Related papers: A Class of Stationary Rotating String Cosmological…
In a flat background, simple non-conducting string loops have no strictly stationary equilibrium states, but for cosmic string loops of superconducting kind such ``vorton'' states will exist, with rotating circular configurations…
We extend the usual gravitational action principle by promoting the bare cosmological constant (CC) from a parameter to a field which can take many possible values. Variation leads to a new integral constraint equation which determines the…
We revisit a scenario in which the cosmological constant is cancelled by the potential energy of a slowly evolving scalar field, or "cosmon". The cosmon's evolution is tied to the cosmological constant by a feedback mechanism. This feedback…
We introduce a dynamical model to reduce a large cosmological constant to a sufficiently small value. The basic ingredient in this model is a distinction which has been made between the two unit systems used in cosmology and particle…
The Cosmological Constant Problem is re-examined from an effective field theory perspective. While the connection between gravity and particle physics has not been experimentally probed in the quantum regime, it is severely constrained by…
A nonlocal gravity model, which does not assume the existence of a new dimensional parameter in the action and includes a function $f(\Box^{-1} R)$, with $\Box$ the d'Alembertian operator, is considered. The model is proven to have de…
We make use of the formalism described in a previous paper [Martins {\it et al.} Phys. Rev. D90 (2014) 043518] to address general features of wiggly cosmic string evolution. In particular, we highlight the important role played by poorly…
We have considered a cosmological model of the FRW universe with variable $G$ and $\Lambda$. The solutions have been obtained for flat model with particular form of cosmological constant. The cosmological parameters have also been obtained…
The mystery of the cosmological constant is probably the most pressing obstacle to significantly improving the models of elementary particle physics derived from string theory. The problem arises because in the standard framework of low…
In the light of the proposal of hep-th/0207195, we discuss in detail the issue of the cosmological constant, explaining how can string theory naturally predict the value which is experimentally observed, without low-energy supersymmetry.
We consider a two-dimensional model of gravity with the cosmological constant as a dynamical variable. The effective cosmological constant is derived when the universe has no initial boundary. It turns out to be extremely small if the…
Exact solutions of the Einstein field equations with cosmic string and space varying cosmological constant, viz., $\Lambda= \Lambda(r)$, in the energy-momentum tensors are presented. Three cases have been studied: where variable…
We consider several model with metric type FRW, with k=0 and with generic equation of state. We take into account too, the conservation principle for the energy-momentum tensor. We find trivially, a set of solutions through dimensional…
We review the current observational status of string cosmology when confronted with experimental datasets. We begin by defining common observational parameters and discuss how they are determined for a given model. Then we review the…
In this paper, we study the dynamical behaviour of the Universe in the $F(R,G)$ theory of gravity, where $R$ and $G$ respectively denote the Ricci scalar and Gauss-Bonnet invariant. Our wide analysis encompasses the energy conditions,…
We introduce a very early universe model based on the thermodynamics of a gas of closed strings in a background which is non-perturbative in $\alpha'$. Upon considering the fully $\alpha'$-corrected equations extended to include certain…
In this study, cosmological models with perfect fluid and a gravitational framework f(Q) will be examined. In this modified theory of gravity, the gravitational force has the form f(Q), where Q stands for the non-metricity scalar. I create…
The cosmological constant problem is examined under the assumption that the extrinsic curvature of the space-time contributes to the vacuum. A compensation mechanism based on a variable cosmological term is proposed. Under a suitable…
In [1], KKLT give a mechanism to generate de Sitter vacua in string theory. And the scenario, \emph{Landscape}, is suggested to explain the problem of the cosmological constant. In this paper, adopting a simple potential describing the…
We present a rich class of exact solutions which contains radiation-dominated and matter-dominated models for the early and late universe. They include a variable cosmological ``constant'' which is derived from a higher dimension and…