Seminars

The seminars are currently held remotely on Friday at 11:00 (Brasilia time, GMT-3). The link is sent by e-mail.
If interested, please join the mailing list below. The seminars are presented in either English or Portuguese.

 The usual audience is from 20 to 40 participants.

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Recorded seminars

Organizers: Júlio Fabris, Paola Seidel, Davi Rodrigues.

2021

Scheduled seminars:

14 May 2021
Fully relativistic predictions in Horndeski gravity from standard Newtonian N-body simulations
Guilherme Brando (PPGCosmo)

The N-body gauge allows the introduction of relativistic effects in Newtonian cosmological simulations. Here we extend this framework to general Horndeski gravity theories, and investigate the relativistic effects that the scalar field introduces in the matter power spectrum at intermediate and large scales. In particular, we show that the kineticity function at these scales enhances the amplitude of the signal of contributions coming from the extra degree of freedom. Using the Quasi-Static Approximation (QSA), we separate modified gravity effects into two parts: one that only affects small-scale physics, and one that is due to relativistic effects. This allows our formalism to be readily implemented in modified gravity N-body codes in a straightforward manner, e.g.,
relativistic effects can be included as an additional linear density field in simulations. We identify the emergence of gravity acoustic oscillations (GAOs) in the matter power spectrum at large scales, $k \sim 10^{-3}-10^{-2}$ Mpc$^{-1}$. GAO features have a purely relativistic origin, coming from the dynamical nature of the scalar field. GAOs may be enhanced to detectable levels by the rapid evolution of the dark energy sound horizon in certain modified gravity models and can be seen as a new test of gravity at scales probed by future galaxy and intensity-mapping surveys.

28 May 2021
Ultra-light dark matter: the light and fuzzy side of dark matter
Elisa G. M. Ferreira (USP)

Among the many possible candidates for the nature of dark matter, one  of the most well-motivated class of models and leading candidate is  the ultra-light dark matter.  This class represents the lightest  possible dark matter candidates and exhibits a wave-like behavior on  galactic scales. This leads to a rich phenomenology on small scales  that can potentially not only reconcile the CDM picture with the  small-scale behavior of dark matter, but offer us the unique  possibility to probe their distinctive predictions and imprints that can reveal clues about the internal properties of dark matter. In this  talk, I will review this class of models, describing and classifying the different constructions and their phenomenology. Given their vast  cosmological and astrophysical effects on observables, I will describe  the ongoing advances in constraining these models using current  gravitational tests, and highlight the strong constraining power of  small-scale astrophysical observations. In the case of the fuzzy dark  matter class of models, I will show the latest constraints and how we  are narrowing down the mass range available for these models.

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Previous seminars:

7 May 2021
Extended theories of gravity to explain the Hubble-Lemaitre tension (slides)
Célia Escamilla-Rivera (UNAM, México)

The current cosmological probes have provided an extraordinary confirmation of the standard LCDM cosmological model, that has been constrained with unprecedented accuracy. However, with the increase of the experimental sensitivity a few statistically significant tensions between different independent cosmological datasets emerged. While these tensions can be in portion the result of systematic errors, the persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the need for new physics. In this talk I will list a few interesting new physics models in the direction of extended theories of gravity that could solve this tension and discuss how the new computational techniques will be crucial in this role.

30 Apr 2021
Tensions of ΛCDM and a gravitational transition (slides)
Leandros Perivolaropoulos (University of Ioannina, Greece)

The standard cosmological model ΛCDM provides a remarkable fit to the vast majority of cosmological data. However some late time data indicate that the Hubble expansion rate H(z) should be rescaled by a factor of 1.09 compared to the form favored by early time data (CMB and BBN) with no change in its functional form (shape). Other cosmological probes indicate that the growth rate of cosmological perturbations is about 10% weaker than the growth rate favored by geometric probe data (data independent of the strength of gravity) in the context of ΛCDM. I will review these tensions of ΛCDM and demonstrate that they may be simultaneously resolved in the presence of a transition of the strength of gravitational interactions which could have occurred 100-150 million years ago. Such a transition would correspond to weaker gravity (lower G_eff) at early times compared to its present value. I will argue that this is a viable and testable conjecture and I will show preliminary results of Tully-Fisher data that show hints for such a transition.​

23 Apr 2021
Propagação de campos bosônicos ao redor de buracos negros  (slides)
Carolina Loureiro Benone (UFPA)

A hipótese de que a matéria escura pode ser descrita por meio de campos bosônicos ultraleves tem recebido atenção considerável nos últimos anos. Essa matéria interagiria com objetos astrofísicos, como buracos negros. Nesse cenário, torna-se interessante estudar como os campos interagem com buracos negros. Neste seminário consideramos a absorção e espalhamento de campos bosônicos por buracos negros numericamente, encontrando também resultados analíticos nos limites de alta e baixa frequência. Também investigamos estados ligados neste contexto, mostrando que modos com uma certa frequência estão presos ao redor do buraco negro. Essas soluções são chamadas de nuvens e indicam a presença de soluções cabeludas. Também estudamos estes estados ligados para análogos de buracos negros, encontrando as chamadas nuvens acústicas.​

