Thiago Santos Pereira - Academia.edu (original) (raw)

Papers by Thiago Santos Pereira

arXiv (Cornell University), Apr 3, 2013

The effect of the scalar spectral index on inflationary super-Hubble waves is to amplify/damp lar... more The effect of the scalar spectral index on inflationary super-Hubble waves is to amplify/damp large wavelengths according to whether the spectrum is red (ns < 1) or blue (ns > 1). As a consequence, the large-scale temperature correlation function will unavoidably change sign at some angle if our spectrum is red, while it will always be positive if it is blue. We show that this inflationary filtering property also affects our estimates of the size of the homogeneous patch of the universe through the Grishchuk-Zel'dovich effect. Using the recent quadrupole measurement of ESA's Planck mission, we find that the homogeneous patch of universe is at least 87 times bigger than our visible universe if we accept Planck's best fit value ns = 0.9624. An independent estimation of the size of the universe could be used to independently constrain ns, thus narrowing the space of inflationary models.

Revista Acadêmica: Ciência Animal, 2008

Este estudo foi conduzido utilizando juvenis de pacu e teve o objetivo de determinar a digestibil... more Este estudo foi conduzido utilizando juvenis de pacu e teve o objetivo de determinar a digestibilidade aparente da energia e da proteína do amido de milho, do glúten de milho, do farelo de girassol e da celulose purificada. Foram utilizados 375 juvenis de pacu com peso médio de 50,53±5,70 gramas, distribuídos em 25 aquários de alimentação com capacidade de 100 litros. As coletas de fezes foram realizadas em cinco coletores cilíndricos (Sistema de Guelph modificado) com capacidade de 80 litros. A determinação dos coeficientes de digestibilidade da energia e da proteína dos alimentos foi realizada pelo método de coleta parcial de excretas e o óxido de cromo utilizado como marcador inerte. O delineamento experimental foi inteiramente casualizado, com quatro tratamentos (alimentos) e cinco repetições. Os resultados da digestibilidade aparente da energia variaram entre 27,62% para a celulose purificada e 99,98% para o amido de milho e as análises estatísticas demonstraram diferenças (P&l...

Journal of Cosmology and Astroparticle Physics, 2022

We present an effective Eulerian description, in the non-relativistic regime, of the growth of co... more We present an effective Eulerian description, in the non-relativistic regime, of the growth of cosmological perturbations around a homogeneous but anisotropic Bianchi I spacetime background. We assume a small deviation from isotropy, sourced at late times for example by dark energy anisotropic stress. We thus derive an analytic solution for the linear dark matter density contrast, and use it in a formal perturbative approach which allows us to derive a second order (non-linear) solution. As an application of the procedure followed here we derive analytic expressions for the power spectrum and the bispectrum of the dark matter density contrast. The power spectrum receives a quadrupolar correction as expected, and the bispectrum receives several angle-dependent corrections. Quite generally, we find that the contribution of a late-time phase of anisotropic expansion to the growth of structure peaks at a finite redshift between CMB decoupling and today, tough the exact redshift value is...

Objetivou-se relatar a experiencia do graduando de enfermagem no processo de ensino/aprendizagem ... more Objetivou-se relatar a experiencia do graduando de enfermagem no processo de ensino/aprendizagem com uso dos mapas mentais.

arXiv: Cosmology and Nongalactic Astrophysics, 2018

To date, no compelling evidence has been found that the universe has non-trivial spatial topology... more To date, no compelling evidence has been found that the universe has non-trivial spatial topology. Meanwhile, anomalies in the observed CMB temperature map, such as the lack of correlations at large angular separations, remain observationally robust. We show that if our universe is flat and has one compact dimension of appropriate size (slab topology), this would suppress large-angle temperature correlations while maintaining a low-$\ell$ angular power spectrum consistent with observations. The optimal length appears to be 1.41.41.4 times the conformal radius of the CMB's last scattering surface ($\chi_{rec}$). We construct the probability distribution function of the statistic S1/2S_{1/2}S1/2 using simulated Sachs-Wolf-only skies for each of several values of Lz/chirecL_z/\chi_{rec}Lz/chirec. For Lzsimeq1.4chirecL_z\simeq1.4\chi_{rec}Lzsimeq1.4chirec the ppp-value of four standard masked Planck maps is psimeq0.15p\simeq0.15psimeq0.15, compared to plesssim0.003p\lesssim0.003plesssim0.003 for the conventional topologically trivial space. The mean angular power spectrum $\lang...

