**Seminar I (Alexander Balakin): Electrodynamics of a cosmic dark fluid**

Abstract:Cosmic Dark Fluid is considered as a non-stationary medium, in which electromagnetic waves propagate, and magneto-electric field structures emerge and evolve. The Dark Fluid is assumed to be formed by a duet of Dark Matter (a pseudoscalar axionic constituent) and Dark Energy (a scalar-vector element). The Dark Fluid consists of electrically neutral particles and thus it does not interact with electromagnetic fields directly. However, the analogs of well-known indirect medium-type effects can exist, such as optical activity, birefringence, pyro-electricity, piezo-magnetism, electro- and magneto-striction and dynamo-optical activity. We discuss, first, exact solutions of the model indicated as Cosmic Axion Electrodynamics, keeping in mind that axions, the pseudo-Goldstone bosons, form (hypothetically) the Dark Matter. Second, we consider models, which describe electrodynamic effects induced by the Dark Energy. The discussion of the structure of these models is accompanied by examples of exact solutions to the master equations, correspondingly extended; applications are considered for cosmology and space-times with spherical and pp-wave symmetries.

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**Seminar II (Roberto Sussman): Structure formation, collapse and black hole formation from exact relativistic non spherical structures**

**Abstract:**We examine the process of structure formation and black hole (BH) formation from the gravitational collapse of multiple pancake-like dust structures surrounding a spheroidal over-density. These configurations are based on superpositions of smoothly matched patches of quasi-spherical Szekeres models of class I. The most external dust layer can always be smoothly matched to a Schwarzschild or LCDM exterior. We show that in a cosmological context shell crossing singularities can always be avoided, but in a black hole context they can only be avoided if the full configuration is very close to spherical symmetry. In a cosmological context these pancake-like structures provide a good coarse grained description of our cosmography in super cluster scale (50-100 Mpc). In the BH formation context large deviations from spherical symmetry require forcing shell crossings to occur in the future of the apparent horizon. However, this condition places strong restrictions on the resulting BH mass and collapse times. Hence, the collapsing times for astrophysical sized BH's (stellar and galactic) are too short (less than $10^4$ years), whereas collapsing times of astrophysical interest ($\sim 1-2$ Gys) yield excessively massive BH's ($M \sim 10^{20) M_\odot$). However, for small mass primordial BH’s the collapsing times are physically plausible. As a consequence, it seems that primordial BH formation is the only physically plausible scenario in which BH’s that can be formed from collapsing non-spherical non-rotating dust structures.

**Seminar III (Gilberto Kremer): Post-Newtonian hydrodynamics from Boltzmann equation**

**Abstract:**The determination of the hydrodynamics equations in the post-Newtonian approximation is an old subject in the literature (see e.g. [1], [2]). Here these equations are deduced from a transfer equation which follows from the post-Newtonian Boltzmann equation. As an application self-gravitating systems of ideal gases in the post-Newtonian approximation are analyzed and static solutions of the gravitational potentials with spherical symmetry are used for the construction of the galaxy rotation curves. In another application a post-Newtonian Lane-Emden equation is applied to show its influence in the values of the fields of the density, pressure and temperature of stars.

[1] S. Chandrasekhar, Ap.J. 142, 1488 (1965).

[2] S. Weinberg, Gravitation and Cosmology (Wiley, New York, 1972).

**Seminar IV (Ronaldo Batista): Spherical Collapse of non-top-hat profiles in the presence of clustering dark energy**

Abstract:We study we spherical collapse of non top-hat matter fluctuations in the presence of dark energy with arbitrary sound speed. The system is described by a set of nonlinear partial differential equations which are solved using the pseudo-spectral method with collocation points. We show how clustering dark energy impacts the matter profiles and estimate its contribution to the mass of virialized halos. We also discuss some issues regarding dark energy with w<-1 that appear in the nonlinear regime.

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**Seminar V (Antonella Cid): Bayesian comparison of interacting scenarios:**

Abstract:We perform a Bayesian model selection analysis for a class of phenomenological interacting scenarios using a combination of some of the latest cosmological data such as Type Ia supernovae (JLA compilation), two different sets of Baryon Acoustic Oscillations (angular measurements denoted as BAO2 and the standard radial and transversal measurements, BAO3), cosmic chronometers (CC) and Cosmic Microwave Background. Among all the models considered in our study we do not find strong evidences in favor of any of the interacting scenarios compared to $\Lambda$CDM when the full joint analysis is considered, nevertheless the evidence remains inconclusive for some of the interacting models after the joint analysis. Our results indicate that either JLA, CC or BAO2 data by themselves are not able to distinguish among interacting models or $\Lambda$CDM but the standard BAO3 measurements and the combination with the CMB data are indeed able to discriminate between them. We find that the evidences disfavoring interacting models are weaker when we use BAO2 (data claimed to be almost model-independent) instead of the standard BAO3 measurements.

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**Seminar VI (Davi Rodrigues): On the cosmology of Renormalization Group-inspired extensions of General Relativity**

**Abstract:**In this short talk I intend to briefly present an ongoing project on the cosmology of Renormalization Group-inspired extensions of General Relativity.

