Navegando por Autor "Daouda, Mahamadou Hamani"
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Item Nonconservative traceless type gravity.(2019) Daouda, Mahamadou Hamani; Fabris, Júlio César; Oliveira, Adriano Mesquita; Smirnov, F.; Velten, Hermano Endlich SchneiderExtensions 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.Item Nonconservative unimodular gravity : a viable cosmological scenario?(2022) Fabris, Júlio César; Alvarenga, Henrique de; Daouda, Mahamadou Hamani; Velten, Hermano Endlich SchneiderIn 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.