Exponential-Potential Scalar Field Universes I: The Bianchi I Models

We obtain a general exact solution of the Einstein field equations for the anisotropic Bianchi type I universes filled with an exponential-potential scalar field and study their dynamics. It is shown, in agreement with previous studies, that for a wide range of initial conditions the late-time behaviour of the models is that of a power-law inflating FRW universe. This property, does not hold, in contrast, when some degree of inhomogeneity is introduced, as discussed in our following paper II.Comment: 16 pages, Plain LaTeX, 1 Figure to be sent on request, to appear in Phys. Rev.

Detailed study of open cluster PIS 20

The very young open cluster Pis 20 has been analyzed by employing CCD UBVR photometry. Down to V = 19 mag, new blue and faint members were found. With them we obtain a better definition of the lower cluster main sequence. Using the fitting method we situate the cluster at a distance of d = 3600 pc (Vo-Mv = 12.80). The main color excess value is = 1.24 mag. In analyzing observed characteristics and spatial positions of the bluest cluster stars we conclude that the star WR 67 has a good chance to be a cluster member. In that case the mass of its progenitor should be greater than 50 M⊙ according to evolutionary models. From isochrone fitting the age of Pis 20 is 5.1 x 10⁶ y. We discuss the cluster mass spectrum whose slope, within the observational uncertainties, appears to be similar to a Salpeter law. We notice that high WR/OB and WC/WN number ratios are present not only within the cluster itself but also in the association where Pis 20 is situated. The cluster distance derived here prevents to assume any spatial connection between Pis 20 and the supernova remnant MSH 15-52. The total cluster mass is around 600 M⊙.Asociación Argentina de Astronomí

Dynamics of Tachyon and Phantom Field beyond the Inverse Square Potentials

We investigate the cosmological evolution of the tachyon and phantom-tachyon scalar field by considering the potential parameter $\Gamma$($=\frac{V V"}{V'^2}$) as a function of another potential parameter $\lambda$($=\frac{V'}{\kappa V^{3/2}}$), which correspondingly extends the analysis of the evolution of our universe from two-dimensional autonomous dynamical system to the three-dimension. It allows us to investigate the more general situation where the potential is not restricted to inverse square potential and .One result is that, apart from the inverse square potential, there are a large number of potentials which can give the scaling and dominant solution when the function $\Gamma(\lambda)$ equals $3/2$ for one or some values of $\lambda_{*}$ as well as the parameter $\lambda_{*}$ satisfies condition Eq.(18) or Eq.(19). We also find that for a class of different potentials the dynamics evolution of the universe are actually the same and therefore undistinguishable.Comment: 8 pages, no figure, accepted by The European Physical Journal C(2010), online first, http://www.springerlink.com/content/323417h708gun5g8/?p=dd373adf23b84743b523a3fa249d51c7&pi=

The solution of tachyon inflation in curved universe

In this paper, we have considered the curved universe which is filled by tachyonic field. We have found the exact solutions for the field, pressure, density, and scale factor and some cosmological parameters. In such universe, we have investigated the role of tachyonic field in different stages of k for the evolution of the universe. Finally we draw the graphs for the scale factor, Hubble's parameter, energy density, pressure, acceleration parameter, equation of state and potential for the different values of k. Also we obtained the exact form of field which shows that the tachyonic field has the kink form.Comment: 8 pages, 21 figure

Tachyonization of the \LaCDM cosmological model

In this work a tachyonization of the $\Lambda$CDM model for a spatially flat Friedmann-Robertson-Walker space-time is proposed. A tachyon field and a cosmological constant are considered as the sources of the gravitational field. Starting from a stability analysis and from the exact solutions for a standard tachyon field driven by a given potential, the search for a large set of cosmological models which contain the $\Lambda$CDM model is investigated. By the use of internal transformations two new kinds of tachyon fields are derived from the standard tachyon field, namely, a complementary and a phantom tachyon fields. Numerical solutions for the three kinds of tachyon fields are determined and it is shown that the standard and complementary tachyon fields reproduces the $\Lambda$CDM model as a limiting case. The standard tachyon field can also describe a transition from an accelerated to a decelerated regime, behaving as an inflaton field at early times and as a matter field at late times. The complementary tachyon field always behaves as a matter field. The phantom tachyon field is characterized by a rapid expansion where its energy density increases with time.Comment: Version accepted for publication in GR

