Phase Portraits of general f(T) Cosmology

We use dynamical system methods to explore the general behaviour of $f(T)$ cosmology. In contrast to the standard applications of dynamical analysis, we present a way to transform the equations into a one-dimensional autonomous system, taking advantage of the crucial property that the torsion scalar in flat FRW geometry is just a function of the Hubble function, thus the field equations include only up to first derivatives of it, and therefore in a general $f(T)$ cosmological scenario every quantity is expressed only in terms of the Hubble function. The great advantage is that for one-dimensional systems it is easy to construct the phase space portraits, and thus extract information and explore in detail the features and possible behaviours of $f(T)$ cosmology. We utilize the phase space portraits and we show that $f(T)$ cosmology can describe the universe evolution in agreement with observations, namely starting from a Big Bang singularity, evolving into the subsequent thermal history and the matter domination, entering into a late-time accelerated expansion, and resulting to the de Sitter phase in the far future. Nevertheless, $f(T)$ cosmology can present a rich class of more exotic behaviours, such as the cosmological bounce and turnaround, the phantom-divide crossing, the Big Brake and the Big Crunch, and it may exhibit various singularities, including the non-harmful ones of type II and type IV. We study the phase space of three specific viable $f(T)$ models offering a complete picture. Moreover, we present a new model of $f(T)$ gravity that can lead to a universe in agreement with observations, free of perturbative instabilities, and applying the Om(z) diagnostic test we confirm that it is in agreement with the combination of SNIa, BAO and CMB data at 1$\sigma$ confidence level.Comment: 39 pages, 12 figures, version published in JCA

New anisotropic star solutions in mimetic gravity

We extract new classes of anisotropic solutions in the framework of mimetic gravity, by applying the Tolman-Finch-Skea metric and a specific anisotropy not directly depending on it, and by matching smoothly the interior anisotropic solution to the Schwarzschild exterior one. Then, in order to provide a transparent picture we use the data from the 4U 1608-52 pulsar. We study the profile of the energy density, as well as the radial and tangential pressures, and we show that they are all positive and decrease towards the center of the star. Furthermore, we investigate the anisotropy parameter and the anisotropic force, that are both increasing functions of the radius, which implies that the latter is repulsive. Additionally, by examining the radial and tangential equation-of-state parameters, we show that they are monotonically increasing, not corresponding to exotic matter. Concerning the metric potentials we find that they have no singularity, either at the center of the star or at the boundary. Furthermore, we verify that all energy conditions are satisfied, we show that the radial and tangential sound speed squares are positive and sub-luminal, and we find that the surface redshift satisfies the theoretical requirement. Finally, in order to investigate the stability we apply the Tolman-Oppenheimer-Volkoff equation, we perform the adiabatic index analysis, and we examine the static case, showing that in all cases the star is stable.Comment: 20 pages 10 figures, will appear in EPJ

Encapsulating soldered electronic components for electronically functional yarn

E-Textiles are fabrics with embedded electronic functions that can be used in many fields, such as clothing, medicine, furniture, safety and many others. The integration of electronics with textiles requires a flexible structure that keeps the garment flexible to ensure the textile retains its physical characteristics and feel. E-textiles in wearable applications are subject to human activities. The integrated electronic components are vulnerable to these stresses, such as bending, torsion, and tensile. These forces can potentially damage the components or their interconnection. Electronics can be integrated into textiles in one of three approaches described as generations of E-Textiles. The Thesis will introduce the three generations of E-textiles through a comprehensive literature review in chapter 2. It will then discuss developing the electronically functional yarn (EFY) as a third generation in E-textiles and garments. The production process of this yarn has three main steps: soldering the semi-conductor on the copper wire, encapsulating it within a micro pod of resin, and covering the micro pod within the filament of the yarn. A detailed study of the encapsulation process and the unit's design is then introduced in the Thesis, where a novel method for packaging electronics using a UV-curable resin was introduced. The design process for the automated encapsulation of soldered semi-conductor has been investigated in Chapter 3 of this Thesis. The novel approach has been evaluated on various electronics and then extended to thin Kapton strips with embedded electronics. The resulting EFY then can be later used in woven or knitted textile. Finite element analysis (FEA) of the soldered semi-conductor on the wire is presented in chapter 4. FEA simulations are used to evaluate the mechanical performance of different electronics and how stresses are distributed after adding the resin and creating the micro pod. This FEA investigation of the materials and micro pod dimensions will understand the packaging method's reliability. The final part of the Thesis included further development added to the design of the encapsulation unit and the electronically functional yarn manufacturing. Developing a reliable, repeatable, and automated electronic packaging method for electronics embedded in the electronically functional yarn (EFY) was achieved in this project. The results were promising for further research

Textiles illuminated with electronic yarn

Poster displayed at the IDTechEx Conference and Exhibition in Berlin, Germany, 11-12 April 2018

Teleparallel Versions of Friedmann and Lewis-Papapetrou Spacetimes

This paper is devoted to investigate the teleparallel versions of the Friedmann models as well as the Lewis-Papapetrou solution. We obtain the tetrad and the torsion fields for both the spacetimes. It is shown that the axial-vector vanishes for the Friedmann models. We discuss the different possibilities of the axial-vector depending on the arbitrary functions $\omega$ and $\psi$ in the Lewis-Papapetrou metric. The vector related with spin has also been evaluated.Comment: 13 pages, accepted for publication in GR

On the energy of charged black holes in generalized dilaton-axion gravity

In this paper we calculate the energy distribution of some charged black holes in generalized dilaton-axion gravity. The solutions correspond to charged black holes arising in a Kalb-Ramond-dilaton background and some existing non-rotating black hole solutions are recovered in special cases. We focus our study to asymptotically flat and asymptotically non-flat types of solutions and resort for this purpose to the M{\o}ller prescription. Various aspects of energy are also analyzed.Comment: LaTe