Leonardo Giani (The University of Queensland)

Gravitational collapse, cosmic web and almost spherical cows

Despite more than a decade of effort we still don't understand why our inference of cosmological parameters from early and late time Universe probes differ, most notably on the value of the Hubble factor today. One of the reasons might be that the Universe, despite being homogeneous and isotropic on very large scales, is highly inhomogeneous at smaller ones, where we live and perform our measurements. In this talk I will review the analytical tools employed to describe the evolution of the cosmic web, from the tiny fluctuations imprinted in the cosmic microwave background to the complex structures we observe today in the sky.I'll focus in particular on our understanding of the non-linear gravitational collapse, which is based on a few simplifying assumptions, the most important one being spherical symmetry. I will show a recently proposed generalisation of the spherical collapse to an arbitrary anisotropic shape (described by the Bianchi IX geometry) and its impact on large-scale-structures physics. The emerging picture is one of a homogeneous Universe composed of almost spherical cows. We speculate on the possible interaction between these, i.e. their collective behaviour, and discuss how the perspective from one of them, Laniakea, might bias our cosmological inference.