Brane World Cosmology with Gauss-Bonnet Interaction

We study a Randall-Sundrum model modified by a Gauss-Bonnet interaction term. We consider, in particular, a Friedmann-Robertson-Walker metric on the brane and analyse the resulting cosmological scenario. It is shown that the usual Friedmann equations are recovered on the brane. The equation of state relating the enery density and the pressure is uniquely determined by the matching conditions. A cosmological solution with negative pressure is found.Comment: 9 pages, revtex styl

Bridging geometries and potentials in DBI cosmologies

We investigate the link between the warp function and the potential in DBI cosmologies in connection with the possibility they represent power-law solutions. A prescription is given to take advantage of the known result that given a warp factor there is always a choice of potential resulting in a constant ratio between pressure and energy density. The method is illustrated with examples with interesting models for either the warp factor or the potential. We complete this investigation by giving a recipe to exploit symmetries in order to generate new solutions from existing ones; this method can be applied, for instance, to the power-law cosmologies obtained using our prescription.Comment: 7 pages, 3 figures, revte

Caustic Formation in Tachyon Effective Field Theories

Certain configurations of D-branes, for example wrong dimensional branes or the brane-antibrane system, are unstable to decay. This instability is described by the appearance of a tachyonic mode in the spectrum of open strings ending on the brane(s). The decay of these unstable systems is described by the rolling of the tachyon field from the unstable maximum to the minimum of its potential. We analytically study the dynamics of the inhomogeneous tachyon field as it rolls towards the true vacuum of the theory in the context of several different tachyon effective actions. We find that the vacuum dynamics of these theories is remarkably similar and in particular we show that in all cases the tachyon field forms caustics where second and higher derivatives of the field blow up. The formation of caustics signals a pathology in the evolution since each of the effective actions considered is not reliable in the vicinity of a caustic. We speculate that the formation of caustics is an artifact of truncating the tachyon action, which should contain all orders of derivatives acting on the field, to a finite number of derivatives. Finally, we consider inhomogeneous solutions in p-adic string theory, a toy model of the bosonic tachyon which contains derivatives of all orders acting on the field. For a large class of initial conditions we conclusively show that the evolution is well behaved in this case. It is unclear if these caustics are a genuine prediction of string theory or not.Comment: 23 pages, 5 figures; accepted for publication in JHEP. Revised derivation of eikonal equation for the DBI action. Added comments concerning the relationship between p-adic string theory and tachyon matter. Added second example of inhomogeneous evolution in p-adic string theory. Misleading statements concerning caustic-free evolution removed, references adde

Cosmology With Non-Minimally Coupled K-Field

We consider non-minimally coupled (with gravity) scalar field with non-canonical kinetic energy. The form of the kinetic term is of Dirac-Born-Infeld (DBI) form.We study the early evolution of the universe when it is sourced only by the k-field, as well as late time evolution when both the matter and k-field are present. For the k-field, we have considered constant potential as well as potential inspired from Boundary String Field Theory (B-SFT). We show that it is possible to have inflationary solution in early time as well as late time accelerating phase. The solutions also exhibit attractor property in a sense that it does not depend on the initial conditions for a certain values of the parameters.Comment: 10 pages, Revtex style, 14 eps figures, to appear in General Relativity and Gravitatio

Cosmological Dynamics of Phantom Field

We study the general features of the dynamics of the phantom field in the cosmological context. In the case of inverse coshyperbolic potential, we demonstrate that the phantom field can successfully drive the observed current accelerated expansion of the universe with the equation of state parameter $w_{\phi} < -1$. The de-Sitter universe turns out to be the late time attractor of the model. The main features of the dynamics are independent of the initial conditions and the parameters of the model. The model fits the supernova data very well, allowing for $-2.4 < w_{\phi} < -1$ at 95 % confidence level.Comment: Typos corrected. Some clarifications and references added. To appear in Physical Review