Navegando por Autor "Velten, Hermano Endlich Schneider"

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Astrobiologia : como modelar o crescimento de organismos na Terra e em exoplanetas.
(2020) Velten, Hermano Endlich Schneider; Silva, Alcides Volpato Carneiro de Castro e; Pinheiro, Carlos Felipe Saraiva
Vamos discutir o chamado modelo de Eigen utilizado para modelar a evolução do código genético de microorganismo. Este modelo determina, em termos do tamanho da cadeia de informação e de sua probabilidade de erro ao se multiplicar, quais configurações genéticas sobrevivem a longo prazo. Mostramos algumas simulações deste modelo e discutimos como utilizar este tema no estudo da astrobiologia.
Astrobiologia: como modelar o crescimento de organismos na Terra e em exoplanetas.
(2020) Velten, Hermano Endlich Schneider; Silva, Alcides Volpato Carneiro de Castro e; Pinheiro, Carlos Felipe Saraiva
Vamos discutir o chamado modelo de Eigen utilizado para modelar a evolução do código genético de micro-organismo. Este modelo determina, em termos do tamanho da cadeia de informação e de sua probabilidade de erro ao se multiplicar, quais configurações genéticas sobrevivem a longo prazo. Mostramos algumas simulações deste modelo e discutimos como utilizar este tema no estudo da astrobiologia.
Constraining the anomalous coupling of gravitational waves with the double pulsar.
(2023) Jesus, Diego Santos de; Velten, Hermano Endlich Schneider; Piazza, Federico
We revisit the decay of the orbital period in binary systems that occurs due to the emission of gravitational waves in the context of modifie- gravity models where the coupling Ggw between matter and on shell gravitons is allowed to differ from the Newton constant GN. Using the most precisely-timed double neutron star binary system, those of the double pulsar, we constrain the ratio Ggw=GN to the level of 10−4, improving by two orders of magnitude the present bound on this quantity.
Early-time thermalization of cosmic components? : a hint for solving cosmic tensions.
(2021) Velten, Hermano Endlich Schneider; Costa, Ingrid; Wilhelm, Zimdahl Winfried Ernst
We study an expanding two-fluid model of nonrelativistic dark matter and radiation, which are allowed to interact during a certain time span and to establish an approximate thermal equilibrium. Such an interaction, which generates an effective bulk viscous pressure at background level, is expected to be relevant for times around the transition from radiation to matter dominance. We quantify the magnitude of this pressure for dark-matter particle masses within the range 1 eV ≲ mχ ≲ 10 eV around the matter-radiation equality epoch (i.e., redshift zeq ∼ 3400) and demonstrate that the existence of a transient bulk viscosity has consequences which may be relevant for addressing current tensions of the standard cosmological model: (i) the additional (negative) pressure contribution modifies the expansion rate around zeq, yielding a larger H0 value, and (ii) large-scale structure formation is impacted by suppressing the amplitude of matter overdensity growth via a new viscous friction-term contribution to the M ́eszáros effect. As a result, the H0 and S8 tensions of the current standard cosmological model are both significantly alleviated.
Early-time thermalization of cosmic components? A hint for solving cosmic tensions.
(2021) Velten, Hermano Endlich Schneider; Costa, Ingrid; Wilhelm, Zimdahl Winfried Ernst
We study an expanding two-fluid model of nonrelativistic dark matter and radiation, which are allowed to interact during a certain time span and to establish an approximate thermal equilibrium. Such an interaction, which generates an effective bulk viscous pressure at background level, is expected to be relevant for times around the transition from radiation to matter dominance. We quantify the magnitude of this pressure for dark-matter particle masses within the range 1 eV ≲ mχ ≲ 10 eV around the matter-radiation equality epoch (i.e., redshift zeq ∼ 3400) and demonstrate that the existence of a transient bulk viscosity has consequences which may be relevant for addressing current tensions of the standard cosmological model: (i) the additional (negative) pressure contribution modifies the expansion rate around zeq, yielding a larger H0 value, and (ii) large-scale structure formation is impacted by suppressing the amplitude of matter overdensity growth via a new viscous friction-term contribution to the M ́eszáros effect. As a result, the H0 and S8 tensions of the current standard cosmological model are both significantly alleviated.
Existence of static spherically-symmetric objects in action-dependent Lagrangian theories.
