12 research outputs found
Clustering Properties of Dynamical Dark Energy Models
We provide a generic but physically clear discussion of the clustering
properties of dark energy models. We explicitly show that in quintessence-type
models the dark energy fluctuations, on scales smaller than the Hubble radius,
are of the order of the perturbations to the Newtonian gravitational potential,
hence necessarily small on cosmological scales. Moreover, comparable
fluctuations are associated with different gauge choices. We also demonstrate
that the often used homogeneous approximation is unrealistic, and that the
so-called dark energy mutation is a trivial artifact of an effective, single
fluid description. Finally, we discuss the particular case where the dark
energy fluid is coupled to dark matter.Comment: 5 page
Linear and non-Linear Instabilities in Unified Dark Energy Models
We revisit the paradigm of unified dark energy discussing in detail the
averaging problem in this type of scenarios, highlighting the need for a full
non-linear treatment. We also address the question of if and how models with
one or several dark fluids can be observationally distinguished. Simpler and
physically clearer derivations of some key results, most notably on the
relation between the generalized Chaplygin gas and the standard (CDM)
`concordance' model and on a Jeans-type small-scale instability of some coupled
dark energy/dark matter models are presented.Comment: 6 pages, accepted for publication in Physical Review
Adsorption of yellow lanasol 4g reactive dye in a simulated textile effluent on gallinaceous feathers
Adsorption is one of the most efficient physicochemical processes known to remove colour in textile industry effluents. Activated charcoal is conventionally the most used material for this purpose and although its efficiency is very high also is its price. The aim of this study was to know the variables that influence the adsorption process of wool reactive dye Yellow Lanasol 4G in a simulated textile effluent, on an inexpensive and abundant material, gallinaceous feathers, so as to optimize the operating conditions. Factorial experimentation within a certain domain was used to determine the influence level of several parameters, such as, temperature, initial pH of the solution, "size" and adsorbent's concentration and their interactions. A statistical analysis of the results showed that within the selected domain all the parameters have influence at a significance level of 1% excepting initial pH that only has influence at a significance level of 5%. Some of the possible interactions between these parameters also have significant influence on the adsorption process specially first order ones, including significance levels of 5 and 1%. In industrial practice it will be possible to take advantage of the prominent effect of a temperature increase on adsorption capacity as effluents of wool dyeing baths are already at an elevated temperature. Besides that as the influence of "granulometry" becomes rather smaller for higher temperatures there will be no need to grind the adsorbent. This means then that optimization of operating conditions in industrial wastewaters treatment using the studied adsorption technique will not imply additional costs except those concerning pH adjustment. This emphasizes the importance of gallinaceous feathers as an alternative to activated carbon.(undefined
Alternatives to Quintessence Model Building
We discuss the issue of toy model building for the dark energy component of
the universe. Specifically, we consider two generic toy models recently
proposed as alternatives to quintessence models, known as Cardassian expansion
and the Chaplygin gas. We show that the former is enteriely equivalent to a
class of quintessence models. We determine the observational constraints on the
latter, coming from recent supernovae results and from the shape of the matter
power spectrum. As expected, these restrict the model to a behaviour that
closely matches that of a standard cosmological constant .Comment: RevTex4; 7 pages, 4 figures. v2: Improved discussion of constraints
on Chaplygin gas models. Other clarifications added. Phys Rev. D (in press
The Role of Baryons in Unified Dark Matter Models
We discuss the importance of including baryons in analyses of unified dark
matter scenarios, focusing on toy models involving a generalized Chaplygin gas.
We determine observational constraints on this unified dark matter scenario
coming from large scale structure, type Ia Supernovae and CMB data showing how
this component can bring about a different behaviour from classical
CDM and thus motivate further studies of this type of models. We also
speculate on interesting new features which are likely to be important on
non-linear scales in this context.Comment: 4 pages, 3 figures. Final version (with typos corrected and a
reference added), to appear in Phys Rev D (Rapid Communications
Bayesian Analysis of the Chaplygin Gas and Cosmological Constant Models using the SNe Ia Data
The type Ia supernovae observational data are used to estimate the parameters
of a cosmological model with cold dark matter and the Chaplygin gas. The
Chaplygin gas model depends essentially on four parameters: the Hubble
constant, the velocity of the sound of the Chaplygin gas, the curvature of the
Universe and the fraction density of the Chaplygin gas and the cold dark
matter. The Bayesian parameter estimation yields , , , , , and . These and other results indicate that a Universe
completely dominated by the Chaplygin gas is favoured, at least as the type Ia
supernovae data are concerned. A closed and accelerating Universe is also
favoured. The Bayesian statistics indicates that the Chaplygin gas model is
more likely than the standard cosmological constant () model at
55.3% confidence level when an integration on all free parameters is performed.
Assuming the spatially flat curvature, this percentage mounts to 65.3%. On the
other hand, if the density of dark matter is fixed at zero value, the Chaplygin
gas model becomes more preferred than the model at 91.8%
confidence level. Finally, the hypothesis of flat Universe and baryonic matter
() implies a Chaplygin gas model preferred over the at a confidence level of 99.4%.Comment: 19 pages, LaTeX 2e, 4 EPS figures, uses graphic
The onset of the non-linear regime in unified dark matter models
We discuss the onset of the non-linear regime in the context of unified dark
matter models involving a generalised Chaplygin gas. We show that the
transition from dark matter-like to dark energy-like behaviour will never be
smooth. In some regions of space the transition will never take place while in
others it may happen sooner or later than naively expected. As a result the
linear theory used in previous studies may break down late in the matter
dominated era even on large cosmological scales. We study the importance of
this effect showing that its magnitude depends on the exact form of the
equation of state in the low density regime. We expect that our results will be
relevant for other unified dark matter scenarios particularly those where the
quartessence candidate is a perfect fluid.Comment: 5 pages, 4 figure