16 Apr 2021
 The Hubble tension from a Supernova perspective (slides)
David Camarena Torres (PPGCosmo/UFES)

 Current cosmological (and astrophysical) data shows that there exists a discrepancy between early and late-time determinations of the Hubble constant. This discrepancy is greater than 4 sigma if one considers the most accurate determinations of H0: CMB analyses by Planck and the cosmic distance ladder by SH0ES. While this mismatch between early and late-time can not be explained by the LambdaCDM model, several new models have been proposed to alleviate this tension, between them those that try to accommodate a higher value of H0 by a phantom transition to low-redshift (z < 0.1). In this talk, I will discuss why this kind of model fails to explain the Hubble tension by analyzing a particular and didactic case: the Hockey-Stick dark energy. The discussion will focus from a Supernova perspective, highlighting the role of the absolute magnitude of Supernovas on the Bayesian inference and the cosmic distance ladder. Some criticisms and alternatives to the cosmic distance ladder will also present during this talk.​
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09 Apr 2021
Matéria Escura tipo WIMP: status atual e cenários alternativos (slides)
Clarissa Siqueira (USP-SC)

Matéria escura continua como um dos principais problemas em aberto no que concerne à física de partículas e cosmologia. Neste seminário, iremos abordar o status atual de uma das mais bem motivadas candidatas do ponto de vista da física de partículas, as WIMPs (do inglês, "Weakly Interacting Massive Particles"), explorando os vínculos atuais sobre estas partículas. Com base neste cenário, introduziremos algumas propostas alternativas, dentre as quais, modelos confinados em um setor escondido e a cosmologia não-padrão, ambos aplicados a modelos concretos da física de partículas. Por fim, exploraremos a complementaridade dos experimentos atuais e futuros, incluindo os de detecção direta, como XENON1T, e os de detecção indireta, como HESS (atual), CTA e SWGO (futuros).

26 Mar 2021
The Connected Universe: Relating Early, Intermediate and Late Universe with cosmological data
Vivian Miranda (University of Arizona)

The standard model of cosmology is built upon on a series of propositions on how the early, intermediate, and late epochs of the Universe behave. In particular, it predicts that dark energy and dark matter currently permeates the cosmos. Understanding the properties of the dark sector is plausibly the biggest challenge in theoretical physics. There is, however, a broad assumption in cosmology that the Universe in its earlier stages is fully understood and that discrepancies between the standard model of cosmology and current data are suggestive of distinct dark energy properties. Uncertainties on this hypothesis's validity are not usually taken into account when forecasting survey capabilities, even though our investigations might be obfuscated if the intermediate and early Universe did behave abnormally. In this colloquium, I propose a program to investigate dark energy and earlier aspects of our Universe simultaneously, through space missions in the 2020s in combination with ground-based observatories. This program will help guide the strategy for the future Rubin and Roman supernovae and weak lensing surveys. My investigations on how properties of the early and intermediate Universe affect inferences on dark energy (and vice-versa) will also support community understanding of how future missions can be employed to test some of the core hypotheses of the standard model of cosmology.

9 Mar 2021
Probing thermal fluctuations through scalar test particles (slides)
Alexsandre Ferreira (PPGCosmo)

 The fundamental vacuum state of quantum fields, related to Minkowski spacetime, produces divergent fluctuations that must be suppressed in order to bring reality to the description of physical systems. As a consequence, negative vacuum expectation values of classically positive-defined quantities can appear. This has been addressed in the literature as subvacuum phenomenon. Here it is investigated how a scalar charged test particle is affected by the vacuum fluctuations of a massive scalar field in D+1 spacetime when the background evolves from empty space to a thermal bath, and also when a perfectly reflecting boundary is included. It is shown that when the particle is brought into a thermal bath it gains an amount of energy by means of positive dispersions of its velocity components. The magnitude of this effect is dependent on the temperature and also on the field mass. However, when a reflecting wall is inserted, dispersions can be positive or negative, showing that subvacuum effect happens even in a finite temperature environment. Furthermore, a remarkable result is that temperature can even improve negative velocity fluctuations. The magnitude of the residual effects depends on the switching interval of time the system takes to evolve between two states.

