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A Relativistic Approch to Gravitational Instability in the Expanding Universe: Second Order Lagrangian Solutions

A Lagrangian relativistic approach to the non--linear dynamics of cosmological perturbations of an irrotational collisionless fluid is considered. Solutions are given at second order in perturbation theory for the relevant fluid and geometric quantities and compared with the corresponding ones in the Newtonian approximation. Specifically, we compute the density, the volume expansion scalar, the shear, the ``electric" part, or tide, and the ``magnetic" part of the Weyl tensor. The evolution of the shear and the tide beyond the linear regime strongly depends on the ratio of the characteristic size of the perturbation to the cosmological horizon distance. For perturbations on sub--horizon scales the usual Newtonian approximation applies, at least at the considered perturbative order; on super--horizon scales, instead, a new picture emerges, which we call ``silent universe", as each fluid element evolves independently of the environment, being unable to exchange signals with the surrounding matter through either sound waves or gravitational radiation. For perturbations inside the Hubble radius particular attention is paid in singling out non--local effects during the non--linear evolution of fluid elements. These non--local effects are shown to be carried by a traceless and divergenceless tensor, contained in the magnetic part of the Weyl tensor, which is dynamically generated as soon as the system evolves away from the linear regime.