Formation of voids in the Universe within the Lemaitre-Tolman model

TL;DR

This paper models the formation of cosmic voids using the Lemaitre-Tolman framework, highlighting the importance of initial velocity profiles and the challenges in matching observed voids with small initial fluctuations.

Contribution

It extends existing algorithms to construct evolving Lemaitre-Tolman models from specified initial and final profiles, emphasizing the role of initial velocity in void formation.

Findings

Initial velocity profiles are more crucial than density profiles for void development.

Small initial fluctuations struggle to produce realistic voids today.

Extrapolation of CMB data to proto-void scales is highly uncertain.

Abstract

We develop models of void formation starting from a small initial fluctuation at recombination and growing to a realistic present day density profile in agreement with observations of voids. The model construction is an extension of previously developed algorithms for finding a Lemaitre-Tolman metric that evolves between two profiles of either density or velocity specified at two times. Of the 4 profiles of concern -- those of density and velocity at recombination and at the present day -- two can be specified and the other two follow from the derived model. We find that, in order to reproduce the present-day void density profiles, the initial velocity profile is more important than the initial density profile. Extrapolation of current CMB observations to the scales relevant to proto-voids is very uncertain. Even so, we find that it is very difficult to make both the initial density and velocity fluctuation amplitudes small enough, and still obtain a realistic void by today.