In the beginning was nothing. Then the Universe was born in a searing hot
fireball called the big bang. Recent developments, such as the discovery of
鈥渃osmic ripples鈥, have helped to refine this dramatic vision of creation
FIFTEEN billion years ago, our Universe erupted out of nothing in a
titanic explosion that we now call the big bang. Everything 鈥 all matter,
energy, even space and time 鈥 came into being at that instant. Ever since,
the stuff of the Universe has been expanding and cooling.
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In the earliest moments of the big bang, the Universe occupied a tiny
volume and was unimaginably hot. It was a blistering fireball of radiation
mixed with microscopic particles of matter. But eventually, the Universe
cooled enough for atoms to form. Gradually, these clumped together under
gravity to make billions of galaxies, great islands of stars of which our own
galaxy, the Milky Way, is but one.
When, in 1992, NASA鈥檚 Cosmic Background Explorer (COBE) satellite detected
the 鈥渟eeds鈥 from which such galaxies grew out of the cooling gas of the big
bang, one of the last jigsaw pieces in the big bang picture was in place.
Cosmologists now believe they can trace the evolution of the Universe from
the first split-second of creation all the way to the present day.
Cosmic shrapnel
Signs of the big bang
ASTRONOMERS have been led to the big bang theory by three principal
observations. The first and most remarkable is that the entire Universe is
expanding. In 1929, Edwin Hubble discovered that the major constituents of
the Universe 鈥 galaxies like our own Milky Way 鈥 are fleeing from each other
like cosmic shrapnel in the aftermath of a titanic explosion.
If the Universe is expanding, one conclusion seems inescapable: it must
have been smaller in the past. There must have been a moment when the
headlong expansion started: the moment of the Universe鈥檚 birth. This is the
real significance of Hubble鈥檚 discovery. Although the Universe is old, it has
not existed forever. By imagining the expansion running backwards, like a
movie in reverse, astronomers can deduce that the Universe was born in a giant
explosion about 15 billion years ago.
The second observation that supports the big bang theory is the existence
of the cosmic background radiation, the cooled 鈥渁fterglow鈥 of the the big
bang fireball. Incredibly, it still permeates every pore of space 15 billion
years after the event. Now cooled to a temperature of 2.726 Kelvin (about 鈥
273 掳C), it appears as shortwavelength radio waves, or microwaves.
Although the cosmic background radiation accounts for 99 per cent of all
the light energy streaming through the Universe at this moment, it was not
found until 1965, and then entirely by accident. For its discovery, Arno
Penzias and Robert Wilson, two young astronomers at AT&T鈥檚 Bell Laboratories
in Holmdel, New Jersey, shared the 1978 Nobel Prize for Physics.
In the beginning
A hot fiery furnace
THE final piece of evidence that supports the big bang theory is the
observation that about 25 per cent of the mass of the Universe is in the form
of the element helium. Although most elements, such as carbon and iron, are
known to be made from hydrogen by nuclear reaction inside stars, the
Universe has simply not existed long enough for stars to have made such a
large quantity of helium. The best explanation for this anomaly is that at
one time in the past the entire Universe passed through a hot dense phase in
which nuclear reactions forged most of its helium from hydrogen.
Calculations show that such a phase 鈥 a hot big bang 鈥 would have turned
roughly 25 per cent of the mass of the Universe into helium, just as
observed.
The first person to realise that the big bang was a hot big bang was the
Russian-American physicist George Gamow. It was hot for the same reason that
air in a bicycle pump heats up when it is compressed. Gamow鈥檚 great insight
was to recognise that it was possible to understand the Universe at an early
epoch simply by applying the laws of physics known to operate at the
corresponding temperature. In the 1940s, Gamow did just that, using nuclear
physics 鈥 the physics of matter at temperatures of millions and billions of
degrees 鈥 to understand what was going on in the Universe when it was just a
few minutes old.
Gamow realised that a natural consequence of a hot big bang was intense
radiation, which would have been mixed in with the matter in a searing hot
fireball. But it was left to his American colleagues, Ralph Alpher and
Robert Herman, to make the remarkable prediction in 1948 that this 鈥渇ireball鈥
radiation might be still around today, greatly cooled by the expansion of
the Universe. This relic radiation would have two striking characteristics.