12 Mar 2021
Do black holes fall in Love? (slides)
Marc Casals (CBPF/PPGCosmo)

An important property of a compact object such as a planet, a star or a black hole is its deformability under an external  gravitational field. In 1909, A. Love introduced some numbers to measure such deformability within Newtonian mechanics  for the case of the Earth under the gravitational pull of the Moon, which creates the well-known ocean tides.
Already within the context of General Relativity, it has been shown that neutron stars also tidally deform, i.e., their Love numbers are nonzero. In fact, LIGO detections of gravitational waves emitted by neutron star binary inspirals have placed constraints on their Love numbers which, in its turn, has served to constrain their equation of state. Turning to black holes, it has been shown that their Love numbers are zero when they are non-rotating.  In contrast, in this talk we shall show that the Love numbers of rotating black holes are nonzero: they deform under an external tidal field.  So, in particular, while inspiralling in a binary system, a rotating black hole "falls in Love" with its companion.

5 Mar 2021
21 cm cosmology and the BINGO radio telescope
Carlos Alexandre Wuensche (Divisão de Astrofísica- INPE)

Cosmology in the XXI century is experiencing a "Golden Age", with observations and theoretical models contributing to a large-scale description of the Universe. The current view is that it can be well described by the so-called Lambda-CDM model, but some open problems challenge physics and cosmology, including the origin and properties of so-called dark energy. The so-called baryonic acoustic oscillations (BAO), detected for the first time in 2005, are considered one of the most effective probes to understand the properties of dark energy. However, given the implications of these measurements, it is important that they are confirmed at other wavelengths and measured over a wide range of redshifts. The radio band provides a unique and complementary observation window, by emitting 21 cm of neutral hydrogen. The redshifted 21 cm (1420 MHz) emission of the hyperfine transition of neutral hydrogen is measured at lower frequencies, so that the observation frequency is converted directly into information about the source's redshift. The BINGO radio telescope (BAO from Integrated Neutral Gas Observations) is a new instrument, designed specifically to observe BAO, mapping a redshift band between 0.13 and 0.45. This seminar will present the basics of 21 cm BAO cosmology, the intensity mapping technique used and describe the current development status of the BINGO radio telescope.

26 Feb 2021
Via Láctea: Ilha Isolada?
Victória Flório Andrade (UFES)
(Cancelled)

Em 26 de abril de 1920, dois astrônomos norte-americanos, Harlow Shapley e Heber Curtis apresentaram, no evento anual da Academia Nacional de Ciências, seus resultados e teorias a respeito do tamanho do Universo, natureza das nebulosas espirais e existência de outras galáxias. Neste artigo, exploramos a apropriação pela imprensa norte-americana dessa disputa sobre o nosso lugar no Universo através do artigo “Existem outros universos além do nosso?”, publicada em 1922 na revista “Popular Science”. A reportagem baseou-se em argumentos de Shapley e Curtis expostos num artigo do Boletim do Conselho Nacional de Pesquisa, em 1921. Além de informar seu público sobre as questões abordadas no artigo do Boletim por meio do uso de metáforas, infográficos e conjecturas, a reportagem transcendeu a astronomia através da imaginação, propondo, inclusive, um mapa para a Via Láctea com base no padrão espiral observado nas nebulosas. Este estudo evidencia a historicidade das discussões científicas do lugar da humanidade no universo e a maneira como tais acontecimentos podem sensibilizar a imaginação, ressaltando o papel e a importância da divulgação científica no processo de formação de uma cultura sobre ciências.

19 Feb 2021
Astrobiologia no novo milênio   (slides)
Jorge Ernesto Horvath (IAG-USP, São Paulo)

A integração de disciplinas e sua fusão na chamada Astrobiologia constitui um dos fatos científicos no novo milênio. Mas, da mesma forma que a definição das disciplinas que hoje conhecemos (Física, Química, etc.) existe um longo caminho a percorrer para que a chamada matriz disciplinar, metodologias e acima de tudo, ponto de vista conjunto, fiquem claros. Estes assuntos serão o objeto do seminário, onde repassaremos também o estado-da-arte de algumas iniciativas conjuntas e forneceremos exemplos de problemas abordados e resolvidos que exigem essa nova compreensão da natureza, a caminho da formação de uma geração de cientistas já formados e familiarizados com a nova disciplina.

12 Feb 2021
Brans-Dicke cosmology with a Lambda-term to alleviate the H0 and sigma8 tensions
Adrià Gomez-Valent (ITP, Heidelberg University, Germany)

The detection of high-redshift supernovae of Type Ia (SNIa) and the measurement of their corresponding light-curves led already more than 20 years ago to the discovery of the current accelerated expansion of the universe and the consolidation of the standard model of cosmology, also known as concordance model or $\Lambda$CDM. It relies (among other things) on the theory of General Relativity (GR) and the presence of a rigid cosmological constant term in the action, $\Lambda$, with repulsive gravitational properties. Nevertheless, in the last years, thanks to the improvement of the observational facilities and the increase of their precision, some tensions have emerged within the GR-$\Lambda$CDM. If systematics do not play a leading role here, these tensions might point to the need of a new change of paradigm. In this talk I will present and discuss the results obtained from a couple of recent studies of Brans-Dicke (BD) cosmology with constant vacuum energy density. I will show how the $H_0$ and $\sigma_8$ tensions that affect the GR-$\Lambda$CDM model can be strongly mitigated in the framework of the BD theory. This fact could be a hint of new physics in the gravitational sector, acting at cosmological scales. The analysis has been carried out taking into account several data set combinations, including SNIa, cosmic chronometers, baryon acoustic oscillations, redshift-space distortions, weak and strong-lensing data and, of course, the last CMB data release from Planck 2018. The talk is based on the letter-type paper arXiv:1909.02554 [ApJL 886, L6 (2019)] and the more extensive study arXiv:2006.04273 [CQG 37, 245003 (2020)].