(2019) Fabris, Júlio César; Velten, Hermano Endlich Schneider; Wojnar, Aneta Magdalena
We study static symmetric solutions in the context of a gravitational theory based on a action-dependent Lagrangian. Such theory has been designed as a setup to implement dissipative effects into the traditional principle of least action. Dissipation appears therefore from the first principles and has a purely geometric origin. An interesting feature of this theory is the existence of a coupling four-vector λμ, which in an expanding background is related to cosmological bulk viscosity. General relativity is recovered with a vanishing λμ. We analyze the existence of equilibrium solutions of static configurations aiming to describe astrophysical objects. We find out that the existence of static spherically symmetric configurations occurs only in the particular scenario with vanishing λt, λr and λφ components i.e, λμ 1⁄4 f0; 0; λθ; 0g. Thus, the component λθ is the unique available parameter of the theory in the astrophysical context. This result severely constrains the existence of this sort of gravitational theories. We proceed then verifying the impact of λθ on the stability and the mass-radius configurations for a reasonable equation of state for the cold dense matter inside compact stars. We further investigate the relativistic spherical collapse in order to track the structure of geometrical singularities appearing in the theory.
Flavour composition and entropy increase of cosmological neutrinos after decoherence.
(2019) Boriero, Daniel Francisco; Schwarz, Dominik J.; Velten, Hermano Endlich Schneider
We propose that gravitational interactions of cosmic neutrinos with the statistically homogeneous and isotropic fluctuations of space-time lead to decoherence. This working hypothesis, which we describe by means of a Lindblad operator, is applied to the system of two- and three-flavour neutrinos undergoing vacuum oscillations and the consequences are investigated. As a result of this decoherence we find that the neutrino entropy would increase as a function of initial spectral distortions, mixing angles and charge-parity (CP)-violation phase. Subsequently we discuss the chances to discover such an increase observationally (in principle). We also present the expected flavour composition of the cosmic neutrino background after decoherence is completed. The physics of two- or three-flavour oscillation of cosmological neutrinos resembles in many aspects two- or three-level systems in atomic clocks, which were recently proposed by Weinberg for the study of decoherence phenomena.
Is the Hubble diagram of quasars in tension with concordance cosmology?
(2020) Velten, Hermano Endlich Schneider; Silveira, Syrios Gomes da
Recently, Risaliti and Lusso [Nat. Astron. 3, 272 (2019)] reported new measurements of the expansion rate of the Universe by constructing the Hubble diagram of 1598 quasars in the redshift range 0.5
Matéria escura e as estruturas cósmicas.
(2021) Velten, Hermano Endlich Schneider
O objetivo deste artigo é mostrar o motivo de se considerar a matéria escura como ingrediente fundamental para o Universo. A existência da matéria escura é indispensável para formar as estruturas cósmicas que observamos como galáxias e aglomerados de galáxias. Apesar de ainda desconhecermos a natureza desta componente cósmica, pois ainda não a detectamos diretamente, o aspecto discutido neste trabalho é considerado um dos principais argumentos a favor da existência da matéria escura.
Matéria escura, energia escura e a busca por uma nova teoria para a gravitação.
(2020) Velten, Hermano Endlich Schneider
Vamos discutir como a teoria da relatividade geral (RG) de Einstein nos conduz aos conceitos de matéria escura e energia escura que, juntas, devem compor cerca de 95% do universo. O desconhecimento da natureza destas componentes escuras nos leva a um delicado questionamento: Será que a TRG é correta? Existiriam outras teorias para a gravitação?
Matter growth in extended ΛCDM cosmology.
(2019) Wilhelm, Zimdahl Winfried Ernst; Velten, Hermano Endlich Schneider; Algoner Jorge, William Celestino
On the basis of a previously established scalar–tensor extension of the ΛCDM model, we develop an effective fluid approach for the matter growth function. This extended ΛCDM (henceforth eΦΛCDM) cosmology takes into account deviations from the Standard Model both via a modified background expansion and by the inclusion of geometric anisotropic stresses as well as of perturbations of the geometric dark-energy equivalent. The background dynamics is governed by an explicit analytic expression for the Hubble rate in which modifications of the Standard Model are given in terms of a single constant parameter [W. C. Algoner, H. E. S. Velten and W. Zimdahl, J. Cosmol. Astropart. Phys. 1611 (2016) 034]. To close the system of fluid-dynamical perturbation equations, we introduce two phenomenological parameters through which the anisotropic stress is related both to the total energy density perturbation of the cosmic substratum and to relative perturbations in the effective two-component system. We quantify the impact of deviations from the standard background, of anisotropic stresses and of nonvanishing perturbations of the effective dark-energy component on the matter growth rate function fσ8 and confront the results with recent redshift-space distortion (RSD) measurements.
Matter growth in imperfect fluid cosmology.
