Bubble Bath Cosmology

By Brian Greene
Newsweek, May 2012

Edited by Andy Ross

Albert Einstein helped us to understand gravity. His equations showed that space is expanding. At some moment in the past, everything we now see must have been compressed to a tiny speck that then blew up. The big bang theory was born.

Inflationary cosmology aimed to explain the bang. A hypothetical cosmic fuel drove an explosion so stupendous that quantum jitters would have been smeared clear across space. This would yield a precise pattern of tiny temperature variations over the night sky. We have detected them. And the big bang was likely not unique. The fuel would power countless other bangs, too, each yielding its own separate, expanding universe. So our universe is just a bubble in a cosmic bubble bath a multiverse.

Dark energy also led to the multiverse. As space expands, the gravitational pull of each galaxy on every other should slow the expansion. But when astronomers measured the rate of this cosmic slowdown, they found that the expansion of space is actually speeding up. In Einstein's theory, if space contains an invisible energy uniformly spread through space, then the gravity exerted by the energy is repulsive. The repulsive gravity of dark energy would push every galaxy away from every other, speeding up the expansion. Astronomers measured a tiny amount of dark energy. Theorists calculated amounts a hundred orders of magnitude larger. This spectacular failure drove some physicists to consider the multiverse.

Inflationary theory naturally gives rise to other universes with different amounts of dark energy. In universes with more dark energy, whenever matter tries to clump into galaxies, the repulsive push of the dark energy is so strong that the clump gets blown apart, thwarting galactic formation. In universes with less, the repulsive push changes to an attractive pull, causing those universes to collapse back on themselves so quickly that again galaxies don't form. No galaxies, no life.

The other universes solve the mystery of dark energy. If the feature you want to explain takes on a wide variety of different values across the landscape of reality, then seeking an explanation for one value is wrongheaded. The multiverse invites us to reevaluate our questions.

String theory ensures that among all the bubbles in the multiverse there's one with the dark energy value we measure. According to string theory, inside every fundamental particle is a tiny string of energy. The different vibrations of these strings yield the different particles. The equations have some 10^500 solutions, each representing a different possible universe. This enormous diversity of bubbles ensures that we can find our bubble. On its own, inflationary cosmology says that many of the bubbles in the bath would look too similar. Together with string theory, it gives us a wide variety of bubbles, blown by one big bang after another. Our universe is sure to appear.

If the multiverse theory makes correct predictions about things in our universe, then we can be more confident in its prediction of other universes. String theory is hypothetical, but more direct evidence for the multiverse might come from collisions between our bubble and its neighbors. Cosmic fender benders would put new temperature variations in the microwave background.

The multiverse can be a cop-out, but ignoring it can leave us chasing unanswerable questions. The science of the multiverse is a risk worth taking.

AR  Nice one, Brian.