Parallel Worlds (15 page)

Eventually, this
sticky problem became known as the "graceful exit problem," that is,
how to inflate the universe long enough so that a single bubble can create the
entire universe. Over the years, at least fifty different mechanisms have been
proposed to solve the graceful exit problem. (This is a deceptively difficult
problem. I've tried several solutions myself. It was relatively easy to
generate a modest amount of inflation in the early universe. But what is extremely
difficult is getting the universe to inflate by a factor of 10
⁵⁰
. Of
course, one might simply put in this 10
⁵⁰
factor by hand, but this
is artificial and contrived.) In other words, the process of inflation was
widely believed to have solved the monopole, horizon, and flatness problems,
but no one knew precisely what drove inflation and what shut it off.

CHAOTIC INFLATION AND PARALLEL UNIVERSES

Physicist Andrei
Linde, for one, was unfazed by the fact that no one agreed on a solution to the
graceful exit problem. Linde confessed, "I just had the feeling that it
was impossible for God not to use such a good possibility to simplify his
work."

Eventually,
Linde proposed a new version of inflation that seemed to eliminate some of the
defects of the early versions. He envisioned a universe in which, at random
points in space and time, spontaneous breaking occurs. At each point where
breaking occurs, a universe is created which inflates a little. Most of the
time, the amount of inflation is minor. But because this process is random, eventually
there will be a bubble where the inflation lasts long enough to create our
universe. Taken to its logical conclusion, this means that inflation is
continuous and eternal, with big bangs happening all the time, with universes
sprouting from other universes. In this picture, universes can "bud"
off into other universes, creating a "multiverse."

In this theory,
spontaneous breaking may occur anywhere within our universe, allowing an entire
universe to bud off our universe. It also means that our own universe might
have budded from a previous universe. In the chaotic inflationary model, the
multiverse is eternal, even if individual universes are not. Some universes may
have a very large Omega, in which case they immediately vanish into a big
crunch after their big bang. Some universes only have a tiny Omega and expand
forever. Eventually, the multiverse becomes dominated by those universes that
inflate by a huge amount.

In retrospect,
the idea of parallel universes is forced upon us. Inflation represents the
merger of traditional cosmology with advances in particle physics. Being a
quantum theory, particle physics states that there is a finite probability for
unlikely events to occur, such as the creation of parallel universes. Thus, as
soon as we admit the possibility of one universe being created, we open the
door to the probability of an endless number of parallel universes being
created. Think, for example, of how the electron is described in the quantum
theory. Because of uncertainty, the electron does not exist at any single
point, but exists in all possible points around the nucleus. This electron
"cloud" surrounding the nucleus represents the electron being many
places at the same time. This is the fundamental basis of all of chemistry
which allows electrons to bind molecules together. The reason why our molecules
do not dissolve is that parallel electrons dance around them and hold them
together. Likewise, the universe was once smaller than an electron. When we
apply the quantum theory to the universe, we are then forced to admit the possibility
that the universe exists simultaneously in many states. In other words, once we
open the door to applying quantum fluctuations to the universe, we are almost
forced to admit the possibility of parallel universes. It seems we have little
choice.

THE UNIVERSE FROM NOTHING

At first, one might object to the notion of a multiverse,
because it seems to violate known laws, such as the conservation of matter and
energy. However, the total matter/energy content of a universe may actually be
very small. The matter content of the universe, including all the stars,
planets, and galaxies, is huge and positive. However, the energy stored within
gravity may be negative. If you add the positive energy due to matter to the
negative energy due to gravity, the sum may be close to zero! In some sense,
such universes are free.
They can spring
out of the vacuum almost effortlessly. (If the universe is closed, then the
total energy content of the universe must be precisely zero.)

(To grasp this,
think of a donkey that falls into a large hole in the ground. We have to add
energy to the donkey in order to pull him out of the hole. Once he is out and
he is standing on the ground, he is considered to have zero energy. Thus,
because we had to add energy to the donkey to get him to a state of zero
energy, he must have had negative energy while in the hole. Similarly, it takes
energy to pull a planet out of a solar system. Once it is out in free space,
the planet has zero energy. Since we have to add energy to extract a planet out
of a solar system to attain a state of zero energy, the planet has negative
gravitational energy while inside the solar system.)

In fact, to
create a universe like ours may require a ridiculously small net amount of
matter, perhaps as little as an ounce. As Guth likes to say, "the universe
may be a free lunch." This idea of creating a universe from nothing was
first introduced by physicist Edward Tryon of Hunter College of the City University
of New York, in a paper published in
Nature
magazine in 1973. He speculated that the universe is
something "which happens from time to time" due to a quantum
fluctuation in the vacuum. (Although the net amount of matter necessary to
create a universe may be close to zero, this matter must be compressed to
incredible densities, as we see in chapter 12.)

Like the P'an Ku
mythologies, this is an example of
creatio ex nihilo
cosmology. Although the universe-from-nothing theory cannot
be proved with conventional means, it does help to answer very practical
questions about the universe. For example, why doesn't the universe spin?
Everything we see around us spins, from tops, hurricanes, planets, and
galaxies, to quasars. It seems to be a universal characteristic of matter in
the universe. But the universe itself does not spin. When we look at the
galaxies in the heavens, their total spin cancels out to zero. (This is quite
fortunate, because, as we see in chapter 5, if the universe did spin, then time
travel would become commonplace and history would be impossible to write.) The
reason why the universe does not spin may be that our universe came from
nothing. Since the vacuum does not spin, we do not expect to see any net spin
arising in our universe. In fact, all the bubble-universes within the
multiverse may have zero net spin.

