The hierarchical structure and dynamics of voids

Contrary to the common view voids have very complex internal structure and dynamics. Here, we show how the hierarchy of structures in the density field inside voids is reflected by a similar hierarchy of structures in the velocity field. Voids defined by dense filaments and clusters can de described as simple expanding domains with coherent flows everywhere except at their boundaries. At scales smaller than the void radius the velocity field breaks into expanding subdomains corresponding to subvoids. These subdomains break into even smaller sub-subdomains at smaller scales resulting in a nesting hierarchy of locally expanding domains. The ratio between the magnitude of the velocity field responsible for the expansion of the void and the velocity field defining the subvoids is approximately one order of magnitude. The small-scale components of the velocity field play a minor role in the shaping of the voids but they define the local dynamics directly affecting the processes of galaxy formation and evolution. The super-Hubble expansion inside voids makes them cosmic magnifiers by stretching their internal primordial density fluctuations allowing us to probe the small scales in the primordial density field. Voids also act like time machines by 'freezing' the development of the medium-scale density fluctuations responsible for the formation of the tenuous web of structures seen connecting protogalaxies in computer simulations. As a result of this freezing haloes in voids can remain 'connected' to this tenuous web until the present time. This may have an important effect in the formation and evolution of galaxies in voids by providing an efficient gas accretion mechanism via coherent low-velocity streams that can keep a steady inflow of matter for extended periods of time.