There is, we are told, an infinite hierarchy of universes. Within it, organized into the local equivalent of galaxies and smaller structures, are an immense number of other, much tinier elementary particles, which are themselves universes at the next level and so on forever - an infinite downward regression, universes within universes, endlessly. And upward as well. Our familiar universe of galaxies and stars, planets and people, would be a single elementary particle in the next universe up, the first step of another infinite regress. What would those other universes be like? Would they be built on different laws of physics? Would they have stars and galaxies and worlds, or something quite different? Might they be compatible with some unimaginably different form of life? To enter them, we would somehow have to penetrate a fourth physical dimension - not an easy undertaking, but perhaps a black hole would provide a way. There may be small black holes in the solar neighborhood. Poised at the edge of forever, we would jump off….
Excerpts from Carl Sagan's book 'Cosmos'
Us and Them