Parallel Worlds (16 page)

The breaking of
O(3) symmetry, however, could have proceeded in a different way. Starfish, for
example, do not have cylindrical or bilateral symmetry; instead, when the
spherical symmetry is broken, they have a C
₅
symmetry (which remains
the same under rotations by 72 degrees), giving it its five-pointed-star shape.
Thus, the way in which the symmetry O(3) breaks determines the shape of the
organism when it is born.

Similarly,
scientists believe the universe started out in a state of perfect symmetry,
with all the forces unified into a single force. The universe was beautiful,
symmetrical, but rather useless. Life as we know it could not exist in this
perfect state. In order for the possibility of life to exist, the symmetry of
the universe had to break as it cooled.

SYMMETRY AND THE STANDARD MODEL

In the same way, to understand what parallel universes might
look like, we must first understand the symmetries of the strong, weak, and
electromagnetic interactions. The strong force, for example, is based on three
quarks, which scientists label by giving them a fictitious "color"
(for example, red, white, and blue). We want the equations to remain the same
if we interchange these three colored quarks. We say that the equations have
SU(3) symmetry, that is, when we reshuffle the three quarks, the equations
remain the same. Scientists believe that a theory with SU(3) symmetry forms the
most accurate description of the strong interactions (called Quantum
Chromodynamics). If we had a gigantic supercomputer, starting with just the
masses of the quarks and the strength of their interactions, we could, in
theory, calculate all the properties of the proton and neutron and all the
characteristics of nuclear physics.

Similarly, let's
say we have two leptons, the electron and the neutrino. If we interchange them
in an equation, we have SU(2) symmetry. We can also throw in light, which has
the symmetry group U(i). (This symmetry group shuffles the various components
or polarizations of light among each other.) Thus, the symmetry group of the
weak and electromagnetic interactions is SU(2)
X
U(i).

If we simply
glue these three theories together, not surprisingly we have the symmetry SU(3)
X
SU(2)
X
U(i), in other
words, the symmetry that separately mixes three quarks among themselves and
two leptons among themselves (but does not mix quarks with leptons). The
resulting theory is the Standard Model, which, as we saw earlier, is perhaps
one of the most successful theories of all time. As Gordon Kane of the
University of Michigan says, "Everything that happens in our world (except
for the effects of gravity) results from Standard Model particle
interactions." Some of its predictions have been tested in the laboratory
to hold within one part in a hundred million. (In fact, twenty Nobel Prizes
have been awarded to physicists who have pieced together parts of the Standard
Model.)

Finally, one
might construct a theory that combines the strong, weak, and electromagnetic
interaction into a single symmetry. The simplest GUT theory that can do this
interchanges all five particles (three quarks and two leptons) into each other
simultaneously. Unlike the Standard Model symmetry, the GUT symmetry can mix
quarks and leptons together (which means that protons can decay into
electrons). In other words, GUT theories contain SU(5) symmetry (reshuffling
all five particles—three quarks and two leptons— among themselves). Over the
years, many other symmetry groups have been analyzed, but SU(5) is perhaps the
minimal group that fits the data.

When spontaneous
breaking occurs, the original GUT symmetry can break in several ways. In one
way, the GUT symmetry breaks down to SU(3)
X
SU(2)
X
U(i) with
precisely 19 free parameters that we need to describe our universe. This gives
us the known universe. However, there are actually many ways in which to break
GUT symmetry. Other universes would most likely have a completely different
residual symmetry. At the very minimum, these parallel universes might have
different values of these 19 parameters. In other words, the strengths of the
various forces would be different in different universes, leading to vast
changes in the structure of the universe. By weakening the strength of the
nuclear force, for example, one might prevent the formation of stars, leaving
the universe in perpetual darkness, making life impossible. If the nuclear
force is strengthened too much, stars could burn their nuclear fuel so fast
that life would not have enough time to form.

The symmetry
group may also be changed, creating an entirely different universe of
particles. In some of these universes, the proton might not be stable and
would rapidly decay into antielectrons. Such universes cannot have life as we
know it, but would rapidly disintegrate into a lifeless mist of electrons and
neutrinos. Other universes could break the GUT symmetry in yet another way, so
there would be more stable particles, like protons. In such a universe, a huge
variety of strange new chemical elements could exist. Life in those universes
could be more complex than our own, with more chemical elements out of which to
create DNA-like chemicals.

We can also
break the original GUT symmetry so that we have more than one U(i) symmetry, so
there is more than one form of light. This would be a strange universe, indeed,
in which beings might "see" using not just one kind of force but
several. In such a universe, the eyes of any living being could have a large
variety of receptors to detect various forms of light-like radiation.

Not surprisingly, there are hundreds, perhaps even an
infinite number of ways to break these symmetries. Each of these solutions, in
turn, might correspond to an entirely separate universe.

TESTABLE PREDICTIONS

Unfortunately,
the possibility of testing the multiverse theory, involving multiple universes
with different sets of physical laws, is at present impossible. One would have
to travel faster than light to reach these other universes. But one advantage
of the inflation theory is that it makes predictions about the nature of our
universe that
are
testable.

Since the
inflationary theory is a quantum theory, it is based on the Heisenberg
uncertainty principle, the cornerstone of the quantum theory. (The uncertainty
principle states that you cannot make measurements with infinite accuracy, such
as measuring the velocity and position of an electron. No matter how sensitive
your instruments are, there will always be uncertainty in your measurements.
If you know an electron's velocity, you cannot know its precise location; if
you know its location, you cannot know its velocity.) Applied to the original
fireball that set off the big bang, it means that the original cosmic explosion
could not have been infinitely "smooth." (If it had been perfectly uniform,
then we would know precisely the trajectories of the subatomic particles
emanating from the big bang, which violates the uncertainty principle.) The
quantum theory allows us to compute the size of these ripples or fluctuations
in the original fireball. If we then inflate these tiny quantum ripples, we can
calculate the minimum number of ripples we should see on the microwave
background 380,000 years after the big bang. (And if we expand these ripples to
the present day, we should find the current distribution of galactic clusters.
Our galaxy itself started out in one of these tiny fluctuations.)