Physics of the Dark Universe, 2021

In this paper, we study a triad of inhomogeneous scalar fields, known as "solid", as a source of ... more In this paper, we study a triad of inhomogeneous scalar fields, known as "solid", as a source of homogeneous but anisotropic dark energy. By using the dynamical system approach, we find that anisotropic accelerated solutions can be realized as attractor points for suitable choices of the parameters of the model. We complement the dynamical analysis with a numerical solution whose initial conditions are set in the deep radiation epoch. The model predicts a non-negligible spatial shear within the observational bounds nowadays, even when it is set to zero at early times. The anisotropic attractor and the ultra slowly varying equation of state of dark energy very close to −1 are key features of this scenario. Following a similar approach, we also analyzed the isotropic version of the model. We find that the solid can be characterized by a nearly constant equation of state and thus being able to simulate the behavior of a cosmological constant.

Physical Review D, 2020

Within cosmological perturbation theory, the cosmic microwave background anisotropies are usually... more Within cosmological perturbation theory, the cosmic microwave background anisotropies are usually computed from a Boltzmann hierarchy coupled to the perturbed Einstein equations. In this setup, one set of multipoles describes the temperature anisotropies, while two other sets, of electric and magnetic types, describe the polarization anisotropies. In order to reduce the number of multipoles types needed for polarization, and thus to speed up the numerical resolution, an optimal hierarchy has been proposed in the literature for Einstein-Boltzmann codes. However, it has been recently shown that the separability between directional and orbital eigenfunctions employed in the optimal hierarchy is not correct in the presence of spatial curvature. We investigate how the assumption of separability affects the optimal hierarchy, and show that it introduces relative errors of order Ω K with respect to the full hierarchy. Despite of that, we show that the optimal hierarchy still gives extremely good results for temperature and polarization angular spectra, with relative errors that are much smaller than cosmic variance even for curvatures as large as |Ω K | = 0.1. Still, we find that the polarization angular spectra from tensor perturbations are significantly altered when using the optimal hierarchy, leading to errors that are typically of order 50|Ω K |% on that component.

Classical and Quantum Gravity, 2020

We investigate a corner of the Bianchi models that has not received much attention: "extended FLR... more We investigate a corner of the Bianchi models that has not received much attention: "extended FLRW models" (eFLRW) defined as a cosmological model with underlying anisotropic Bianchi geometry that nevertheless expands isotropically and can be mapped onto a reference FLRW model with the same expansion history. In order to investigate the stability and naturalness of such models in a dynamical systems context, we consider spatially homogeneous models that contain a massless scalar field ϕ and a non-tilted perfect fluid obeying an equation of state p = wρ. Remarkably, we find that matter anisotropies and geometrical anisotropies tend to cancel out dynamically. Hence, the expansion is asymptotically isotropic under rather general conditions. Although extended FLRW models require a special matter sector with anisotropies that are "fine-tuned" relative to geometrical anisotropies, our analysis shows that such solutions are dynamically preferred attractors in general relativity. Specifically, we prove that all locally rotationally symmetric Bianchi type III universes with space-like ∇ µ ϕ are asymptotically shear-free, for all w ∈ [−1, 1]. Moreover, all shear-free equilibrium sets with anisotropic spatial curvature are proved to be stable with respect to all homogeneous perturbations for w ≥ −1/3.

Physical Review D, 2019

A powerful result in theoretical cosmology states that a subset of anisotropic Bianchi models can... more A powerful result in theoretical cosmology states that a subset of anisotropic Bianchi models can be seen as the homogeneous limit of (standard) linear cosmological perturbations. Such models are precisely those leading to Friedmann spacetimes in the limit of zero anisotropy. Building on previous works, we give a comprehensive exposition of this result, and perform the detailed identification between anisotropic degrees of freedom and their corresponding scalar, vector, and tensor perturbations of standard perturbation theory. In particular, we find that anisotropic models very close to open (i.e., negatively curved) Friedmann spaces correspond to some type of super-curvature perturbations. As a consequence, provided anisotropy is mild, its effects on all types of cosmological observables can always be computed as simple extensions of the standard techniques used in relativistic perturbation theory around Friedmann models. This fact opens the possibility to consistently constrain, for all cosmological observables, the presence of large scale anisotropies on the top of the stochastic fluctuations.