(2019) Wilhelm, Zimdahl Winfried Ernst; Velten, Hermano Endlich Schneider; Algoner Jorge, William Celestino
Extensions of Einstein’s General Relativity (GR) can formally be given a GR structure in which additional geometric degrees of freedom are mapped on an effective energy-momentum tensor. The corresponding effective cosmic medium can then be modeled as an imperfect fluid within GR. The imperfect fluid structure allows us to include, on a phenomenological basis, anisotropic stresses and energy fluxes which are considered as potential signatures for deviations from the cosmological standard Λ-cold-dark-matter (ΛCDM) model. As an example, we consider the dynamics of a scalar-tensor extension of the standard model, the eΦΛCDM model. We constrain the magnitudes of anisotropic pressure and energy flux with the help of redshift-space distortion (RSD) data for the matter growth function f σ8.
Modified gravity away from a ΛCDM background.
(2019) Oliveira, Guilherme Brando de; Falciano, Felipe Tovar; Linder, Eric; Velten, Hermano Endlich Schneider
Within the effective field theory approach to cosmic acceleration, the background expansion can be specified separately from the gravitational modifications. We explore the impact of modified gravity in a background different from a cosmological constant plus cold dark matter (ΛCDM) on the stability and cosmological observables, including covariance between gravity and expansion parameters. In No Slip Gravity the more general background allows more gravitational freedom, including both positive and negative Planck mass running. We examine the effects on cosmic structure growth, as well as showing that a viable positive integrated Sachs-Wolfe effect crosscorrelation easily arises from this modified gravity theory. Using current data we constrain parameters with a Monte Carlo analysis, finding a bound on the running |αM,max| . 0.03 (95% CL) for the adopted form at all cosmic times. We provide the modified hi class code publicly on GitHub, now enabling computation and inclusion of the redshift space distortion observable fσ8 as well as the No Slip Gravity modifications.
Newtonian-like gravity with variable G.
(2021) Fabris, Júlio César; Gomes, Tales; Toniato, Júnior Diniz; Velten, Hermano Endlich Schneider
We propose a Lagrangian formulation for a varying G Newtonian-like theory inspired by the Brans–Dicke gravity. Rather than imposing an ad hoc dependence for the gravitational coupling, as previously done in the literature, in our proposal, the running of G emerges naturally from the internal dynamical structure of the theory. We explore the features of the resulting gravitational field for static and spherically symmetric mass distributions as well as within the cosmological framework.
Nonconservative traceless type gravity.
(2019) Daouda, Mahamadou Hamani; Fabris, Júlio César; Oliveira, Adriano Mesquita; Smirnov, F.; Velten, Hermano Endlich Schneider
Extensions of the gravity theory in order to obtain traceless field equations have been widely considered in the literature. The leading example of such class of theories is the unimodular gravity, but there are other possibilities like the mimetic gravity and the Rastall gravity with a coupling parameter λ = 1/2. The unimodular gravity proposal is a very interesting approach in order to address the cosmological constant problem. When coupled to matter, such theories may imply that the energy–momentum tensor is not divergence free anymore. In this paper, a unimodular type theory will be developed by evading the conservation T μν ;μ = 0. The cosmological consequences of the later, both at background as well as for scalar and tensor perturbations, are explored. Possible further extensions of this approach are discussed as well as its connection with the traditional unimodular gravity.
Nonconservative unimodular gravity : a viable cosmological scenario?
(2022) Fabris, Júlio César; Alvarenga, Henrique de; Daouda, Mahamadou Hamani; Velten, Hermano Endlich Schneider
In this work we review the issue of imposing the conservation of the energy–momentum tensor as a necessary condition to recover the equivalence between the unimod- ular gravity and General Relativity (GR) equipped with a cosmological constant. This procedure is usually interpreted as an ad hoc imposition on the unimodular theory’s struc- ture. Whereas the consequences of avoiding the conserva- tion of the total energy–momentum tensor has been already introduced in the literature, it has been not widely explored so far. We study an expanding universe sourced by a sin- gle effective perfect fluid such that the null divergence of its energy–momentum tensor is not imposed. As we shall show, in this scheme, the unimodular theory has its own conservation equation obtained from the Bianchi identities. We explore the evolution of the homogeneous and isotropic expanding background and show that a viable cosmologi- cal scenario exists. Also, we consider scalar perturbations with particular attention given to the gauge issue. We show that contrary to the traditional unimodular theory where the synchronous and longitudinal (newtonian) gauge for cosmo- logical perturbations are not permitted, if the conservation of the energy–momentum is relaxed the scalar perturbations in the synchronous condition survive and present a growing mode behavior. We study therefore a new cosmological sce- nario in which the dynamics of the universe transits from the radiative phase directly to a accelerated one but allowing thus for structure formation.
Nonconservative unimodular gravity : gravitational waves.
