Parallel Worlds (11 page)

If primordial
helium was mainly created in the stars, as Hoyle believed, then it should be
quite rare and found near the cores of stars. But all the astronomical data
showed that helium was actually quite plentiful, making up about 25 percent of
the mass of the atoms in the universe. It was found to be uniformly distributed
around the universe (as Gamow believed).

Today, we know
that both Gamow and Hoyle had pieces of the truth concerning nucleosynthesis.
Gamow originally thought that all the chemical elements were fallout or ashes
of the big bang. But his theory fell victim to the 5-particle and 8-particle
gap. Hoyle thought he could sweep away the big bang theory altogether by
showing that stars "cook" all the elements, without any need to resort
to a big bang at all. But his theory failed to account for the huge abundance
of helium we now know exists in the universe.

In essence,
Gamow and Hoyle have given us a complementary picture of nucleosynthesis. The
very light elements up to mass 5 and 8 were indeed created by the big bang, as
Gamow believed. Today, as the result of discoveries in physics, we know that
the big bang did produce most of the deuterium, helium-3, helium-4, and
lithium-7 we see in nature. But the heavier elements up to iron were mostly
cooked in the cores of the stars, as Hoyle believed. If we add the elements
beyond iron (such as copper, zinc, and gold) that were blasted out by the
blistering heat of a supernova, then we have a complete picture explaining the
relative abundances of all the elements in the universe. (Any rival theory to
modern-day cosmology would have a formidable task: to explain the relative
abundances of the hundred- odd elements in the universe and their myriad
isotopes.)

HOW STARS ARE BORN

One by-product
of this intense debate over nucleosynthesis is that it has given us a rather
complete description of the life cycle of stars. A typical star like our Sun
begins its life as a large ball of diffuse hydrogen gas called a protostar and
gradually contracts under the force of gravity. As it begins to collapse, it
begins to spin rapidly (which often leads to the formation of a double-star
system, where two stars chase each other in elliptical orbits, or the formation
of planets in the plane of rotation of the star). The core of the star also
heats up tremendously until it hits approximately 10 million degrees or more,
when the fusion of hydrogen to helium takes place.

After the star
ignites, it is called a main sequence star and it may burn for about 10 billion
years, slowly turning its core from hydrogen to waste helium. Our Sun is
currently midway through this process. After the era of hydrogen burning ends,
the star begins to burn helium, whereupon it expands enormously to the size of
the orbit of Mars and becomes a "red giant." After the helium fuel in
the core is exhausted, the outer layers of the star dissipate, leaving the core
itself, a "white dwarf" star about the size of Earth. Smaller stars
like our Sun will die in space as hunks of dead nuclear material in white
dwarf stars.

But in stars,
perhaps ten to forty times the mass of our Sun, the fusion process proceeds
much more rapidly. When the star becomes a red supergiant, its core rapidly
fuses the lighter elements, so it resembles a hybrid star, a white dwarf
inside a red giant. In this white dwarf star, the lighter elements up to iron
on the periodic table of elements may be created. When the fusion process
reaches the stage where the element iron is created, no more energy can be
extracted from the fusion process, so the nuclear furnace, after billions of
years, finally shuts down. At this point, the star abruptly collapses, creating
huge pressures that actually push the electrons into the nuclei. (The density
can exceed 400 billion times the density of water.) This causes temperatures to
soar to trillions of degrees. The gravitational energy compressed into this
tiny object explodes outward into a supernova. The intense heat of this process
causes fusion to start once again, and the elements beyond iron on the periodic
table are synthesized.

The red
supergiant Betelgeuse, for example, which can be easily seen in the
constellation Orion, is unstable; it can explode at any time as a supernova,
spewing large quantities of gamma rays and X rays into the surrounding
neighborhood. When that happens, this supernova will be visible in daytime and
might outshine the Moon at night. (It was once thought that the titanic energy
released by a supernova destroyed the dinosaurs 65 million years ago. A
supernova about ten light-years away could, in fact, end all life on Earth.
Fortunately, the giant stars Spica and Betelgeuse are 260 and 430 light-years
away, respectively, too far to cause much serious damage to Earth when they
finally explode. But some scientists believe that a minor extinction of sea
creatures 2 million years ago was caused by a supernova explosion of a star 120
light-years away.)

