Tracking the validity of the quasi-static and sub-horizon approximations in modified gravity

Within the framework of modified gravity (MG), the quasi-static (QS) and sub-horizon (SH) approximations are widely used in analyses aiming to identify departures from the concordance model at late-times. In general, it is assumed that time derivatives are subdominant with respect to spatial derivatives given that the relevant physical modes are those well inside the Hubble radius. In practice, the perturbation equations under these approximations are reduced to a tractable algebraic system in terms of the gravitational potentials and the perturbations of involved matter fields. Here, in the framework of $f(R)$ theories, we revisit standard results when these approximations are invoked using a new parameterization scheme that allows us to track the relevance of each time-derivative term in the perturbation equations. This new approach unveils terms which are neglected in the standard procedure. We assess the relevance of these differences by comparing results from both approaches against full numerical solutions for two well-known toy-models: the designer $f(R)$ model and the Hu-Sawicki model. We find that: i) the SH approximation can be safely applied to linear perturbation equations for scales $0.06 h/$Mpc $\lesssim k \lesssim 0.2 h/$Mpc, ii) in this "safety region", the QS approximation provides a very accurate description of the late-time cosmological dynamics even when dark energy significantly contribute to the cosmic budget, and iii) our new methodology performs better than the standard procedure, even for several orders of magnitude in some cases. Although, the impact of this major improvement on the linear observables is minimal for the studied cases, this does not represent an invalidation for our approach. Instead, our findings indicate that the perturbation expressions derived under these approximations in more general MG theories, such as Horndeski, should be also revisited.Comment: 28 pages, 18 figures. Changes match published versio

Using machine learning to compress the matter transfer function T(k)T(k)

The linear matter power spectrum $P(k,z)$ connects theory with large scale structure observations in cosmology. Its scale dependence is entirely encoded in the matter transfer function $T(k)$, which can be computed numerically by Boltzmann solvers, and can also be computed semi-analytically by using fitting functions such as the well-known Bardeen-Bond-Kaiser-Szalay (BBKS) and Eisenstein-Hu (EH) formulae. However, both the BBKS and EH formulae have some significant drawbacks. On the one hand, although BBKS is a simple expression, it is only accurate up to $10\%$, which is well above the $1\%$ precision goal of forthcoming surveys. On the other hand, while EH is as accurate as required by upcoming experiments, it is a rather long and complicated expression. Here, we use the Genetic Algorithms (GAs), a particular machine learning technique, to derive simple and accurate fitting formulae for the transfer function $T(k)$. When the effects of massive neutrinos are also considered, our expression slightly improves over the EH formula, while being notably shorter in comparison.Comment: 12 pages, 7 figures, 2 tables. Changes match published versio

Anisotropic Dark Energy from String Compactifications

We explore the cosmological dynamics of a minimalistic yet generic string-inspired model for multifield dark energy. Adopting a supergravity four-dimensional viewpoint, we motivate the model's structure arising from superstring compactifications involving a chiral superfield and a pure $U(1)$ gauge sector. The chiral sector gives rise to a pair of scalar fields, such as the axio-dilaton, which are kinetically coupled. However, the scalar potential depends on only one of them, further entwined with the vector field through the gauge kinetic function. The model has two anisotropic attractor solutions that, despite a steep potential and thanks to multifield dynamics, could explain the current accelerated expansion of the Universe while satisfying observational constraints on the late-times cosmological anisotropy. Nevertheless, justifying the parameter space allowing for slow roll dynamics together with the correct cosmological parameters, would be challenging within the landscape of string theory. Intriguingly, we find that the vector field, particularly at one of the studied fixed points, plays a crucial role in enabling geodesic trajectories in the scalar field space while realizing slow-roll dynamics with a steep potential. This observation opens a new avenue for exploring multifield dark energy models within the superstring landscape.Comment: Submitted to JHEP. Comments are welcome

Reconstructing the parameter space of non-analytical cosmological fixed points

Dynamical system theory is a widely used technique in the analysis of cosmological models. Within this framework, the equations describing the dynamics of a model are recast in terms of dimensionless variables, which evolve according to a set of autonomous first-order differential equations. The fixed points of this autonomous set encode the asymptotic evolution of the model. Usually, these points can be written as analytical expressions for the variables in terms of the parameters of the model, which allows a complete characterization of the corresponding parameter space. However, a thoroughly analytical treatment is impossible in some cases. In this work, we give an example of a dark energy model, a scalar field coupled to a vector field in an anisotropic background, where not all the fixed points can be analytically found. Then, we put forward a general scheme that provides a numerical description of the parameter space. This allows us to find interesting accelerated attractors of the system with no analytical representation. This work may serve as a template for the numerical analysis of highly complicated dynamical systems.Comment: 13 pages, 13 figures, 1 table. Changes match the published versio

El cáncer de mama y su relación con los factores de riesgo modificables en mujeres de Armenia- Quindío

El cáncer de mama es el cáncer ginecológico más frecuente en mujeres en todo el mundo, con 1,67 millones de nuevos casos reportados. Existen factores de riesgo modificables, como la nuliparidad o la primiparidad luego de los treinta años, no haber lactado, utilizar terapia de reemplazo hormonal, consumo regular de bebidas alcohólicas, obesidad, inactividad física y consumo de tabaco.Objetivo: describir los factores de riesgo modificables en mujeres de Armenia y su relación con cáncer de mama.Metodología: estudio exploratorio con mujeres de una institución prestadora de salud (IPS) privada de Armenia, con cáncer de mama y que consultaron en el segundo semestre de 2013 previo consentimiento informado; se aplicó el instrumento Step V 2.1 previamente validado. Se realizó una prueba piloto, se utilizó la base de datos SPSS 19 y se hizo un análisis univariado y bivariado

The occurrence of the red-handed howler monkey (Alouatta belzebul) in Amazonian savannas is related to forest patch area and density of flooded area palms

All Neotropical primates are arboreal and thus depend on forests for their survival. Arboreality puts many Neotropical primates at risk of extinction due to the high rates of deforestation in the tropics. We assessed the influence of vegetation structure and forest patch attributes on the occurrence of the threatened red‐handed howler monkey (Alouatta belzebul) in an Amazonian savanna. Using a sample of 38 forest patches in a region of approximately 2000 km2 in the state of Amapá, northern Brazil, we used logistic regression to find the best predictors of the occurrence of A. belzebul. We assessed patch area, patch isolation, the proportion of seasonally flooded forest in the patch, the density of flooded area palms, forest height, canopy cover, and diameter at breast height of trees. Patch area and palm density were the best predictors of the occurrence of A. belzebul in forest patches, both having a positive effect on the probability of occurrence. Our results indicate that areas of flooded forest in forest patches may be keystone habitats for A. belzebul living in Amazonian savannas. The observed effect of palm density on A. belzebul suggests that this variable is useful for planning conservation actions, including the selection of areas for protection and management strategies for areas inhabited by this primate