235 research outputs found
Reactividad del cemento aluminóso en agua: Caracterización de la fase acuosa de los poros y de la fase sólida
The hydration reaction of high alumina cement at a temperature of 20 ºC and water/cement ratio of 0.5 has been studied over a period of one month. The changes of the solid phase were followed by X-ray diffraction and thermal analysis. Besides, pore-solution was expressed by application of high mechanical pressure (500 MPa) during different periods of the reaction in order to study its Chemical composition evolution as a result of the hydration process.La reacción de hidratación del cemento aluminoso (CA), a la temperatura de 20 ºC y relación agua/cemento de 0,5, ha sido estudiada durante un mes. Los cambios de la fase sólida se siguieron por difracción de rayos X y análisis térmico. Además, la fase acuosa de los poros (FAP) fue extraída mediante aplicación de altas presiones mecánicas (500 MPa) en diferentes etapas de la reacción, con el fin de estudiar la evolución de su composición química durante el proceso de hidratación
Large scale structure and the generalised Chaplygin gas as dark energy
The growth of large scale structure is studied in a universe containing both
cold dark matter (CDM) and generalized Chaplygin gas (GCg). GCg is assumed to
contribute only to the background evolution of the universe while the CDM
component collapses and forms structures. We present some new analytical as
well as numerical results for linear and non-linear growth in such model. The
model passes the standard cosmological distance test without the need of a
cosmological constant (LCDM). But we find that the scenario is severely
constrained by current observations of large scale structure. Any small
deviations of the GCg parameters away from the standard Lambda dominated
cosmology (LCDM) produces substantial suppression for the growth of structures.Comment: 6 pages, matches version accepted for publication in Phys.Rev.D (in
press
Noninteracting dark matter
Since an acceptable dark matter candidate may interact only weakly with
ordinary matter and radiation, it is of interest to consider the limiting case
where the dark matter interacts only with gravity and itself, the matter
originating by the gravitational particle production at the end of inflation.
We use the bounds on the present dark mass density and the measured large-scale
fluctuations in the thermal cosmic background radiation to constrain the two
parameters in a self-interaction potential that is a sum of quadratic and
quartic terms in a single scalar dark matter field that is minimally coupled to
gravity. In quintessential inflation, where the temperature at the end of
inflation is relatively low, the field starts acting like cold dark matter
relatively late, shortly before the epoch of equal mass densities in matter and
radiation. This could have observable consequences for galaxy formation. We
respond to recent criticisms of the quintessential inflation scenario, since
these issues also apply to elements of the noninteracting dark matter picture.Comment: 37 pages, 3 figure
Measuring our universe from galaxy redshift surveys
Galaxy redshift surveys have achieved significant progress over the last
couple of decades. Those surveys tell us in the most straightforward way what
our local universe looks like. While the galaxy distribution traces the bright
side of the universe, detailed quantitative analyses of the data have even
revealed the dark side of the universe dominated by non-baryonic dark matter as
well as more mysterious dark energy (or Einstein's cosmological constant). We
describe several methodologies of using galaxy redshift surveys as cosmological
probes, and then summarize the recent results from the existing surveys.
Finally we present our views on the future of redshift surveys in the era of
Precision Cosmology.Comment: 82 pages, 31 figures, invited review article published in Living
Reviews in Relativity, http://www.livingreviews.org/lrr-2004-
Probing Planckian physics: resonant production of particles during inflation and features in the primordial power spectrum
The phenomenon of resonant production of particles {\it after} inflation has
received much attention in the past few years. In a new application of resonant
production of particles, we consider the effect of a resonance {\em during}
inflation. We show that if the inflaton is coupled to a massive particle,
resonant production of the particle during inflation modifies the evolution of
the inflaton, and may leave an imprint in the form of sharp features in the
primordial power spectrum. Precision measurements of microwave background
anisotropies and large-scale structure surveys could be sensitive to the
features, and probe the spectrum of particles as massive as the Planck scale.Comment: 19 pages, 11 eps figure
Natural Double Inflation in Supergravity
We propose a natural double inflation model in supergravity. In this model,
chaotic inflation first takes place by virtue of the Nambu-Goldstone-like shift
symmetry, which guarantees the absence of the exponential factor in the
potential for the inflaton field. During chaotic inflation, an initial value of
the second inflation (new inflation) is set. In this model, the initial value
of new inflation can be adequately far from the local maximum of the potential
for new inflation due to the small linear term of the inflaton in the K\"ahler
potential. Therefore, the primordial fluctuations within the present horizon
scale may be attributed to both inflations; that is, the first chaotic
inflation produces the primordial fluctuations on the large cosmological scales
while the second new inflation on the smaller scales. The successive decay of
the inflaton for new inflation leads to a reheating temperature low enough to
avoid the overproduction of gravitinos in a wide range of the gravitino mass.Comment: 13 pages, to appear in Phys. Rev.