Physical Review D, 2019

We present a comprehensive construction of scalar, vector and tensor harmonics on maximally symme... more We present a comprehensive construction of scalar, vector and tensor harmonics on maximally symmetric three-dimensional spaces. Our formalism relies on the introduction of spin-weighted spherical harmonics and a generalized helicity basis which, together, are ideal tools to decompose harmonics into their radial and angular dependencies. We provide a thorough and self-contained set of expressions and relations for these harmonics. Being general, our formalism also allows to build harmonics of higher tensor type by recursion among radial functions, and we collect the complete set of recursive relations which can be used. While the formalism is readily adapted to computation of CMB transfer functions, we also collect explicit forms of the radial harmonics which are needed for other cosmological observables. Finally, we show that in curved spaces, normal modes cannot be factorized into a local angular dependence and a unit norm function encoding the orbital dependence of the harmonics, contrary to previous statements in the literature.

Journal of Cosmology and Astroparticle Physics, 2017

Besides expanding anisotropically, the universe can also be anisotropic at the level of its (spat... more Besides expanding anisotropically, the universe can also be anisotropic at the level of its (spatial) curvature. In particular, models with anisotropic curvature and isotropic expansion leads both to a ΛCDM-like phenomenology and to an isotropic and homogeneous CMB at the background level. Thus, they offer an interesting and viable example where the cosmological principle does not follow from the isotropy of observational data. In this paper we extract the linear dynamics of tensor perturbations in two classes of cosmologies with anisotropic spatial curvature. Two difficulties arise in comparison to the same computation in isotropic cosmologies. First, the two tensor polarizations do not behave as a spin-2 field, but rather as the spin-0 and spin-1 irreducible components of a symmetric, traceless and transverse tensor field, each with its own dynamics. Second, because metric perturbations are algebraically coupled, one cannot ignore scalar and vector modes and focus just on tensors-even if one is only interested in the latter-under the penalty of obtaining the wrong equations of motion. We illustrate our results by finding analytical solutions and evaluating the power-spectra of tensor polarizations in a radiation dominated universe. We conclude with some comments on how these models could be constrained with future experiments on CMB polarization.

Journal of Cosmology and Astroparticle Physics, 2017

Two-point correlation functions are ubiquitous tools of modern cosmology, appearing in disparate ... more Two-point correlation functions are ubiquitous tools of modern cosmology, appearing in disparate topics ranging from cosmological inflation to late-time astrophysics. When the background spacetime is maximally symmetric, invariance arguments can be used to fix the functional dependence of this function as the invariant distance between any two points. In this paper we introduce a novel formalism which fixes this functional dependence directly from the isometries of the background metric, thus allowing one to quickly assess the overall features of Gaussian correlators without resorting to the full machinery of perturbation theory. As an application we construct the CMB temperature correlation function in one inhomogeneous (namely, an off-center LTB model) and two spatially flat and anisotropic (Bianchi) universes, and derive their covariance matrices in the limit of almost Friedmannian symmetry. We show how the method can be extended to arbitrary N-point correlation functions and illustrate its use by constructing three-point correlation functions in some simple geometries.

Revista De Iniciacao Cientifica Da Ulbra, Jan 16, 2013

A aplicação de células-tronco em retalhos cutâneos trouxe novas perspectivas ao campo da microcir... more A aplicação de células-tronco em retalhos cutâneos trouxe novas perspectivas ao campo da microcirurgia reconstrutiva. Este estudo visou desenvolver um modelo experimental para avaliar a utilização de célulastronco derivadas de tecido adiposo (ADSCs) em lesão por isquemia-reperfusão de retalhos cutâneos. Dez camundongos foram operados e divididos em Grupo 1 (isquemia e reperfusão) e Grupo 2 (controle). Foram avaliados temperatura, coloração e microangiografia. ADSCs foram isoladas de camundongos C57BL/6. Observou-se queda da temperatura e perda da coloração no Grupo 1, com recuperação no período de rep

Astronomy & Astrophysics, 2016

Aims. We use the angular power spectrum of B-modes of the weak-lensing shear as a tool for constr... more Aims. We use the angular power spectrum of B-modes of the weak-lensing shear as a tool for constraining late-time deviations of spatial isotropy in our Universe. Methods. We used the formalism of weak lensing in arbitrary spacetimes. Results. We find that off-diagonal correlations must exist between E-modes, B-modes, and convergence of the weak-lensing field, which allow one to reconstruct the eigendirections of anisotropic expansion. Focusing on future surveys, such as Euclid and the Square Kilometer Array (SKA), we find that observations can constrain the geometrical shear in units of the Hubble rate at the percent level or better. These observations offer a new and powerful method to probe our cosmological model, however, the power of this new technique still requires further investigations and a full analysis of signal-to-noise ratio.

