The Big Bang
How did the universe begin? Were we
created by some higher being, or is our entire universe in all it’s beauty a
fluke of evolution? These are questions
humans have been attempting to answer for all humanity. Many people
disagree on what happened in the beginning moments of our universe. Some
believe that God created the universe with a divine hand, other say that the
universe evolved after a massive explosion.
We may never be able to prove beyond a doubt how the universe came into
existence, but science can at least point us in the right direction. The
most widely accepted theory in science today dealing with how the universe came
into existence is the big bang theory, specifically the "hot big
bang" model. The big bang theory tells us that millions of
years ago there was a point when the universe had zero size and was infinitely
hot. The big bang was a giant explosion that caused the matter in the
universe to expand and cool down, eventually forming the planets and stars we
see today. It was at the big bang that time itself began. What
proof do we have that suggests that the big bang is a probable theory?
Redshift
All light gives off
electromagnetic radiation. This electromagnetic radiation travels
through space as a wave, with a wavelength and frequency. On the
spectrum waves with the lowest frequency are red, waves with the highest
frequency are blue. 
The Doppler effect tells us that waves
propagate out from a stationary source of light at a constant frequency.
When the light source (a star or galaxy, in this case) is moving away
from us, the frequency of the light is lower.
In 1929, using these principles, Edwin Hubble demonstrated that the
universe is expanding. He observed that
light from the distant parts of the galaxies was not at the frequency he knew
it should be. Instead, it was shifted towards the red end of the spectrum when
compared to the wavelengths of the nearer galaxies. This shift in
wavelength is called the redshift. This means that the frequency of the
light is lower than expected, therefore these galaxies are moving away from
us. It has also been shown that the
further a galaxy is away from us, the faster it is moving. The expanding
universe is just one of the reasons science points to the big bang.
Another strong proof is the comic microwave background radiation.
Cosmic Microwave Background Radiation
The universe is filled
with microwaves, or short wavelength radio waves. In 1965 Penzias and
Wilson were making a series of measurements with a radio telescope when they
discovered background radio noise that appeared to be independent of direction.
Initially, there was much uncertainty as to the origin of this microwave
radiation. But Robert Dicke, another physicist, realized that the background
radiation provided clues to the origin of the universe and was able to begin to
put the pieces together.
Years earlier George Gamow had predicted that
radiation should be left over from the big bang, currently at a temperature of
about 10 degrees Kelvin, just above absolute zero. Until Penzias and
Wilson’s discovery, this idea had all but been forgotten about.
Experiments showed that the cosmic microwave background radiation found
by Penzias and Wilson was highly uniform, more than one part in a thousand.
If this radiation was from a nearby star or galaxy it would not be evenly
distributed throughout the universe at it was. Since it was so uniform we
can assert that it came from the most distant parts of the universe and is left
over from the big bang. The microwaves
corresponds to a characteristic temperature of about three degrees Kelvin,
close to what Gamow predicted. The
cosmic microwave background radiation is one of the best arguments for the big
bang.
Helium
The existence and
amount of helium in the universe today also points scientists to the
possibility of a big bang at some point in the past. Today the universe consists of about 88.6% hydrogen and 11.3%
helium. The rest of the elements are all heavier than helium and are
formed when trace impurities are added to a mixture of helium and hydrogen.
In 1957 it was shown that all elements heavier than helium could be built
up inside of stars and other explosions. So where did helium come from?
Scientists argue that helium could have only come from the big bang for
two basic reasons. The first is that older stars contain no less helium
than younger stars. The second reason is that a large amount of energy is
required to form helium, if this much energy were present things would
currently be different in the universe.
For example, the stars would shine brighter than they do. The
redshift, cosmic background radiation, and helium in the universe give us
reason to believe that the big bang is a possible answer to the problem of the
beginning of the universe. Before we can take a deeper look at the big
bang itself and what happened shortly afterwards, we need to understand the
particles that were involved in the early stages of the big bang.
For many years, the atom was considered to be the fundamental building block of
all matter. By combining with other atoms to form molecules, the atom
makes up everything in the universe. 
But in 1911, Rutherford discovered
that the atom had a nucleus. This caused scientists to realize that the
atom itself could be divided into smaller parts and is not the smallest
building block of life. Later, the electron with its negative
charge was discovered to be part of the atom, as well as the positively charged
proton and the zero charge neutron. These particles were considered the
most basic particles until the 1960's when once again even smaller particles,
called quarks were discovered to make up the proton and neutron. Scientists now know that all particles in
the universe are composed of either quarks or leptons, called elementary
particles.
