Approximation of tensor fields on surfaces of arbitrary topology based on local Monge parametrizations

We introduce a new method, the Local Monge Parametrizations (LMP) method, to approximate tensor fields on general surfaces given by a collection of local parametrizations, e.g.~as in finite element or NURBS surface representations. Our goal is to use this method to solve numerically tensor-valued partial differential equations (PDE) on surfaces. Previous methods use scalar potentials to numerically describe vector fields on surfaces, at the expense of requiring higher-order derivatives of the approximated fields and limited to simply connected surfaces, or represent tangential tensor fields as tensor fields in 3D subjected to constraints, thus increasing the essential number of degrees of freedom. In contrast, the LMP method uses an optimal number of degrees of freedom to represent a tensor, is general with regards to the topology of the surface, and does not increase the order of the PDEs governing the tensor fields. The main idea is to construct maps between the element parametrizations and a local Monge parametrization around each node. We test the LMP method by approximating in a least-squares sense different vector and tensor fields on simply connected and genus-1 surfaces. Furthermore, we apply the LMP method to two physical models on surfaces, involving a tension-driven flow (vector-valued PDE) and nematic ordering (tensor-valued PDE). The LMP method thus solves the long-standing problem of the interpolation of tensors on general surfaces with an optimal number of degrees of freedom.Comment: 16 pages, 6 figure

Simulating gravitational collapse with arbitrary-precision arithmetic

The collapse of smooth initial conditions into Black Holes is an important phenomenon to unlock fundamental aspects of the gravitational theory. In this paper we go closer to the formation of the apparent horizon using arbitrary-precision arithmetic (MPFR library) for examining the finer structure that forms during the collapse

Numerical Relativity studies in Anti-de Sitter spacetimes: Gravitational Collapse and the AdS/CFT correspondence

[eng] In this thesis we study several open problems using Numerical Relativity on asymptotically Anti-de Sitter (AdS) spacetimes. The understanding of the dynamics of AdS is interesting not only because of pure theoretical reasons but also because of its importance in the correspondence gauge/gravity. In the thesis we present three different topics. The first is our research on the gravitational collapse of massless scalar fields in AAdS spacetimes. We have developed a new method that combines two different formulations of the Einstein Field Equations to get closer and with more accuracy to the collapse. The simulation starts with a Cauchy evolution with pseudo-spectral methods and when the collapse is taking place, it performs a change of coordinates to a characteristic one to track the formation of the apparent horizon. The collapse of the scalar field happens after a number of bounces with the critical points being the separation between the different branches. We have numerical evidence that in the separation of the branches there is a power law for subcritical configurations in addition to the one for supercritical ones. This new power law confirms that there is a gap in the mass of the apparent horizon. In the second part, we introduce a shock waves model in AdS to study the far-from-equilibrium regime in the heavy ion collisions through the holographic correspondence in a non-conformal theory. Holographic collisions have attracted a lot of attention in the last few years because of the possibility of simulating strongly coupled systems but, as a drawback, we do not know yet the exact dual of the QCD that should explain the phenomena. In the models used until now, the shock waves correspond to conformal gauge theories while QCD is not conformal. In order to get closer to a description of the actual physical collisions we present the first shock wave collisions in a non-conformal theory. With this, we show how the non-conformality increases the hydrodynamisation time and also that this can happen before the equation of state is fulfilled. In the last part, we propose the use of spectral methods as a very strong option for high precision computations. Arbitrary precision arithmetic has two main problems. The first is the necessity of increasing a lot the discretisation units to reach the precision we want. The other one is the slowing down in the computational performance due to the fact that we need to emulate the fundamental operations with software because current processors are not adapted to carry out computations with precision different from the standard one. The exponential convergence of spectral methods can approximate functions to a very high accuracy with a few hundred terms in our spectral expansion while in other numerical methods it would be a few orders of magnitude larger. This makes these methods very attractive because they facilitate the accessibility to very small error simulations, removes the bottleneck of the memory demand and also help in the computational speed because fewer points are needed for the computation. We have tested this idea with the ANETO library for simulations in AdS spacetimes and the gravitational collapse in an asymptotically flat spacetime with very promising results. This library has been developed as a direct result of this thesis and that can be downloaded as Free Software