Why do positive
and negative electrical charges balance out exactly? Normally, when we think
of the cosmic forces governing the universe, we think more about gravity than
the electromagnetic force, even though the gravitational force is
infinitesimally small compared to the electromagnetic force. The reason for
this is the perfect balance between positive and negative charges. As a
result, the net charge of the universe appears to be zero, and gravity
dominates the universe, not the electromagnetic force.

Although we take
this for granted, the cancellation of positive and negative charges is quite
remarkable, and has been experimentally checked to 1 part in 10
²¹
.
(Of course, there are local imbalances between the charges, and that's why we
have lightning bolts. But the total number of charges, even for thunderstorms,
adds up to zero.) If there were just 0.00001 percent difference in the net
positive and negative electrical charges within your body, you would be ripped
to shreds instantly, with your body parts thrown into outer space by the
electrical force.

The answer to
these enduring puzzles may be that the universe came from nothing. Since the
vacuum has net zero spin and charge, any baby universe springing forth from
nothing must also have net zero spin and charge.

There is one
apparent exception to this rule. That exception is that the universe is made of
matter rather than antimatter. Since matter and antimatter are opposites (with
antimatter having exactly the opposite charge from matter), we might assume
that the big bang must have created equal amount of matter and antimatter. The
problem, however, is that matter and antimatter will annihilate each other on
contact into a burst of gamma rays. Thus, we should not exist. The universe
should be a random collection of gamma rays instead of teeming with ordinary
matter. If the big bang were perfectly symmetrical (or if it came from
nothing), then we should expect equal amounts of matter and antimatter to be
formed. So why do we exist? The solution proposed by Russian physicist Andrei
Sakharov is that the original big bang was not perfectly symmetrical at all.
There was a tiny amount of symmetry breaking between matter and antimatter at
the instant of creation, so that matter dominated over antimatter, which made
possible the universe we see around us. (The symmetry that was broken at the
big bang is called CP symmetry, the symmetry that reverses charges and the
parity of matter and antimatter particles.) If the universe came from
"nothing," then perhaps nothing was not perfectly empty but had a
slight amount of symmetry breaking, which allows for the slight dominance of
matter over antimatter today. The origin of this symmetry breaking is still not
understood.

WHAT MIGHT OTHER UNIVERSES LOOK LIKE?

The multiverse
idea is appealing, because all we have to do is assume that spontaneous
breaking occurs randomly. No other assumptions have to be made. Each time a
universe sprouts off another universe, the physical constants differ from the
original, creating new laws of physics. If this is true, then an entirely new
reality can emerge within each universe. But this raises the intriguing
question: what do these other universes look like? The key to understanding the
physics of parallel universes is to understand how universes are created, that
is, to understand precisely how spontaneous breaking occurs.

When a universe
is born and spontaneous breaking takes place, this also breaks the symmetry of
the original theory. To a physicist, beauty means symmetry and simplicity. If a
theory is beautiful, this means it has a powerful symmetry that can explain a
large body of data in the most compact, economical manner. More precisely, an
equation is considered to be beautiful if it remains the same when we
interchange its components among themselves. One great advantage to finding
the hidden symmetries of nature is that we can show that phenomena that are
seemingly distinct are actually manifestations of the same thing, linked
together by a symmetry. For example, we can show that electricity and magnetism
are actually two aspects of the same object, because there is a symmetry that
can interchange them within Maxwell's equations. Similarly, Einstein showed
that relativity can turn space into time and vice versa, because they are part
of the same object, the fabric of space-time.

Think of a
snowflake, which has a beautiful six-fold symmetry, a source of endless
fascination. The essence of its beauty is that it remains the same if we
rotate the snowflake by 60 degrees. This also means that any equation we write
down to describe the snowflake should reflect this fact, that it remains
invariant under rotations of multiples of 60 degrees. Mathematically, we say
that the snowflake has C
₆
symmetry.

Symmetries then
encode the hidden beauty of nature. But in reality, today these symmetries are
horribly broken. The four great forces of the universe do not resemble each
other at all. In fact, the universe is full of irregularities and defects;
surrounding us are the fragments and shards of the original, primordial
symmetry shattered by the big bang. Thus, the key to understanding possible
parallel universes is to understand "symmetry breaking"—that is, how
these symmetries might have broken after the big bang. As physicist David Gross
has said, "The secret of nature is symmetry, but much of the texture of
the world is due to mechanisms of symmetry breaking."

Think of the way
a beautiful mirror shatters into a thousand pieces. The original mirror
possessed great symmetry. You can rotate a mirror at any angle and it still
reflects light in the same way. But after it is shattered, the original
symmetry is broken. Determining precisely how the symmetry is broken determines
how the mirror shatters.

SYMMETRY
BREAKING

To see this,
think of the development of an embryo. In its early stages, a few days after
conception, an embryo consists of a perfect sphere of cells. Each cell is no
different from the others. It looks the same no matter how we rotate it.
Physicists say that the embryo at this stage has O(3) symmetry—that is, it
remains the same no matter how you rotate it on any axis.

Although the
embryo is beautiful and elegant, it is also rather useless. Being a perfect
sphere, it cannot perform any useful functions or interact with the
environment. In time, however, the embryo breaks this symmetry, developing a
tiny head and torso, so it resembles a bowling pin. Although the original
spherical symmetry is now broken, the embryo still has a residual symmetry; it
remains the same if we spin it along its axis. Thus, it has cylindrical symmetry.
Mathematically, we say that the original O(3) of the sphere has now been broken
down to the O(2) symmetry of the cylinder.