme tal que requereix l’existència d’una quantitat enorme d’energia desconeguda. Aquests treballs, val a dir, han estat reconeguts amb l'atorgament del premi Nobel de Física d'enguany. S’estima actualment que aquesta energia ‘fosca’ donaria compte del 75% del contingut energètic total del Cosmos, un 20% ho seria la matèria fosca, i tan sols el restant 5% ho constituiria la matèria normal coneguda per la física actual i de la qual estan fets els planetes, estrelles i galàxies que observem

Informe relatiu als informes del primer i segon semestre de 1984 del Consejo de Seguridad Nuclear

Còpia en paper oxidat. Marques d'òxid d'un clip i part del text marcat amb rotulador fluorescent a la primera pàgina.Publicació al Boletín Oficial de las Cortes Generales de l'informe realitzat per la Ponencia Especial encarregada d'analitzar els informes de 1984 del Consejo de Seguridad Nuclear i de les resolucions aprovades per aquesta. També es fa una relació de tota la documentació sol·licitada al CSN

Sistemes BCI i 3D per control de drones

Nowadays man-machine interaction is so common in people's lives that we do not even realize how many daily operations we conduct using electronic devices, such as phones, personal computers, ATMs, tablets, televisions... As technology develops it tends to separate such interaction using less interconnected elements, reaching sometimes the communication with body gestures or just biological signals from the body. An important part of lifetime is leisure and entertainment in which currently many electronic devices are involved. For example, Quadrotors or Drones are recently been sold just for the fun of flying them around. This project has tried to take a step beyond the control of a drone or any other mobile device, removing direct interaction with the usual controls; replacing them with a control through a BCI (Brain Computer Interface) system and reinforcing it with an acceleration sensor. In addition, it has been integrated a viewing system, simulating 3D which provides the user a much more realistic and deep environment. The aim of the whole is to create a user-friendly and fast control system, focused specifically on disabled and elderly people. And by positioning the devices with the viewing system it provides control to those who denote a greater disability. This project aims to equal conditions between people with and without disabilities; expanding the diversity of leisure activities often limited in this sector despite the fact that day by day progresses are been made. It facilitates the access to new technologies for these people; to new forms of leisure and entertainment.A día de hoy la interacción hombre máqui na está tan presente en la vida de las personas , que ni si quiera nos damo s cuenta de c uantas ope raciones c otidianas realizamos utilizando dispositivos electrónicos , ya sean móviles, ordenadores personales, cajeros automáticos, tabletas, televisores... A m edida que avanza la tecnología , é sta tiende a separar dicha interacción utilizando cada vez menos elementos conectados entre sí, llegando a veces simplemente a gestos o simples señales biológicas del cuerpo. Una parte importante del día a día es el ocio de las personas y actualmente en dicho ocio se involucran muchos de éstos dispositivos electrónicos. Reciente mente, se h an comercializado quadroto res o d rones con el fin de que el usuario pueda disfrutar de un rato ameno y distraído , haciendo volar est os aparatos . Este proyecto ha tratado de llevar un paso más allá el control de un drone o cualquier otro disposi tivo teledirigido , eliminando la interacción directa mediante los controles habituales, sustituyéndolos por un control de sistema BCI (Brain Computer Interface) y reforzando este sistema c on un sensor inercial . Así mismo, se ha integrado junto con el sistema de con trol un sistema de visionado , simulando el 3D; para así dotar al usuario de un entorno y una profundidad mucho más realista. Con el conjunto del sistema se pretende crear un método de control fácil y r ápido, enfocado específicamente para pe rsonas discapacitadas o de la tercera edad . Y mediante el sistema 3D se facilita el seguimiento de dichos apara tos para aquellas personas que padecen una mayor incapacitación. Con este proyecto s e pretende contribuir a la igualdad de condiciones entre pe rsonas con y sin discapacidad, ampliando la oferta de ocio en un sector muchas veces limitado, pero en el que día a día se consiguen nuevos progresos. Se facilita así, el acceso a nuevas tecnologías para estas personas; a nuevas formas de ocio y entretenim ient