Apresentamos neste trabalho uma generalização da teoria de perturbações cosmológicas para o caso ... more Apresentamos neste trabalho uma generalização da teoria de perturbações cosmológicas para o caso de universos homogêneos e anisotrópicos, caracterizados por um espaço-tempo do tipo Bianchi I. Como aplicação da teoria, investigamos as conseqüências de uma fase inacionária e anisotrópica do universo dos pontos de vista clássico e quântico. Após uma discussão da evolução do espaço-tempo de fundo nós quantizamos os modos perturbativos para, em seguida, construir o espectro de potências das perturbações de curvatura e de ondas gravitacionais do m da inação. Nossos resultados mostram que as principais características de uma fase anisotrópica primordial do universo são: (1) dependência direcional dos espectros de potências, (2) acoplamento entre as perturbações de curvatura e as ondas gravitacionais e (3) espectros distintos para as diferentes polarizações das ondas gravitacionais em grandes escalas cosmológicas. Todos esses efeitos são importantes apenas em grandes escalas cosmológicas e, localmente, recuperamos a teoria isotrópica de perturbações cosmológicas. Nossos resultados dependem de uma escala característica que pode, embora não seja estritamente necessário, ser ajustada a alguma escala observável.

Volume 10: Petroleum Technology, 2015

The gas hydrates problem has been growing in offshore deep water condition where due to low tempe... more The gas hydrates problem has been growing in offshore deep water condition where due to low temperature and high pressure hydrate formation becomes more favorable. Several studies have been done to predict the influence of gas hydrate formation in natural gas flow pipeline. However, the effects of multiphase hydrodynamic properties on hydrate formation are missing in these studies. The use of CFD to simulate gas hydrate formation can overcome this gap. In this study a computational fluid dynamics (CFD) model has been developed for mass, heat and momentum transfer for better understanding natural gas hydrate formation and its migration into the pipelines using ANSYS CFX-14. The problem considered in this study is a three-dimensional multiphase-flow model based on Simon Lo (2003) study, which considered the oil-dominant flow in a pipeline with hydrate formation around water droplets dispersed into the oil phase. The results obtained in this study will be useful in designing a multipha...

Comptes Rendus Physique, 2015

A primordial inflationary phase allows one to erase any possible anisotropic expansion thanks to ... more A primordial inflationary phase allows one to erase any possible anisotropic expansion thanks to the cosmic no-hair theorem. If there is no global anisotropic stress, then the anisotropic expansion rate tends to decrease. What are the observational consequences of a possible early anisotropic phase? We first review the dynamics of anisotropic universes and report analytic approximations. We then discuss the structure of dynamical equations for perturbations and the statistical properties of observables, as well as the implication of a primordial anisotropy on the quantization of these perturbations during inflation. Finally we briefly review models based on primordial vector field which evade the cosmic no-hair theorem.

Physical Review D, 2015

When searching for deviations of statistical isotropy in CMB, a popular strategy is to write the ... more When searching for deviations of statistical isotropy in CMB, a popular strategy is to write the two-point correlation function (2pcf) as the most general function of four spherical angles (i.e., two unit vectors) in the celestial sphere. Then, using a basis of bipolar spherical harmonics, statistical anisotropy will show up if and only if any coefficient of the expansion with non-trivial bipolar momentum is detected-although this detection will not in general elucidate the origin of the anisotropy. In this work we show that two new sets of four angles and basis functions exist which completely specifies the 2pcf, while, at the same time, offering a possible geometrical interpretation of the mechanisms generating the signal. Since the coefficients of these expansions are zero if and only if isotropy holds, they act as a simple and geometrically motivated null test of statistical isotropy, with the advantage of allowing cosmic variance to be controlled in a systematic way. We report the results of the application of these null tests to the latest temperature data released by the Planck collaboration.

Journal of Cosmology and Astroparticle Physics, 2015

If one is willing to give up the cherished hypothesis of spatial isotropy, many interesting cosmo... more If one is willing to give up the cherished hypothesis of spatial isotropy, many interesting cosmological models can be developed beyond the simple anisotropically expanding scenarios. One interesting possibility is presented by shear-free models in which the anisotropy emerges at the level of the curvature of the homogeneous spatial sections, whereas the expansion is dictated by a single scale factor. We show that such models represent viable alternatives to describe the large-scale structure of the inflationary universe, leading to a kinematically equivalent Sachs-Wolfe effect. Through the definition of a complete set of spatial eigenfunctions we compute the two-point correlation function of scalar perturbations in these models. In addition, we show how such scenarios would modify the spectrum of the CMB assuming that the observations take place in a small patch of a universe with anisotropic curvature.

