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.
120897327
VACUUMLESS COSMIC STRINGS IN BRANS–DICKE THEORY
The gravitational fields of vacuumless global and gauge strings have been studied in Brans–Dicke theory under the weak field assumption of the field equations. It has been shown that both global and gauge string can have repulsive as well as attractive gravitational effect in Brans–Dicke theory which is not so in General Relativity.
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.
118586619
Cosmology with non-minimally coupled k-field
We consider a non-minimally coupled (with gravity) scalar field with non-canonical kinetic energy. The form of the kinetic term is of Dirac–Born–Infeld 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. We show that it is possible to have an inflationary solution in early time as well as late time accelerating phases. The solutions also exhibit attractor properties in a sense that they do not depend on the initial conditions for certain values of the parameters.
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).
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.
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.