Ribamar R. R. Reis

Dark interactions and cosmological fine-tuning

Cosmological models involving an interaction between dark matter and dark energy have been proposed in order to solve the so-called coincidence problem. Different forms of coupling have been studied, but there have been claims that observational data seem to narrow (some of) them down to something annoyingly close to the

From cosmic deceleration to acceleration: new constraints from SN Ia and BAO/CMB

\Lambda

THE SDSS CO-ADD: COSMIC SHEAR MEASUREMENT

CDM model, thus greatly reducing their ability to deal with the problem in the first place. The smallness problem of the initial energy density of dark energy has also been a target of cosmological models in recent years. Making use of a moderately general coupling scheme, this paper aims to unite these different approaches and shed some light as to whether this class of models has any true perspective in suppressing the aforementioned issues that plague our current understanding of the universe, in a quantitative and unambiguous way.

Entropy perturbations in quartessence Chaplygin models

We use type Ia supernovae (SN Ia) data in combination with recent baryonic acoustic oscillations (BAO) and cosmic microwave background (CMB) observations to constrain a kink-like parametrization of the deceleration parameter (q). This q-parametrization can be written in terms of the initial (qi) and present (q0) values of the deceleration parameter, the redshift of the cosmic transition from deceleration to acceleration (zt) and the redshift width of such transition (τ). By assuming a flat space geometry, qi = 1/2 and adopting a likelihood approach to deal with the SN Ia data we obtain, at the 68% confidence level (C.L.), that: zt = 0.56+0.13−0.10, τ = 0.47+0.16−0.20 and q0 = −0.31+0.11−0.11 when we combine BAO/CMB observations with SN Ia data processed with the MLCS2k2 light-curve fitter. When in this combination we use the SALT2 fitter we get instead, at the same C.L.: zt = 0.64+0.13−0.07, τ = 0.36+0.11−0.17 and q0 = −0.53+0.17−0.13. Our results indicate, with a quite general and model independent approach, that MLCS2k2 favors Dvali-Gabadadze-Porrati-like cosmological models, while SALT2 favors ΛCDM-like ones. Progress in determining the transition redshift and/or the present value of the deceleration parameter depends crucially on solving the issue of the difference obtained when using these two light-curve fitters.

The SDSS Coadd: A Galaxy Photometric Redshift Catalog

Stripe 82 in the Sloan Digital Sky Survey was observed multiple times, allowing deeper images to be constructed by coadding the data. Here we analyze the ellipticities of background galaxies in this 275 square degree region, searching for evidence of distortions due to cosmic shear. The E-mode is detected in both real and Fourier space with > 5-{sigma} significance on degree scales, while the B-mode is consistent with zero as expected. The amplitude of the signal constrains the combination of the matter density {Omega}{sub m} and fluctuation amplitude {sigma}{sub 8} to be {Omega}{sub m}{sup 0.7} {sigma}{sub 8} = 0.276{sub -0.050}{sup +0.036}.

Skewness as a test for quartessence

We show that entropy perturbations can eliminate instabilities and oscillations in the mass power spectrum of the quartessence Chaplygin models. Our results enlarge the current parameter space of models compatible with large-scale structure and cosmic microwave background observations.

Type Ia supernova parameter estimation: a comparison of two approaches using current datasets

We present and describe a catalog of galaxy photometric redshifts (photo-zs) for the Sloan Digital Sky Survey (SDSS) Coadd Data. We use the Artificial Neural Network (ANN) technique to calculate photo-zs and the Nearest Neighbor Error (NNE) method to estimate photo-z errors for {approx} 13 million objects classified as galaxies in the coadd with r < 24.5. The photo-z and photo-z error estimators are trained and validated on a sample of {approx} 89, 000 galaxies that have SDSS photometry and spectroscopic redshifts measured by the SDSS Data Release 7 (DR7), the Canadian Network for Observational Cosmology Field Galaxy Survey (CNOC2), the Deep Extragalactic Evolutionary Probe Data Release 3(DEEP2 DR3), the SDSS-IIIs Baryon Oscillation Spectroscopic Survey (BOSS), the Visible imaging Multi-Object Spectrograph - Very Large Telescope Deep Survey (VVDS) and the WiggleZ Dark Energy Survey. For the best ANN methods we have tried, we find that 68% of the galaxies in the validation set have a photo-z error smaller than {sigma}{sub 68} = 0.036. After presenting our results and quality tests, we provide a short guide for users accessing the public data.

Constraining the cosmic deceleration-acceleration transition with type Ia supernova, BAO/CMB and H(z) data

Quartessence is one of the alternatives to

The Sloan Nearby Cluster Weak Lensing Survey

\ensuremath{\Lambda}\mathrm{CDM}

Matter power spectrum for convex quartessence

that has lately attracted considerable interest. According to this unifying dark matter/energy scenario, the Universe evolved from an early nonrelativistic matter-dominated phase to a more recent accelerated expansion phase, driven by a single fluid component. Recently, it has been shown that some problems of the quartessence model, such as the existence of instabilities and oscillations in the matter power spectrum, can be avoided if a specific type of intrinsic entropy perturbation is considered. In the present article we explore the role of skewness in constraining this nonadiabatic scenario. We show that nonadiabatic quartessence and quintessence have different signatures for the skewness of the density distribution on large scales and suggest that this quantity might prove helpful to break possible degeneracies between them.