Towards understanding the structure of voids in the cosmic web

Context. According to the modern cosmological paradigm, cosmic voids form in low density regions between filaments of galaxies and superclusters. Aims. Our goal is to see how density waves of different scale combine to form voids between galaxy systems of various scales. Methods. We perform numerical simulations of structure formation in cubes of size 100, and 256 h(-1)Mpc, with resolutions 256(3) and 512(3) particles and cells. To understand the role of density perturbations of various scale, we cut power spectra on scales from 8 to 128 h(-1) Mpc, using otherwise in all cases identical initial random realisations. Results. We find that small haloes and short filaments form all over the simulation box, if perturbations only on scales as large as 8 h(-1) Mpc are present. We define density waves of scale >= 64 h(-1) Mpc as large, waves of scale similar or equal to 32 h(-1) Mpc as medium scale, and waves of scale similar or equal to 8 h(-1) Mpc as small scale, within a factor of two. Voids form in regions where medium- and large-scale density perturbations combine in negative parts of the waves because of the synchronisation of phases of medium- and large-scale density perturbations. In voids, the growth of potential haloes (formed in the absence of large-scale perturbations) is suppressed by the combined negative sections of medium- and large-scale density perturbations, so that their densities are less than the mean density, and thus during the evolution their densities do not increase. Conclusions. The phenomenon of large multi-scale voids in the cosmic web requires the presence of an extended spectrum of primordial density perturbations. The void phenomenon is due to the action of two processes: the synchronisation of density perturbations of medium and large scales, and the suppression of galaxy formation in low-density regions by the combined action of negative sections of medium- and large-scale density perturbations.