arXiv (Cornell University), Apr 3, 2013

The effect of the scalar spectral index on inflationary super-Hubble waves is to amplify/damp lar... more The effect of the scalar spectral index on inflationary super-Hubble waves is to amplify/damp large wavelengths according to whether the spectrum is red (ns < 1) or blue (ns > 1). As a consequence, the large-scale temperature correlation function will unavoidably change sign at some angle if our spectrum is red, while it will always be positive if it is blue. We show that this inflationary filtering property also affects our estimates of the size of the homogeneous patch of the universe through the Grishchuk-Zel'dovich effect. Using the recent quadrupole measurement of ESA's Planck mission, we find that the homogeneous patch of universe is at least 87 times bigger than our visible universe if we accept Planck's best fit value ns = 0.9624. An independent estimation of the size of the universe could be used to independently constrain ns, thus narrowing the space of inflationary models.

Revista Acadêmica: Ciência Animal, 2008

Este estudo foi conduzido utilizando juvenis de pacu e teve o objetivo de determinar a digestibil... more Este estudo foi conduzido utilizando juvenis de pacu e teve o objetivo de determinar a digestibilidade aparente da energia e da proteína do amido de milho, do glúten de milho, do farelo de girassol e da celulose purificada. Foram utilizados 375 juvenis de pacu com peso médio de 50,53±5,70 gramas, distribuídos em 25 aquários de alimentação com capacidade de 100 litros. As coletas de fezes foram realizadas em cinco coletores cilíndricos (Sistema de Guelph modificado) com capacidade de 80 litros. A determinação dos coeficientes de digestibilidade da energia e da proteína dos alimentos foi realizada pelo método de coleta parcial de excretas e o óxido de cromo utilizado como marcador inerte. O delineamento experimental foi inteiramente casualizado, com quatro tratamentos (alimentos) e cinco repetições. Os resultados da digestibilidade aparente da energia variaram entre 27,62% para a celulose purificada e 99,98% para o amido de milho e as análises estatísticas demonstraram diferenças (P&l...

Journal of Cosmology and Astroparticle Physics, 2022

We present an effective Eulerian description, in the non-relativistic regime, of the growth of co... more We present an effective Eulerian description, in the non-relativistic regime, of the growth of cosmological perturbations around a homogeneous but anisotropic Bianchi I spacetime background. We assume a small deviation from isotropy, sourced at late times for example by dark energy anisotropic stress. We thus derive an analytic solution for the linear dark matter density contrast, and use it in a formal perturbative approach which allows us to derive a second order (non-linear) solution. As an application of the procedure followed here we derive analytic expressions for the power spectrum and the bispectrum of the dark matter density contrast. The power spectrum receives a quadrupolar correction as expected, and the bispectrum receives several angle-dependent corrections. Quite generally, we find that the contribution of a late-time phase of anisotropic expansion to the growth of structure peaks at a finite redshift between CMB decoupling and today, tough the exact redshift value is...

Objetivou-se relatar a experiencia do graduando de enfermagem no processo de ensino/aprendizagem ... more Objetivou-se relatar a experiencia do graduando de enfermagem no processo de ensino/aprendizagem com uso dos mapas mentais.

arXiv: Cosmology and Nongalactic Astrophysics, 2018

To date, no compelling evidence has been found that the universe has non-trivial spatial topology... more To date, no compelling evidence has been found that the universe has non-trivial spatial topology. Meanwhile, anomalies in the observed CMB temperature map, such as the lack of correlations at large angular separations, remain observationally robust. We show that if our universe is flat and has one compact dimension of appropriate size (slab topology), this would suppress large-angle temperature correlations while maintaining a low-$\ell$ angular power spectrum consistent with observations. The optimal length appears to be 1.41.41.4 times the conformal radius of the CMB's last scattering surface ($\chi_{rec}$). We construct the probability distribution function of the statistic S1/2S_{1/2}S1/2 using simulated Sachs-Wolf-only skies for each of several values of Lz/chirecL_z/\chi_{rec}Lz/chirec. For Lzsimeq1.4chirecL_z\simeq1.4\chi_{rec}Lzsimeq1.4chirec the ppp-value of four standard masked Planck maps is psimeq0.15p\simeq0.15psimeq0.15, compared to plesssim0.003p\lesssim0.003plesssim0.003 for the conventional topologically trivial space. The mean angular power spectrum $\lang...

