  In a previous work the authors have solved the Einstein equations of General
Relativity for a class of metrics with constant spatial curvature, where it was
found a non vanishing Weyl tensor in the presence of a primordial magnetic
field with an anisotropic pressure component. Here, we perform the perturbative
analysis of this model in order to study the gravitational stability under
linear scalar perturbations. For this purpose, we take the Quasi-Maxwellian
formalism of General Relativity as our framework, which offers a naturally
covariant and gauge-invariant approach to deal with perturbations that are
directly linked to observational quantities.
