No abstract

In this paper we study emergent universe model in the context of a self interacting Jordan-Brans-Dicke theory. The model presents a stable past eternal static solution which eventually enters a phase where the stability of this solution is broken leading to an inflationary period. We also establish constraints for the different parameters appearing in our model.

We study the properties of a non-abelian gauge theory subjected to a gauge invariant constraint given by the classical equations of motion. The constraint is not imposed by hand, but appears naturally when we study a particular type of local gauge transformations. In this way, all standard techniques to treat gauge theories are available. We will show that this theory lives at one-loop. Also this model retains some quantum characteristic of the usual non-abelian gauge theories as asymptotic freedom. * jalfaro@puc.cl † plabrana@puc.cl

The dilaton-gravity sector of a linear in the scalar curvature, scale invariant Two Measures Field Theory (TMT), is explored in detail in the context of closed FRW cosmology and shown to allow stable emerging universe solutions. The model possesses scale invariance which is spontaneously broken due to the intrinsic features of the TMT dynamics. We study the transition from the emerging phase to inflation, and then to a zero cosmological constant phase. We also study the spectrum of density perturbations and the constraints that impose on the parameters of the theory.PACS numbers: 98.80. Cq, 04.20.Cv, 95.36.+x As a way to address the cosmological constant (CC) problem [1]-[3], the accelerated expansion of the late time universe [4], the cosmic coincidence [5] (see also reviews on dark energy[6]-[8], dark matter [9] and references therein), many models have been proposed with the aim to find answer to these puzzles, for example: the quintessence[10], coupled quintessence [11], k-essence[12], [13].One can add to the list of puzzles the problem of initial singularity [14], [15], including the singularity theorems for scalar field-driven inflationary cosmology [16], resolution of which is perhaps a crucial criteria for the true theory. The avoidance of the initial singularity will be the central question that we will address in this paper, exploring the idea of the "emerging universe", where the universe has a non singular origin, such that the Einstein Universe. Although the original proposal for the emerging Universe [17] suffered an instability, several proposals to formulate a stable model have been given [18], in particular one is obtained by invoking Jordan Brans Dicke models [19] In this paper we explore a model including gravity and a single scalar field φ in the framework of the so called Two Measures Field Theory (TMT) [20]-[25]. In TMT, many cosmological issues can be addressed: zero vacuum energy is obtained without fine tuning, the fifth force problem is resolved and the Einstein's GR is restored when the local fermion matter energy density (i.e in the space-time regions occupied by matter) is much larger than the vacuum energy density. In this paper, we will address the existence and stability of the emerging universe in TMT. In a previous work, [26] in which a square curvature term was almost of the TMT structure (except for a contribution that gave rise to a cosmological term), an emergent universe was described.TMT is a generally coordinate invariant theory, where the action has to be of the form[21]-[25]including two Lagrangians L 1 and L 2 and two measures of integration √ −g and Φ. One is the usual measure of integration √ −g in the 4-dimensional space-time manifold equipped with the metric g µν . The other is the new *

In this work we study the stability of the Jordan-Brans-Dicke (JBD) static universe. This is motivated by the possibility that the universe might have started out in an asymptotically JBD static state, in the context of the so called emergent universe scenario. We extent our previous results on stability of JBD static universe by considering spatially homogeneous Bianchi type IX anisotropic perturbation modes and by including more general perfect fluids. Contrary to general relativity, we have found that the JBD static universe, dominated by a standard perfect fluid, could be stable against isotropic and anisotropic perturbations. The implications of these results for the initial state of the universe and its pre-inflationary evolution are discussed.