16133253
Local cosmic string in generalized scalar tensor theory
A recent investigation shows that a local gauge string with a phenomenological energy momentum tensor, as prescribed by Vilenkin, is inconsistent in Brans-Dicke theory. In this work it is shown that such a string is consistent in a more general scalar tensor theory where $\ensuremath{\omega}$ is a function of the scalar field. A set of solutions of full nonlinear Einstein equations for the interior region of such a string is presented.
118614329
Reconstructing the interaction rate in holographic models of dark energy
Abstract We reconstruct the interaction rate of the holographic dark energy model recently proposed by Zimdahl and Pavon [W. Zimdahl, D. Pavon, Class. Quantum Grav. 24 (2007) 5461] in the redshift interval 0 z 1.8 with observational data from supernovae type Ia, baryon acoustic oscillations, gas mass fraction in galaxy clusters, and the growth factor. It shows a reasonable behavior in the sense that it increases with expansion from a small or vanishing value in the long past but starts decreasing at recent times. The later feature suggests that the equation of state parameter of dark energy does not cross the phantom divide.
118513128
Thawing quintessence with a nearly flat potential
The thawing quintessence model with a nearly flat potential provides a natural mechanism to produce an equation of state parameter, w, close to -1 today. We examine the behavior of such models for the case in which the potential satisfies the slow-roll conditions: [(1/V)(dV/d{phi})]{sup 2}<<1 and (1/V)(d{sup 2}V/d{phi}{sup 2})<<1, and we derive the analog of the slow-roll approximation for the case in which both matter and a scalar field contribute to the density. We show that in this limit, all such models converge to a unique relation between 1+w, {omega}{sub {phi}}, and the initial value of (1/V)(dV/d{phi}). We derive this relation and use it to determine the corresponding expression for w(a), which depends only on the presentday values for w and {omega}{sub {phi}}. For a variety of potentials, our limiting expression for w(a) is typically accurate to within {delta}w < or approx. 0.005 for w<-0.9. For redshift z < or approx. 1, w(a) is well fit by the Chevallier-Polarski-Linder parametrization, in which w(a) is a linear function of a.
119358901
WMAP constraints on Cardassian model
We have investigated the constraints on the Cardassian model using the recent results from the Wilkinson Microwave Anisotropy Probe (WMAP) for the locations of the peaks of the Cosmic Microwave Background (CMB) anistropy spectrum. We find that the model is consistent with the recent observational data for certain range of the model parameter $n$ and the cosmological parameters. We find that the Cardassian model is favoured compared to $\Lambda$CDM model, for a higher spectral index ($n_s \approx 1$) together with lower value of Hubble parameter $h$ ($h\leq 0.71$). But for smaller values of $n_s$, both $\Lambda$CDM and Cardassian are equally favoured. Also, irrespective of Supernova constraint, CMB data alone predicts current acceleration of the universe in this model. We have also studied the constraint on the $\sigma_{8}$, the rms density fluctuations at $8h^{-1}$ Mpc scale.
119177992
The thawing dark energy dynamics: Can we detect it?
We consider different classes of scalar field models including quintessence and tachyon scalar fields with a variety of generic potentials belonging to the thawing type. We focus on observational quantities like Hubble parameter, luminosity distance as well as quantities related to the Baryon Acoustic Oscillation measurement. Our study shows that with present state of observations, one cannot distinguish amongst various models which in turn cannot be distinguished from cosmological constant. Our analysis indicates that there is a small chance to observe the dark energy metamorphosis in near future.
118981720
The weak energy condition and the expansion history of the Universe
We examine flat models containing a dark matter component and an arbitrary dark energy component, subject only to the constraint that the dark energy satisfies the weak energy condition. We determine the constraints that these conditions place on the evolution of the Hubble parameter with redshift, H(z), and on the scaling of the coordinate distance with redshift, r(z). Observational constraints on H(z) are used to derive an upper bound on the current matter density. We demonstrate how the weak energy condition constrains fitting functions for r(z).
118381804
An Introduction to Duality Symmetries in String Theory
In this review I discuss some basic aspects of non-perturbative string theory. The topics include test of duality symmetries based on the analysis of the low energy effective action and the spectrum of BPS states, relationship between different duality symmetries, and an introduction to M-theory.