Quarks
Quarks come in pairs: up and
down, strange and charm, bottom and top. Protons and neutrons are
composed of three quarks each. A proton has two ups and a down (u,u,d).
A neutron made up of the opposite: two downs and an up (d,d,u).
Since protons have a positive charge and neutrons zero charge, the quarks
themselves must have individual charges to make the larger particles
charged. By solving for “u” and “d” in
these equations:
Proton = u + u + d = +1
Neutron = d + d + u = 0
we find that the charge on an up quark is +2/3, while the charge on a
down quark is -1/3. The other quarks also have the same charges -
up, charm, and top quarks all have a charge of +2/3. Down, strange, and
bottom quarks all have a charge of -1/3. Quarks are grouped into
generations based on how they respond to the fundamental forces. The
quarks in the first generation (up and down) are the most common in the
universe because these are the quarks that make up the nucleus in the atom.
The mass of the quarks increase as you move across the generations.
The second and third generation quarks are much more rare because of
their higher masses. For these quarks to be created, particles must
collide at high velocities. These second and third generation quarks were
much more common in the earlier universe, because matter was more energetic it
traveled at higher velocities, yielding more of these heavier quarks to be
produced in collisions. 
Leptons
As well as the
elementary particles called quarks, there are six kinds of leptons. The
electron is a well-known lepton and is very similar to two other leptons, the
muon and the tau. Essentially the only things that distinguish an
electron from a muon or a tau is it's mass. Both particles have the same
charge as an electron and respond to the fundamental forces in the same way.
But unlike the electron, the muon and the tau are allowed to decay
into other particles whereas the electron is a stable particle. The other
three leptons are called neutrinos because they are electrically neutral.
This is different from the zero charge on a neutron - the neutron is made
up of fundamental particles with charges adding up to zero, but the neutrinos
themselves have no charge and are called “genuinely neutral”.
An Elementary Particle Example: The electron
The electron is one
example of a lepton, it is the earliest known elementary particle.
In 1891, George Stoney named the theoretical unit that would link
electricity and matter the electron. In 1897, through a series of cathode
ray tube experiments, J.J. Thomson actually discovered the electron. The
electron has a mass of 9x10-31 kilograms, the same mass as one
million millionth of a spec of dust. The electron has a charge of 1.6x10-19
Coulombs. The radius of the electron is so small that it cannot be
measured, although it is known to be less than 10-18 meters in
radius. In fact, the electron's properties are consistent with a radius
equal to zero! It is known that
electrons can only exist in certain discrete energy levels in an atom.
DeBroglie concluded that electrons act not only like a particle but also
as a wave. This is called wave-particle duality. The implications
of wave-particle duality are astonishing. Because a wave spreads through
space, we cannot know the exact velocity and location of a particle at the same
time.
Electrons, as well as all
other elementary particles display rotational motion, or spin. The spin
of a particle shows us what the particle looks like when viewed from different
directions. If a particle has a spin 0 it looks the same from all
directions; it is like a dot. A particle with a spin 1 is like an arrow,
it looks the same only if rotated 360 degrees. A particle with a spin 2
is like a double-headed arrow, it looks the same after every 180-degree
rotation. Some particles (electrons for example) look the same only after
rotating them through two complete revolutions or 720 degrees. These
particles are said to have a spin 1/2.
Now that we have a basic understanding of the elementary particles that
played a role in the big bang, we need to understand the fundamental forces
that allow these particles to interact.
Fundamental Forces
There are many
different forces in our world today, but all of them can be summed up as a
combination of the four fundamental forces. These four are gravity, the
electromagnetic force, the strong force, and the weak force. Gravity is a
familiar force. This is the force that works only between massive
objects. It has no role at the microscopic level. But it effects
essentially everything that has mass. Gravity is also the weakest of the
four forces. The electromagnetic force is also a fairly well known force.
The electromagnetic force is the force that holds electrons to the
nucleus. This force exists only between charges. The
strong force is what causes quarks to stick together. The strong force
allows protons and neutrons to form. The range of the strong force is
only 10 -15 meters, or approximately the diameter of an atom.
This force allows the nucleus to resist being blown apart by
electrostatic repulsion. The final of the four fundamental forces is the
weak force. The weak force is the most difficult to understand. It
facilitates the decay of heavy particles into smaller particles. The
range of the weak force is 10 -17 m. The weak force changes
particles from one type to another. Leptons can only be changed to other
leptons of the same generation. For example, an electron could be changed
to an electron neutrino, or a muon neutrino to a muon. But in quarks, the
weak force not only changes the quarks within generations, but also across
generations. For example a bottom quark could be changed to
an up quark or a top quark.
