Multiversal truths

THE expansion of the universe, starting with the Big Bang, is a well-attested physical phenomenon. But over the past 400 years the universe has also undergone a different sort of expansion—a mental one. This began with a big bang too, the shattering in the early 17th century by astronomers such as Galileo Galilei and Johannes Kepler of the crystal spheres hitherto supposed to have held heavenly bodies on their proper courses. That led people to realise the so-called fixed stars, the celestial backdrop on which the movement of the planets is played out, are vastly farther away than had been believed, which led in turn to an understanding that the Milky Way, as seen in the night sky, is actually the view from Earth of a gigantic system of stars, of which the sun is a single, lowly member.