Physics of the Dark Universe, 2021

In this paper, we study a triad of inhomogeneous scalar fields, known as "solid", as a source of ... more In this paper, we study a triad of inhomogeneous scalar fields, known as "solid", as a source of homogeneous but anisotropic dark energy. By using the dynamical system approach, we find that anisotropic accelerated solutions can be realized as attractor points for suitable choices of the parameters of the model. We complement the dynamical analysis with a numerical solution whose initial conditions are set in the deep radiation epoch. The model predicts a non-negligible spatial shear within the observational bounds nowadays, even when it is set to zero at early times. The anisotropic attractor and the ultra slowly varying equation of state of dark energy very close to −1 are key features of this scenario. Following a similar approach, we also analyzed the isotropic version of the model. We find that the solid can be characterized by a nearly constant equation of state and thus being able to simulate the behavior of a cosmological constant.

Physical Review D, 2020

Within cosmological perturbation theory, the cosmic microwave background anisotropies are usually... more Within cosmological perturbation theory, the cosmic microwave background anisotropies are usually computed from a Boltzmann hierarchy coupled to the perturbed Einstein equations. In this setup, one set of multipoles describes the temperature anisotropies, while two other sets, of electric and magnetic types, describe the polarization anisotropies. In order to reduce the number of multipoles types needed for polarization, and thus to speed up the numerical resolution, an optimal hierarchy has been proposed in the literature for Einstein-Boltzmann codes. However, it has been recently shown that the separability between directional and orbital eigenfunctions employed in the optimal hierarchy is not correct in the presence of spatial curvature. We investigate how the assumption of separability affects the optimal hierarchy, and show that it introduces relative errors of order Ω K with respect to the full hierarchy. Despite of that, we show that the optimal hierarchy still gives extremely good results for temperature and polarization angular spectra, with relative errors that are much smaller than cosmic variance even for curvatures as large as |Ω K | = 0.1. Still, we find that the polarization angular spectra from tensor perturbations are significantly altered when using the optimal hierarchy, leading to errors that are typically of order 50|Ω K |% on that component.

Classical and Quantum Gravity, 2020

We investigate a corner of the Bianchi models that has not received much attention: "extended FLR... more We investigate a corner of the Bianchi models that has not received much attention: "extended FLRW models" (eFLRW) defined as a cosmological model with underlying anisotropic Bianchi geometry that nevertheless expands isotropically and can be mapped onto a reference FLRW model with the same expansion history. In order to investigate the stability and naturalness of such models in a dynamical systems context, we consider spatially homogeneous models that contain a massless scalar field ϕ and a non-tilted perfect fluid obeying an equation of state p = wρ. Remarkably, we find that matter anisotropies and geometrical anisotropies tend to cancel out dynamically. Hence, the expansion is asymptotically isotropic under rather general conditions. Although extended FLRW models require a special matter sector with anisotropies that are "fine-tuned" relative to geometrical anisotropies, our analysis shows that such solutions are dynamically preferred attractors in general relativity. Specifically, we prove that all locally rotationally symmetric Bianchi type III universes with space-like ∇ µ ϕ are asymptotically shear-free, for all w ∈ [−1, 1]. Moreover, all shear-free equilibrium sets with anisotropic spatial curvature are proved to be stable with respect to all homogeneous perturbations for w ≥ −1/3.

Physical Review D, 2019

A powerful result in theoretical cosmology states that a subset of anisotropic Bianchi models can... more A powerful result in theoretical cosmology states that a subset of anisotropic Bianchi models can be seen as the homogeneous limit of (standard) linear cosmological perturbations. Such models are precisely those leading to Friedmann spacetimes in the limit of zero anisotropy. Building on previous works, we give a comprehensive exposition of this result, and perform the detailed identification between anisotropic degrees of freedom and their corresponding scalar, vector, and tensor perturbations of standard perturbation theory. In particular, we find that anisotropic models very close to open (i.e., negatively curved) Friedmann spaces correspond to some type of super-curvature perturbations. As a consequence, provided anisotropy is mild, its effects on all types of cosmological observables can always be computed as simple extensions of the standard techniques used in relativistic perturbation theory around Friedmann models. This fact opens the possibility to consistently constrain, for all cosmological observables, the presence of large scale anisotropies on the top of the stochastic fluctuations.