Holographic Collisions in Non-conformal Theories

We numerically simulate gravitational shock wave collisions in a holographic model dual to a non-conformal four-dimensional gauge theory. We find two novel effects associated to the non-zero bulk viscosity of the resulting plasma. First, the hydrodynamization time increases. Second, if the bulk viscosity is large enough then the plasma becomes well described by hydrodynamics before the energy density and the average pressure begin to obey the equilibrium equation of state. We discuss implications for the quark-gluon plasma created in heavy ion collision experiments.Comment: 10 pages, 3 figures; published versio

Paths to equilibrium in non-conformal collisions

We extend our previous analysis of holographic heavy ion collisions in non-conformal theories. We provide a detailed description of our numerical code. We study collisions at different energies in gauge theories with different degrees of non-conformality. We compare four relaxation times: the hydrodynamization time (when hydrodynamics becomes applicable), the EoSization time (when the average pressure approaches its equilibrium value), the isotropization time (when the longitudinal and transverse pressures approach each other) and the condensate relaxation time (when the expectation value of a scalar operator approaches its equilibrium value). We find that these processes can occur in several different orderings. In particular, the condensate can remain far from equilibrium even long after the plasma has hydrodynamized and EoSized. We also explore the rapidity distribution of the energy density at hydrodynamization. This is far from boost-invariant and its width decreases as the non-conformality increases. Nevertheless, the velocity field at hydrodynamization is almost exactly boost-invariant regardless of the non-conformality. This result may be used to constrain the initialization of hydrodynamic fields in heavy ion collisions

Thermodynamics, transport and relaxation in non-conformal theories

We study the equilibrium and near-equilibrium properties of a holographic five-dimensional model consisting of Einstein gravity coupled to a scalar field with a non-trivial potential. The dual four-dimensional gauge theory is not conformal and, at zero temperature, exhibits a renormalisation group flow between two different fixed points. We quantify the deviations from conformality both in terms of thermodynamic observables and in terms of the bulk viscosity of the theory. The ratio of bulk over shear viscosity violates Buchel's bound. We study relaxation of small-amplitude, homogeneous perturbations by computing the quasi-normal modes of the system at zero spatial momentum. In this approximation we identify two different relaxation channels. At high temperatures, the different pressures first become approximately equal to one another, and subsequently this average pressure evolves towards the equilibrium value dictated by the equation of state. At low temperatures, the average pressure first evolves towards the equilibrium pressure, and only later the different pressures become approximately equal to one another

hiperlife

hiperlife is a high performance parallel library for finite elements. The aim of this library is to provide a computational framework to address problems of cell and tissue mechanobiology for a wide range of cases and users, with special focus on curved surfaces (the cell membrane, the cell cortex, epithelial monolayers,etc.). It is designed to handle the multiphysic nature of problems in mechanobiology, to manage unstructured grids that deal with complex geometries and to allow arbitrary higher-order basis functions that describe the curvature of interfaces. hiperlife is written in C++, uses the Message Passage Interface (MPI) paradigm for parallelism, and is built on top of several packages of the Trilinos Project. These particular choices set the basis for the main features of the library, namely, parallelism, flexibility, user-centered design and sustainability.4.

Approximation of tensor fields on surfaces of arbitrary topology based on local Monge parametrizations

We introduce a new method, the Local Monge Parametrizations (LMP) method, to approximate tensor fields on general surfaces given by a collection of local parametrizations, e.g. as in finite element or NURBS surface representations. Our goal is to use this method to solve numerically tensor-valued partial differential equations (PDEs) on surfaces. Previous methods use scalar potentials to numerically describe vector fields on surfaces, at the expense of requiring higher-order derivatives of the approximated fields and limited to simply connected surfaces, or represent tangential tensor fields as tensor fields in 3D subjected to constraints, thus increasing the essential number of degrees of freedom. In contrast, the LMP method uses an optimal number of degrees of freedom to represent a tensor, is general with regards to the topology of the surface, and does not increase the order of the PDEs governing the tensor fields. The main idea is to construct maps between the element parametrizations and a local Monge parametrization around each node. We test the LMP method by approximating in a least-squares sense different vector and tensor fields on simply connected and genus-1 surfaces. Furthermore, we apply the LMP method to two physical models on surfaces, involving a tension-driven flow (vector-valued PDE) and nematic ordering (tensor-valued PDE), on different topologies. The LMP method thus solves the long-standing problem of the interpolation of tensors on general surfaces with an optimal number of degrees of freedom