Close your eyes and visualize the universe, and you may see endless space with stars scattered unimaginably far apart, or great spiral galaxies, or exotic scenes such as the rising of an ember-red moon over an unknown planet. But actually, the expanding universe cannot be pictured. First, the universe outside us. Second, a picture captures a moment in time, but the universe encompasses time and no slice of time can even suggest that. And third, the universe cannot be pictured because it is almost all either invisible dark matter or radiation that is outside the tiny band of frequencies that we can see.
To get some visual sense of where we are in the grand scheme of things, the best image we have found is one of the oldest symbols for the universe known to humankind, the serpent swallowing its tail—an “uroboros” as the Greeks called it. Earlier people used it to represent eternity, or the waters surrounding the world. But for us, the Cosmic Uroboros represents the universe as a continuity of vastly different size scales.
Traveling around the serpent from head to tail, the largest scale is the “cosmic horizon”—the farthest distance away, in all directions, from which light has had time to travel to us since the beginning. On a somewhat smaller scale is a galaxy supercluster, then a single galaxy, then the solar system, the sun, the moon, a mountain, a human, a single-celled creature, a strand of DNA, an atom, a nucleus, and the scale of weak interactions. Approaching the tail we reach the extremely small-size scales associated with Supersymmetry (SUSY), dark matter particles such as the axion, and perhaps a Grand Unified Theory that brings together everything from the very large to the very small so that the serpent swallows its tail.
The universe exists on all size scales, everywhere, all the time, and largeness is by no means its most important characteristic. Focusing on largeness makes people feel small, not because we are, but because we are simply ignoring all scales smaller than ourselves. In fact, humans fall more or less at the center of all the size scales of the universe. When we explain this to our students, many of them are so stunned by this apparently special place that they refuse to believe it and insist that it must be a result of some tricky choice of units. It’s not. We don’t know if the human place is in fact special, but it harkens back to the soul-satisfying cosmology of the Middle Ages, in which earth was the center of the universe.