Physical Review D, 2019

We present a comprehensive construction of scalar, vector and tensor harmonics on maximally symme... more We present a comprehensive construction of scalar, vector and tensor harmonics on maximally symmetric three-dimensional spaces. Our formalism relies on the introduction of spin-weighted spherical harmonics and a generalized helicity basis which, together, are ideal tools to decompose harmonics into their radial and angular dependencies. We provide a thorough and self-contained set of expressions and relations for these harmonics. Being general, our formalism also allows to build harmonics of higher tensor type by recursion among radial functions, and we collect the complete set of recursive relations which can be used. While the formalism is readily adapted to computation of CMB transfer functions, we also collect explicit forms of the radial harmonics which are needed for other cosmological observables. Finally, we show that in curved spaces, normal modes cannot be factorized into a local angular dependence and a unit norm function encoding the orbital dependence of the harmonics, contrary to previous statements in the literature.

Journal of Cosmology and Astroparticle Physics, 2017

Besides expanding anisotropically, the universe can also be anisotropic at the level of its (spat... more Besides expanding anisotropically, the universe can also be anisotropic at the level of its (spatial) curvature. In particular, models with anisotropic curvature and isotropic expansion leads both to a ΛCDM-like phenomenology and to an isotropic and homogeneous CMB at the background level. Thus, they offer an interesting and viable example where the cosmological principle does not follow from the isotropy of observational data. In this paper we extract the linear dynamics of tensor perturbations in two classes of cosmologies with anisotropic spatial curvature. Two difficulties arise in comparison to the same computation in isotropic cosmologies. First, the two tensor polarizations do not behave as a spin-2 field, but rather as the spin-0 and spin-1 irreducible components of a symmetric, traceless and transverse tensor field, each with its own dynamics. Second, because metric perturbations are algebraically coupled, one cannot ignore scalar and vector modes and focus just on tensors-even if one is only interested in the latter-under the penalty of obtaining the wrong equations of motion. We illustrate our results by finding analytical solutions and evaluating the power-spectra of tensor polarizations in a radiation dominated universe. We conclude with some comments on how these models could be constrained with future experiments on CMB polarization.

Journal of Cosmology and Astroparticle Physics, 2017

Two-point correlation functions are ubiquitous tools of modern cosmology, appearing in disparate ... more Two-point correlation functions are ubiquitous tools of modern cosmology, appearing in disparate topics ranging from cosmological inflation to late-time astrophysics. When the background spacetime is maximally symmetric, invariance arguments can be used to fix the functional dependence of this function as the invariant distance between any two points. In this paper we introduce a novel formalism which fixes this functional dependence directly from the isometries of the background metric, thus allowing one to quickly assess the overall features of Gaussian correlators without resorting to the full machinery of perturbation theory. As an application we construct the CMB temperature correlation function in one inhomogeneous (namely, an off-center LTB model) and two spatially flat and anisotropic (Bianchi) universes, and derive their covariance matrices in the limit of almost Friedmannian symmetry. We show how the method can be extended to arbitrary N-point correlation functions and illustrate its use by constructing three-point correlation functions in some simple geometries.

Revista De Iniciacao Cientifica Da Ulbra, Jan 16, 2013

A aplicação de células-tronco em retalhos cutâneos trouxe novas perspectivas ao campo da microcir... more A aplicação de células-tronco em retalhos cutâneos trouxe novas perspectivas ao campo da microcirurgia reconstrutiva. Este estudo visou desenvolver um modelo experimental para avaliar a utilização de célulastronco derivadas de tecido adiposo (ADSCs) em lesão por isquemia-reperfusão de retalhos cutâneos. Dez camundongos foram operados e divididos em Grupo 1 (isquemia e reperfusão) e Grupo 2 (controle). Foram avaliados temperatura, coloração e microangiografia. ADSCs foram isoladas de camundongos C57BL/6. Observou-se queda da temperatura e perda da coloração no Grupo 1, com recuperação no período de rep

Astronomy & Astrophysics, 2016

Aims. We use the angular power spectrum of B-modes of the weak-lensing shear as a tool for constr... more Aims. We use the angular power spectrum of B-modes of the weak-lensing shear as a tool for constraining late-time deviations of spatial isotropy in our Universe. Methods. We used the formalism of weak lensing in arbitrary spacetimes. Results. We find that off-diagonal correlations must exist between E-modes, B-modes, and convergence of the weak-lensing field, which allow one to reconstruct the eigendirections of anisotropic expansion. Focusing on future surveys, such as Euclid and the Square Kilometer Array (SKA), we find that observations can constrain the geometrical shear in units of the Hubble rate at the percent level or better. These observations offer a new and powerful method to probe our cosmological model, however, the power of this new technique still requires further investigations and a full analysis of signal-to-noise ratio.