Initially, a
superficial glance at the data from the COBE satellite found no deviations or
fluctuations in the microwave background. This caused some anxiety among physicists,
because a perfectly smooth microwave background would violate not just
inflation but the entire quantum theory as well, violating the uncertainty
principle. It would shake physics to its very core. The entire foundation of
twentieth-century quantum physics might have to be thrown out.

Much to
scientists' relief, a painstakingly detailed look at the computer-enhanced data
from the COBE satellite found a blurry set of ripples, variations in
temperature of i part in 100,000—the minimum amount of deviation tolerated by
the quantum theory. These infinitesimal ripples were consistent with the
inflationary theory. Guth confessed, "I'm completely snowed by the cosmic
background radiation. The signal was so weak it wasn't even detected until 1965,
and now they're measuring fluctuations of one part in 100,000."

Although the
experimental evidence being gathered was slowly favoring inflation, scientists
still had to resolve the nagging problem of the value of Omega—the fact that
Omega was 0.3 rather than 1.0.

SUPERNOVAE—RETURN OF LAMBDA

While inflation
turned out to be consistent with the COBE data scientists gathered,
astronomers still grumbled in the i990s that inflation was in flagrant
violation of the experimental data on Omega. The tide first began to turn in
1998, as a result of data from a totally unexpected direction. Astronomers
tried to recalculate the rate of expansion of the universe in the distant past.
Instead of analyzing Cepheid variables, as Hubble did in the 1920s, they begin
to examine supernovae in distant galaxies billions of light-years into the
past. In particular, they examined type Ia supernovae, which are ideally suited
for being used as standard candles.

Astronomers know
that supernovae of this type have nearly the same brightness. (The brightness
of type Ia supernovae is known so well that even small deviations can be
calibrated: the brighter the supernova, the slower it declines in brightness.)
Such supernovae are caused when a white dwarf star in a binary system slowly
sucks matter from its companion star. By feeding off its sister star, this
white dwarf gradually grows in mass until it weighs 1.4 solar masses, the
maximum possible for a white dwarf. When they exceed this limit, they collapse
and explode in a type Ia supernova. This trigger point is why type Ia
supernovae are so uniform in brightness—it is the natural result of white dwarf
stars reaching a precise mass and then collapsing under gravity. (As
Subrahmanyan Chandrasekhar showed in i935, in a white dwarf star the force of
gravity crushing the star is balanced by a repulsive force between the
electrons, called electron degeneracy pressure. If a white dwarf star weighs
more than 1.4 solar masses, then gravity overcomes this force and the star is
crushed, creating the supernova.) Since distant supernovae took place in the
early universe, by analyzing them one can calculate the rate of expansion of
the universe billions of years ago.

Two independent
groups of astronomers (led by Saul Perlmutter of the Supernova Cosmology Project
and Brian P. Schmidt of the High-Z Supernova Search Team) expected to find that
the universe, although still expanding, was gradually slowing down. For several
generations of astronomers, this was an article of faith, taught in every
cosmology class—that the original expansion was gradually decelerating.

After analyzing
about a dozen supernovae each, they found that the early universe was not
expanding as fast as previously thought (that is, the redshifts of the
supernovae and hence their velocity were smaller than originally suspected).
When comparing the expansion rate of the early universe to today's expansion,
they concluded that the expansion rate was relatively greater today. Much to
their shock, these two groups came to the astounding conclusion that the
universe is
accelerating.

Much to their
dismay, they found that it was impossible to fit the data with any value of
Omega. The only way to make the data fit the theory was to reintroduce Lambda,
the energy of the vacuum first introduced by Einstein. Moreover, they found
that Omega was overwhelmed by an unusually large Lambda that was causing the
universe to accelerate in a de Sitter-type expansion. The two groups
independently came to this startling realization but were hesitant to publish
their findings because of the strong historical prejudice that the value of
Lambda was zero. As George Jacoby of the Kitt's

Peak Observatory
has said, "The Lambda thing has always been a wild-eyed concept, and
anybody crazy enough to say it's not zero was treated as kind of nuts."

Schmidt recalls,
"I was still shaking my head, but we had checked everything ... I was very
reluctant about telling people, because I truly thought that we were going to
get massacred." However, when both groups released their results in 1998,
the sheer mountain of data they amassed could not be easily dismissed. Lambda,
Einstein's "biggest blunder," which had been almost completely
forgotten in modern cosmology, was now staging a remarkable comeback after
ninety years of obscurity!

Physicists were
dumbfounded. Edward Witten of the Institute for Advanced Study at Princeton
said it was "the strangest experimental finding since I've been in
physics." When the value of Omega, 0.3, was added to the value of Lambda,
0.7, the sum was (to within experimental error) equal to 1.0, the prediction
of the inflationary theory. Like a jigsaw puzzle being assembled before our
eyes, cos- mologists were seeing the missing piece of inflation. It came from
the vacuum itself.

This result was
spectacularly reconfirmed by the WMAP satellite, which showed that the energy
associated with Lambda, or dark energy, makes up 73 percent of all matter and
energy in the universe, making it the dominant piece of the jigsaw puzzle.