One, it would appear to be coming from every direction in the sky, and, two,
it would have a thermal, or black body, spectrum. The humped-back spectrum
of a black body is characterised by a single temperature and is as familiar to
every physicist as the face of Albert Einstein.
Unfortunately, no one took much notice of Alpher and Herman鈥檚 prediction
and it was left to Penzias and Wilson to stumble on the cosmic background
radiation 17 years later and confirm Gamow鈥檚 vision of a hot big bang.
Primordial accelerator
Processes of particles
GAMOW had shown physicists how to 鈥渓ook鈥 into earliest moments of the big
bang. Today鈥檚 physicists, following his lead, apply their knowledge of matter
at trillions of degrees and hotter, which they have gleaned from experiments
in giant particle accelerators. But whereas Gamow probed the era a few
minutes after the big bang, they confidently predict the conditions in the
first thousandths of a second and earlier. This is Gamow鈥檚 legacy: we now
recognise that the ultimate questions of where the Universe came from can
only be answered by high-energy particle physics.
The early Universe was a seething cauldron of electromagnetic radiation 鈥
in the form of tiny packets of energy called photons 鈥 and microscopic
particles of matter. As the temperature dropped, the mix of particles which
dominated the Universe changed continuously because less and less energy was
available to make them from photons.
The reason for this is that energy and matter, as Einstein discovered,
are merely different faces of the same coin, connected by the famous
equation E=mc2, where c is the speed of light. This means
that particles of a particular mass, m, can be created from photons only if
the photons have an energy of at least mc2. Because photons become
less energetic as the temperature falls, it follows that there is a threshold
temperature below which a particle of mass m cannot be produced. So as the
Universe cooled the particles mix changed, to a soup of less and less massive
particles.
In the earliest moments of the Universe the particles which were dominant
were unlike any that exist in today鈥檚 Universe. In fact, theorists can only
guess at what they were like. There is no reliable guide because physicists
are unable to achieve comparable temperatures on Earth.
But by the time the Universe was about a thousandth of a second old and
the temperature had dropped to about 1000 billion degrees, it was filled by
exotic particles which today can be briefly created in particle accelerators
(see Inside Science No. 55). Among these were quarks, the building blocks of
the familar neutron and proton. Unfortunately, there is no satisfactory theory
which explains how a soup of quarks behaves, so we know little about this era
(see Inside Science No. 63).
It was not until the Universe was about a hundredth of a second old that
it was dominated by familiar particles: photons, electrons, positrons and
neutrinos. Quarks came together in threes to make neutrons and protons, but
these particles were a minor contaminant of the Universe. By around 1
second, when the temperature had fallen to about 10 billion degrees, the
photons no longer had sufficient energy to easily conjure up particles from
energy.
The next important event in the history of the Universe occurred when the
Universe was about 100 seconds old and its temperature had dropped to a mere
billion degrees. Protons and neutrons were now moving so slowly that they
stayed long enough in each other鈥檚 vicinity for their nuclear glue to take
hold. This is the force which sticks protons and neutrons together in an
atomic nucleus. So began the epoch of 鈥渘ucleosynthesis鈥, in which light
atomic nuclei were made. These included helium, the second lightest
atom in nature with two protons and two neutrons, together with other light
elements such as lithium.
However, the process of element building was short-lived. To make even
heavier elements, such as carbon and oxygen, higher temperatures were
needed, but the Universe was cooling rapidly as it expanded. The Universe鈥檚
heavy elements would be created much later, in the nuclear furnaces deep
inside stars.
After the formation of helium any neutrons that were left over became
protons 鈥 solitary neutrons decay in about 11 minutes. According to
physicists鈥 calculations, roughly 10 protons were left over for every helium
nucleus that formed, making 25 per cent of the mass of the Universe helium.
Cosmic ripples
Oldest fossil
BY ABOUT half an hour after the big bang, almost all the electrons and
positrons had annihilated each other to form photons. But a slight
lopsidedness in the laws of physics had ensured that early on slightly more
electrons had been created than positrons. This meant that, at the end of
the annihilation, there was a tiny remnant of electrons. For every proton
and neutron in the Universe there were roughly a billion photons, a ratio
which persists to this day. The next significant event in the early history of
the Universe was the formation of atoms.
About 300 000 years after the big bang, when the temperature of the
Universe had dropped to about 3000 degrees, roughly the temperature of the
surface of the Sun, it was finally cool enough for atoms to form. Nuclei of
hydrogen and helium promptly mopped up all the Universe鈥檚 free electrons.