 05 Feb 2021
Microscopia de buracos negros: teoria conforme no fundo da garganta do horizonte (slides)
André Alves Lima (UFES)

Um dos maiores sucessos da teoria de cordas enquanto uma teoria quântica da gravitação é a possibilidade de dedução da entropia de Bekenstein-Hawking a partir de uma descrição microscópica dos estados quânticos de um buraco negro. Faremos uma breve exposição do sistema de branas D1-D5 – a construção mais simples de um buraco negro extremo 5-dimensional no limite de baixas energias da teoria de cordas – e da teoria bidimensional superconforme que mora holograficamente no seu horizonte e descreve seus microestados quânticos. Apresento alguns resultados recentes sobre a renormalização de estados nessa teoria. Como conclusão, comento sobre a possibilidade de que buracos negros sejam 'bolas difusas' (fuzzballs) formadas pela superposição de geometrias de microestados.

2020

December 18, 2020 -  Gastão Krein (IFT/UNESP) - Origin of most of the visible mass in the universe: theory and experiment (slides)

Most of the visible mass in the universe is due to the strong interaction; it is contained in the protons and neutrons that make up atomic nuclei. There is by now overwhelming evidence that quantum chromodynamics (QCD), the theory of the quarks and gluons, is indeed the correct theory of the strong interaction and yet we do not understand how it actually works. Brute-force, large-scale lattice QCD calculations have reproduced the experimental values of the masses of ground-state hadrons with a single quark-antiquark pair or three valence quarks, but those calculations do not tell us the mechanisms QCD uses to produce those masses. Our ignorance in this matter is profound. Experimentally, information on the origin of mass can be accessed with near-threshold quarkonium-proton scattering, as I will explain in the talk. I will also discuss in the talk our recent theoretical progress in using effective field theory techniques to explore this issue and relate those theoretical developments to upcoming experiments in different laboratories worldwide.
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December 11, 2020 - 10:00h (Exceptional time) - Yi-Fu Cai (Univ.  Science and Technology of China) - Towards a healthy bouncing cosmology from general scalar tensor theories

The hot big bang cosmology, while is a successful theory in describing our Universe, suffers from several conceptual problems if we trace back to the earliest moments near the big bang. These severe issues have been delicately addressed by inflationary cosmology, which was proposed in 1980s and now has become the prevailing paradigm of the very early Universe. However, the spacetime singularity, which appeared at the moment of the birth of the Universe, remains to be a long-standing question in modern cosmology. 
Non-singular bounce cosmology is a novel theoretical paradigm of the very early Universe, which aims at solving the initial big bang singularity. Although as beautiful as it looks like, the associated model building is a long-termed journey full of challenges from the theoretical and observational aspects. I will give an overview of this cosmological paradigm and introduce the major efforts that cosmologists have made in past research from the perspective of general scalar tensor theories. At the end I would make some comment on the future development in this field.

December 04, 2020 - Zahra Davari (Sharif Univ., Iran) - Testing MOG theory and CDM model in Milky Way galaxy using observational data

In this talk, I will discuss one of the alternative theories to dark matter named MOdified Gravity (MOG) on the galactic scale. I have investigated MOG theory and the dark matter model by testing their ability to describe the local dynamics of the Milky Way in vertical and transverse directions by using observation data such as the vertical dispersion, rotation curve, surface density, and number density of stars. The results show that two models of MOG and CDM are able to describe equally well the rotation curve and the vertical dynamics of stars in the local MW.
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November 27, 2020 - Richard Kerner (Sorbonne, França) - Inertial motion and mass from Aristotle to our days (download slides here)

We present in this talk the ideas concerning the motion of massive bodies on Earth and in Heavens prevailing in ancient Greek science and in Aristotelean physics and in Ptolemaic system of the Universe. Next, we mention certain medieval concepts, to arrive at the dawn of modern science - Copernicus, Descartes, Galileo, Kepler, Pascal, and finally the Newtonian approach to motion and mass. Further on, we shall present Cavendish's and Eotvos' experiments determining Earth's mass and equivalence between inertial and gravitational masses, and the ideas of Mach and Einstein on the subject. The modern theory of mass generation will be shortly presented, too, on the example of the Higgs'-Kibble mechanism. Finally,  shall show how some points of view and mathematical treatments persist until the modern times on the example of Newton's approach to celestial motion and Feynman's approach to quantum scattering processes.


