The Crowded Emptiness of the Observable Universe: A 70-Mile Scale Model of Cosmic Structure

Space is commonly described as overwhelmingly empty. At local scales this description is justified: the distances separating stars, galaxies, and galaxy clusters are enormous compared with the dimensions of the objects themselves. Yet this characterization can obscure another equally remarkable property of the universe: when cosmic structure is considered collectively, the observable universe contains an extraordinary number of objects, particles, radiation fields, and other forms of matter and energy. This paper develops a simple scale model to illustrate this apparent contradiction. If the approximately 92-billion-light-year diameter of the observable universe were compressed to a sphere only 70 miles across, and if galaxies were represented by bowling balls, an estimated population of two trillion galaxies would correspond to approximately one galaxy-marker for every 20–25 feet of characteristic three-dimensional spacing. The model therefore presents a universe that is simultaneously sparse in volume and extraordinarily crowded in population. The analogy is extended to include cosmic microwave background photons, relic neutrinos, intergalactic gas, dark matter, electromagnetic radiation, and other components that permeate regions commonly regarded as “empty space.” The resulting picture suggests that cosmic emptiness is strongly scale-dependent: the universe is exceedingly dilute in average mass density while nevertheless containing structure, particles, radiation, and fields throughout essentially every observable region.