Covariant fluid dynamics: a long wavelength approximation

In this paper, we consider the long wavelength approximation scheme (LWAS) in the framework of the covariant fluid approach to general relativistic dynamics, specializing to the particular case of irrotational dust matter. We discuss the dynamics of these models during the approach to any spacelike singularity where an evolution of the Belinski–Khalatnikov–Lifshitz (BKL) type is expected, studying the validity of this approximation scheme and the role of the magnetic part of the Weyl tensor, Hab. Our analytic results confirm a previous numerical analysis: it is Hab that destroys the pure Kasner-like approach to the singularity and eventually produces the bounce to another Kasner phase. Expanding regions evolve as separate universes where inhomogeneities and anisotropies die away.