Center of Mathematics and Applications of University of Beira Interior

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Publications

Interacting 3-form dark energy models: distinguishing interactions and avoiding the Little Sibling of the Big Rip

Publication . Morais, João; Bouhmadi Lopez, Mariam; Kumar, K. Sravan; Marto, João; Tavakoli, Yaser

In this paper we consider 3-form dark energy (DE) models with interactions in the dark sector. We aim to distinguish the phenomenological interactions that are defined through the dark matter (DM) and the DE energy densities. We do our analysis mainly in two stages. In the first stage, we identify the non-interacting 3-form DE model which generically leads to an abrupt late-time cosmological event which is known as the little sibling of the Big Rip (LSBR). We classify the interactions which can possibly avoid this late-time abrupt event. We also study the parameter space of the model that is consistent with the interaction between DM and DE energy densities at present as indicated by recent studies based on BAO and SDSS data. In the later stage, we observationally distinguish those interactions using the statefinder hierarchy parameters {S3(1),S4 (1)},{S3 (1),S5 (1)} . We also compute the growth factor parameter ε(z) for the various interactions we consider herein and use the composite null diagnostic (CND) {S3(1), ε(z)} as a tool to characterise those interactions by measuring their departures from the concordance model. In addition, we make a preliminary analysis of our model in light of the recently released data by SDSS~III on the measurement of the linear growth rate of structure.

2017-03Journal article

Open access

Cosmic infinity: a dynamical system approach

Publication . Bouhmadi Lopez, Mariam; Marto, João; Morais, João; Silva, César M.

Dynamical system techniques are extremely useful to study cosmology. It turns out that in most of the cases, we deal with finite isolated fixed points corresponding to a given cosmological epoch. However, it is equally important to analyse the asymptotic behaviour of the universe. On this paper, we show how this can be carried out for 3-forms model. In fact, we show that there are fixed points at infinity mainly by introducing appropriate compactifications and defining a new time variable that washes away any potential divergence of the system. The richness of 3-form models allows us as well to identify normally hyperbolic non-isolated fixed points. We apply this analysis to three physically interesting situations: (i) a pre-inflationary era; (ii) an inflationary era; (iii) the late-time dark matter/dark energy epoch.

2017-03-21Journal article

Open access

Estimation of variance components in models with positive commutativity orthogonal block structure

Publication . Ferreira, Sandra S.; Ferreira, Dário; Nunes, Célia; Mexia, João T.

The article addresses alternative estimators for estimable vectors. The relative performances of the proposed estimators are studied through numerical analysis. The major motivation behind this article is providing an optimal estimator through applying simple modifications in models with Orthogonal Block Structure. In addition, an illustrative example is presented to demonstrate the efficiency and accuracy of the proposed method.

2018Journal article

Restricted access

ECIR 2018: Text2Story Workshop-Narrative Extraction from Texts

Publication . Jorge, Alípio Mário Guedes; Campos, Ricardo; Jatowt, Adam; Nunes, Sergio; Rocha, Conceiçeição; Cordeiro, João; Pasquali, Arian; Mangaravite, Vitor

The 1st International Workshop on Narrative Extraction from Texts (Text2Story 2018) was held in conjunction with the 40th European Conference on Information Retrieval, ECIR 2018, Grenoble on the 26th March 2018. The workshop aimed to help foster the collaboration of researchers on a wide range of multidisciplinary issues related to the text-to-narrative- structure. The program consisted of two keynote talks, six research presentations, a poster session and a slot for demo presentations. This report briefly summarizes the workshop.

2018-03-26Report

Open access

Non-Gaussianity in multiple three-form field inflation

Publication . Kumar, K. Sravan; Mulryne, David J.; Nunes, Nelson J.; Marto, João; Moniz, Paulo

In this work, we present a method for implementing the δN formalism to study the primordial non-Gaussianity produced in multiple three-form field inflation. Using a dual description relating three-form fields to noncanonical scalar fields, and employing existing results, we produce expressions for the bispectrum of the curvature perturbation in terms of three-form quantities. We study the bispectrum generated in a two three-form field inflationary scenario for a particular potential that for suitable values of the parameters was found in earlier work to give values of the spectral index and ratio of tensor to scalar perturbations compatible with current bounds. We calculate the reduced bispectrum for this model, finding an amplitude in equilateral and orthogonal configurations of δ(1) and in the squeezed limit of δ(10-3). We confirm, therefore, that this three-form inflationary scenario is compatible with present observational constraints.

2016-06-22Journal article

Open access