Lagrangian theory of structure formation in relativistic cosmology. VI. Comparison with Szekeres exact solutions

Abstract

We examine the relation between the Szekeres models and relativistic Lagrangian perturbation schemes, in particular the Relativistic Zel'dovich Approximation (RZA). We show that the second class of the Szekeres solutions is exactly contained within the RZA when the latter is restricted to an irrotational dust source with a flow-orthogonal foliation of spacetime. In such a case, the solution is governed by the first principal scalar invariant of the deformation field, proving a direct connection with a class of Newtonian three-dimensional solutions without symmetry. For the second class, a necessary and sufficient condition for the vanishing of cosmological backreaction on a scale of homogeneity is expressed through integral constraints. Domains with no backreaction can be smoothly matched, forming a lattice model, where exact deviations average out at a given scale of homogeneity, and the homogeneous and isotropic background is recovered as an average property of the model. Although the connection with the first class of Szekeres solutions is not straightforward, this class allows for the interpretation in terms of a spatial superposition of nonintersecting fluid lines, where each world line evolves independently and under the RZA model equations, but with different associated `local backgrounds'. This points to the possibility of generalizing the Lagrangian perturbation schemes to structure formation models on evolving backgrounds, including global cosmological backreaction.

27 pages, 1 figure; matches published version in PRD

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• Including relativistic and primordial Non-Gaussianity contributions in cosmological simulations by modifying the initial condition
• On the maximum volume of collapsing structures
• Comment on "Szekeres universes with homogeneous scalar fields"
• Backreaction and the Role of Spatial Curvature in the Cosmic Neighborhood
• Direct correspondence between Newtonian gravitation and general relativity
• An effective $\mathbf{\Lambda }$-Szekeres modelling of the local Universe with Cosmicflows-4
• Cosmological perturbations on an averaged background