Nonconservative unimodular gravity : a viable cosmological scenario?

Resumo
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.
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Citação
FABRIS, J. C. et al. Nonconservative unimodular gravity: a viable cosmological scenario?. The European Physical Journal C, v. 82, artigo 522, 2022. Disponível em: <https://link.springer.com/article/10.1140/epjc/s10052-022-10470-2>. Acesso em: 06 jul. 2023.