Apresentamos neste trabalho uma generalização da teoria de perturbações cosmológicas para o caso ... more Apresentamos neste trabalho uma generalização da teoria de perturbações cosmológicas para o caso de universos homogêneos e anisotrópicos, caracterizados por um espaço-tempo do tipo Bianchi I. Como aplicação da teoria, investigamos as conseqüências de uma fase inacionária e anisotrópica do universo dos pontos de vista clássico e quântico. Após uma discussão da evolução do espaço-tempo de fundo nós quantizamos os modos perturbativos para, em seguida, construir o espectro de potências das perturbações de curvatura e de ondas gravitacionais do m da inação. Nossos resultados mostram que as principais características de uma fase anisotrópica primordial do universo são: (1) dependência direcional dos espectros de potências, (2) acoplamento entre as perturbações de curvatura e as ondas gravitacionais e (3) espectros distintos para as diferentes polarizações das ondas gravitacionais em grandes escalas cosmológicas. Todos esses efeitos são importantes apenas em grandes escalas cosmológicas e, localmente, recuperamos a teoria isotrópica de perturbações cosmológicas. Nossos resultados dependem de uma escala característica que pode, embora não seja estritamente necessário, ser ajustada a alguma escala observável.

Volume 10: Petroleum Technology, 2015

The gas hydrates problem has been growing in offshore deep water condition where due to low tempe... more The gas hydrates problem has been growing in offshore deep water condition where due to low temperature and high pressure hydrate formation becomes more favorable. Several studies have been done to predict the influence of gas hydrate formation in natural gas flow pipeline. However, the effects of multiphase hydrodynamic properties on hydrate formation are missing in these studies. The use of CFD to simulate gas hydrate formation can overcome this gap. In this study a computational fluid dynamics (CFD) model has been developed for mass, heat and momentum transfer for better understanding natural gas hydrate formation and its migration into the pipelines using ANSYS CFX-14. The problem considered in this study is a three-dimensional multiphase-flow model based on Simon Lo (2003) study, which considered the oil-dominant flow in a pipeline with hydrate formation around water droplets dispersed into the oil phase. The results obtained in this study will be useful in designing a multipha...

Comptes Rendus Physique, 2015

A primordial inflationary phase allows one to erase any possible anisotropic expansion thanks to ... more A primordial inflationary phase allows one to erase any possible anisotropic expansion thanks to the cosmic no-hair theorem. If there is no global anisotropic stress, then the anisotropic expansion rate tends to decrease. What are the observational consequences of a possible early anisotropic phase? We first review the dynamics of anisotropic universes and report analytic approximations. We then discuss the structure of dynamical equations for perturbations and the statistical properties of observables, as well as the implication of a primordial anisotropy on the quantization of these perturbations during inflation. Finally we briefly review models based on primordial vector field which evade the cosmic no-hair theorem.

Physical Review D, 2015

When searching for deviations of statistical isotropy in CMB, a popular strategy is to write the ... more When searching for deviations of statistical isotropy in CMB, a popular strategy is to write the two-point correlation function (2pcf) as the most general function of four spherical angles (i.e., two unit vectors) in the celestial sphere. Then, using a basis of bipolar spherical harmonics, statistical anisotropy will show up if and only if any coefficient of the expansion with non-trivial bipolar momentum is detected-although this detection will not in general elucidate the origin of the anisotropy. In this work we show that two new sets of four angles and basis functions exist which completely specifies the 2pcf, while, at the same time, offering a possible geometrical interpretation of the mechanisms generating the signal. Since the coefficients of these expansions are zero if and only if isotropy holds, they act as a simple and geometrically motivated null test of statistical isotropy, with the advantage of allowing cosmic variance to be controlled in a systematic way. We report the results of the application of these null tests to the latest temperature data released by the Planck collaboration.

Journal of Cosmology and Astroparticle Physics, 2015

If one is willing to give up the cherished hypothesis of spatial isotropy, many interesting cosmo... more If one is willing to give up the cherished hypothesis of spatial isotropy, many interesting cosmological models can be developed beyond the simple anisotropically expanding scenarios. One interesting possibility is presented by shear-free models in which the anisotropy emerges at the level of the curvature of the homogeneous spatial sections, whereas the expansion is dictated by a single scale factor. We show that such models represent viable alternatives to describe the large-scale structure of the inflationary universe, leading to a kinematically equivalent Sachs-Wolfe effect. Through the definition of a complete set of spatial eigenfunctions we compute the two-point correlation function of scalar perturbations in these models. In addition, we show how such scenarios would modify the spectrum of the CMB assuming that the observations take place in a small patch of a universe with anisotropic curvature.