This also means
that our Sun is not Earth's true "mother." Although many peoples of
Earth have worshipped the Sun as a god that gave birth to Earth, this is only
partially correct. Although Earth was originally created from the Sun (as part
of the ecliptic plane of debris and dust that circulated around the Sun 4.5
billion years ago), our Sun is barely hot enough to fuse hydrogen to helium.
This means that our true "mother" sun was actually an unnamed star or
collection of stars that died billions of years ago in a supernova, which then
seeded nearby nebulae with the higher elements beyond iron that make up our
body. Literally, our bodies are made of stardust, from stars that died billions
of years ago.

In the aftermath
of a supernova explosion, there is a tiny remnant called a neutron star, which
is made of solid nuclear matter compressed to the size of Manhattan, almost 20
miles in size. (Neutron stars were first predicted by Swiss astronomer Fritz
Zwicky in 1933, but they seemed so fantastic that they were ignored by scientists
for decades.) Because the neutron star is emitting radiation irregularly and is
also spinning rapidly, it resembles a spinning lighthouse, spewing radiation as
it rotates. As seen from Earth, the neutron star appears to pulsate and is
hence called a pulsar.

Extremely large
stars, perhaps larger than 40 solar masses, when they eventually undergo a
supernova explosion, might leave behind a neutron star that is larger than 3
solar masses. The gravity of this neutron star is so large that it can
counteract the repulsive force between neutrons, and the star will make its
final collapse into perhaps the most exotic object in the universe, a black
hole, which I discuss in chapter 5.

BIRD DROPPINGS
AND THE BIG BANG

The final stake
in the heart of the steady state theory was the discovery of Arno Penzias and
Robert Wilson in 1965. Working on the 20-foot Bell Laboratory Holmdell Horn
Radio Telescope in New Jersey, they were looking for radio signals from the
heavens when they picked up an unwanted static. They thought it was probably an
aberration, because it seemed to be coming uniformly from all directions,
rather than from a single star or galaxy. Thinking the static might have come
from dirt and debris, they carefully cleaned off what Penzias described as
"a white coating of dieletric material" (commonly known as bird
droppings) that had covered the opening of the radio telescope. The static
seemed even larger. Although they did not yet know it, they had accidentally
stumbled upon the microwave background predicted by Gamow's group back in
1948.

Now the
cosmological history reads a little bit like the Keystone cops, with three
groups groping for an answer without any knowledge of the others. On one hand,
Gamow, Alpher, and Hermann had laid out the theory behind the microwave
background back in 1948; they had predicted the temperature of the microwave
radiation to be 5 degrees above absolute zero. They gave up trying to measure
the background radiation of space, however, because the instruments back then
were not sensitive enough to detect it. In 1965, Penzias and Wilson found this
black body radiation but didn't know it. Meanwhile, a third group, led by
Robert Dicke of Princeton University, had independently rediscovered the theory
of Gamow and his colleagues and were actively looking for the background
radiation, but their equipment was too woefully primitive to find it.

This comical
situation ended when a mutual friend, astronomer Bernard Burke, informed
Penzias of the work of Robert Dicke. When the two groups finally connected, it
became clear that Penzias and Wilson had detected signals from the big bang
itself. For this momentous discovery, Penzias and Wilson won the Nobel Prize
in 1978.

In hindsight,
Hoyle and Gamow, the two most visible proponents of the opposite theories, had
a fateful encounter in a Cadillac in 1956 that could have changed the course of
cosmology. "I recall George driving me around in a white Cadillac,"
recalled Hoyle. Gamow repeated his conviction to Hoyle that the big bang left
an afterglow that should be seen even today. However, Gamow's latest figures
placed the temperature of that afterglow at 50 degrees. Then Hoyle made an
astounding revelation to Gamow. Hoyle was aware of an obscure paper, written
in 1941 by Andrew McKellar, that showed that the temperature of outer space
cannot exceed 3 degrees. At higher temperatures, new reactions can occur which
would create excited carbon-hydrogen (CH) and carbon-nitrogen (CN) radicals in
outer space. By measuring the spectra of these chemicals, one could then
determine the temperature of outer space. In fact, he found that the density of
CN molecules he detected in space indicated a temperature of about 2.3 degrees
K. In other words, unknown to Gamow, the 2.7 K background radiation had already
been indirectly detected in 1941.

