Almost anybody who has ever thought deeply about the universe sooner or later wonders if there is more than one of them. Whether a multiplicity of universes—known as a multiverse—actually exists has been a contentious issue since ancient times. Greek philosophers who believed in atoms, such as Democritus, proposed the existence of an infinite number of universes. But Aristotle disagreed, insisting that there could be only one.
Today a similar debate rages over whether multiple universes exist. In recent decades, advances in cosmology have implied (but not proved) the existence of a multiverse. In particular, a theory called inflation suggests that in the instant after the Big Bang, space inflated rapidly for a brief time and then expanded more slowly, creating the vast bubble of space in which the Earth, sun, Milky Way galaxy, and billions of other galaxies reside today. If this inflationary cosmology theory is correct, similar big bangs occurred many times, creating numerous other bubbles of space like our universe.
Properties such as the mass of basic particles and the strength of fundamental forces may differ from bubble to bubble. In that case, the popular goal pursued by many physicists of finding a single theory that prescribes all of nature’s properties may be in vain. Instead, a multiverse may offer various locales, some more hospitable to life than others. Our universe must be a bubble with the right combination of features to create an environment suitable for life, a requirement known as the anthropic principle.