November 20, 2020 - Mario Novello (CBPF) - Teoria spinorial da gravitação (arXiv:2006.16810 and arXiv:2007.14393 )

Em uma conversa particular em Genève (1971) o professor Stueckelberg comentou sobre sua hipótese de que haveria uma conexão íntima entre interação nuclear fraca (Fermi) e a gravitação, simbolizada pela relação entre a constante de Fermi g_{F} a constante de Newton k. Isso leva a pensar que o efeito gravitacional da matéria pode ser entendido como gerador de uma geometria efetiva. Dito de outro modo, seguindo os passos de Feynmann, consideramos que o processo gravitacional nada mais é do que a ação de um campo no espaço-tempo de Minkowski. No caso de Feynmann e vários outros que lhe seguiram, esse campo teria spin 2. Nós iremos pensar no cenário sugerido por Stuckelberg e tratar esse campo gravitacional como constituído de spins semi-inteiro. O resultado da interação desses férmions-gravitacionais com a matéria é descrito pela construção de uma métrica efetiva, como Volovik e outros propuseram recentemente. Para mostrar como esse procedimento pode ser implementado, vou apresentar alguns exemplos usando somente um G-neutrino: duas configurações estáticas e esfericamente simétricas e uma cosmológica representando um universo dinâmico isotrópico. Um dos casos obtidos coincide com a métrica de Schwarzschild. Um outro resultado (dependente do processo de auto-interação do spinor) gera um novo buraco negro gravitacional.


November 13, 2020 - Raul Abramo (USP) - Cosmologia com o levantamento astrofísico J-PAS (download slides here)
 
Diversos novos levantamentos astrofísicos têm seu início planejado para os próximos meses e anos, com o objetivo principal de estudar a aceleração cósmica e compreender se ela seria devida a uma nova substância chamada "energia escura", ou se é a Relatividade Geral que precisa ser alterada. Neste seminário eu vou dar um status do levantamento J-PAS, que teve sua "primeira luz" recentemente e deve começar a operar em plena capacidade dentro de alguns meses. Vou também mostrar como novas ferramentas de inteligência artificial (Machine Learning) estão definitivamente entrando para a caixa de ferramentas de físicos e astrônomos. Finalmente, vou mostrar uma simulação “end-to-end” de um mapa de galáxias e quasares a altos redshifts que o levantamento J-PAS deverá realizar dentro de 3-4 anos.

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November 06, 2020 - Alberto Saa (Unicamp) - Soluções de Robinson-Trautman em (2+1) dimensões (download slides here)

O espaço-tempo de Robinson-Trautman (RT) é a solução mais simples da Relatividade Geral (RG) que pode ser interpretada como uma fonte compacta rodeada por ondas gravitacionais. Como um problema de valor inicial, a evolução do espaço-tempo da RT é um problema bem posto. As equações dinâmicas pertinentes são equivalentes ao chamado fluxo de Calabi, e dados iniciais regulares evoluem suavemente em direção a um estado final assintótico correspondente a um buraco negro de Schwarzschild. Extensões do espaço-tempo de RT para dimensões mais altas (D>4) foram recentemente propostas, e a essência da evolução é a mesma. A situação para D=3 é bem diferente devido a algumas peculiaridades bem conhecidas da RG em dimensões mais baixas. Apresentaremos um fluxo de RT para D=3 que reproduz as propriedades essenciais do fluxo de Calabi. Em particular, os dados iniciais regulares evoluem assintoticamente para um buraco negro BTZ, e as possíveis assimetrias nos dados iniciais são expelidas como um fluido de radiação, imitando os mecanismos de emissão de ondas gravitacionais convencionais em D=4.


October 09, 2020 - Jérôme Martin (IAP, França) - Quantum-mechanical Perturbations and the pioneering role of John Bell in Cosmology

According to the theory of cosmic inflation, the large scale structures observed in our Universe (galaxies, clusters of galaxies, Cosmic Background Microwave - CMB - anisotropy...) are of quantum mechanical origin. They are nothing but vacuum fluctuations, stretched to cosmological scales by the cosmic expansion and amplified by gravitational instability. At the end of inflation, these perturbations are placed in a two-mode squeezed state with the strongest squeezing ever produced in Nature (much larger than anything that can be made in the laboratory on Earth). In this talk, one studies whether astrophysical observations could unambiguously reveal this quantum origin by borrowing ideas from quantum information theory. It is argued that some of the tools needed to carry out this task have been discussed long ago by J. Bell in a, so far, largely unrecognized contribution. Although J. Bell could not have realized it when he wrote his letter since the quantum state of cosmological perturbations was not yet fully characterized at that time, it is also shown that Cosmology and cosmic inflation represent the most interesting frameworks to apply the concepts he investigated. This confirms that cosmic inflation is not only a successful paradigm to understand the early Universe. It is also the only situation in Physics where one crucially needs General Relativity and Quantum Mechanics to derive the predictions of a theory and, where, at the same time, we have high-accuracy data to test these predictions, making inflation a playground of utmost importance to discuss foundational issues in Quantum Mechanics.