**Seminar VII (Ilya Shapiro): Anomaly and induced effective action of gravity**

Abstract:The effective action of gravity is supposed to contain the main information about quantum corrections to gravity. However, in many cases it can not be calculated exactly. A remarkable and important exception is the effective action of vacuum for massless and conformal-invariant matter fields. In this case the effective action can be easily derived by integrating trace anomaly. The integration constant is an unknown conformal functional of the background metric, but for zero-order cosmology this functional is irrelevant and the solution becomes exact. Until recently the integrated anomaly was known only in dimensions d=2 and d=4, but recently we achieved the explicit result for d=6, which confirms the universal functional structure of the effective action. The most important applications in d=4 include systematic classification of vacuum states in the vicinity of the black hole and the extended version of Starobinsky model of inflation. It is remarkable that one can write down the general form of the anomaly-induced action even in an arbitrary even dimension, if assuming the correctness of two conjectures about the conformal features of the corresponding topological and surface terms.

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**Seminar VIII (Oliver Piattella): Stability of neutron stars in R + \alpha R^2 gravity**

**Abstract:**We study the stability of spherically symmetric configurations of matter described by a Sly equation of state in the context of f(R) = R + \alpha R^2 gravity. We stress how different definitions of mass lead to different results.

**Seminar IX (Saulo Carneiro): Interactions in the dark sector - theoretical and observational aspects**

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**Seminar X (Luís Ureña): Ultra-light boson particles as dark matter candidates: boson mass and self-interaction**

**Abstract:**I will present details about the possibility that ultra-light bosons could be the dark matter, from large scales down to galactic scales in the universe. Using numerical solutions, and the classical scalar field approximation, I will show the constraints that cosmological observations impose upon the boson mass and its quartic self-interaction in the case of an axion potential. I will also comment on the consequences of this scalar field hypothesis for the formation of galaxies in three aspects: the growth of linear density perturbations, the non-linear formation process via modified gravity models, and the case of satellite galaxies in the Milky Way. At the end, I will present an assessment of the viability of this model to be the dark matter.

**Seminar XI (Hermano Velten): Quasi-degeneracy between nonadiabatic dark energy models and the $\Lambda$CDM model**

**Abstract:**Supporting the standard cosmological model there exist a number of assumptions. One of them concerns the adiabatic nature of the dark components. We study widely known dark energy equation of state parameter parameterizations by assuming that at the perturbative level the dark energy fluid behaves as a nonadiabatic component. By analysing the first order scalar observables like the growth of matter structures and the integrated Sachs-Wolfe effect we have found that such nonadiabatic dark energy models become quasi-degenerated to the LCDM model. This talk is based on Velten et al Phys.Rev. D96 (2017) no.8, 083502 and Velten et al Phys.Rev. D97 (2018) no.10, 103514.

**Seminar XII (Rodrigo Gonçalves): Measuring the Homogeneity Scale with Matter Tracers**

**Given the rapidly increasing amount of cosmological data, not only we can test the standard cosmological model (SCM), i.e., the LCDM model with unprecedented precision, but also assess the validity of its fundamental pillars in light of these observations. Among these pillars, we have the Cosmological Principle (CP), which states that the Universe is isotropic and homogeneous on large scales. In this presentation we explore the cosmic homogeneity in the spatial distribution from many different matter observables, as Luminous Red Galaxies, Quasars, among others. We also show a model independent approach where we can acess the homogeneity via a tomographic 2D analysis. The results show that large scale homogeneity assumption is consistent with the latest observational data available.**

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**Seminar XIII (Luís Chimento): 2, 3, … fluidos acoplados en un universo de FRW plano**

Abstract:Estudiaremos un sistema de 2, 3, … fluidos acoplados con una interacción que depende linealmente de las densidades de energía y sus derivadas en un universo de FRW espacialmente plano. Asumiremos que estos fluidos tienen ecuaciones de estado lineales con índices barotrópicos constantes, en este caso encontraremos la "ecuación de la fuente" que determina la densidad total de energía efectiva. Obtendremos la solución general, que alivia el problema de la coincidencia, para un conjunto amplio de interacciones. Analizaremos con detalle un modelo de materia oscura acoplada con energía oscura holográfica de Ricci modificada. También, veremos como la ecuación de estado del fluido efectivo de un sector oscuro acoplado con un interacción no lineal incluye los distintos tipos de gases de Chaplygin.

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Para un sistema de tres fluidos, introduciremos un espacio interno tridimensional donde el vector interacción Q = (Q_1, Q_2, Q_3) está contenido en el "plano de interacción". Esta visión tridimensional simplifica notablemente los cálculos cuando seleccionamos una "interacción transversal Q_t", perpendicular al vector índice barotrópico γ = (γ_1, γ_2, γ_3). Para una interacción lineal obtendremos la solución general de la "ecuación de la fuente" y las densidades de energía de las tres componentes del modelo. Finalmente mostraremos como un universo lleno de materia oscura en interacción con un campo escalar puede plantearse y resolverse en el contexto de tres fluidos acoplados.

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**Seminar XIV**

**(Júlio Fabris): Viscous Cosmology**

Abstract:It is analysed the effects of both bulk and shear viscosities on the perturbations, relevant for structure formation in late time cosmology. It is shown that shear viscosity can be as effective as the bulk viscosity on suppressing the growth of perturbations and delaying the nonlinear regime. The possible degeneracy of the results with respect to the modified gravity framework is discussed. A dissipation term of geometric origin is also presented at background and perturbative level.

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**OUTREACH seminars. Tuesday 05/June**

19:00 (Walter Có): Terra, uma gota azul no oceano cósmico

20:00 (Hermano Velten): Busca por vida fora da Terra: Novas formas de vida ou nossa nova casa?

19:00 (Walter Có): Terra, uma gota azul no oceano cósmico

20:00 (Hermano Velten): Busca por vida fora da Terra: Novas formas de vida ou nossa nova casa?