(2022) Fabris, Júlio César; Alvarenga, Marcelo Henrique de; Mahamadou, Hamani Daouda; Velten, Hermano Endlich Schneider
Unimodular gravity is characterized by an extra condition with respect to general relativity, i.e., the determinant of the metric is constant. This extra condition leads to a more restricted class of invariance by coordinate transformation: The symmetry properties of unimodular gravity are governed by the transverse diffeomorphisms. Nevertheless, if the conservation of the energy– momentum tensor is imposed in unimodular gravity, the general relativity theory is recovered with an additional integration constant which is associated to the cosmological term Λ. However, if the energy–momentum tensor is not conserved separately, a new geometric structure appears with potentially observational signatures. In this text, we consider the evolution of gravitational waves in a nonconservative unimodular gravity, showing how it differs from the usual signatures in the standard model. As our main result, we verify that gravitational waves in the nonconservative version of unimodular gravity are strongly amplified during the evolution of the universe.
On (non-)dynamical dark energy.
(2020) Wilhelm, Zimdahl Winfried Ernst; Fabris, Júlio César; Velten, Hermano Endlich Schneider; González Herrera, Manuel Ramón
The current Universe is composed by a mixture of relativistic species, baryonic matter, dark matter and dark energy which evolve in a non-trivial way at perturbative level. An advanced description of the cosmological dynamics should include non-standard features beyond the simplistic approach idealized by the standard cosmology in which cosmic components do not interact, are adiabatic and dissipationless. We promote a full perturbative analysis of linear scalar perturbations of a non- interacting cosmological model containing baryons, dark matter (both pressureless) and a scalar field allowing for the presence of relative entropic perturbations between the three fluids. Assuming an effective scalar-field sound speed equal to one and neglecting anisotropic stresses we establish a new set of equations for the scalar cosmological perturbations. As a consequence of this new approach, we show that tiny departures from a constant scalar field equation of state wS = −1 damage structure formation in a non-acceptable manner. Hence, by strongly constraining wS our results provide compelling evidence in favor of the standard cosmological model and rule out a large class of dynamical dark energy models.
Ondas ultrassônicas para o controle de Limnoperna fortunei – o mexilhão dourado.
(2022) Araújo, Gislaine Soares; Assis, Paulo Santos; Pelli, Afonso; Assis, Paulo Santos; Velten, Hermano Endlich Schneider; Hayashi, Carmino
O presente estudo se caracteriza como uma pesquisa experimental realizada em laboratório, com o objetivo de verificar possível efeito de ondas ultrassônicas no controle da invasão do Limnoperna fortunei (mexilhão-dourado). Foram realizados três experimentos contendo quantidades distintas de indivíduos e, para cada experimento, o aparelho sonicador foi utilizado na frequência de 40kHz. Adicionalmente, os procedimentos realizados variaram em tempo e dias de exposição dos indivíduos às ondas ultrassônicas. Os resultados apresentaram números variados para mortalidade e desacoplamento das amostras analisadas, havendo diferenças significativas referentes ao tempo de exposição por experimento e dias em que as amostras foram submetidas. Assim, o uso do ultrassom para a desincrustação e morte dos mexilhões dourados foi eficiente e pode ser uma alternativa ao controle da invasão do L. Fortunei.
Polítropa quântica anisotrópica.
(2023) Trindade, Nadson de Jesus da Silva; Velten, Hermano Endlich Schneider; Velten, Hermano Endlich Schneider; Batista, Ronaldo Junio Campos; Fabris, Júlio César
A mecânica quântica de De Broglie-Bohm pode estabelecer uma conexão entre a Mecânica Quântica não-relativística com o movimento de partículas em Dinâmica dos Fluidos. De fato, essa interpretação mostra que um sistema quântico pode ser decomposto em um sistema hidrodinâmico, submetido a um potencial adicional, de origem quântica. Neste tipo de sistema podemos observar possíveis manifestações de fenômenos quânticos em objetos macroscópicos. Sabemos que para um fluido que obedece ao princípio de Pascal (tensões principais iguais), a pressão é tida como isotrópica. No entanto, conhecemos objetos com desvio de isotropia local como, por exemplo, Polítropas Anisotrópicas. Polítropas são configurações em equilíbrio hidrostático como, por exemplo, estrelas ideais. Podemos considerar uma grande diversidade de eventos físicos que dão origem a configurações anisotrópicas. Um possível grau de anisotropia em um sistema pode acontecer tanto em regimes de baixa ou alta densidade. A anisotropia pode provocar certas instabilidades em estruturas astrofísicas. Neste trabalho estudamos as propriedades físicas de um objeto que denominamos Polítropa Quântica Anisotrópica.