October 02, 2020 - Caio Macedo (UFPA) - Escalarização espontânea de buracos negros (download slides here)

Na Relatividade Geral, buracos negros são objetos extremamente simples, e no vácuo são completamente descritos por apenas dois parâmetros: massa e momento angular. Estas soluções simples também fazem parte do espectro de várias teorias que possuem campos adicionais. Recentemente, foi apontado que certos tipos de acoplamento entre a gravitação e estes campos adicionais podem gerar soluções com "cabelos" adicionais, sendo dinamicamente mais favoráveis do que as soluções existentes na Relatividade, possibilitando, por exemplo, a escalarização espontânea. Neste seminário, discutiremos algumas teorias que possibilitam escalarização espontânea de buracos negros e analisaremos possíveis assinaturas astrofísicas de tais objetos.
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September 25, 2020 - Pedro Baqui (UFES) - Separação de Estrelas-Galáxias usando algoritmos de Machine Learning aplicados aos dados preliminares do Survey MiniJPAS.

Futuros levantamentos em astronomia/astrofísicas como o J-PAS, SDSS e LSST produzirão conjuntos de dados enormes chegando à uma taxa de 150 TB por dia. Portanto, novas ferramentas para processamento dessa quantidade de dados devem ser empregadas. De preferência que nos forneçam uma resposta quase que em tempo real, de forma eficiente e precisa. Cenário ideal para a aplicação de métodos de Machine Learning. Neste trabalho, analisamos dados do Pathfinder miniJPAS Survey, que observou ∼ 1deg² sobre o campo AEGIS com 56 filtros de banda estreita e 4 filtros de banda larga ugri. Nesta apresentação, discutiremos a classificação de fontes observadas pelo miniJPAS em estrelas e galaxias, uma etapa necessária para estudos científicos subsequentes. Nosso objetivo é desenvolver um classificador de Machine Learning (ML) complementar às ferramentas tradicionais baseadas em outras modelagem. Treinamos e testamos diferentes algoritmos de ML. Como resultado encontramos algoritmos competitivos com os classificadores padrões adotados pelo survey. Por fim construímos um catálogo para a faixa de magnitude 15 ≤ r ≤ 23.5 utilizando o método a partir da fusão de rótulos entre os surveys SDSS e HSC-SSP.

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September 18, 2020 - André Alves (UFES) - Inversão do fator de escala em paredes de domínio holográficas

Paredes de domínio são geometrias com d+1 dimensões, em que uma coordenada radial z atravessa infinitos espaços de Minkowski d-dimensionais cujos tamanhos são regulados por um fator de escala a(z). Através da dualidade holográfica AdS/CFT e suas generalizações para teorias não-conformes, as paredes de domínio correspondem ao fluxo do grupo de renormalização de uma QFT d-dimensional com escala de energia a. Neste trabalho (arXiv:1911.08392), nós apresentamos uma relação entre paredes ligadas por uma inversão do fator de escala conforme: ã = 1/a. A inversão preserva a forma das equações de Einstein, relaciona os limites IR e UV de certas teorias de campo holográficas, e preserva a propriedade de uma teoria possuir 'simetria conforme generalizada'. Em d = 3, a inversão do fator de escala tem aplicações para uma descrição holográfica da inflação.


September 04, 2020 - Oliver Piattella (UFES) - Uma revisão do problema da constante cosmológica

A constante cosmológica foi introduzida por Einstein no contexto puramente clássico da teoria da relatividade geral e constitui hoje um bloco fundamental do modelo padrão da cosmologia, o $\Lambda$CDM, que é chamado também de “modelo de concordância” por causa do seu sucesso em se ajustar às várias amostras de dados advindos de fenômenos cosmológicos de naturezas bem distintas. Um candidato fundamental e natural para o papel de constante cosmológica é a densidade de energia do vácuo quântico. Porém, simples cálculos mostram que o valor predito para a constante cosmológica, tomando em conta os campos quânticos materiais e as transições de fase conhecidas do modelo padrão das partículas, é pelo menos 56 ordens de grandeza maior do valor observado. Esse é o problema da constante cosmológica. Neste seminário apresentarei uma revisão do problema e algumas das muitas tentativas propostas para resolvê-lo.


August 21, 2020 - Emmanuel Frion (UFES) - Primordial magnetogenesis in bouncing cosmology

I will present a model of primordial magnetogenesis and the evolution of the electromagnetic field through a quantum bounce. In this model, the initial conditions are taken in the far past in a contracting phase where only dust is present and the electromagnetic field is in the adiabatic quantum vacuum state. By including a coupling between curvature and electromagnetism of the form RFµνFµν, we find acceptable magnetic field seeds within the current observational constraints at 1 mega-parsec (Mpc), and that the magnetic power spectrum evolves as a power-law with spectral index nB = 6. It is also shown that the electromagnetic backreaction is not an issue in the model under scrutiny.


