Parallel Worlds (14 page)

Similarly, in
GUT theory, the universe originally started out in the state of the false
vacuum, with the three forces unified into a single force. However, the theory
was unstable, and the theory spontaneously broke and made the transition from
the false vacuum, where the forces were unified, to the true vacuum, where the
forces are broken.

This was already
known before Guth began to analyze GUT theory. But Guth noticed something that
had been overlooked by others. In the state of the false vacuum, the universe
expands exponentially, just the way de Sitter predicted back in 1917. It is
the cosmo- logical constant, the energy of the false vacuum, that drives the
universe to expand at such an enormous rate. Guth asked himself a fateful
question: can this exponential de Sitter expansion solve some of the problems
of cosmology?

MONOPOLE PROBLEM

One prediction
of many GUT theories was the production of copious numbers of monopoles at the
beginning of time. A monopole is a single magnetic north or south pole. In
nature, these poles are always found in pairs. If you take a magnet, you
invariably find both a north pole and a south pole bound together. If you take
a hammer and split a magnet in half, then you do not find two monopoles; instead,
you find two smaller magnets, each with its own pair of north and south poles.

The problem,
however, was that scientists, after centuries of experiments, had found no
conclusive evidence for monopoles. Since no one had ever seen a monopole, Guth
was puzzled why GUT theories predicted so many of them. "Like the
unicorn, the monopole has continued to fascinate the human mind despite the
absence of confirmed observations," Guth remarked.

Then it suddenly
hit him. In a flash, all the pieces fit together. He realized that if the
universe started in a state of false vacuum, it could expand exponentially, as
de Sitter had proposed decades earlier. In this false vacuum state, the
universe could suddenly inflate by an incredible amount, thereby diluting the
density of monopoles. If scientists had never seen a monopole before, it was
only because monopoles were spread out over a universe that was much larger
than previously thought.

To Guth, this
revelation was a source of amazement and joy. Such a simple observation could
explain the monopole problem in a single stroke. But Guth realized that this
prediction would have cosmolog- ical implications far beyond his original idea.

FLATNESS
PROBLEM

Guth realized
that his theory solved another problem, the flatness problem, discussed
earlier. The standard picture of the big bang could not explain why the
universe was so flat. In the 1970s, it was believed that the matter density in
the universe, called Omega, was around 0.1. The fact that this was relatively
close to the critical density of 1.0 so many billions of years after the big
bang was deeply disturbing. As the universe expanded, Omega should have
changed with time. This number was uncomfortably close to the value of 1.0,
which describes a perfectly flat space.

For any
reasonable value of Omega at the beginning of time, Einstein's equations show
that it should almost be zero today. For Omega to be so close to 1 so many
billions of years after the big bang would require a miracle. This is what is
called in cosmology the fine- tuning problem. God, or some creator, had to
"choose" the value of Omega to within fantastic accuracy for Omega to
be about 0.1 today. For Omega to be between 0.1 and 10 today, it means that
Omega had to be 1.00000000000000 one second after the big bang. In other words,
at the beginning of time the value of Omega had to be "chosen" to
equal the number 1 to within one part in a hundred trillion, which is difficult
to comprehend.

Think of trying
to balance a pencil vertically on its tip. No matter how we try to balance the
pencil, it usually falls down. In fact, it requires a fine-tuning of great
precision to start the pencil balanced just right so it doesn't fall over. Now
try to balance the pencil on its tip so that it stays vertical not just for one
second but for years! You see the enormous fine-tuning that is involved to get
Omega to be 0.1 today. The slightest error in fine-tuning Omega would have
created Omega vastly different from 1. So why is Omega so close to 1 day, when
by rights it should be astronomically different?

To Guth, the
answer was obvious. The universe simply inflated by such a remarkable degree
that it flattened the universe. Like a person concluding that Earth is flat
because he cannot see the horizon, astronomers concluded that Omega is around 1
because inflation flattened the universe.

HORIZON PROBLEM

Not only did
inflation explain the data supporting the flatness of the universe, it also
solved the horizon problem. This problem is based on the simple realization
that the night sky seems to be relatively uniform, no matter where you look. If
you turn your head 180 degrees, you observe that the universe is uniform, even
though you have just seen parts of the universe separated by tens of billions
of light-years. Powerful telescopes scanning the heavens can find no
appreciable deviation from this uniformity either. Our space satellites have
shown that the cosmic microwave radiation is also extremely uniform. No matter
where we look in space, the temperature of the background radiation deviates no
more than a thousandth of a degree.

But this is a
problem, because the speed of light is the ultimate speed limit in the
universe. There is no way, in the lifetime of the universe, that light or
information could have traveled from one part of the night sky to the other
side. For example, if we look at the microwave radiation in one direction, it
has traveled over 13 billion years since the big bang. If we turn our heads
around and look in the opposite direction, we see microwave radiation that is
identical that has also traveled over 13 billion years. Since they are at the
same temperature, they must have been in thermal contact at the beginning of
time. But there is no way that information could have traveled from opposite
points in the night sky (separated by over 26 billion light-years) since the
big bang.

The situation is
even worse if we look at the sky 380,000 years after the big bang, when the
background radiation was first formed. If we look in opposite points in the
sky, we see that the background radiation is nearly uniform. But according to
calculations from the big bang theory, these opposite points are separated by
90 million light- years (because of the expansion of space since the
explosion). But there is no way that light could have traveled by 90 million
light- years in just 380,000 years. Information would have had to travel much
faster than the speed of light, which is impossible.

