The Peculiar-Velocity-Field in Structure Formation Theories with Cosmic Strings

We investigate the peculiar velocity field due to long cosmic strings in several cosmological models and analyse the influence of a nonscaling behaviour of the string network, which is expected in open cosmological models or models with a cosmological constant. It is shown that the deviation of the propability distribution of the peculiar velocity field from the normal distribution is only weak in all models. It is further argued that one can not necessarily obtain the parameter $\beta=\Omega_{0}^{0.6}/b$ from density and velocity fields, where $\Omega_0$ is the density parameter and $b$ the linear biasing parameter, if cosmic strings are responsible for structure formation in the universe. An explanation for this finding is given.Comment: 11 pages, 5 figure

On the Origin of Very Wide Lyα\alpha-Absorption-Lines in Quasar Spectra

We present a new explanation for the very wide absorption features in quasar spectra. In our model, a very wide absorption feature will originate, when the line of sight crosses a bubble wall tangentially. We demonstrate this on the quasar pair (2138-4427), (2139-4434). Both show two very wide absorption lines in their spectra at the same redshift. The bubble wall model can explain these observations in low density Friedmann-Lemaitre models with spherical metric. It contradicts models with euclidian or hyperbolic metric.Comment: Latex, 6 pages, Figures available at [email protected]

Quintessence dynamics with two scalar fields and mixed kinetic terms

The dynamical properties of a model of dark energy in which two scalar fields are coupled by a non-canonical kinetic term are studied. We show that overall the addition of the coupling has only minor effects on the dynamics of the two-field system for both potentials studied, even preserving many of the features of the assisted quintessence scenario. The coupling of the kinetic terms enlarges the regions of stability of the critical points. When the potential is of an additive form, we find the kinetic coupling has an interesting effect on the dynamics of the fields as they approach the inflationary attractor, with the result that the combined equation of state of the scalar fields can approach -1 during the transition from a matter dominated universe to the recent period of acceleration.Comment: 14 pages, 9 figures. Includes a extra section describing the case of a field-dependent coupling with two additional figures and minor changes to match published versio