August 07, 2020 - Jhonny​
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July 31, 2020 - Hermano Velten (UFOP) - No room for dark energy perturbations.

Contrarily to the cosmological constant scenario, dynamical dark energy fields have intrinsic density fluctuations. The evolution of the latter depends on the dark energy speed of sound value remaining very small in quintessence fields since its speed of sound is equal to one. However, there is a long debate in the literature on whether or not dark energy perturbations are (ir-)relevant to large scale structure formation. The goal of this talk is threefold: i) we review the techniques used to study cosmological dark energy perturbations, ii) we discuss some recent results available in the literature and iii) present new own results indicating that the viability of dynamical dark energy fields is challenged.

July 24, 2020 - Leonardo Giani (UFES) - Testing gravity with strong Lensing time delay

In this talk I will briefly discuss how Strong lensing time delay measurements provide a valuable and almost model-independent tool for cosmological investigations. I will also assess their potential  for testing fundamental gravity, in particular to measure deviations from the equivalence principle  on  extragalactic  scales. Finally, I will outline some interesting possible future developments.
July 17, 2020 - Kyle Oman (Durham U., UK) - Observational constraints on the slope of the radial acceleration relation at low accelerations (slides)

The radial acceleration relation (RAR) locally relates the `observed' acceleration inferred from the dynamics of a system to the acceleration implied by its baryonic matter distribution. The relation as traced by galaxy rotation curves is one-to-one with remarkably little scatter, implying that the dynamics of a system can be predicted simply by measuring its density profile as traced by e.g. stellar light or gas emission lines. Extending the relation to accelerations below those usually probed by practically observable kinematic tracers is challenging, especially once accounting for faintly emitting baryons, such as the putative warm-hot intergalactic medium, becomes important. We show that in the low-acceleration regime, the (inverted) RAR predicts an unphysical, declining enclosed baryonic mass profile for systems with `observed' acceleration profiles steeper than gobs∝r^−1 (corresponding to density profiles steeper than isothermal - ρ(r)∝r^−2). If the RAR is tantamount to a natural law, such acceleration profiles cannot exist. We apply this argument to test the compatibility of an extrapolation of the rotation curve-derived RAR to low accelerations with data from galaxy-galaxy weak lensing, dwarf spheroidal galaxy stellar kinematic, and outer Milky Way dynamical measurements, fully independent of the uncertainties inherent in direct measurements of the baryonic matter distribution. In all cases we find that the data weakly favour a break to a steeper low-acceleration slope. Improvements in measurements and modelling of the outer Milky Way, and weak lensing, seem like the most promising path toward stronger constraints on the low-acceleration behaviour of the RAR.

July 10, 2020 - Guilherme Brando (UFES) - Correções (Efeitos) relativísticos no crescimento de estruturas em gravitação modificada (slides)

Simulações newtonianas são ferramentas importantes no cenário da cosmologia atual. A partir delas, grandes levantamentos cosmológicos são capazes de simular a formação de estruturas do Universo, em média e pequena escalas, para testar as ferramentas estatísticas da análise dos dados coletados, bem como de calcular observáveis cosmológicos, como o espectro de potência da matéria. No entanto, a teoria gravitacional que descreve o Modelo Padrão Cosmológico é relativística. Sendo assim, é necessário introduzir "correções" relativísticas de espécies com pressão não-nula (fótons, neutrinos, etc.), assim como efeitos da própria estrutura do espaço-tempo (problema do calibre, backreaction), para aumentar a precisão dos levantamentos cosmológicos futuros, uma vez que a Teoria Newtoniana é incapaz de acomodar tais efeitos. Neste seminário será apresentado um método para introduzir essas correções em simulações newtonianas (em ordem linear) para o caso de teorias escalares tensoriais. Será descrito o campo escalar responsável pela atual expansão acelerada do Universo como um fluido efetivo. Este formalismo tem como base fundamental o chamado calibre de N-corpos, um sistema de coordenadas no qual a densidade de partículas Newtoniana é a mesma que a relativística, possibilitando o mapeamento entre a contagem de partículas em cada teoria.

July 03, 2020 - Leonardo Giani (UFES) - O princípio cosmológico, quando e porque

Neste seminário revisarei de maneira pedagógica as motivações teóricas e observacionais por trás de um dos pilares da cosmología moderna, o princípio cosmológico. Em particular tentaremos discutir quando e porque ele se torna uma suposição razoável para a descrição do nosso Universo.

