Statistics of tidal and deformation eigenvalue fields in the primordial Gaussian matter distribution: the two-dimensional case

Abstract

We study the statistical properties of the eigenvalues of the primordial tidal and deformation tensor for random Gaussian cosmic density fields. With the tidal and deformation tensors, Hessians of the gravitational and velocity potential, being Gaussian, the corresponding eigenvalue fields are distinctly non-Gaussian. Following the extension of the Doroshkevich formula for the joined distribution of eigenvalues to two-dimensional fields, we evaluate the two- and three-point correlation functions of the eigenvalue fields. In addition, we assess the number densities of singular points of the eigenvalue fields and find their corresponding two- and three-point correlation functions. The role of tidal forces and the resulting mass element deformation in shaping the prominent anisotropic wall-like and filamentary components of the cosmic web has since long been recognized based on the Zel'dovich approximation. Less well-known is that the weblike spatial pattern is already recognizable in the primordial tidal and deformation eigenvalue field, even while the corresponding Gaussian density and the potential field appear merely as a spatially incoherent and unstructured random field. Furthermore, against the background of a full phase-space assessment of structure formation in the Universe, the caustic skeleton theory entails a fully analytical framework for the nonlinear evolution of the cosmic web. It describes the folding of the dark matter sheet and the emerging caustic singularities, fully specified by the deformation eigenvalues and eigenvectors. Finally, tidal tensor eigenvalues are of central importance, and understanding their distribution is critical in predicting the resulting rotation and orientation. The current study applies to two-dimensional Gaussian random fields and will be generalized to a three-dimensional analysis in an upcoming study.