Curvas de rotação galácticas em teorias métricas da gravitação
Resumo
From a Parametrized Post-Newtonian (PNP) perspective we attempt to partially answer
the question, by studying some models, of whether or not the new degrees of freedom,
represented by the PNP potentials, can lead to significant modifications in the dynamics of
galaxies in the direction of rendering dark matter obsolete. In the PNP, several parameters
classify each theory and their possible values are tightly constrained mainly due to solar
system experiments [ 1 , 2 , 3 ]. Such restrictions renders the modifications of gravitational
effects, with respect to General Relativity, to be insignificant, making attempts to find
alternative metrical theories rather fruitless. In recent years a new kind of metric theories
have arisen to popularity and are characterized by "Screening" mechanisms [ 4 ]. These lead
to the possibility of modifications in larger scales than the solar system while retaining the
success of GR in the latter, allowing for violations of the constraints of the Post-Newtonian
parameters. With this new paradigm we consider two kinds of solutions: (i) When no
matter field is present, or in other words, vacuum; (ii) or a Politropic distribution of matter.
For (i) we find that the corrections don’t lead to the needed modifications as required
to be an alternative to dark matter. In (ii) for most choices of the parameters (when
changing γ , β , ξ , α 3 , ζ j , with j = 1 , 2, from their values in GR) the need for dark matter
is unavoidable, in order to find a flat rotation curve. It’s only for theories in which ζ 3 > 0
that some resemblance of flat rotation curves is found. The latter suggests, at least for the
models considered, that these are the only theories capable of replacing dark matter as a
possible explanation for the dynamics of galaxies.