the one people know, but each one may
also have been different from the others. In combining eternal inflation with
string theory, an idea that has become
popular because it could help unify
the four known forces in nature (see
Page 26), each inflated universe would
have its own set of physical properties.
Although the known universe is chocka-block with galaxies, for example, gravity
in another, earlier universe could have
been too weak to form galaxies.
Bounce not bang
String theory itself— which calls for
a space with many rolled-up dimen-
sions— may suggest a different type of
pre–Big Bang picture. In a model devel-
oped by Steinhardt, now at Princeton
University, and Turok, now director of
the Perimeter Institute in Waterloo,
Canada, the Big Bang is replaced with
an endless cycle of contractions and
bounces; 13. 7 billion years is merely the
time since the last “Big Bounce.”
In this picture, the known universe
resides on a three-dimensional version
of a sheet, called a brane, which can
In the beginning
not knowing what came before the Big
Bang doesn’t stop physicists from
theorizing. in the eternal inflation sce-
nario, the known universe bubbled
out of a larger multiverse (right).
another model (below) suggests
that the universe cycles through a
series of contractions and bounces.
in the cyclic model,
the known universe
occupies a sheetlike
surface, a “brane.”
another brane sits a
small distance away.
source: p. steinhardt
an interbrane force
pulls the two sheets
quantum ripples and
creating wrinkles in
travel along an extra dimension. Another
brane resides a tiny distance away.
When they are separated, the two
branes are perfectly wrinkle-free, representing a universe nearly devoid of matter.
As the two branes pull closer, they develop
tiny wrinkles. These wrinkles are the
seeds of galaxies. When the branes finally
collide and bounce apart, they unleash
an enormous amount of energy, some of
which is converted to matter and radiation. To an observer on one of the branes,
this Big Bounce would look just like a Big
Bang (SN: 9/22/01, p. 184).
While the branes are separated, they
stretch and smooth out; the cosmos is
expanding just as it is today. But eventually, the two branes are pulled back
together for another round of collisions
and bounces. Each cycle may last a trillion years or more.
In the Big Bounce model, the universe
not only existed before the Big Bang, it
retains the memory of what came before.
All of the stars, galaxies, and large-scale
structures now present owe their existence to the composition of the universe
in the previous cycle. Though the details
the branes collide
and then rebound,
releasing energy in
what looks like a
once the branes
and other cosmic
structures form. the
matter spreads out
and the cycle repeats.
might be different, the underlying physical laws would remain the same.
Whether the Big Bounce or the multiverse captures reality—if either one
does at all—remains a mystery. One
observation, though, could distinguish
between the Big Bounce and any inflationary scenario, Steinhardt notes.
Gravitational waves, tiny ripples in the
fabric of spacetime, are generated during
each cycle of the Big Bounce. But in this
scenario, the waves would be too weak to
be detected. Inflation, in contrast, would
produce a much more powerful set of the
waves — strong enough to leave a noticeable imprint on the cosmic microwave
background, the radiation left over from
the Big Bang.
The European Space Agency’s Planck
spacecraft is now searching for the telltale signs that gravitational waves would
leave in the cosmic microwave background (SN: 4/11/09, p. 16). If the imprint
is found, “we’re done,” says Steinhardt.
The Big Bounce would fall flat.
Whether or not inflation implies a
multiverse is another story, but Planck
may offer clues about that too.
As bubble universes expand, they can
collide with each other. If another universe happened to have struck the one
in which people reside, Planck might be
able to detect a particular pattern of
hot and cold spots in the microwave
Even if no sign of a collision can
be spotted, though, other bubble universes may still exist. Bumps could be so
infrequent that observers might have to
wait a millennium to find the pattern.
If that prolonged uncertainty about
cosmic genesis sounds a bit like pur-
gatory, consider the words of an
unnamed man quoted in St. Augustine’s
Confessions. When asked what God was
doing before making heaven and Earth,
the man replied: “He was preparing Hell
for those who pry too deep.”
St. Augustine, himself, found the
answer facetious: “More willingly would
I have answered, ‘I do not know what I do
not know.’ ” s