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.


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