Hoyle recalled,
"Whether it was the too-great comfort of the Cadillac, or because George
wanted a temperature higher than 3 K, whereas I wanted a temperature of zero
degrees, we missed the chance of spotting the discovery made nine years later
by Arno Penzias and Bob Wilson." If Gamow's group had not made a numerical
error and had come up with a lower temperature, or if Hoyle had not been so
hostile to the big bang theory, perhaps history might have been written
differently.

PERSONAL AFTERSHOCKS OF THE BIG BANG

The discovery of
the microwave background by Penzias and Wilson had a decided effect on the
careers of Gamow and Hoyle. To Hoyle, the work of Penzias and Wilson was a
near-death experience. Finally, in
Nature
magazine in 1965, Hoyle officially conceded defeat, citing
the microwave background and helium abundance as reasons for abandoning his
steady state theory. But what really disturbed him was that the steady state
theory had lost its predictive power: "It is widely believed that the
existence of the microwave background killed the 'steady state' cosmology, but
what really killed the steady-state theory was psychology . . . Here, in the
microwave background, was an important phenomenon which it had not predicted
. . . For many years, this knocked the stuffing out of me." (Hoyle later
reversed himself, trying to tinker with newer variations of the steady state
theory of the universe, but each variation became less and less plausible.)

Unfortunately,
the question of priority left a bad taste in Gamow's mouth. Gamow, if one reads
between the lines, was not pleased that his work and the work of Alpher and
Hermann were rarely mentioned, if at all. Ever polite, he kept mum about his
feelings, but in private letters he wrote that it was unfair that physicists
and historians would completely ignore their work.

Although the
work of Penzias and Wilson was a huge blow to the steady state theory and
helped put the big bang on firm experimental footing, there were huge gaps in
our understanding of the structure of the expanding universe. In a Friedmann
universe, for example, one must know the value of Omega, the average distribution
of matter in the universe, to understand its evolution. However, the
determination of Omega became quite problematic when it was realized that most
of the universe was not made of familiar atoms and molecules but a strange new
substance called "dark matter," which outweighed ordinary matter by a
factor of 10. Once again, the leaders in this field were not taken seriously by
the rest of the astronomical community.

OMEGA AND DARK MATTER

The story of
dark matter is perhaps one of the strangest chapters in cosmology. Back in the
1930s, maverick Swiss astronomer Fritz Zwicky of Cal Tech noticed that the
galaxies in the Coma cluster of galaxies were not moving correctly under
Newtonian gravity. These galaxies, he found, moved so fast that they should fly
apart and the cluster should dissolve, according to Newton's laws of motion.
The only way, he thought, that the Coma cluster can be kept together, rather
than flying apart, was if the cluster had hundreds of times more matter than
could be seen by telescope. Either Newton's laws were somehow incorrect at
galactic distances or else there was a huge amount of missing, invisible matter
in the Coma cluster that was holding it together.

This was the
first indication in history that there was something terribly amiss with regard
to the distribution of matter in the universe. Astronomers universally
rejected or ignored the pioneering work of Zwicky, unfortunately, for several
reasons.

First,
astronomers were reluctant to believe that Newtonian gravity, which had
dominated physics for several centuries, could be incorrect. There was a
precedent for handling crises like this in astronomy. When the orbit of Uranus
was analyzed in the ninteenth century, it was found that it wobbled—it deviated
by a tiny amount from the equations of Isaac Newton. So either Newton was
wrong, or there must be a new planet whose gravity was tugging on Uranus. The
latter was correct, and Neptune was found on the first attempt in 1846 by
analyzing the location predicted by Newton's laws.