June 26, 2020 - Tays Miranda (UFES) - Colapso estocástico (slides)

Modelos cosmológicos de contração originam flutuações quânticas de vácuo das quais as perturbações primordiais presentes em nosso universo poderiam ser geradas. A existência de uma fase cosmológica antes do Big Bang nos leva a uma perspectiva diferente em relação às condições iniciais para o Universo. Neste seminário, descreveremos um universo em colapso, baseado em um campo escalar com um potencial exponencial, e aplicaremos o formalismo estocástico, o qual nos permite estudar como as flutuações quânticas originam ruídos estocásticos que modificam a dinâmica clássica do campo escalar em largas escalas, acima de uma escala de corte que chamamos de coarse-graining scale. Em particular, vamos explorar como essas flutuações quânticas podem perturbar a equação de estado em largas escalas levando à violação da solução clássica de colapso.

June 19, 2020 - Davi Rodrigues (UFES) - General relativity with running couplings and cosmological implications (slides)

There are currently different frameworks for gravity with running couplings. I will comment on some of them and present our recent results, which are based on possible renormalization group (RG) effects at large scales. Our approach has two key differences with respect to other RG approaches that consider large distances phenomena: i) it is explicitly described by a covariant effective action that contains all the relevant information; ii) the runnings of G and \Lambda are sensitive to the wavenumber k from the cosmological perturbations, not the Hubble parameter or the time since the big bang (as the majority of the approaches consider). Consequently, in this picture, the background evolution is essentially the same of LCDM, the new effects are in the perturbative sector. To disclose its consequences, we evaluate its PPF parameters and f\sigma_8. We comment about a possible impact on alleviating the \sigma_8 tension found in LCDM cosmology. A complete CMB analysis with Class and Monte Python is under evaluation.


​June 12, 2020 - Saulo Carneiro (UFBA) - On the value of the Immirzi parameter and the horizon entropy
The absence of driving experimental tests is one of the main challenges for quantum gravity theories. In Loop Quantum Gravity (LQG) the quantisation of General Relativity leads to precise predictions for the eigenvalues of geometrical observables like volume and area, up to the value of the only free parameter in the theory, the Barbero-Immirzi (BI) parameter. With the help of the eigenvalues equation for the area operator, LQG successfully derives the Bekenstein-Hawking entropy of large black holes with isolated horizons, fixing in this limit the BI parameter as γ ≈ 0.274. Any further empirical test should give this same value. In this paper we show that a black hole with angular momentum J =1 and Planck mass is eigenstate of the area operator provided that γ = √3/6 ≈ 1.05 × 0.274. As the black hole is extremal, there is no Hawking radiation and the horizon is isolated. We then show that such a black hole can be formed in the head-on scattering of two parallel Standard Model neutrinos in the mass state m2 (assuming m1 = 0). Furthermore, we use the obtained BI parameter to numerically compute the entropy of isolated horizons with areas ranging up to 250 lP2 , by counting the number of micro-states associated to a given area. The resulting entropy has a leading term S ≈ 0.25 A, in agreement to the Bekenstein-Hawking entropy.

​June 5, 2020 - Olesya Galkina (UFES) - Future soft singularities, Born-Infeld-like fields and particles (slides).

We consider different scenarios of the evolution of the universe, where the singularities or some non-analyticities in the geometry of the spacetime are present, trying to answer the following question: is it possible to conserve some kind of notion of particle corresponding to a chosen quantum field present in the universe when the latter approaches the singularity? We study scalar fields with different types of Lagrangians, writing down the second-order differential equations for the linear perturbations of these fields in the vicinity of a singularity. If at least one of two independent solutions has a singular asymptotic behavior, then we cannot define the creation and the annihilation operators and construct the vacuum and the Fock space. This means that the very notion of particle loses sense. In the case of the model of the universe described by the tachyon field with a special trigonometric potential, where the Big Brake singularity occurs, we see that the (pseudo)
tachyon particles do not pass through this singularity. Adding to this model some quantity of dust, we slightly change the characteristics of this singularity and tachyon particles survive. Finally, we consider a model with the scalar field with the cusped potential, where the phantom divide line crossing occurs. Here the particles are well defined in the vicinity of this crossing point.

March 20, 2020 - CANCELED due to COVID19- Tays Miranda de Andrade (UFES) - Stochastic formalism applied to a collapsing universe

Contracting cosmologies give rise to quantum vacuum fluctuations from which the primordial perturbations present in our universe could be generated. The existence of a cosmological phase before the Big Bang leads to a different perspective regarding the initial conditions for the universe. In this talk I will describe a collapsing universe, based on a scalar field and an exponential potential, and apply the stochastic formalism, which allows us to study how quantum fluctuations give rise to stochastic noise which modifies the classical dynamics of the scalar field at large scales, above a coarse-graining scale. In particular I will explore how quantum fluctuations can perturb the equation of state on large scales leading to the break down of the classical collapse solution.

March 12, 2020 - Carla Rodrigues Almeida (Max Planck Institute for the History of Science, Berlin) - Descobrindo os buracos negros
Thursday, at 17:00
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Previous seminars (2012-2020)