Sub-horizon perturbation behavior in extended quintessence

Abstract

In the general context of scalar-tensor theories, we consider a model in which a scalar field coupled to the Ricci scalar in the gravitational sector of the Lagrangian, is also playing the role of an "Extended Quintessence" field, dominating the energy content of the Universe at the present time. In this framework, we study the linear evolution of the perturbations in the Quintessence energy density, showing that a new phenomenon, named here "gravitational dragging", can enhance the scalar field density perturbations as much as they reach the non-linear regime. The possibility of dark energy clumps formation is thus discussed.1. The growth of quintessence linear perturbations.The recent growing evidences for a vacuum energy component in the Universe [1,2] have lead to the development of a number of models, most of which tend to alleviate the theoretical difficulties of the well known "cosmological constant problem" (see [3] for a review). All these models are replacing the cosmological constant with a "dynamical vacuum energy" provided by an evolving scalar field, often named "Quintessence". This field should provide the present cosmic accelerating phase through its negative equation of state. Such "dark energy" component has also been modeled in the general framework of scalar-tensor theories of Gravity: in these "Extended Quintessence" (EQ) scenarios [4]-[11], the Quintessence scalar field is directly coupled to the Ricci scalar R in the Lagrangian of the theory: the canonical Ricci scalar term κ −1 R, with κ = 8πG * , is replaced by F (φ)R, where F (φ) is a function of the quintessence field φ. Here, G * is a "bare" gravitational constant, generally different from the Newton's constant G as it is measured by Cavendish-type experiments [12,13]. In EQ models, a scalar field has thus a double role: at any epoch, its value determines the effective gravitational constant, and the contribution in dark energy density. Of course, such a coupling is not arbitrary: it is instead subject to a number of constraints, mainly arising from the bounds on the time variations of the constants of nature, which must be taken into account. EQ scenarios have been analyzed from different points of view, and there are robust predictions about their possible imprints on the power spectrum of CMB anisotropies [4,5]. However, the important role of Dark Energy in modern cosmology opens the question of its relation with the other main dark component currently under investigation, the Dark Matter. One major point with Quintessence scenarios is whether a Dark Energy component could affect, in some way, the formation of Dark Matter clumps. In particular, one fundamental question is: can the Dark Energy itself form clumps on certain scales? If we restrict ourselves to minimally-coupled scalar fields, we have to deal with a very relativistic component: indeed, the sound speed of such component turns out to be identical to that of radiation [14]. As a consequence, an "ordinary" (i.e., minimally coupled) Quintessence component becomes homogeneou...

Relaxing nucleosynthesis constraints on Brans-Dicke theories

“…For φ ≫ 1 we recover the case considered previously of a dominant BD contribution to H, and correspondingly the bound on y reported in Eq. (27), while for y ≪ 1, we obtain the "trivial" bound on the effective Newton constant during BBN, namely 0.9 φ 1.2. This result shows that not only both regimes are viable, but also that intermediate situations are phenomenologically allowed, although the details of these cases are more model dependent, and the parameters in the BD sector require a high degree of fine tuning.…”

“…This component may be a manifestation of a large-scale modification of General Relativity [5,6,7,8,9,10,11,12], or a new dynamical field, such as a scalar field [13,14,15,16,17]. It is quite natural to consider scalar fields that are non-minimally coupled to gravity [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. Such fields arise both in the context of string theory, and in more general theories with extra spatial dimensions.…”

Visualization of cosmological density fluctuations with phase space analysis: case study: Brans–Dicke theory

“…Scalar perturbation models have been widely studied in fourth order gravity mainly in the metric formalism [5][6][7][8][9][10][11][12][13][14]. These models originally developed for General relativity by Bardeen [15] and then expanded in the 1 + 3 covariant approach [16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Many attempts have been implemented to numerically compute the matter power spectrum and its numerical analysis for classes of modified gravity theories, rather solving the full set of field equations [30][31][32][33].…”

“…Perturbations in scalar-tensor theories have been considered from different approaches, for example: the reconstruction problem was addressed in [36], the density contrast evolution was studied in [20][21][22][23][24][25][26][27][28][29]37,38] and the second order perturbations were presented in [39]. In all these approaches the gravitational instability is a fundamental assumption.…”

Baryogenesis after hyperextended inflation

“…Such models arise naturally, for example, in the context of extra-dimensional models, for which the radion field describing the size of the extra dimensions is a scalar-tensor (ST) scalar field. These ST fields have been exploited to model dark energy behavior in a quite large variety of models [18,19,20,21,22,23,24,25,26,27,28,29,30] and are a natural complement to other modified gravity theories which lead to cosmic acceleration [31,32,33,34,35].…”