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.
17013560
Constraints on the Generalized Chaplygin Gas Model
The generalized Chaplygin gas (GCG) model explains the recent accelerated expansion of the Universe via an exotic background fluid whose equation of state is given by p = −A/ρ, where A is a positive constant and 0 < α ≤ 1. The model is an interesting alternative to scenarios involving scalar field potentials, with the ensuing unnatural fine tuning conditions for the underlying particle physics theories. We derive constraints on the parameter space of the model from bounds on the location of the first few peaks and troughs of the the Cosmic Microwave Background Radiation (CMBR) power spectrum arising from recent WMAP and BOOMERanG data. PACS number(s): 98.80.Cq
119357493
WMAP Constraints on the Generalized Chaplygin Gas Model
Abstract The generalized Chaplygin gas (GCG) model explains the recent accelerated expansion of the Universe via an exotic background fluid whose equation of state is given by p =− A / ρ α , where A is a positive constant and 0 α ⩽1. The model is an interesting alternative to scenarios involving scalar field potentials, with the ensuing unnatural fine tuning conditions for the underlying particle physics theories. We derive constraints on the parameter space of the model from bounds on the location of the first few peaks and troughs of the cosmic microwave background radiation (CMBR) power spectrum arising from recent WMAP and BOOMERanG data.
15686768
Structure formation and CMBR anisotropy spectrum in the inflessence model
The inflessence model has recently been proposed in an attempt to explain both early inflation and present day accelerated expansion within a single mechanism. The model has been successfully tested against the Hubble diagram of Type Ia Supernovae, the shift parameter, and the acoustic peak parameter. As a further mandatory test, we investigate here structure formation in the inflessence model determining the evolution of matter density contrast $\delta \equiv \delta \rho_{\rm M}/\rho_{\rm M}$ in the linear regime. We compare the growth factor $D(a) \equiv \delta/a$ and the growth index $f(z) \equiv {\rm dln}{\delta}/{\rm dln}{a}$ to these same quantities for the successful concordance Λ CDM model with a particular emphasis on the role of the inflessence parameters $(\gamma, z_Q)$. We also evaluate the anisotropy spectrum of the cosmic microwave background radiation (CMBR) to check whether the inflessence model may be in agreement with the observations. We find that, for large values of $(\gamma, z_Q)$, structure formation proceeds in a similar way to that in the Λ CDM scenario, and it is also possible to nicely fit the CMBR spectrum.
119357931
Generalized Chaplygin gas, accelerated expansion and dark energy matter unification
We consider the scenario emerging from the dynamics of a generalized d-brane in a (d +1 ; 1) spacetime. The equation of state describing this system is given in terms of the energy density, , and pressure, p, by the relationship p = A= ,w hereA is a positive constant and 0 < 1. We discuss the conditions under which homogeneity arises and show that this equation of state describes the evolution of a universe evolving from a phase dominated by non-relativistic matter to a phase dominated by a cosmological constant via an intermediate period where the eective equation of state is given by p = .
119288549
Hořava-Lifshitz cosmology with generalized Chaplygin gas
We investigate cosmological scenarios of generalized Chaplygin gas in a universe governed by Hořava-Lifshitz gravity. We consider both the detailed and non-detailed balance versions of the gravitational background, and we include the baryonic matter and radiation sectors. We use observational data from Type Ia Supernovae, Baryon Acoustic Oscillations, and Cosmic Microwave Background, along with requirements of Big Bang Nucleosynthesis, to constrain the parameters of the model, and we provide the corresponding likelihood contours. We deduce that the present scenario is compatible with observations. Additionally, examining the evolution of the total equation-of-state parameter, we find in a unified way the succession of the radiation, matter, and dark energy epochs, consistently with the thermal history of the universe.
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.