1,722,031 research outputs found
Non-minimally coupled dark matter: effective pressure and structure formation
No full textWe propose a phenomenological model in which a non-minimal coupling between gravity and dark matter is present in order to address some of the apparent small scales issues of \lcdm model. When described in a frame in which gravity dynamics is given by the standard Einstein-Hilbert action, the non-minimal coupling translates into an effective pressure for the dark matter component. We consider some phenomenological examples and describe both background and linear perturbations. We show that the presence of an effective pressure may lead these scenarios to differ from \lcdm at the scales where the non-minimal coupling (and therefore the pressure) is active. In particular two effects are present: a pressure term for the dark matter component that is able to reduce the growth of structures at galactic scales, possibly reconciling simulations and observations; an effective interaction term between dark matter and baryons that could explain observed correlations between the two components of the cosmic fluid within Tully-Fisher analysis. © 2012 IOP Publishing Ltd and Sissa
Scaling solutions in scalar tensor cosmologies
No full textThe possibility of a connection between dark energy and gravity through a direct coupling in the Lagrangian of the underlying theory has acquired an increasing interest due to the recently discovered capability of the extended quintessence model to encompass the fine-tuning problem of the cosmological constant. The gravity-induced 'R-boost' mechanism is indeed responsible for an early, enhanced scalar field dynamics, by virtue of which the residual imprint of a wide set of initial field values is cancelled out. The initial conditions problem is particularly relevant, as the most recent observations indicate that the dark energy equation of state approaches, at the present time, the cosmological constant value, wDE = −1; if confirmed, such observational evidence would cancel the advantage of a standard, minimally coupled scalar field as a dark energy candidate instead of the cosmological constant, because of the huge fine tuning it would require. We give here a general classification of the scalar–tensor gravity theories admitting R-boost solutions scaling as a power of the cosmological redshift, outlining those behaving as an attractor for the quintessence field. In particular, we show that all the R-boost solutions with the dark energy density scaling as the relativistic matter or shallower represent attractors. This analysis is exhaustive as for the classification of the couplings which admit R-boost and the subsequent enlargement of the basin of attraction enclosing the initial scalar field values
Extended quintessence with an exponential coupling
No full textWe study a class of extended quintessence cosmologies where the scalar field playing the role of the dark energy is exponentially coupled to the Ricci scalar. We find that the dynamics induced by the effective gravitational potential in the Klein–Gordon equation dominates the motion of the field in the early universe. The resulting 'R-boost' trajectory is characterized by a kinetic dark energy density, given by [3ρmnr0(1+z)]2[32ρr0ωJBD0]−1, where ωJBD0, ρr0 and ρmnr0 are calculated at present, and represent the Jordan–Brans–Dicke parameter, the density of relativistic matter and of those species which are non-relativistic at redshift z, respectively. We show that such a trajectory represents an attractor, equivalent to a tracking solution with equation of state w = −1/3, providing a large basin of attraction for the initial dark energy density regardless of the properties of the potential energy yielding acceleration today. We derive the up to date constraints from big bang nucleosynthesis (BBN) on the present scenario, and we show that they are largely satisfied for interesting trajectories of the dark energy scalar field in the early universe. We compute the cosmological perturbation spectra in these cosmologies. For a fixed value of ωJBD 0, the projection and integrated Sachs–Wolfe effects on the cosmic microwave background anisotropy are considerably larger in the exponential case with respect to a quadratic non-minimal coupling, reflecting the fact that the effective gravitational constant depends exponentially on the dynamics of the field
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Nonlinear matter spectra in coupled quintessence
Full text linkWe consider cosmologies in which a dark-energy scalar field interacts with cold dark matter. The growth of perturbations is followed beyond the linear level by means of the time-renormalization-group method, which is extended to describe a multicomponent matter sector. Even in the absence of the extra interaction, a scale-dependent bias is generated as a consequence of the different initial conditions for baryons and dark matter after decoupling. The effect is enhanced significantly by the extra coupling and can be at the 2%–3% level in the range of scales of baryonic acoustic oscillations. We compare our results with N-body simulations, finding very good agreement
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