Characteristics of the Universe
Gases are made up of
particles moving at a variety of speeds.
The temperature of a gas can be defined by the way the particles are
colliding with one another. The particles interact with one another in
certain ways because of the different fundamental forces that control them.
But the particles interact in such a way that the average energy of the
gas is always constant. This is called thermal equilibrium. Our
universe is in thermal equilibrium.
Also, each particle in the universe has a threshold temperature.
When the temperature of the universe drops below this threshold
temperature for the particle, these particles disappear from the universe.
They will be annihilated due to particle/antiparticle collisions faster
than they will be produced. But the energy of the universe will remain
constant.
The Big Bang Itself
The
point in time just before the big bang is called singularity; this is the point
where the laws of physics breaks down. At this point the universe
was zero in size and infinitely hot. Because all of our laws of physics break
down at singularity, scientists cannot possibly understand what caused the big
bang to happen. The moment after the
big bang happened, the universe began to expand and the matter and radiation in
the universe began to cool. In fact, when the size of the universe
doubles, the temperature falls by one half.
At 10 –43
seconds after the big bang, the universe was at 10 33 degrees
Kelvin. At this point all four
fundamental forces were equal in strength.
The universe was filled with quarks and leptons in approximately equal
numbers. The universe continued to
expand and cool. At 10 –35
seconds after the big bang, the electromagnetic and weak forces were equal in
strength, gravity and the strong force were weaker. As time continued, the universe is expanded enough for quarks
and anitquarks to bond, which means that protons and neutrons are forming. At 7x10 –7 seconds after the big
band the universe is at a temperature of about 1013 Kelvin. This is below the threshold for protons and
neutrons, so these particles stop being created. But the remaining protons and neutrons are kept in thermal
equilibrium with the rest of the particles by neutrino interactions. One
hundred seconds after the big bang, the universe was at one thousand million
degrees. At this point, protons and neutrons could no longer escape the
strong nuclear force so they would start to come together to form deuterium
(heavy hydrogen). The earliest stars condensed from this deuterium. Finally, about 3.2 minutes after the big
bang occurred, the deuterium begins to interact with other particles to produce
helium.
D + D → ³He + n
³He + n → ³H + p
³H + d → 4He + n
Reactions similar to these continue to occur
as hydrogen and helium combine to produce the heavier elements. As time
continued, the deuterium combined with more protons to produce other elements,
especially helium. A few hours later, the production of helium stopped.
Over the next million years the universe continued to expand without much
other major activity.
The
big bang theory gives us a probable explanation for how the universe came into
existence. The amount of information we
can learn about things that happened so long ago is incredible. Of course, we still don’t know for sure
whether the big bang is even the correct theory of how the universe came to be.
We many never know. And
even if we can prove that the big bang is correct, scientists still have no
idea how or why the big bang actually happened. Stephen Hawking, in his book A Brief History of Time
writes, “It would be very difficult to explain why the universe should have
begun in just this way, except as the act of a God who intended to create
beings like us.”
Works
Cited
Allday, Jonathan. Quarks, Leptons and the
Big Bang. Philadelphia, PA: Institute of Physics Publishing. 1998.
Fermi National Accelerator Laboratory. What is the world made of? http://www.fnal.gov/pub/inquiring/matter/madeof/index.html Internet.
February 28, 2001.
Greenwood, N.N. and A. Earnshaw. Chemistry of the Elements.
Pergamon Press: Tarringtown, New York. 1984.
Hawking, Stephen W. A Brief History of Time. New York, New
York: Bantam Books. 1988.
NSSDC Photo Gallery. Galaxies and
Globular Clusters. http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-astro-galaxy.html Internet.
May 31, 2001
Parker, Barry. Creation: The Story of the Origin and
Evolution of the Universe. New York. Plenum Press. 1988.
The Particle Data Group. The Particle Adventure. http://www.particleadventure.org/particleadventure/frameless/index.html
Internet. 2000.
Silk, Joseph. The Big Bang: The Creation and Evolution
of the Universe. San Francisco. W. H. Freeman and Company.
1980.
Science Museum and Institute of Physics. Life, the Universe, and the
Electron. http://www.iop.org/Physics/Electron/Exhibition/
Internet. 1997.
Diagrams
(1) Science Museum and Institute of Physics
(2) The Particle Data Group
(3) The Particle Data Group
(4) NSSDC Photo Gallery