By rights, the
universe should appear quite lumpy, with one part too distant to have made
contact with another distant part. How can the universe appear so uniform, when
light simply did not have enough time to mix and spread information from one
distant part of the universe to the other? (Princeton physicist Robert Dicke
called this the horizon problem, since the horizon is the farthest point you
can see, the farthest point that light can travel.)

But Guth
realized that inflation was the key to explain this problem, as well. He
reasoned that our visible universe was probably a tiny patch in the original
fireball. The patch itself was uniform in density and temperature. But
inflation suddenly expanded this tiny patch of uniform matter by a factor of 10
⁵⁰
,
much faster than the speed of light, so that the visible universe today is
remarkably uniform. So the reason why the night sky and the microwave
radiation is so uniform is that the visible universe was once a tiny but
uniform patch of the original fireball that suddenly inflated to become the
universe.

REACTION TO INFLATION

Although Guth
was confident the inflationary idea was correct, he was a bit nervous when he
first began to give talks publicly. When he presented his theory in 1980,
"I was still worried that some consequence of theory might be
spectacularly wrong. There was also the fear that I would reveal my status as a
greenhorn cosmologist," he confessed. But his theory was so elegant and
powerful that physicists around the world immediately saw its importance.
Nobel laureate Murray Gell-Mann exclaimed, "You've solved the most important
problem in cosmology!" Nobel laureate Sheldon Glashow confided to Guth
that Steven Weinberg was "furious" when he heard about inflation.
Anxiously, Guth asked, "Did Steve have any objections to it?"
Glashow replied, "No, he just didn't think of it himself."

How could they
have missed such a simple solution, scientists asked themselves. The reception
to Guth's theory was enthusiastic among theoretical physicists, who were amazed
at its scope.

It also had an
impact on Guth's job prospects. One day, because of the tight job market, he
was staring unemployment in the face. "I was in a marginal situation on
the job market," he confessed. Suddenly, job offers began to pour in from
top universities, but not from his first choice, MIT. But then he read a
fortune cookie that said, "An exciting opportunity lies just ahead of you
if you are not too timid." This gave him the nerve to boldly phone MIT and
inquire about a job. He was stunned when MIT called a few days later and offered
him a professorship. The next fortune cookie he read said, "You should not
act on the impulse of the moment." Ignoring its advice, he decided to
accept the MIT position. "What would a Chinese fortune cookie know,
anyhow?" he asked himself.

However, there
were still serious problems. The astronomers were less than impressed by Guth's
theory, since it was glaringly deficient in one area: it gave the wrong
prediction for Omega. The fact that Omega was roughly close to 1 could be
explained by inflation. However, inflation went much further and predicted that
Omega (or Omega plus Lambda) should be precisely 1.0, corresponding to a flat
universe. In the following years, as more and more experimental data were
collected locating vast amounts of dark matter in the universe, Omega budged
slightly, rising to 0.3. But this was still potentially fatal for inflation.
Although inflation would generate over three thousand papers in the next decade
among physicists, it continued to be a curiosity for astronomers. To them, the
data seemed to rule out inflation.

Some astronomers
complained privately that particle physicists were so obsessed with the beauty
of inflation that they were willing to ignore experimental fact. (Astronomer
Robert Kirshner of Harvard wrote, "This 'inflation' idea sounds crazy. The
fact that it is taken seriously by people who sit firmly in endowed chairs
doesn't automatically make it right." Roger Penrose of Oxford called inflation
"a fashion the high-energy physicists have visited on the cos- mologists .
. . Even aardvarks think their offspring are beautiful.")

Guth believed
that sooner or later the data would show that the universe was flat. But what
did bother him was that his original picture suffered from a small but crucial
defect, one that is still not completely understood today. Inflation was
ideally suited to solving a series of deep cosmological problems. The problem
was he didn't know how to turn inflation off.

Think of heating
up a pot of water to its boiling point. Just before it boils, it is momentarily
in the state of high energy. It wants to boil, but it can't because it needs
some impurity to start a bubble. But once a bubble starts, it quickly enters a
lower energy state of the true vacuum, and the pot becomes full of bubbles.
Eventually, the bubbles become so large that they coalesce, until the pot is
uniformly full of steam. When all the bubbles merge, the phase of transition
from water to steam is complete.

In Guth's
original picture, each bubble represented a piece of our universe that was
inflating out of the vacuum. But when Guth did this calculation, he found that
the bubbles did not coalesce properly, leaving the universe incredibly lumpy.
In other words, his theory left the pot full of steam bubbles that never quite
merged to become a uniform pot of steam. Guth's vat of boiling water never seemed
to settle down to the universe of today.

In 1981, Andrei
Linde of the P. N. Lebedev Institute in Russia and Paul J. Steinhardt and
Andreas Albrecht, then at the University of Pennsylvania, found a way around
this puzzle, realizing that if a single bubble of false vacuum inflated long
enough, it would eventually fill up the entire pot and create a uniform
universe. In other words, our entire world could be the by-product of a single
bubble that inflated to fill up the universe. You did not need a large number
of bubbles to coalesce in order to create a uniform pot of steam. Just a single
bubble would do, if it inflated long enough.

Think back to
the analogy of the dam and the false vacuum. The thicker the dam, the longer it
takes for water to tunnel through the dam. If the wall of the dam is thick
enough, then the tunneling will be delayed arbitrarily long. If the universe is
allowed to inflate by a factor of 10
⁵⁰
, then a single bubble has
enough time to solve the horizon, flatness, and monopole problem. In other
words, if tunneling is sufficiently delayed, the universe inflates long enough
to flatten the universe and dilute the monopoles. But this still leaves the
question: what mechanism can prolong inflation that huge amount?