game, for the entropy of the universe to be perceptibly less
than the maximum amount possible. So by all odds, everything should already be all messed up — and there should
therefore be no arrow of time.
proposes, will require reaching even farther back into time,
to before the Big Bang.
But that’s not the way the universe is. As messy as things
are, they aren’t as messy as they could be, and so the fuse of
cosmic time can continue to burn. In other words, entropy
in the universe was low enough in the past to have plenty of
room to keep getting higher, and it is that quest toward disarray that drives time’s arrow in its singular direction. Explaining time’s arrow requires not only
the second law, then, but also some
reason why entropy used to be so
much lower — specifically, why it
was low when the universal clock
began ticking with the lighting of
the cosmic fuse in the Big Bang.
“You often hear cosmologists say that the Big Bang is the
moment when space and time began, there’s no such thing as
before the Big Bang,” Carroll said at the AAAS meeting. “The
truth is the Big Bang is the moment where our understanding ends. We don’t know what happened before the Big Bang,
but it’s absolutely possible that something did.”
“Trying to understand why you
can mix cream into coffee but not
unmix them takes us back to the
Big Bang, takes us back to questions
of the origin of our observable universe,” Carroll said in February in
San Diego at the annual meeting of
the American Association for the
TIME
In fact, many cosmologists today seriously study the pos-
sibility that all sorts of things happened before the Big Bang,
and that the universe it created is
just one among a multitude of dis-
tinct spacetime bubbles, coating the
Baby universe
surface of eternity like the froth on a
mug of beer (SN: 6/6/09, p. 26). This
complex “multiverse” could contain
countless individual universes, each
born in a Big Bang of its own in the
form of a baby bubble that then sev-
ered the umbilical wormhole linking
it to a primordial emptiness.
de Sitter space
Advancement of Science.
Before the Bang
From the instant of the Big Bang,
about 13. 7 billion years ago, space
has been expanding. Invoking
this expansion to explain the flow
of time in daily life has become a
standard strategy for solving time’s
mystery. That approach dates to
half a century ago, when astronomer Thomas Gold was apparently
the first to link the thermodynamic
arrow of time defined by the second
law to the cosmic arrow defined by
the Big Bang–induced expansion.
Some theories propose that the known universe
is just a baby bubble of spacetime that budded
off a preexisting space. Other baby universes
might have formed the same way, but some
with time flowing in the opposite direction, preserving time symmetry for the entire cosmos.
That emptiness, Carroll suggests,
would be a high-entropy environment technically known as de Sitter
space. “Empty,” however, does not
convey a precisely correct description. Because of quantum physics — specifically, the Heisenberg
uncertainty principle — an utterly
empty space is impermissible. Fluctuations of energy are unavoidable,
and on rare occasion one such fluctuation will be huge enough to burst
a whole new spacetime bubble into
existence — a baby universe. That
baby could expand into just the sort
of thing that human physicists see
in the one bubble they can examine
from within.
In various forms, this approach argues that expanding space
allows entropy to increase however low or high it started.
Even if entropy starts high, expansion permits it to grow
even higher. Consequently it continues to rise, and the universal clock keeps on ticking.
“Every so often a fluctuation will
make a little dollop of universe here,
dominated by energy that makes it expand really, really fast,”
Carroll explained. “That energy can stick around for a while
before it turns into ordinary matter and radiation, and the
whole scenario would look just like our Big Bang.”
KELLY ANN MCCANN
Carroll, though, in his new book From Eternity to Here,
points out (as others have before him) that this solution
simply assumes the existence of time’s direction without explaining it (see Page 30). Basically it just defines the
Big Bang as a point in the “past” from which time flows in
one direction. That scenario does not preserve the parity
between the two time directions found in the universe’s
basic equations. Finding a complete explanation, Carroll
In this way, the high-entropy empty spacetime that
existed before the Big Bang can always increase its entropy
even more — by giving birth to a baby universe. Although
the baby would have low entropy, the total entropy of the
system (mother de Sitter space plus baby) would be higher,
preserving the second law. After pinching itself away from
the mother space, the low-entropy baby will expand and
the second law will drive a direction of time as the baby’s
entropy rises. Eventually, the baby universe’s entropy will
