J. P. M. de Carvalho

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, respectively known as Cardassian expansion and the Chaplygin gas. We show that the former is entirely 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 �.

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 behavior from the classical cold dark matter model with a cosmological constant and thus motivate further studies of this type of models. We also speculate on interesting new features which are likely to be important on nonlinear scales in this context.

Onset of the nonlinear 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.

The ΛCDM limit of the generalized Chaplygin gas scenario

We explicitly demonstrate that, contrary to recent claims, the dynamics of a generalized Chaplygin gas model with an equation of state

Spherical `Top-Hat' Collapse in general Chaplygin gas dominated universes

p=-C

Dark matter from cosmic defects on galactic scales

(where

Can our Universe be inhomogeneous on large sub-horizon scales?

C

Can we predict the fate of the Universe

is a positive constant) is equivalent to that of a standard

COSMIC STRINGS WITH SELF-INTERACTING HOT DARK MATTER

\Lambda

Formation of Galaxies in a Dark Matter Dominated Universe - A New Approach

CDM model to first order in the metric perturbations. We further argue that the analogy between the two models goes well beyond linear theory and conclude that they cannot be distinguished based on gravity alone.