This had a dramatic consequence because free electrons had been very good at
scattering, or redirecting, photons.
Until this 鈥渆poch of last scattering鈥, photons had zigzagged their way
across the Universe, constantly ricocheting off electrons. But afterwards,
photons were suddenly free to fly across space unhindered. Today, we
can see those very same photons as the cosmic background radiation, greatly
cooled by the expansion of the Universe in the past 15 billion years.
The significance of the epoch of last scattering is that it signalled the
moment when matter and radiation went their separate ways. Until that time,
photons had blasted apart particles of matter as fast as they could come
together. Only when matter was finally freed from the tyranny of
radiation, could it begin clumping under gravity to form galaxies.
Incredibly, we can glimpse the beginning of this process of galaxy
formation by looking closely at the cosmic background radiation, the oldest
鈥渇ossil鈥 in creation. At first glance, it appears to be the same temperature
everywhere in the sky: 2.726 Kelvin. But, in 1992, NASA鈥檚 COBE satellite
detected 鈥渞ipples鈥 鈥 regions of the sky where it was slightly hotter than
average and regions where it was slightly colder.
The difference was tiny 鈥 only a few parts in 100 000 鈥 but by mapping
the hot spots and cold spots COBE produced a 鈥渂aby photo鈥 of the Universe
when it was just 300 000 years old (see Figure on p1). The cold spots
corresponded to regions of the Universe that were slightly denser than
average. They were the 鈥渟eeds鈥 from which have grown the great collection of
galaxies in today鈥檚 Universe.
Since COBE鈥檚 discovery, many scientists using telescopes lifted high into
the atmosphere by balloons, have reported finding smaller hotspots and
coldspots in the cosmic background radiation. None of the reports have yet
been confirmed. But the hope is that these 鈥渕iniripples鈥 are the seeds from
which individial galaxies, such as our own Milky Way, formed.
1: What happened before the big bang? Inflation
ACCORDING to a popular but very speculative theory, first proposed by the
American physicist Alan Guth, when the Universe was about 3 脳 10
-44 seconds it began to expand exponentially fast. This so-called
inflationary epoch ended at about 3脳10
-35 seconds. Afterwards, the Universe continued to expand but at a
rate which was enormously slower. It is this slower expansion which we have
come to call the big bang.
So violent was inflation that some have likened the big bang to a hand
grenade going off in the middle of a nuclear explosion. During inflation,
space blew up, or 鈥渋nflated鈥, until it was 10
50 times bigger than it had been. The entire observable Universe,
with all its stars and galaxies, would in this picture have inflated from a
region of space smaller than an atom. Incredibly, the energy that drove this
enormous expansion came from the vacuum itself 鈥 empty space.
Inflation is a bizarre theory, which is almost impossible to understand
unless you are a particle physicist! It is an outgrowth of our modern picture
of the vacuum. Far from being empty, the vacuum is now known to be seething
with 鈥渜uantum fluctuations鈥, subatomic particles and their antiparticles which
the Heisenberg uncertainty principle allows to appear out of nothing and
annihilate each other in the blink of an eye.
The principle arises because the act of measuring the energy of a
microscopic entity is intrusive, changing the energy in an unpredictable way.
So, the energy of the vacuum fluctuates and this energy can create particles
because of E = mc2.
The 鈥渧irtual鈥 particles of the vacuum have real and peculiar consequences.
Not only do they create a small mass density even in empty space but
associated with this is a 鈥渘egative pressure鈥, a sort of tension in space. In
normal circumstances, this pressure is miniscule but when the Universe was
3脳10-44 seconds old, and the density of both matter and the
vacuum was 1094 grams per cubic centimetres, a very special
鈥渧acuum-like state鈥 was created in which the negative pressure was
overwhelming. It manifested itself as a kind of gravitation repulsion. This
provided the enormous 鈥減ush鈥 which started the expansion of the Universe.
But the vacuum was unstable. Like an atom in an excited state, it wanted
to return to the ground state. After about 3脳10-35 seconds,
it decayed into the familiar vacuum we know today. The consequences of this
were dramatic. During its exponential expansion, the density and pressure had
remained constant 鈥 another bizarre property of the special vacuum state. This
meant there was an enormous amount of energy available. When the vacuum
decayed, the energy went into the creation of ordinary particles moving near
the speed of light. The Universe became hot, its temperature leaping to 10
27 K. This was the hot big bang.
Only a tiny 鈥渟eed鈥 of matter was needed in the Universe before inflation.
