Ruth Lazkoz

Constructing phantom cosmologies from standard scalar field universes.

We illustrate how form-invariance transformations can be used for constructing phantom cosmologies from standard scalar field universes. First we discuss how to relate two flat Friedmann-Robertson-Walker cosmologies with different barotropic indexes gamma and gamma;. Then we consider the particular case gamma;=-gamma, and we show that, if the matter content is interpreted in terms of self-interacting scalar fields, then the corresponding transformation provides the link between a standard and a phantom cosmology. After that, we illustrate the method by considering models with exponential potentials. Finally, we also show that the mentioned duality persists even if the typical brane-world modifications to the Friedmann equation are considered.

Dynamics of dark energy with a coupling to dark matter

Dark energy and dark matter are the dominant sources in the evolution of the late universe. They are currently only indirectly detected via their gravitational effects, and there could be a coupling between them without violating observational constraints. We investigate the background dynamics when dark energy is modeled as exponential quintessence and is coupled to dark matter via simple models of energy exchange. We introduce a new form of dark sector coupling, which leads to a more complicated dynamical phase space and has a better physical motivation than previous mathematically similar couplings.

Geodesic behavior of sudden future singularities

In this paper we analyze the effect of recently proposed classes of sudden future singularities on causal geodesics of FLRW spacetimes. Geodesics are shown to be extendible and just the equations for geodesic deviation are singular, although tidal forces are not strong enough to produce a Big Rip. For the sake of completeness, we compare with the typical sudden future singularities of phantom cosmologies.

Tracking solutions in tachyon cosmology

We perform a thorough phase-plane analysis of the flow defined by the equations of motion of a FRW Universe filled with a tachyonic fluid plus a barotropic one. The tachyon potential is assumed to be of inverse square form, thus allowing for a two-dimensional autonomous system of equations. The Friedmann constraint, combined with a convenient choice of coordinates, renders the physical state compact. We find the fixed-point solutions, and discuss whether or not they represent attractors. The way the two fluids contribute at late times to the fractional energy density depends on how fast the barotropic fluid redshifts. If it does it fast enough, the tachyonic fluid takes over at late times, but if the opposite happens, the situation will not be completely dominated by the barotropic fluid; instead there will be a residual non-negligible contribution from the tachyon subject to restrictions coming from nucleosynthesis.

Quintom cosmologies admitting either tracking or phantom attractors

Abstract In this Letter we investigate the evolution of a class of cosmologies fueled by quintom dark energy and dark matter. Quintom dark energy is a hybrid of quintessence and phantom which involves the participation of two real scalar fields playing the roles of those two types of dark energy. In that framework we examine, from a dynamical systems perspective, the possibility that those fields are coupled among them by considering an exponential potential with an interesting functional dependence similar but not identical to others studied before. The model we consider represents a counterexample to the typical behavior of quintom models with exponential potentials because it admits either tracking attractors ( w = 0 ), or phantom attractors ( w − 1 ).

Quintom cosmologies with arbitrary potentials

In this Letter we investigate quintom cosmologies with arbitrary potentials from the dynamical systems perspective. The dynamical systems analysis is complete in the sense that it includes the asymptotic regime where both scalar fields diverge, which proves to be particularly relevant in connection with the existence of tracking phases. The results of the present study indicate that the existence of phantom attractors is not generic: for quintom models there may exist either de Sitter attractors associated with the saddle points of the potential, or tracking attractors in the asymptotic regime where the scalar fields diverge.

Crossing the phantom divide without phantom matter

A class of braneworld models can lead to phantom-like acceleration of the late universe, but without the need for any phantom matter. In the simplest models, the universe contains only cold dark matter and a cosmological constant. We generalize these models by introducing a quintessence field. The new feature in our models is that quintessence leads to a crossing of the phantom divide, w = −1. This is a purely gravitational effect, and there is no phantom instability. Furthermore, the Hubble parameter is always decreasing, and there is no big rip singularity in the future.

Classification of cosmological milestones

In this paper causal geodesic completeness of Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological models is analyzed in terms of generalized power expansions of the scale factor in coordinate time. The strength of the found singularities is discussed following the usual definitions due to Tipler and Krolak. It is shown that while classical cosmological models are both timelike and lightlike geodesically incomplete, certain observationally allowed models which have been proposed recently are lightlike geodesically complete.

Phantom k-essence cosmologies

We devise a method to obtain a phantom version of FRW k-essence cosmologies with homogeneous k fields by applying form-invariance transformations. It can be seen that the transformation performs the maps

ON BIG RIP SINGULARITIES

\stackrel{\ensuremath{\rightarrow}}{H}\ensuremath{-}H