Standing waves in the Universe
  At first, a review of our knowledge on the distribution of galaxies at large-scale, leading to a foam-like large-scale structure of the Universe, is presented in the Introduction. Then, it is shown how, according to the present theory for the formation of superclusters, wave scalar perturbations of the same frequency traveling in opposite directions give rise to standing waves, which cause a motion of the cosmic material towards the nodes, resulting in the concentration of the cosmic material around the nodes. Generalizing this effect to two (three) dimensions, the cosmic material is concentrated around the node lines (node surfaces). It is proposed that the three-dimensional effect is responsible for the foam-like large-scale structure of the Universe. 
Evangelos Chaliasos