The Multiscale Morphology Filter: Identifying and Extracting Spatial Patterns in the Galaxy Distribution
We present here a new method, MMF, for automatically segmenting cosmic
structure into its basic components: clusters, filaments, and walls.
Importantly, the segmentation is scale independent, so all structures are
identified without prejudice as to their size or shape. The method is ideally
suited for extracting catalogues of clusters, walls, and filaments from samples
of galaxies in redshift surveys or from particles in cosmological N-body
simulations: it makes no prior assumptions about the scale or shape of the
structures.}Comment: Replacement with higher resolution figures. 28 pages, 17 figures. For
Full Resolution Version see:
http://www.astro.rug.nl/~weygaert/tim1publication/miguelmmf.pd
The fundamental constants and their variation: observational status and theoretical motivations
This article describes the various experimental bounds on the variation of
the fundamental constants of nature. After a discussion on the role of
fundamental constants, of their definition and link with metrology, the various
constraints on the variation of the fine structure constant, the gravitational,
weak and strong interactions couplings and the electron to proton mass ratio
are reviewed. This review aims (1) to provide the basics of each measurement,
(2) to show as clearly as possible why it constrains a given constant and (3)
to point out the underlying hypotheses. Such an investigation is of importance
to compare the different results, particularly in view of understanding the
recent claims of the detections of a variation of the fine structure constant
and of the electron to proton mass ratio in quasar absorption spectra. The
theoretical models leading to the prediction of such variation are also
reviewed, including Kaluza-Klein theories, string theories and other
alternative theories and cosmological implications of these results are
discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy
Gravitational Lensing
Gravitational lensing has developed into one of the most powerful tools for
the analysis of the dark universe. This review summarises the theory of
gravitational lensing, its main current applications and representative results
achieved so far. It has two parts. In the first, starting from the equation of
geodesic deviation, the equations of thin and extended gravitational lensing
are derived. In the second, gravitational lensing by stars and planets,
galaxies, galaxy clusters and large-scale structures is discussed and
summarised.Comment: Invited review article to appear in Classical and Quantum Gravity, 85
pages, 15 figure
Large Scale Structure of the Universe
Galaxies are not uniformly distributed in space. On large scales the Universe
displays coherent structure, with galaxies residing in groups and clusters on
scales of ~1-3 Mpc/h, which lie at the intersections of long filaments of
galaxies that are >10 Mpc/h in length. Vast regions of relatively empty space,
known as voids, contain very few galaxies and span the volume in between these
structures. This observed large scale structure depends both on cosmological
parameters and on the formation and evolution of galaxies. Using the two-point
correlation function, one can trace the dependence of large scale structure on
galaxy properties such as luminosity, color, stellar mass, and track its
evolution with redshift. Comparison of the observed galaxy clustering
signatures with dark matter simulations allows one to model and understand the
clustering of galaxies and their formation and evolution within their parent
dark matter halos. Clustering measurements can determine the parent dark matter
halo mass of a given galaxy population, connect observed galaxy populations at
different epochs, and constrain cosmological parameters and galaxy evolution
models. This chapter describes the methods used to measure the two-point
correlation function in both redshift and real space, presents the current
results of how the clustering amplitude depends on various galaxy properties,
and discusses quantitative measurements of the structures of voids and
filaments. The interpretation of these results with current theoretical models
is also presented.Comment: Invited contribution to be published in Vol. 8 of book "Planets,
Stars, and Stellar Systems", Springer, series editor T. D. Oswalt, volume
editor W. C. Keel, v2 includes additional references, updated to match
published versio
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