The balance was created from the vacuum. This has led to the suggestion that
the seed was a quantum fluctuation, making, as Guth put it, the Universe the
鈥渦ltimate free lunch鈥.
As for what happened before inflation, here we are into even more
speculative realms. In some inflationary scenarios, the observable Universe is
no more than a bubble inflating in one corner of a greater universe.
Elsewhere, forever inaccessible to us, may be an infinity of other expanding
bubbles like the froth on a great sea.
The principal problem that inflation solves is the so-called horizon
problem. This is that the temperature of the cosmic background radiation is
virtually the same in regions of the sky which could not have been in contact
at the epoch of last scattering. They were beyond each other鈥檚 鈥渉orizon鈥.
Inflation solves this problem because our entire Universe inflated from a
miniscule region of space, different portions of which could easily have been
in touch.
2: The future of the Universe
THE WAY in which the Universe evolves henceforth will depend on how much
matter it contains, because the combined gravity of all the galaxies is
constantly acting to brake its headlong expansion. If the Universe contains
sufficient mass, then sooner or later the expansion will be slowed to a halt
and reversed. Then all of creation will embark on a runaway collapse down to a
鈥渂ig crunch鈥, a sort of mirror image of the big bang. However, if the Universe
contains insufficient mass to halt its expansion, it may grow in size forever
until the burnt out hulks of galaxies become isolated islands in an infinite
ocean of space.
Some scientists have even speculated that we might in fact be living in an
oscillating or bouncing, universe. Each big crunch would then be followed by
another big bang. Like a giant beating heart, the Universe may have been
swelling and contracting throughout eternity.
So, will the Universe expand forever or will it eventually collapse once
more in a big crunch? Unfortunately, no one is sure. When astronomers add up
the amount of matter in all the visible stars and galaxies, they find that it
is woefully inadequate to stop the expansion. But clusters of galaxies are
known to contain large quantities of invisible matter, which reveals itself
only when its gravity tugs at the visible galaxies.
No one knows what makes up the 鈥渄ark matter鈥 but it may account for
between 90 and 99 per cent of the mass of the Universe. If it is nearer the
upper limit than the lower one, then the Universe may indeed collapse again
one day. The jury is still out on this one.
3. The moment of creation itself
IF WE run the expansion of the Universe backwards to the moment of
creation itself, we find that the Universe was compressed into an impossibly
small volume, was infinitely dense and infinitely hot. In mathematical jargon,
the Universe was a singularity. But singularities are a disaster in any
physical theory. They are a warning that we have gone terribly wrong with our
description of nature.
We need a better theory. And that theory is a 鈥渜uantum鈥 theory of gravity.
Quantum theories have been hugely successful in describing the other three
forces of nature 鈥 the electromagnetic, weak and strong nuclear forces (see
Inside Sciences Nos. 15 and 17) 鈥 so there is great confidence that they will
be successful in describing gravity as well. But whereas quantum theories of
the other three forces can be tested in particle accelerators, this is
impossible with gravity. The reason is quite simply that gravity is much
weaker.
However, experiments in particle accelerators have suggested that the
forces of nature are merely different facets of a single 鈥渟uperforce鈥. As the
temperature of the Universe fell after the big bang, the forces 鈥渇roze out鈥
from this superforce, one at a time. The belief is that when the Universe was
only 10-43 seconds old all the forces were one, including gravity,
hence the birth of the Universe cannot be understood without an adequate
theory of quantum gravity.
In any quantum theory, the idea of an exact position in space is
jettisoned because of the Heisenberg uncertainy principle. The implication is
that if the Universe were to be run backwards in time it would never quite
reach the stage when all of creation was compressed into a single point.
Something would happen to prevent the formation of a singularity. However, its
impossible to say what without a quantum theory of gravity.
Further reading
The First Three Minutes, by Steven Weinberg (Fontana). Afterglow of
Creation, by Marcus Chown (Arrow). In Search of the Big Bang, by John Gribbin
(Corgi). The New Physics, edited by Paul Davis (Cambridge UP).